Design of nano-sized model systems for multi

Transcription

Design of Nano-Sized Model
Systems for Multi-Functional Materials
with Hierarchical Structure
Dissertation
zur Erlangung des Grades eines Doktors der Naturwissenschaften
Vorgelegt der Fakultät für Chemie und
Biochemie der Ruhr-Universität Bochum
von
Carolina Neudeck
aus Wermelskirchen
Bochum 2014
Die vorliegende Arbeit wurde in der Zeit von August 2010 bis Mai 2014 am Max-Planck-Institut
für Kohlenforschung in Mülheim an der Ruhr unter der Leitung von Herrn Prof. Dr. Ferdi Schüth
angefertigt.
Referent: Prof. Dr. Ferdi Schüth
Korreferent: Prof. Dr. Wolfgang Grünert
Meinen Eltern
und Herrn Dr. Sackmann
in tiefer Dankbarkeit.
Acknowledgements:
I would like to express my sincerest gratitude to my supervisor Prof. Dr. Ferdi Schüth. Only
based on the trust he had in me and the scientific freedom he granted me, it was possible for
me to take the steps that brought me to the point I am now. The time I spent as a Ph.D.
student in his group was truly inspiring and tremendously important to my development on a
professional and personal level.
I sincerely thank Prof. Dr. Wolfgang Grünert for taking an interest in my research and being the
second referee.
I would like to acknowledge the essential support of Dr. Wolfgang Schmidt throughout my
time in Mülheim. I am deeply grateful for his help and advice on synthetic challenges, making
sense of analytical results, proof reading parts of my thesis and beyond.
I am deeply thankful to Dr. Claudia Weidenthaler, Jan Ternieden and Ulli Holle. Their generous
help and support in all X-ray-related regards was essential to this work.
Furthermore I would like to express my gratitude to Axel Dreier, Hans Bongart, Bernd Spliethoff
and Silvia Palm for their help in visualizing the numerous synthesized materials and their
generous support throughout my time at the MPI.
I am very grateful to Prof. Krijn de Jong and Dr. Marianna Casavola for sharing my enthusiasm
for the Fischer-Tropsch reaction and for a highly interesting collaboration.
I thank Dr. Eckhart Bill and Bernd Mienert for their help with with Mößbauer spectroscopy and
many helpful discussions on nano-sized iron.
I am thankful to Dr. Harun Tüysüz for proof reading parts of my thesis and for sharing his
scientific experience.
A special thanks goes to Sandra Kestermann for her support of my work and a wonderful time
spent in the lab. Furthermore the support of Andre Pommerin and Laila Sahraoui cannot be
left unnoticed: I thank both for their every-single-day support in the lab and for always having
a friendly ear for any small Ph.D.-student-problem.
I would like to thank all of my colleagues and friends from the AK Schüth for making the time
at the MPI truly unforgettable. I especially want to thank my sister Ivy Lim, Julia vom Stein,
Tobias Zimmermann, Mano Passas-Lagos, and Dr. Hequing Jiang for a wonderful time in our
office and being such wonderful friends. Additionally I want to thank Dr. Felix Richter for
countless moments of true understanding – on the golf course and beyond. I want to express
my sincerest gratitude to Dr. Kameh Tajvidi for being such a good friend and for being my
kindred spirit. Finally I thank Jean Pascal Schulte for the great time as neighbours in the lab and
for being such a reliable friend.
I sincerely thank Markus Neumann and Dr. Robin Kolvenbach for the wonderful time we had in
Munich and ever since. Their essential support and friendship helped me to keep my sanity
during my studies and especially over the last years.
My final thanks are addressed to my family:
My gratitude towards my parents is beyond words- their support and trust made every single
step I took possible. I would like to thank my sister Christina for her encouragement and
guidance throughout my life.
Finally I thank Andreas, Ulrike and Friedhelm for their ever loving support during times of
struggle and for having their share in making the extraordinary Neudeck/Becker-clan
extraordinary!
Abstract
Abstract
The focus of the studies described in the following is the development of variable model
systems of functional materials in the nano-range with a hierarchical structure. Many
examples prove the beneficial influence of specific organisation and structure on the
properties of composite materials applied in various application fields ranging from catalysis to
sensing and biomedical applications. Central to the introduced systems is the development of
synthetic concepts with which the organisation of composite materials can be determined
while at the same time allowing versatility regarding the chemical composition of the final
materials. Yet, the developed preparation approaches vary greatly in tackling this challenge.
In the 1st developed model system the requirement of hierarchical order is met by the coreshell structure of the materials. Core-shell nanoparticles consisting of an Fe2O3 core and a SiO2
shell are the starting point of several modification reactions with which a variety of core and
shell materials can be combined. The initial Fe2O3@SiO2 nanoparticles are chosen as the
starting point because their preparation pathway is straightforward and allows for easy
changes in the particles’ properties. In order to modify the shell material of the particles, the
initial SiO2 shell is complemented with another layer consisting of TiO2 or ZrO2. After the SiO2
template is removed by leaching, nanoparticles with a yolk-shell structure are obtained with a
Fe2O3 nanoparticle in the core of the surrounding TiO2 or ZrO2 shell. The chemical nature of the
core material can be varied, as well. The initial Fe2O3 core can be replaced by small noble metal
nanoparticles in a metal exchange reaction. Core-shell nanoparticles consisting of a SiO2 shell
encapsulating noble metal nanoparticles like palladium, platinum, rhodium and ruthenium can
be prepared following this approach based on the initial Fe2O3@SiO2 nanoparticles.
The great advantage of the developed system is the possibility to prepare core-shell
nanoparticles with various chemical compositions under very similar conditions for the
application in heterogeneous catalysis. Many studies prove the influence of the preparation
pathway on the catalytic activity, making the comparison of the catalytic results of different
materials very problematic.
In the 2nd part of this work, transition metal oxide and noble metal nanoparticles are
incorporated in hierarchical zeolite crystals. A widely-used approach to prepare composite
materials consisting of zeolite crystals and nanoparticles is the impregnation technique.
Compared to conventional zeolites, hierarchical zeolites allow a better distribution of
nanoparticles and other species, and are additionally distinguished by improved mass
transport properties. Consequently, hierarchical zeolite crystals are ideal candidates for the
preparation of composite catalysts. At low metal and metal oxide loadings the impregnation of
hierarchical zeolites is an easy and straightforward method to prepare composite materials.
I
Abstract
For the incorporation of Fe2O3 nanoparticles in zeolite crystals a novel synthetic approach was
developed which is based on the impregnation of carbon templates. The successful
incorporation of Fe2O3 nanoparticles is facilitated by the preparation of Fe2O3@C nanoparticles
that are used as template in the preparation of hierarchical zeolites.
The synthesis of Fe2O3@SiO2 nanoparticles is modified to prepare Fe2O3@C nanoparticles in a
nanocasting procedure: the 1st SiO2 layer is used as an anchor point for the addition of a
mesoporous SiO2 shell which is used as a template for the impregnation with a monomer
mixture. The monomer is polymerised within the pores of the porous SiO2 template and
carbonised. After the SiO2 template is removed, the Fe2O3@C nanoparticles are used as
template in the preparation of hierarchical zeolites. The zeolite crystals grow around the
carbon shell during the hydrothermal treatment, and after the calcination step, Fe2O3
nanoparticles are left within the zeolite crystals. Fe2O3 loadings of up to 55 wt% can be realised
with this novel preparation concept. In addition, the acidity of the zeolite matrix can be easily
tuned by changing the Si/Al ratio. The possibility of changing both, the transition metal content
and the acidity of the zeolite phase, leads to highly versatile composite materials that can be
obtained by this method.
Furthermore, the concept of encapsulating transition metal oxide nanoparticles in carbon
shells for the successful incorporation of nanoparticles in zeolite crystals can be applied to
various systems. The synthesis was developed in a way that a variety of nanoparticles can be
incorporated into zeolite crystals, leading to possibly numerous novel composite materials.
II
Content
Content
1 Introduction ___________________________________________________________________ 1
1.1 Towards the rational design of functional materials in the nano-range _________________ 1
1.2 Hierarchical materials in nature ________________________________________________ 2
1.3 Hierarchical materials in chemistry _____________________________________________ 4
2 Model Core-Shell System with Variable Core and Shell Material _________________________ 7
2.1 Introduction and motivation ___________________________________________________ 7
2.2 State of the art _____________________________________________________________ 9
2.2.1 Metal and metal oxide nanoparticles ________________________________________ 9
2.2.1.1 Properties of nano-sized materials _____________________________________ 11
2.2.1.2 Preparation pathways for nanoparticles _________________________________ 15
2.2.2 Core-shell materials _____________________________________________________ 21
2.2.2.1 Properties and application as catalysts __________________________________ 23
2.2.2.2 Preparation principles of catalysts with a core-shell structure ________________ 26
2.3 Synthetic strategy __________________________________________________________ 32
2.4 Results and discussion ______________________________________________________ 34
2.4.1 Preparation of Fe2O3@SiO2 nanoparticles as starting point of model system ________ 34
2.4.2 Variation of shell material ________________________________________________ 42
2.4.3 Variation of core metal via metal exchange __________________________________ 49
2.4.4 Critical assessment of metal exchange reaction _______________________________ 54
2.5 Summary and conclusion ____________________________________________________ 61
3 Modification of Hierarchical Zeolites ______________________________________________ 64
3.1 Introduction and motivation __________________________________________________ 64
3.2 State of the art ____________________________________________________________ 65
3.2.1 Zeolites in industry and catalysis ___________________________________________ 65
3.2.1.1 Properties of zeolite materials _________________________________________ 67
3.2.1.2 Synthetic principles for the preparation of zeolites _________________________ 71
3.2.1.3 Industrial applications of zeolites _______________________________________ 73
3.2.2 Hierarchical zeolites_____________________________________________________ 77
3.2.2.1 Removal of framework atoms _________________________________________ 80
3.2.2.2 Hard-templating methods ____________________________________________ 84
3.2.2.3 Supramolecular templating ___________________________________________ 89
3.2.3 Multi-functional zeolitic materials__________________________________________ 94
3.2.3.1 Modification of zeolite crystals ________________________________________ 96
3.2.3.2 Incorporation during zeolite crystallisation _______________________________ 98
3.2.3.3 Fields of application of bi-functional catalysts ____________________________ 102
i
Content
3.2.3.4 Fischer- Tropsch reaction and its industrial relevance ______________________ 107
3.3 Synthetic strategy _________________________________________________________ 110
3.3.1 Synthetic strategy for the incorporation of transition metal oxides in zeolites ______ 110
3.3.2 Impregnation of zeolites as versatile strategy to prepare composite materials ______ 113
3.4 Results and discussion _____________________________________________________ 115
3.4.1 Pre-synthetic modification of hierarchical zeolites ____________________________ 115
3.4.1.1 Synthesis of Fe2O3@mpSiO2 nanoparticles ______________________________ 115
3.4.1.2 Preparation and characterisation of Fe2O3@C nanoparticles ________________ 122
3.4.1.3 Iron oxide containing ZSM-5 crystals ___________________________________ 128
3.4.1.4 Deactivation of iron-based FT catalysts with core-shell geometry ____________ 141
3.4.2 Post-synthetic modification of hierarchical zeolites ___________________________ 148
3.4.2.1 Modification with noble metal nanoparticles ____________________________ 148
3.4.2.2 Post-synthetic addition of transition metal oxides ________________________ 154
3.5 Summary and conclusion ___________________________________________________ 158
4 Final Remarks _______________________________________________________________ 161
5 Experimental ________________________________________________________________ 164
5.1 Chemicals _______________________________________________________________ 164
5.2 Syntheses _______________________________________________________________ 164
5.2.1 Preparation of Fe2O3 nanoparticles ________________________________________ 164
5.2.2 Preparation of Fe2O3@SiO2 nanoparticles___________________________________ 165
5.2.3 Preparation of Fe2O3@TiO2 nanoparticles __________________________________ 166
5.2.4 Preparation of Fe2O3@ZrO2 nanoparticles __________________________________ 166
5.2.5 Metal exchange _______________________________________________________ 166
5.2.6 Preparation of Fe2O3@SiO2@mpSiO2 ______________________________________ 167
5.2.7 Preparation of Fe2O3@C nanoparticles _____________________________________ 168
5.2.8 Preparation of Fe2O3 containing ZSM-5_____________________________________ 168
5.2.9 Synthesis of hierarchical zeolites using carbon templates ______________________ 169
5.2.10 Post-synthetic modification of hierarchical zeolites __________________________ 169
5.2.11 Activity and deactivation processes of iron oxide based FT catalysts _____________ 170
5.2.11.1 Preparation of catalysts ____________________________________________ 170
5.2.11.2 Catalytic testing __________________________________________________ 170
5.3 Characterisation techniques _________________________________________________ 170
6 Appendix ___________________________________________________________________ 173
6.1 List of Figures ____________________________________________________________ 173
6.2 Sample references_________________________________________________________ 179
6.3 List of Tables _____________________________________________________________ 181
6.4 Publications and contributions to conferences ___________________________________ 181
7 References __________________________________________________________________ 182
ii
1 Introduction
1 Introduction
1.1 Towards the rational design of functional materials in the nano-range
Many of the challenges that humankind has to face will only be mastered by utilising highly
functionalised materials with finely tuned properties in the nano-range. Independent of
whether it is the world’s energy supply, the conversion of toxic exhaust gases, the construction
of energy harvesting buildings, or adjusting the texture of ketchup – functional nanomaterials
will be an integral part of the solution due to their unique properties. The soaring scientific
interest in nano-sized materials began in the 1990s, leading to today’s successful application of
nanoparticles in numerous fields, such as life science, electronics and environmental science.
Even though nanomaterial sciences are still a young and continuously evolving field, the great
potential of materials in the nano- range was predicted already over 50 years ago. Richard P.
Feynman emphasized the importance of establishing techniques that would allow the
preparation and characterisation of materials of a very small size and combining the
knowledge of physics, chemistry and biology. He used materials found in nature as powerful
examples for the potential of nanoparticles: “A biological system can be exceedingly small.
Many of the cells are very tiny, but they are very active; they manufacture various substances;
they walk around; they wiggle; and they do all kinds of marvellous things – all on a very small
scale. Also, they store information. Consider the possibility that we too can make a thing very
small, which does what we want – that we can manufacture an object that manoeuvres at that
level!”[1] Nature is indeed a great pool of inspiration for material scientists for its precise
structures on a very small scale and tailored functions (see Chapter 1.2) – as predicted by
Richard P. Feynman in the year 1959, only 6 years after the discovery of the double-helix
structure of DNA by Crick and Watson, and with the advancements of scanning probe
microscopy techniques still around 20 years ahead.
In order to meet the current and future demands of functional materials, nanocomposites
combining the properties of the parent constituents and generating new properties will be of
paramount importance. Apart from the chemical composition of the nanocomposites, their
properties strongly depend on their structure and organisation
[2]
. As observed in nature, the
assembly of small building blocks can be successfully used to create systems with a complex
structure. The full potential of functional nanocomposites, that is combining the properties of
the parent constituents and evoking novel properties, can only be realised by tailoring the
structure as well as the hierarchy of the nanoparticles (see Chapter 1.3). The components must
be in close proximity in order to have synergistic effects leading to novel properties. This only
becomes possible by developing synthetic approaches with which structure and hierarchy can
be introduced to the material over various length scales in a desired manner. Even more
1
1 Introduction
important is the development of synthetic pathways not just to a singular system, but to
model systems of various chemical compositions, particle sizes and shapes.
1.2 Hierarchical materials in nature
The art of preparing nanomaterials with unique structures and highly defined functions is not
merely a strictly human accomplishment: a vast number of composite materials with precise
structures and distinct functions can be found in nature. In many materials discovered in
nature the properties are directly dependent on the structure of the materials. The highly
specialised inorganic materials found in nature are mostly composite materials consisting of
inorganic minerals intricately combined with organic polymers
[3]
. Composition, organisation
and interfacial chemistry of the materials are optimised in order to meet the specific
requirements: in magnetotactic bacteria chains of magnetite nanoparticles are found that act
as nano-sized magnets. The alignment of nanoparticles in a single chain imparts a magnetic
dipole to the bacterial cell, allowing the cells to align with the Earth’s geomagnetic field (Figure
1.1, I and II)[4].
Figure 1.1: Structured crystals and minerals found in nature: Magnetotactic bacteria (I) synthesise chains of
magnetite nanoparticles of various shapes (II) that function as a compass needle
marine alga (III and IV)
[6-7]
[4-5]
. Calcite structures prepared by
[8]
. SEM image of nacre (V) .
The observed crystals often stand out because of their complex form bearing no resemblance
to the equilibrium form of the crystals (see Figure 1.1, III to V). Shape and texture of a material
determine properties like long-term stability, transport behaviour, separation efficiency, and
adhesion. The synthesis of crystals and minerals with the precise structure that is necessary to
2
1 Introduction
procure certain properties is only possible with the precise control of the crystallisation
conditions.
In order to control nucleation, crystal growth and organisation of materials, the reaction field
is highly regulated. Relatively simple inorganic minerals can be formed into complex
architectures
by
three
constructional
processes
occurring
in
biomineralisation [9].
Supramolecular pre-organisation is used to create an organised reaction environment: vesicles
form fluid-filled microenvironments restricted by a bilayer of lipid molecules, providing a mean
to control the size and location of the formation of minerals. The physical and chemical
properties of the microenvironment can be precisely controlled by ion-pumps leading to highly
specialised reaction fields for nucleation and crystal growth. Secondly, interfacial molecular
recognition leads to controlled nucleation and specific structuring of crystals. Pre-organised
structures with functional groups on the surface act as templates by enhancing the nucleation
of a specific crystal face. The 3rd involved constructional process in biomineralisation is cellular
processing: in the absence of cellular intervention the growth of minerals within the vesicle is
dependent on the regular crystallographic morphology. Cellular processing leads to the
formation of various textures and structures by dynamic shaping of the vesicles. The form of
the reaction vessel can be modelled into elongated shapes up to the formation of twodimensional networks
[10]
. Based on these principles an enormous variety of materials with
unique properties is formed in nature.
Figure 1.2: Examples for the application of bio-mimetic materials: surface structures inspired by lotus leaves (I) are
used in ceramics; the structure of nacre is mimicked to improve the strength of materials (II).
In Figure 1.2 two examples are given for materials possessing a unique structure, which is the
foundation of their sought-after properties. The leaves of the lotus plant are highly
hydrophobic and self-cleaning (Figure 1.2, I). The so-called lotus-effect is based on the
hierarchical surface structure of the leaves consisting of wax-covered papillae as shown in
3
1 Introduction
Figure 1.2 on the lower left side. The shape and density of the papillae reduce the contact area
between the leave surface and water drops leading to superhydrophobicity [11]. The analysis of
the unique structure of lotus leaves and other plants led to the development of self-cleaning
materials with various applications ranging from construction to clothing.
Nacre is a composite material consisting of small aragonite platelets and organic biopolymers
like chitin (Figure 1.2, II). A much sought-after characteristic is the high mechanical strength of
the CaCO3- based material: nacre exhibits the two times higher mechanical strength and a
1000-times higher toughness than its compositional phases [12]. The structure and organisation
of nacre is mimicked to enhance the mechanical stability of materials used in construction [13].
The preparation and application of biomimetic materials show that strategies for the synthesis
of highly functionalised inorganic materials can be deduced from principles observed in nature.
Even though the preparation of naturally occurring materials like nacre is highly challenging in
vitro, and the chemical composition of uniquely specialised materials in nature is rather
limited, general principles found in biomineralisation can be applied in the field of materials
chemistry. Molecular design and self-assembly can be powerful tools in the preparation of
multifunctional materials, which are based on the interplay between structure, organisation
and composition. Particularly the degree of organisation across a wide range of length scales
that is observed in materials prepared by biomineralisation make them archetypes for
synthetic strategies of highly functionalised materials. Various principles applied in the
preparation of functional materials in the lab are based on naturally occurring processes
including the use of templates and surfactants, or spatial confinement of reactions in
microemulsions.
1.3 Hierarchical materials in chemistry
Templating is an often used approach to impart hierarchy to materials which are not
obtainable by direct synthetic approaches. The principle of templating allows great control
over particle size, shape, and spatial arrangement, and thermodynamically unfavoured
structures can be obtained. Considered as template are structures within or around which a
material is formed whose morphological and/or stereochemical features are related to those
of the template even after its removal
[14]
. Two different types of templates can be
distinguished, and the classification is made according to their structural relationship to the
growing material. Templates that are fully occluded in the growing material are labelled
endotemplate (see Figure 1.3, Ia), whereas exotemplates contain pores and voids within which
the particle formation is occurring (see Figure 1.3, IIa) [15].
Endotemplates are incorporated in the growing material as single entities that leave behind
pores after removal of the template (Figure 1.3, Ib). In contrast, exotemplates form a scaffold
in which the formation of the material takes place. Depending on the connectivity of the
exotemplate the final product either contains pores (see Figure 1.3, IIb), or small particles are
obtained [15].
4
1 Introduction
Figure 1.3: Comparison of the endo- and exotemplating procedure for the preparation of porous solids: I) Scheme
illustrating the endotemplating approach (a)
[15]
[16]
and SEM picture of thusly prepared, porous TiO2
. II) Preparation
of porous materials using exotemplates (a) and carbon replica of SBA-15 with two pore systems (b)
[15]
.
All templating methods generally consist of three steps in order to exert the high degree of
synthetic control connected with these approaches: first, the template is prepared, and its
surface modified if necessary. Reaction precursors are mixed with the template in order to
synthesise the targeted material. In the last step, the final material is obtained after template
removal by leaching or calcination
[17]
. The field of suitable templates is very wide, and the
nature of the template can range from macroscopic objects to nanoparticles and
supramolecular assemblies. Diverse materials with fascinating structures were developed
based on templating approaches [18].
A well-established templating technique to prepare porous materials is nanocasting. The
synthesis parameters are tuned so the product can be considered a one-to-one replica of the
template that was used for the preparation
[15]
. One of the requirements of a successful
nanocasting process is a three-dimensionally connected, ordered template that is chemically
suitable for the formation of the desired material within its pores. After impregnation and
removal of the template the highly ordered and porous product is obtained [19].
Apart from hollow structures that are obtained upon removal of the template, also hybrid
structures can be prepared by established templating approaches. If the template remains
within the newly formed phase, a hybrid material is created combining the physical and
chemical properties of both components
[17]
. Following this concept, the diverse approaches
developed in the field of templating can be applied and adjusted to prepare bi- or
multifunctional materials. Due to the intimate contact between the phase acting as template
and the newly formed material, high dispersion and proximity of both phases can be realised
leading to novel properties based on synergistic effects. In addition, synthetic approaches to
5
1 Introduction
hybrid materials based on templating often have the advantage of leading to a high degree of
ordering.
Despite the fact that template-assisted syntheses are long known and applied in material
chemistry, the field is continuously growing, and still novel concepts are developed to prepare
hierarchical materials with unique properties [20]. Templating is influenced by diverse scientific
disciplines, such as bioinorganic chemistry and polymer science. Advances in nanoparticle
preparation supply a variety of building blocks with different sizes, shapes and functions that
can be applied as templates in the preparation of both porous and hybrid materials
[17]
. The
development of novel templating approaches is continuously stimulated by advances in these
separate fields as well as by the potential of novel applications of hierarchical materials.
6
2 Model Core-Shell System with Variable Core and Shell Material
2.1 Introduction and motivation
The principle introduced in the previous chapter that the structure and size of particles and
materials can greatly influence their properties, is central to the field of nanoparticles as well.
The research interest in nano-sized materials has been growing strongly over the last decade
and continues up to date, driven by their unique properties and potential of novel applications.
Figure 2.1: Comparative size scale over several orders of magnitude
[21-22]
.
The unique properties that are observed for nano-structured systems stem from the small
dimensions found in their structure. The prefix nano is of Greek origin meaning dwarf and is
used to describe the 10-9th value of the accompanied unit. The scale shown in Figure 2.1 ranges
7
from macroscopic objects of several millimetres to particles of only a few nanometres
emphasizing the different orders of magnitude encountered: nanoparticles are an order of
magnitude smaller than cells in the human body and in the same size range- yet still smallerthan most enzymes.
While the preparation and application of materials in the nano-range is a rather young
scientific field, many structures and materials known and applied in nature are of the same
size range (see Figure 2.1). The reason for the increased interest of the scientific community in
nanoparticles is based on their unique properties which are directly based on their small size
and structure (see Chapter 2.2.1.1).
Within the area of nano-sized materials, the class of materials with a core-shell structure has
been gaining much attention, due to their superior properties in various fields of application
(see Chapters 2.2.2.1 and 2.2.2.2). The fundamental idea of core-shell materials is to prepare a
composite material with a distinct hierarchical order: the core material is entirely encapsulated
by the 2nd constituent, creating a complete shell. In most cases the core material is still
accessible for reactants because of the porous nature of the shell. In other cases the shell
forms a protective layer in order to prevent chemical and physical changes of the nanoparticles
in the core.
Numerous approaches for the preparation of nanoparticles and core-shell particles were and
are developed up to date so that a wide range of chemical compositions and structures can be
realised (see Chapters 2.2.1.2 and 2.2.2.2). Yet, most synthetic approaches are specifically
tailored for the preparation of only one unique material. If a material with a different chemical
composition is to be prepared by the same method, the synthesis parameters must be
adjusted to the new system in a time- and work- intensive process. This makes the screening of
core-shell materials with different chemical compositions for known and novel applications
and their optimisation very difficult. A model system with core-shell geometry in which the
chemical composition can be adjusted over a wide range by minimally changing the reaction
parameters, is therefore an interesting and promising tool for the preparation and application
of highly functionalised materials.
The development and synthetic realisation of such a model system is in the focus of the
following chapters. Core-shell nanoparticles with an Fe2O3 core and a SiO2 shell constitute a
platform on which basis a variety of chemical compositions can be realised. The Fe2O3 core can
be replaced with small noble metal nanoparticles that are proven to be valuable catalysts in
various catalytic processes (Chapter 2.4.3). The initial shell made of porous SiO2 can be used as
an anchor for the synthesis of another shell with a different chemical composition. Yolk-shell
nanoparticles are obtained when the initial SiO2 template is removed. Following this approach
Fe2O3 nanoparticles encapsulated by ZrO2, TiO2 and carbon can be prepared. Thus, the
developed approach can be used to prepare materials of different chemical composition with
distinct core-shell hierarchy starting with Fe2O3@SiO2 nanoparticles.
8
2.2 State of the art
2.2.1 Metal and metal oxide nanoparticles
The development and characterisation of nanoparticles is one of the most fascinating and
promising research areas of the last few decades combining the expertise of various fields such
as chemistry, physics, biology, medicine, and engineering. Materials with sizes in the nanorange have very different properties compared to their bulk analogue, which leads to different
preparation methods, characterisation techniques, and applications
[23]
. Because of the small
size of particles, a high proportion of atoms are in or near the surface layer of the particles,
leading to the observed, unique properties. Those properties cannot be extrapolated from the
corresponding bulk material. Moreover, they are dependent on various material parameters:
the adjustable composition, shape, dispersion state, surface modification and dispersion
medium of nanoparticles are the basis for the diversity of nanomaterials [21].
Nobel laureate Richard P. Feynman is considered one of the theoretical founders of
nanotechnology due to his visionary speech given in the year 1959 in which he focused on the
potentials of materials and processes at a very small scale
[1]
. The research on nanomaterials
intensified in the 1980’s driven by the development of improved microscopy techniques and
therefore improved visualisation of materials with sizes in the nano-range. In the 1990’s the
research field gained momentum, and within 10 years the number of paper published
increased by two orders of magnitude [24].
The unique properties of nanoparticles and their use is not merely a discovery made by todays
advanced scientific community, but the application of nanoparticles can be found in ancient
medical approaches as well as Mayan paints. Metal nanoparticles were added to glass to
achieve the vibrant coloured windows displayed in cathedrals and palaces [23]. Gold and silver
nanoparticles embedded in the glass of the Lycurgus cup, which is dated back to the 4th
century AD, lead to the magnificent effect shown in Figure 2.2: whereas the cup seems to be
light green when examined in reflected light, the colour changes to bright orange in
transmitted light.
Figure 2.2: Gold and silver nanoparticles embedded in the glass of the so called Lycurgus cup lead to the different
colouring in reflected (I) and transmitted (II) light
[25]
.
9
Naturally, the application of nanoparticles during these times was the result of exact
observations of coincidental findings, and the fundamental basis of the effect was unknown.
The definition and deliberate exploration of nanoparticles and their properties started later, as
described above.
Today, nanoparticles are defined as particles with a size ranging from 1 to 100 nm in at least
one of the three dimensions. This criterion was chosen because materials in this specific range
have unique properties. In particles below 1 nm the dominating species are atoms, molecules
and elementary particles. The properties vary strongly from the ones of bulk material,
analysed and described in established scientific disciplines. On the contrary, particles with a
size above 100 nm react similar to their bulk counterparts: the properties are very similar
because the effect of the quantum confinement and the surface-to-volume ratio on the
electrical, thermal and optical properties decreases. Nanoparticles containing multiple atoms
or molecules behave differently from single atoms as well as the corresponding bulk material,
and a separate scientific and technological field was established to characterise the properties
and explore possible applications of nanoparticles [21].
A classification of nanoparticles can be made based on the dimension of their “nano-nature”,
as illustrated in Figure 2.3. Spherical nanoparticles are considered zero-dimensional (0-D)
because they possess nanometer dimensions in every direction. 1-D materials are nano-sized
in two directions but have larger dimensions in the other direction.
Figure 2.3: Illustration of the classification of nanomaterials according to their dimensionality
[26]
.
Thin sheets are classified as 2-D nanomaterials and have sizes between 1 and 100 nm only in
one direction, as for example graphene. Finally, nanoporous or nano-structured materials like
zeolites fall into the category of 3 -D nanomaterials [21].
Because of their unique properties and small size, specific preparation methods were
developed in order to prepare nanoparticles and tailor their structure and properties (see
Chapter 2.2.1.2). The numerous preparation pathways are highly specialised and diverse. Yet,
all approaches can be divided in two classes based on the fundamental idea of the methods,
which are top-down and bottom-up approaches. In the case of top-down approaches, bulk
10
material is processed by sophisticated techniques to fabricate small particles. On the other
hand, bottom-up approaches can be differentiated, in which nanoparticles are formed from
atoms and molecules via self-assembly.
Nanoparticles find diverse applications in many important fields such as medicine, agriculture
and environment protection
[25]
. As building blocks for nanotechnology, nanoparticles are of
significant economical and societal impact, and in reference to the classification of human
historical ages like the Stone Age, Bronze and Iron Ages, the time we are living in today is often
called the Nano Age to emphasize the importance of nanotechnology
[21]
. Nanoparticles have
the potential not only to advance well-established products but also to lead to the
development of completely new products and techniques. Fields in which nanoparticles are
already applied today are widespread and range from medical applications to energy-related
processes [24]. Exemplarily, silver nanoparticles are used for antibacterial creams and powders.
Other applications in the field of medicine include the preparation of biocompatible coatings
for implants as well as cancer diagnostics and targeted drug delivery using magnetic
nanoparticles. More and more consumer goods contain nanoparticles to improve known
characteristics and to include novel functionalities: the application of ZnO and TiO2
nanoparticles in sunscreen and the preparation of water- and stain-repellent textiles are just
two examples of this wide field. The catalytic activity of nanoparticles is used in the treatment
of waste gas and other energy-related applications (see Chapter 2.2.1.1). In the field of
alternative energy production and storage, nano-sized materials are developed and applied in
fuel cells and hydrogen storage devices [27].
Even against the background of the significant potential of nanotechnology described above
and in the following chapters, the great importance of the public opinion and assessment of
nanomaterials and nanotechnology has to be emphasised. An open and clear communication
of the impacts and merits of nanotechnology is crucial for public trust and acceptance of novel
products and techniques, and has to be pursued alongside the development of novel materials
and applications. Without a general approval of nanotechnology by the public, even highly
beneficial applications are in risk of failing economically [28].
2.2.1.1 Properties of nano-sized materials
The scientific interest in and fascination with nanoparticles is essentially based on the diverse
and unique properties of materials in this size range, which are so different from the observed
characteristics of bulk material. Properties generally thought to be constant for a specific
material, like melting point, colour, and hardness, begin to change when approaching the
nano-range. Two main aspects are considered to be central for the change of behaviour when
decreasing the size of a particle below this threshold: the surface-to-volume ratio of
nanoparticles compared to the corresponding bulk materials and quantum confinement
effects.
With decreasing particle size the surface-to-volume ratio increases proportional to the inverse
particle size for simple geometries like spheres, cubes and long cylinders. As a consequence of
the increasing surface-to-volume ratio of small particles, higher values for the dispersion are
11
found. The dispersion describes the fraction of atoms in the surface shell of the particle, and
the dependency of this factor on the cluster size is illustrated in Figure 2.4 on the left. The
fraction of atoms on the surface of the particle compared to the total number of atoms in the
particles decreases drastically with increasing particle size, having a strong influence on the
properties of the material [29].
Figure 2.4: I) Plot illustrating the dispersion as a function of the number of atoms along the edge of a particle n.
st
II) Possible positions of atoms on the surface of a particle with numbers indicating the 1 coordination shell
[29]
.
The reason for the great impact of the fraction of surface atoms on the materials
characteristics is that atoms in the surface shell of a particle behave differently compared to
atoms within a particle. With increasing dispersion the contribution of these surface atoms to
the overall particle properties gains importance and is more pronounced. The different
behaviour of atoms in the surface shell can be explained by the coordination number of the
single atoms, as described in Figure 2.4 on the right side. The atoms at the surface layer have
fewer neighbours than atoms within the particle and are less strongly bound and stabilised.
The surface energy of small particles is therefore higher and many properties observed for the
bulk material change. As illustrated in Figure 2.4 several atom sites on a particle surface can be
distinguished depending on the number of neighbouring atoms resulting in an energetically
heterogeneous surface [29]. In an attempt to lower the surface energy, surface reconstruction is
often observed in nano-sized materials. Surface reconstruction occurs in the form of formation
of dimers, or in a reduction of interatomic distances which can lead to a change in packing of
the surface layer in some cases. If nanoparticles consist of more than one component another
pathway to lower the surface energy is surface segregation: a different surface composition is
generated because atoms with a lower surface energy are accumulated on the surface. In the
presence of adsorbates, the adsorption of molecules is a possibility to stabilise the high energy
surface of a particle, as well [21].
The 2nd important factor leading to unique properties of nanoparticles is the quantum
confinement effect; a model describing the size dependent electronic structure of materials in
the nano-range. With decreasing particle size the dimensions of the particles get closer to the
wavelength of electrons and their characteristics start to contribute to the overall properties
12
of the material. In contrast to the bulk equivalent, small clusters have discreet energy levels
which close up to some extent when more atoms are added to the particle [30].
Both the different surface-to-volume ratio and the quantum confinement effect influence a
wide range of properties and lead to the observed change in the characteristics of
nanoparticles compared to their bulk counterparts.
Figure 2.5: Silver nanoparticles with cubic (I), cuboctahedral (II), and octahedral (III) shapes
[22]
.
As illustrated above, the surface atoms strongly influence the behaviour of nanoparticles.
Hence, the morphology of nanoparticles is very important (Figure 2.5); the aspect ratio,
porosity and surface roughness determine the atomic sites available on the particle surface
and influence physical and chemical properties of nanoparticles.
Properties that are affected by the small size include the conductivity of the materials, physical
properties like magnetic momentum, thermal expansion and hardness, as well as optical and
catalytic characteristics.
The mechanical properties change due to the lowering of the high surface energy of nanosized particles by shortening the bond lengths, which is observed for various metals (Figure
2.6).
Figure 2.6: Lattice contraction ε of metal clusters as a function of particle size
[29, 31]
.
13
The effect of lattice contraction increases with decreasing particle size, and bond contractions
between 4 and 30% can be observed. Moreover, the phenomenon is universal to all kinds of
bonds and is observed for ionic, covalent, and metallic bonds
[31]
. As a consequence, physical
properties vary with the particle size such as magnetic momentum, thermal expansion, and
hardness. Due to shorter bond lengths stronger, stiffer materials are obtained. Another factor
that influences the mechanical properties of nanoparticles is the decreased probability of
defects like grain boundaries [21].
The nanoparticles characteristics are also influenced by the dimensionality of their nanostructured nature, as already introduced in Chapter 2.2.1. Especially 1-D materials conduct an
electrical current differently due to the restricted size of the material perpendicular to the flow
axis and the degree of organisation of the material [26].
The dependency of optical properties on the size of nanoparticles is probably the best known
and demonstrative characteristic of materials in the nano-range, as shown in Figure 2.7. The
vibrant colours of small semiconductor particles are observed because of the discrete energy
levels present in small clusters.
Figure 2.7: Tuning of the emitting colour of quantum dots by changing the particle size
[32]
.
Energy differences between neighbouring energy levels increase when the particle size is
decreased, causing a colour shift from red to blue as illustrated in Figure 2.7 on the right
side[32]. Among metal nanoparticles, especially the various vibrant colours of gold nanoparticles
of different sizes are known. The absorption spectra have a strong absorption band at a
specific wavelength that is dependent on the gold cluster size, the so-called surface plasmon
resonance wavelength which describes the collective oscillation of surface electrons. At this
specific wavelength the interaction of the photons with the electronic structure of the particle
is the strongest. Apart from the size of the nanoparticles, the optical properties of a nano-sized
material depend also on the composition and shape of the nanoparticles [21].
Nanoparticles find application as catalysts in a wide variety of reactions because of their
unique chemical reactivity that can vary strongly from the bulk material
[33]
. The chemical
reactivity of nanoparticles is mainly based on the high fraction of surface atoms. As already
introduced above, the atoms at the surface of the particle are less strongly bound, leading to
novel catalytic behaviour. These different bonding conditions have a strong impact on the
14
reactivity; the difference is so large that even different crystal faces can be distinguished from
each other in their catalytic activity
[34]
. In addition, materials considered to be inert in a
catalytic reaction as bulk can be highly active due to the higher dispersion and different
bonding conditions at the nano-range. Since catalysis with solid catalysts is realised via surface
reactions, a high surface area is essential for the efficient utilisation of the catalyst, favouring
the application of nano-sized materials. Especially in the case of noble metal- based catalysts, a
high fraction of bulk atoms would be an unnecessary weight and cost, which must be avoided.
In conclusion, properties of materials with sizes approaching the nano-range are truly unique
and fascinating in their diversity. The impact of the high surface-to-volume-ratio and the
observed quantum confinement effect lead to changes in their chemical and physical
characteristics compared to the bulk phase. Many observed effects are the consequence of the
high surface energy of nanoparticles, such as surface reconstruction and segregation,
decreasing bond lengths and different melting points. Due to the different bonding of surface
atoms, nano-sized materials show good catalytic activity for various reactions. Even materials
that are inert in the bulk phase are active in catalysis when the particle size is decreased.
Consequently, the field of nano-sized materials is still a fast-developing area driven by the
discovery of novel properties and the potential of novel applications.
2.2.1.2 Preparation pathways for nanoparticles
The preparation pathways developed for the synthesis of nanoparticles differ from the ones
used to prepare bulk materials, because nanoparticles are metastable systems, which grow
into the corresponding bulk material over a certain time span. Granted that, nanoparticles are
kinetically stable in suitable media for some time. In order to obtain a stable dispersion of
nano-sized particles, molecules with specific functional groups bind on the surface of the
nanoparticles as a protective thin shell, enabling the dispersibility and prevent agglomeration.
The stability of those dispersions depends on the type of molecule adsorbed on the surface
(e.g. ions, molecules, polymers) and their bond strength, and is sensitive to changes in pH and
type of solvent used. The field of preparation methods for nanoparticles is wide and very
versatile, but all approaches can be either categorised as “top-down” or “bottom-up”
approaches. The 1st category (top-down) describes the formation of nanoparticles by
mechanical destruction of bulk material (Figure 2.8). Examples for top-down approaches are
high-energy ball milling, photolithography, laser ablation techniques, and sonication-assisted
techniques. The great challenge of most top-down approaches are the preparation of particles
with a distinct size and narrow size distribution as well as the prevention of agglomeration of
small particles. On the contrary, the precursors of bottom-up approaches are low molecular
weight species which form initial clusters that grow into nanoparticles (Figure 2.8). Even
though most of the occurring nucleation and condensation reactions are not yet fully
understood, bottom-up approaches are very well-suited for preparing well-defined
nanoparticles with a variety of chemical compositions. Many different approaches have been
developed to control the physical and chemical properties of nanoparticles and to meet the
requirements of different applications with high precision.
15
Figure 2.8: Comparison of top-down and bottom-up approaches for preparing nanoparticles.
Precipitation is a common bottom-up approach to prepare metal and metal oxide
nanoparticles of diverse composition. Precipitation in general describes the formation of solid
phases, which is induced by the addition of reagents that start a chemical reaction or that
lower the solubility of the precursor, leading to the formation of solid particles. If the intended
product is nano-sized metal particles, usually a reducing agent is added to the precursor salt
solution
[35]
. The approach offers the possibility to produce a wide variety of products with
different chemical composition, shape and size of the nanoparticles
[36]
. Shape and size of the
formed nanoparticles can be tuned by varying the concentrations of reactants, pH of the
reaction solution, chosen solvent and the presence of additives. The particle formation by
precipitation is based on the formation of precursor species initiated by the occurring chemical
reaction. The concentration of these species increases until a critical threshold is reached at
which nuclei are formed, which grow into nanoparticles. The concentration of the precursor
species is lowered due to this consumption, and no more nuclei are formed. The reaction
continues, but only the already existing particles grow larger
[35]
. Because of the size-
dependence of many properties of nanoparticles (see Chapter 2.2.1.1) a narrow size
distribution is usually desirable. In order to prepare particles of same size, the nucleation
phase of the reaction must be short with the consequence that all existing particles start
growing at the same time. The size distribution of some nanoparticles can be altered after
completing the reaction by aging the material. Aging describes the time in which the product is
left in the mother liquor. During this time Ostwald ripening occurs which is a recrystallization
process in which larger particles grow at the expense of smaller ones. Small nanoparticles tend
to agglomerate due to their high surface energy. This must be prevented, if single particles are
targeted. In using electrostatic stabilisation, the attractive and repulsive forces of the particles
are manipulated to stabilise the nanoparticles. A slightly easier way is the steric stabilisation,
which is based on the adsorption of surfactants or polymers on the surface of nanoparticles.
16
Various metal oxide nanoparticles like SiO2, TiO2, Al2O3, ZnO and ZrO2 can be prepared using
precipitation-based methods, which are even carried out on an industrial level. The obtained
products are usually amorphous due to the low preparation temperature.
The preparation of metal nanoparticles differs slightly from the formation of metal oxide
nanoparticles due to the necessary electron transfer during the reaction. Usually, the synthesis
conditions resemble those of precipitation processes, with the modification that a reducing
agent is added to prepare zero-valent particles The reducing agent must be carefully chosen
because the difference in redox potential of metal precursor and reducing agent influences the
reaction: a strong reducing agent leads to a spontaneous, fast reaction, so that many nuclei are
formed, and small nanoparticles are the reaction product. Apart from the size of the particles,
the crystallinity decreases with increasing strength of reducing agent
[37]
. Consequently, the
choice of reducing agent is an excellent tool to tailor the formed nanoparticles. The redox
reaction is furthermore influenced by the pH of the reaction solution and the solvent. The
chosen solvent determines not only the solubility of the metal precursor, but also its
dissociation and the electrostatic stabilisation. Furthermore, the chosen metal precursor itself
can change the occurring reaction, because the ligands strongly influence its redox
potential
[37-38]
. As in the preparation of metal oxide nanoparticles, surfactants and capping
agents are added to the reaction solution to influence the size and shape of metal
nanoparticles.
Figure 2.9: TEM images of gold nanoparticles prepared by the well-known Turkevich method
[21]
.
The gold nanoparticles shown in Figure 2.9 are prepared by the Turkevich method, in which
sodium citrate is used as both capping agent and reducing agent
[39]
. The size of the gold
nanoparticles can be easily controlled and tuned by varying the amount of sodium citrate.
In general, it is also possible to first prepare the metal oxide nanoparticles by precipitation
followed by reduction to obtain the metal equivalent. Yet, a great disadvantage of this method
is the poor control over retaining size and shape of the initial particles during the reduction
step, so often a protective shell is necessary (see Chapter 2.2.2). Precipitation and reductionbased syntheses are easy and straight-forward methods to prepare nanoparticles with a wide
range of adjustable parameters such as chemical composition, size and shape. The products of
coprecipitation stand out because of their homogeneous distribution of components, which is
crucial for various applications (compare Chapter 3.2.3). However, the parameters determining
shape and size of the obtained nanoparticles, like the effect of additives, must be tested for
each new system due to the complex network of reactions [40].
17
The preparation of nanoparticles using microemulsions is fundamentally a precipitation
reaction as well. A microemulsion is a thermodynamically stable dispersion of two immiscible
fluids that is stabilised by surfactants
[41]
. The micelles form small reactors in which the
nanoparticle formation takes place, restricting their size and changing their shape. The most
common microemulsion used for preparing nanoparticles is based on water droplets in a
hydrocarbon phase, so-called water-in-oil microemulsions. The addition of surfactant is
necessary to form micelles which are usually of spherical shape because of its lowest surface
energy, but other geometries are possible as well. The surfactant layer stabilising the micelles
during nucleation and growth of the nanoparticles can be used in later stages to stabilise the
formed nanoparticles, leading to stable dispersions[42].
Figure 2.10: Illustration of the general preparation method of metal nanoparticles using microemulsions.
The preparation of nanoparticles using microemulsion-based techniques is usually divided in
three stages (Figure 2.10) [43]: in the 1st step two microemulsions are prepared, one containing
the metal precursor while the other contains the reducing agent or reaction catalysts.
Afterwards both microemulsions are mixed and the micelles collide, starting the nucleation of
reaction species on the edges of the micelles. The growth of the particles moves then towards
the core of the micelle
[44]
. Because of the slow intermicellar exchange, the reactions are
slowed down compared to reactions in native aqueous solutions. Furthermore, the growth of
particles is restricted to the confines of the micelles. Another possibility is the preparation of
only one microemulsion containing the metal precursor, followed by the addition of the
reducing or oxidising agent directly to the microemulsion. Since the reactions occurring in
microemulsions are different compared to homogeneous solutions, not all systems can be
carried out in micelles.
The size of the formed nanoparticles can be tuned by changing critical parameters, like chosen
solvent and surfactant, addition of electrolytes, and the water-surfactant ratio. The great
18
potential of microemulsion-based preparation of nanoparticles is mainly based on the
possibility to influence the particle shape, as illustrated in Figure 2.11 in the transmission
electron microscopy (TEM) micrographs II and III. The assembly of surfactants can be varied
into a great range of shapes. Yet, it is not necessarily certain whether the shape of the
surfactant assembly leads to a distinct templating effect
[45]
. For each system the parameters
leading to specifically shaped nanoparticles must be tested. Often, the addition of anions is
important for generating special shapes: while the surfactant assembly stays unchanged,
anions affect the crystallisation reactions by the preferential adsorption onto specific crystal
facets [33].
Figure 2.11: Nanoparticles with different crystal shapes prepared by microemulsion-based preparation techniques:
I) BaCrO4 nanoparticles
[46]
, II) and III) copper nanoparticles with elongated and spherical shape
[47]
.
While the shape control in some systems is truly impressive, microemulsion-based syntheses
must be carefully analysed for each new system, and no general templating effect is observed.
Nevertheless, the preparation of metal and metal oxide nanoparticles within micelle-based
microreactors offers a great chance to obtain particles of a defined size and narrow size
distribution. The stabilisation of the formed nanoparticles by surfactants is highly desirable,
because it prevents agglomeration of the small particles. This becomes especially important if
further processing is necessary to obtain the desired product (compare Chapter 2.2.2).
Additionally, nanoparticles can be assembled in a specific hierarchy dependent on the
surfactant after their formation, as shown for BaCrO4 nanoparticles in Figure 2.11 (picture
I) [46].
Another powerful tool for the preparation of crystalline nanoparticles is the hydrothermal
synthesis. All preparation techniques using water as solvent, temperatures above 100 °C, and
pressures above 1 bar are considered as hydrothermal. The solubility and reactivity of
chemical reactants change drastically as a consequence of these conditions. In general, two
different modes of operation can be distinguished: in the autogeneous pressure mode the
pressure inside the reaction vessel results from the equilibrium between the gaseous and
liquid water phase. The pressure is therefore dependent on the temperature of the synthesis.
In the 2nd possible operation mode an additional external pressure is applied to the system,
and the pressure usually exceeds the values reached at equilibrium water vapour pressure.
Parameters influencing the reaction and product properties are mostly temperature, pressure
and reaction time. The temperature has a major influence on the kinetics of product formation
and the stability of formed products. The thermodynamic stability of the products is also
19
dependent on the pressure during the hydrothermal reaction. Moreover, choosing the right
pressure range is essential for tuning the solubility of the reactants and the supersaturation
range necessary for the formation of nanoparticles. The reaction time determines, whether
kinetically favoured products are formed (short term synthesis) or the formation of
thermodynamically stable phases is encouraged in choosing longer reaction times [48].
Furthermore, the size of the particles and their size distribution is dependent on the reaction
time: the particles grow larger with increasing time, and Ostwald ripening can be observed
under hydrothermal conditions, as well
[49]
. The presence of additives in the reaction solution
can lead to differently shaped products. If the additives preferentially adsorb on certain crystal
planes the growth in this direction is hindered, while the growth of others continues
[50]
.
Among the advantages of the hydrothermal preparation of nanoparticles is the high
crystallinity of the formed nanoparticles, as well as the low degree of agglomeration. The
versatile process can be used to prepare nanoparticles of various chemical compositions with a
very narrow product distribution and precise crystal shape. On the other hand, the yield of the
process can be very small, depending on the chosen reaction parameters. As in the
aforementioned approaches for nanoparticle preparation, the influence of reaction
parameters and presence of additives must be tested for every new system, which can be a
time- and work-consuming process [49].
A high-temperature method for the preparation of nanoparticles is pyrolysis, which is also
used on an industrial scale. In pyrolysis methods either a gaseous or a liquid precursor is
converted into nanoparticles by combustion in a flame. The variability of the process allows
the preparation of a wide range of nanoparticles, ranging from large agglomerations of optical
fibres to small nanoparticles for catalytic applications. In Figure 2.12 metal oxide nanoparticles
of different crystal shape prepared by flame spray pyrolysis are shown. Flame-assisted
methods also allow the preparation of multi-component nanoparticles by varying the reactant
composition.
Figure 2.12: TEM images of metal oxide nanoparticles prepared by flame spray pyrolysis
[51]
.
The overall set-up generally consists of an atomiser, a burner, and finally a collecting system
[52]
. In the 1st part of the set-up, the prior prepared precursor gas or solution is atomised and
mixed with a fuel that sustains the flame in the 2nd step. Different geometries and
configurations can be applied to the burner unit: in the diffusion mode the fuel and oxidiser
are not mixed before entering the flame zone but diffuse into each other
[53]
. On the other
hand, the gases can be mixed before entering the burner unit, resulting in a faster reaction.
20
The prepared nanoparticles are collected in the last part of the experimental set-up
[52]
. The
products of the pyrolysis process are influenced by various process parameters: the flame
temperature and the residence time of the reactants in the burner unit are the most influential
parameters of the process. The temperature of the flame is dependent on the reactants, their
mixing and the flame geometry because of the heat released during the reaction. By changing
the gas velocity, the residence time can be varied. The reactant mixing, possible presence of
additives and electric fields influence the properties of the resulting nanoparticles, because of
their impact on the size of the primary particles formed and degree of agglomeration.
Generally it is a great challenge to prepare unagglomerated nanoparticles in pyrolysis-based
processes. On the other hand, the obtained nanoparticles are of high purity, and especially
based on the combustion of vapours, it is very easy to prepare multicomponent nanoparticles.
In industry the simplicity of the process and the machinery without moving parts are of great
advantage alongside the high yields achievable with this process [53].
The great challenge of preparing nanoparticles remains until now, that is the prediction of
particle size and shape in dependence on the used precursors, surfactants, solvents, and
temperature on a more general basis. In all described methods the parameters for preparing
nanoparticles with a desired size and shape must be experimentally determined for each new
system. In order to complete and expand the tool box of materials chemistry the development
of more general preparation systems is essential.
Already with the existing methods it is possible to prepare nanoparticles of distinct size and
shape. Precipitation is the most basic method to prepare nanoparticles, and by adding suitable
surfactants various properties can be tuned. A modification of the precipitation method is the
preparation of nanoparticles within micelles. The micelles act as microreactors and
nanoparticles with a narrow size distribution can be obtained. Higher temperatures and
pressures are used for the preparation of nanoparticles by hydrothermal treatment. The
synthetic approach allows the adjustment of various parameters to influence the properties of
the final nanoparticles. At last, pyrolysis was introduced as a preparation method for
nanoparticles by combustion of gaseous or liquid precursors.
2.2.2 Core-shell materials
Only in very few applications nanoparticles are used in their pure form. Most often nanoparticles are supported on an inert material or immobilised in matrix materials, in order to
enable the incorporation into a functional devise or to stabilise the nanoparticles. As described
above, nanoparticles have an exceptionally high surface energy which can lead to
agglomeration of particles. A possibility to protect nanoparticles from agglomeration is the
addition of a thin layer of another material, resulting in materials with a core-shell structure.
The number of publications over the last years addressing the topic of core-shell materials is
evidence for the continuously high interest in this material class, since the pioneering work on
gold nanoparticles encapsulated in SiO2 published in the year 1995 [54]. One of the reasons for
the ever high interest in the preparation of composite materials with a core-shell structure is
21
the large number of possible combinations, and the resulting possibility to prepare materials
with precisely tailored properties. Additionally, the combination of core and shell material in
such close proximity can lead to novel synergistic properties that go beyond the properties of
both the core and shell material. Apart from the additional functionality simply by the
presence of the shell material, the shell can be used as an anchor point for organic moieties.
Different angles can be used in order to tailor nanoparticles with a core-shell structure as
illustrated in Figure 2.13 [55]. In manipulating the composition and shape of the nanoparticles in
the core of the material, various properties can be realised (see Chapter 2.2.1.1). The 2nd
vantage point is preparing the shell according to the requirements of the application. The
chemical composition as well as the structure of the shell is of importance in order to tailor the
properties of the final product. The shell can have different functions depending on the
application: if reactants should reach the core, e.g. for a catalytic conversion, the shell must be
porous, whereas if the shell acts as a protective layer against dissolution, it must be dense.
Figure 2.13: Possibilities to tailor properties of core-shell nanoparticles
[55]
.
Different architectural designs can be prepared, like spherical particles, anisotropic core-shell
particles, or yolk-shell particles, resulting in different properties and applications. At last, the
interaction between the nanoparticle in the core and the shell material influence the
properties of the overall material, offering yet another vantage point to tailor the properties of
materials with a core-shell structure.
Due to this great potential of tuneable properties, the fields of application of core-shell
nanoparticles is wide spread, ranging from sensing, microwave absorption and water
treatment. In addition, various biomedical applications are analysed and developed, including
biomedical imaging, drug delivery, gene delivery and targeted drug release [56]. Another major
field in which materials with a core-shell structure are intensively studied is catalysis, as
summarised in the following chapter. The high expectations in this field are based on the
potential of preparing catalysts with tailored properties leading to high activities and superior
stability. Research on the properties and possible applications of nanoparticles with a coreshell structure is accompanied by intense efforts to improve existing and develop new
synthetic pathways to prepare highly functionalised core-shell nanoparticles (see Chapter
2.2.2.2). Only with sophisticated synthetic approaches will it be possible to realise the high
22
expectations of core-shell nanoparticles, and to tailor their properties to meet the
requirements of the envisioned applications.
2.2.2.1 Properties and application as catalysts
Interest in nano-sized particles with a core-shell structure and the development of suitable
syntheses are based on the numerous possibilities to tailor the properties of this composite
material. While naked nanoparticles already show great potential in various applications, the
combination with a 2nd or even more components multiplies the possibilities of materials
design. While core-shell nanoparticles have wide-spread applications as discussed before, the
use of core-shell structured particles in heterogeneous catalysis has raised a lot of interest
over the last years
[57]
. The great versatility regarding the chemical composition of catalysts
with a core-shell structure led to the development of various systems, which show superior
behaviour compared to conventional catalysts. Especially the distinct hierarchy of core-shell
nanoparticles has proven to be of great advantage in heterogeneous catalysis
[58]
. The
functions of the core and the shell component of a solid catalyst can vary, and therefore,
catalysts can be categorised according to those differences: the 1st group consists of materials
in which the core serves as support for the catalytic active shell, whereas catalysts of the 2 nd
category contain a catalytically active core which is encapsulated by an inert material. The final
3rd group summarises catalysts which are distinguished by the presence of catalytic active sites
on both the core as well as the shell component
[56]
. While in many cases the classification of
materials is clear, for some reactions the influence of components of the core-shell catalyst on
the reaction is not fully understood and clear. Hence, for some systems it can be debatable
whether the support material is chemically inert (group 1 or 2) or whether support effects exist
that can be interpreted as interference in the reaction (group 3).
The idea behind core-shell structured catalysts in which the shell contains the catalytically
active sites, is the improved utilisation of expensive noble metal particles. Only surface atoms
partake in reactions (compare Chapter 2.2.1.1), and atoms in the core of the nanoparticle are
of no catalytic use, which limits the application of noble metal catalysts due to high costs. In
order to economise the catalyst production, an inexpensive metal like nickel or iron is covered
with a thin layer of catalytically active noble metal
[56]
. A core-shell catalysts composed of a
nickel core and a thin palladium shell demonstrates a similar activity to pure palladium
nanoparticles in the oxidation of CO
[59]
. The catalytic activity of the core-shell catalysts stays
stable even at high temperatures, and the formation of an alloy is not observed during the
reaction. The presence of a 2nd metal in the core of the catalyst can even lead to an enhanced
activity compared to pure noble metals. The system made of a nickel core and a palladium
shell shows an enhanced activity in Sonogashira coupling reactions when compared to the
pure palladium catalyst [60].
In contrast to afore described systems, the catalytically active part of core-shell nanoparticles
can also be the core, which is encapsulated by a chemically inert shell. Nanoparticles stand out
because of their superior catalytic activity due to their small size and are usually not used in
pure form, but supported on an inert matrix. The advantage of using such a solid catalyst is the
23
ease of separation compared to nanoparticles. Yet, supported nanoparticles can suffer from
poor long-term stability in solution or at elevated temperatures due to the sintering and
melting of small nanoparticles. By encapsulating metal or metal oxide nanoparticles in an inert
shell, changes in the particles size due to migration and sintering during the catalytic reaction
can be prevented
[55-56]
. The catalyst is more stable and has a longer lifetime because of its
improved structural stability. Several requirements for sinter-stable catalysts with a core-shell
structure must be met: in order to maintain the structure of the catalyst the shell materials
must be thermally stable. Additionally the shell material has to be chemically inert and has to
have no negative impact on the catalytic activity under reaction conditions. The final
requirement concerns the porosity of the shell: for an efficient catalytic cycle, mass transport
limitations must be minimal so that reactants can penetrate the shell easily to react on the
surface of the core particle
[57]
. The most common used shell materials are SiO2, TiO2, Al2O3,
ZrO2, SnO2 and carbon– also often used as support materials for the deposition of
nanoparticles in conventional catalysts.
Figure 2.14: Comparison of encapsulated (I, II) and non-encapsulated Fe2O3 nanoparticles as catalysts for the NH3
decomposition
[58]
.
Iron oxide nanoparticles encapsulated in SiO2 shells can be used for the decomposition of NH3
which is an important reaction for the utilisation of NH3 as a fuel for fuel cells [58]. While other
metals like ruthenium, molybdenum and nickel also show catalytic activity, iron oxide is the
preferred catalyst due to low costs. The reaction is carried out at high temperatures posing
difficulties on conventional catalyst systems because of agglomeration and loss in activity. The
core-shell structure makes it possible to carry out the reaction at high temperature while still
maintaining the catalysts structural stability and activity as shown in Figure 2.14. A reference
catalyst containing the same Fe2O3 nanoparticles and empty SiO2 shells (compare Figure 2.14
III and IV) confirms the importance of the core-shell structure of the catalyst. Without the
protective shell, the Fe2O3 particles sinter during the reaction leading to a rapid deactivation in
24
the NH3 decomposition. The porous SiO2 shell does not lead to mass transport limitations, but
still protects the catalytically active core from sintering even at a temperature of 800 °C [58].
Another metal that is very sensitive to sintering if nano-sized is gold. Gold nanoparticles raised
enormous interest over the last years due to their catalytic activity in a wide range of
reactions. Nevertheless, the poor thermal stability limits the application of gold-based
catalysts. The principle of core-shell structured catalysts is ideal to prevent the sintering of
gold nanoparticles and the resulting loss in activity. Various materials are used to encapsulate
gold nanoparticles, making it possible to compare and analyse the support effect on the
catalyst activity in the oxidation of CO. Carbon and TiO2 are two materials that are commonly
used to encapsulate gold nanoparticles [61]. When the catalytic activity of those two catalysts is
compared in the CO oxidation, the beneficial effect of the oxidic support is demonstrated,
leading to an improved catalyst performance. Since the catalytic activity is strongly dependent
on the preparation method, this comparison is only possible due to the developed synthesis
pathway which enables the preparation of different shell materials under similar conditions
(for synthesis details see Chapter 2.2.2.2). Palladium nanoparticles encapsulated in various
shell materials like CeO2 and SiO2 also show remarkable activity in the oxidation of CO
[62-63]
.
SiO2 encapsulated palladium nanoparticles show no signs of sintering and loss in activity even
after heating to 700 °C. Compared to an impregnated SiO2 support the catalyst with core-shell
structure leads to an improved activity in the oxidation of CO
[63]
. The positive impact of the
core-shell hierarchy in noble metal containing catalysts is proven by the application of
platinum nanoparticle encapsulated in SiO2 in the CO oxidation, as well [64].
The final category of solid catalysts with a core-shell structure combines materials with a
catalytically active core covered by a shell which also contains catalytically active sites. Bifunctional materials with a core-shell geometry are ideal to catalyse consecutive reactions that
are conventionally carried out separately. Catalysts with a core-shell structure have to fulfil
several requirements for a successful realisation of consecutive reactions in one set-up: the
dispersion of both active sites is high due to the nano-sized dimensions, while the migration of
reactant from one active site to the next is ensured by the core-shell hierarchy. Especially the
combination of catalytically active metal sites with acid sites find wide-spread applications in
the direct synthesis of dimethyl ether (DME) and the Fischer-Tropsch reaction, which will be in
the focus in the 2nd part of this thesis (compare Chapters 3.1 and 3.2.3).
Another field of application for bi-functional catalysts with a core-shell structure is photocatalysis. TiO2 is known for its superior photo-catalytic performance and is applied in various
photo-assisted reactions
[65]
. While it is established that the combination of TiO2 with noble
metals enhances the photo-catalytic activity, dissolution and corrosion limit the long-term
stability and therefore the application of such systems which usually consist of a noble metal
dispersed on the surface of the TiO2 support
[66]
. A novel system with a core-shell structure
meets these challenges (Figure 2.15): a noble metal core is covered by a photo-catalytically
active TiO2 shell, which is not only catalytically active but also acts as a protective layer against
dissolution and corrosion when in contact with the reaction solution [67]. The core-shell
hierarchy of the catalyst not only protects the silver core but demonstrates enhanced photo25
catalytic activity, proving once again the importance not only of the composition but also of
the structure of a catalyst.
Figure 2.15: Improved photo-active TiO2 catalyst due to noble metal core
[67]
.
The field of nanoparticles with a core-shell structure used as catalysts is rather diverse and
new fields continue to emerge, profiting from the tuneable properties of this material class.
While the protection from sintering and the consequential loss in activity are the main driving
force for the application of core-shell catalysts in reactions at elevated temperatures, in other
reactions the synergistic effect of the two components is in the focus of interest. Above all, the
combination of two materials in a core-shell hierarchy offers the great potential to tailor the
properties of the material according to the requirements of the specific catalytic process,
leading to the realisation of highly functionalised composite materials.
2.2.2.2 Preparation principles of catalysts with a core-shell structure
Many synthetic approaches have been developed in order to tailor the properties of core-shell
materials according to the specific requirements of heterogeneous catalysis. Most core-shell
catalysts consist of a metal or metal oxide core encapsulated by an inert and thermally stable
shell through which the core is still accessible for reactants.
All preparation methods of core-shell nanoparticles must balance heterogeneous nucleation
and controlled growth of the shell materials on the one hand, while preventing selfnucleation
on the other. The vast number of different preparation methods can be divided into three
groups: two-step methods, one-pot approaches and surface treatment of metal nanoparticles.
In two-step-methods the metal or metal oxide core is prepared first followed by encapsulation,
while core and shell are formed simultaneously in one-pot-methods. Finally, nanoparticles
with a core-shell structure can be obtained by surface treatment, e. g partial oxidation, of
metal nanoparticles [68]. The latter approach is rather restricted, and mostly binary metal nanoparticles are used for these methods because of the possible phase separation upon surface
oxidation, yet the control over the resulting products is rather poor.
More examples can be found for one-pot-methods in which the precursors for the core
materials as well as the reactants for the shell formation are mixed together. In order to
succeed in preparing materials with a core-shell hierarchy, the nucleation and growth rates of
both core and shell must be balanced. In addition, the assembly of core and shell must be
ensured. Because of the complex network of crystallisation reactions followed by hydrolysis
26
and condensation, the developed synthetic approaches are highly case-specific and no general
methods are found, yet [68].
The by-far most applied methods belong to the group of two-step-approaches in which
nanoparticles are prepared in a 1st step followed by the synthesis of the shell. The sequence of
preparation steps allows a high degree of control over size, shape and structure of the coreshell nanoparticles.
A possible approach is using hydrothermal conditions to form a metal oxide shell around noble
metal nanoparticles in a two-step process. Exemplary, after the preparation of noble metal
nanoparticles following the citrate route (see Chapter 2.2.1.2), TiO2 precursors were added to
the dispersion alongside suitable stabilisers and placed in an autoclave. After heating the
reaction mixture up to 180 °C and keeping it at this temperature for 48 h, the sample was
cleaned and the final product was obtained as shown in Figure 2.2.16. The noble metal core (in
this case platinum) is encapsulated by a TiO2 shell of around 150 to 200 nm thickness
[69]
.
Hydrothermal syntheses can be used to encapsulate noble metal nanoparticles with various
metal oxides, yet the separate formation of metal oxide nanoparticles is difficult to supress.
Figure 2.2.16: SEM images (I and II) and TEM image (III) of TiO2 covered platinum nanoparticles
[69]
.
In most cases the inert shell is placed on stabilised core nanoparticles by conventional wet
chemical approaches. Since the shell material of catalysts has to meet several requirements
(see Chapter 2.2.2.1) - most of all chemical inertness and thermal stability - mostly metal
oxides like SiO2, TiO2, CeO2, and ZrO2 are used, but also carbon encapsulated nanoparticles find
application in catalytic reactions. The by far most studied system is SiO2 due to its properties
(high chemical and thermal stability, optical transparency) and its easy processibility (easy
regulation of coating process, controlled porosity). Additionally, the SiO2 surface is a great
platform for further functionalisation steps: organic species like dyes and ligands can be
grafted on the silica shell
[70]
. Another advantage is the manipulation of the interaction
potential which allows the dispersion of the core-shell nanoparticles in polar as well as apolar
solvents. The SiO2 shell is placed onto the surface of nanoparticles by hydrolysis and following
condensation of reactive silica precursors like tetraethylorthosilicate (TEOS). Stöber et al.
described a general method to prepare mono-disperse SiO2 nanoparticles catalysed by
ammonia in ethanol as solvent in 1968, which can also be used to encapsulate nanoparticles
into a SiO2 shell in a very controlled manner [71]. The general procedure includes the transfer of
stabilised nanoparticles in an ethanol/water mixture that contains NH3 as catalyst and finally
27
the addition of the reactive silica precursor. The reaction mixture is stirred at room
temperature, and the reaction completes within several hours. Various properties like shell
thickness or porosity can be easily tuned by changing the precursors and reactant
concentrations. Upon contact with water the silica precursors are first hydrolysed catalysed by
NH3, and the following condensation and polycondensation reactions lead to the formation of
extended SiO2 networks. The silica shells are formed by aggregation of small subparticles
which are only few nm in size [72].
Some nanoparticles can be directly coated with SiO2 because of their chemical affinity, but
most of the initially prepared nanoparticles are functionalised with ligands or surfactants
before the SiO2 shell can be formed
[73]
. The surfactants enhance the affinity of the
nanoparticle surface to the shell precursors and initiate nucleation and growth of the shell
around the nanoparticles, so that no separate nucleation of the shell materials occurs.
Moreover, the transfer of the colloids into ethanol or other alcohols is a critical step in the
Stöber process. Ligands and surfactants stabilise the nanoparticles and prevent aggregation
which would occur due to the changed chemical environment. Polyvinylpyrrolidone (PVP) is an
amphiphilic surfactant (see Figure 2.17, I) that is often used for stabilising nanoparticles before
transferring into new media.
Figure 2.17: I) Structure of PVP. II) Stabilisation of nanoparticles with amphiphilic polymer PVP.
The stabilisation effect is based on electrostatic interactions between the polar amid group of
PVP and the surface of the nanoparticles, leading to the formation of a protective layer of
polymer around the nanoparticles (see Figure 2.17, II). The amphiphilic nature of the
surfactant allows the stabilisation of nanoparticles in a wide range of media [73]. The length of
the polymer chain influences the stability of the colloids in solution and can be tailored to
meet the requirements of a specific reaction environment: PVP molecules with higher
molecular weights enhance the stabilisation of the nanoparticles on the one hand, but the
formed shell is more inhomogeneous, and more than one core can be encapsulated in one
shell. If the chain length is decreased nanoparticle aggregation can occur resulting in the
formation of rod-like agglomerations. Thus, choosing the right surfactant is a key factor to
control the encapsulation of nanoparticles with an inert metal oxide shell.
Apart from PVP, other surfactants can be used as well to stabilise metal nanoparticles during
SiO2 shell formation: to prepare platinum nanoparticles encapsulated in a porous SiO2 shell
28
tetradecyltriammonium bromide (TTAB) is used as stabilising agent (Figure 2.18). The overall
synthesis includes 3 steps: the platinum nanoparticles are prepared and stabilised by TTAB
first, then a SiO2 shell around the Platinum cores is formed, and finally all organic compounds
are removed by calcination and the porous SiO2 shell is obtained [64].
Figure 2.18: Encapsulation of TTAB-stabilised Platinum nanoparticles in porous SiO2 shells
[64]
.
The ratio of Platinum nanoparticles and the amount of added TEOS influences the quality of
the core-shell nanoparticles: if the amount of TEOS is too low, several Platinum cores are
encapsulated in one SiO2 shell, whereas higher amounts of TEOS lead to the formation of SiO2
spheres without platinum cores alongside the desired core-shell particles
[64]
. Also transition
metal nanoparticles can be encapsulated with a SiO2 shell following the Stöber method
[74]
.
Nickel nanoparticles covered by SiO2 shells are desirable catalysts due to the low cost of nickel,
and the stabilisation of the nanoparticles in a core-shell hierarchy expands the field of
prospective catalytic reactions. To prepare catalysts for CH4 reforming, nickel nanoparticles
were dispersed in a mixture containing ethanol and aqueous NH3 without the addition of a
stabilising agent. After addition of TEOS the reaction proceeded at room temperature for
60 min. The lack of stabilising agent led to formation of separate SiO2 particles together with
the desired core-shell nanoparticles. After separating the Ni@SiO2 nanoparticles using a
magnet, the nickel core size was reduced by selective etching with HCl, resulting in yolk-shell
nanoparticles [74].
Apart from SiO2 other metal oxides are often used as inert protective layers in the preparation
of catalysts with a core-shell structure. The reaction steps generally resemble the Stöber
process, yet with some alterations. The pre-formed nanoparticles are stabilised prior to
encapsulation, which is realised by hydrolysis and condensation of reactive metal oxide
precursors. Since the reactivities of the precursors can vary drastically from the ones used in
the Stöber reaction, the synthetic parameters have to be adjusted accordingly.
TiO2 is an important shell material due to its thermal stability and possible support effect on
the catalytic activity of the core material. Additionally, TiO2 materials can be used in photoassisted reactions (see Chapter 2.2.2.1). For this specific application, gold nanoparticles
stabilised with hydroxypropyl-cellulose (HPC) were transferred from an aqueous reaction
environment into an alcoholic solvent, and after adding the titanium precursor dropwise, a
TiO2 shell was formed. In order to control the nucleation and growth of the TiO2 shell, the less
reactive titanium diisopropoxide bi(acetylacetate) was used as compared to the generally used
tetrabutoxidetitanium precursor (TBOT). The thickness of the TiO2 shell can be varied over a
29
wide range by changing the precursor amounts, resulting in particles ranging from 25 to
125 nm [75].
A very versatile strategy to encapsulated gold nanoparticles with various metal oxides was
developed by Sun et al [76]. The approach is generally based on the strong stabilising effect of
PVP. Because of the different reaction rates of the metal oxide precursors, solvents, reactant
concentrations and seed concentration had to be varied for each system. If the reaction rate is
too high, selfnucleation cannot be prevented, and control over the growth of the shell material
is difficult. But by changing the reactant concentrations or solvent, the reaction rates can be
slowed down to some extent.
Figure 2.19: Versatile PVP-based method to encapsulate gold nanoparticles with metal oxides
[76]
.
Core-shell nanoparticles can also be used to prepare sandwiched structures, in which a 2nd
shell is added on top of the 1st. The motives for adding a 2nd shell can be very diverse: the 1st
shell might be easier to functionalise (compared to the core nanoparticle) or the 1st shell can
act as a spacer to increase the overall particle dimensions.
Because it is very difficult to create thin TiO2 layers on small nanoparticles, a possible approach
for the synthesis of photocatalysts is the preparation of a 1st, thicker SiO2 shell which is then
coated with a very thin TiO2 layer [77]. The reduced thickness of the TiO2 is necessary because
the catalytic activity of the TiO2 layer is inversely proportional to the shell thickness.
Additionally, the prepared photocatalyst contained a Fe3O4 core that allowed the easy
separation of the materials with a magnet but does not partake in the photo-assisted reaction.
The SiO2 shell stabilised the thin TiO2 shell, and the shell thickness can be reduced to an extent
impossible by coating naked nanoparticles.
A continuing step is the preparation of yolk-shell materials for which the 1st shell is leached
after the formation of the 2nd layer to obtain nanoparticles with a core freely moveable inside
the shell. SiO2 is widely used as sacrificial template because of its convenient removal by
etching with alkaline solutions and hydrofluoric acid, and because of its easy surface
functionalisation. Yet, it has to be taken into account that the metal or metal oxide core is
exposed to the harsh conditions during the leaching step, which can lead to dissolution or
damage of the core. Additionally, the synthesis parameters must be carefully chosen to
prevent the formation of highly stable phases like silicates
[68]
. Noble metal nanoparticles like
gold are stable in alkaline solution and ideal candidates for the preparation of yolk-shell
catalysts
[57]
. In the 1st step of the synthesis, the Au@SiO2 nanoparticles were dispersed in
ethanol using ultrasonication to facilitate the encapsulation of single particles. The core-shell
particles were then stabilised by Lutensol AO5, a non-ionic surfactant, during the reaction, and
30
the reaction mixture was heated up to 30 °C. Afterwards, zirconium butoxide (ZBOT) was
added dropwise to the dispersion, and the hydrolysis and condensation of the zirconium
precursor proceeded overnight. After cleaning up the Au@SiO2@ZrO2 particles using
centrifugation, the particles were kept in water at a temperature of 20 °C for 3 days before the
sample was calcined at 900 °C. Finally, the SiO2 template was removed by leaching in alkaline
solution. The resulting yolk-shell particles consist of a small gold core (15 nm) in a ZrO2 shell of
7 nm shell thickness and a diameter of around 100 nm [57].
Two SiO2 shells of different porosity can be used to prepare carbon encapsulated nanoparticles
with a yolk-shell structure based on nanocasting. The 1st, dense SiO2 layer acts as an anchor
point for the 2nd, mesoporous SiO2 shell. After calcination the pores of the outer SiO2 shell are
impregnated with a monomer mixture that is polymerised within the pores (Figure 2.20, step
iii). After carbonisation of the polymer (Figure 2.20, step iv), the SiO2 template is removed by
leaching (step v) and the final product is obtained. Various carbon encapsulated noble metals
can be prepared following this preparation approach [78-79].
Figure 2.20: Illustration of the preparation of a carbon shell using a mesoporous SiO2 layer as template and TEM
images of Au@mpSiO2, C (I) and the final product Au@C
[61]
.
As shown in the TEM images in Figure 2.20, gold nanoparticles were first encapsulated in two
SiO2 layers to finally result in Au, @C particles used as catalysts [61].
The preparation of both SiO2 shells follows the conventional Stöber process, but by choosing
different silica precursors, differently porous shells can be obtained: for the 1st dense layer,
only TEOS was used as silica precursor. The porous SiO2 layer results from the use of a mixture
of TEOS and octadecyltrimethoxysilane (OTMS). The long hydrocarbon chain of OTMS is then
burned off during calcination, leaving pores behind. The porous SiO2 shell was then
impregnated with divinylbenzene, and the polymerisation was initiated by increasing the
temperature. After the polymerisation was complete, the polymer was carbonised in an inert
atmosphere and finally the SiO2 template was removed with NaOH [78]. The obtained yolk-shell
nanoparticles contain a carbon shell that is the exact replica of the outer, porous SiO2 shell.
All described methods to prepare composite nanoparticles with a core-shell or yolk-shell
structure form a powerful toolbox to meet the challenge of preparing highly functionalised
materials. The most commonly used methods include two steps: first, the nanoparticles are
31
prepared that are then covered by a shell in the 2nd step. Even though core-shell nanoparticles
can also be obtained by one-pot methods, the degree of control over the materials properties
is much greater in two-step approaches. As discussed, the catalytically active core is usually
encapsulated in a chemically inert and thermally stable material such as SiO2, TiO2, CeO2 and
ZrO2. Approaches following the Stöber protocol are widely used to encapsulate various
nanoparticles in SiO2. The Stöber process is characterised by a high degree of control of the
SiO2 shell properties such as thickness and porosity. Similar to the SiO2 formation, the
encapsulation of nanoparticles with other transition metal oxide shells is based on the
hydrolysis and condensation of reactive transition metal precursors. Yet, the reactivity is
increased compared to SiO2 precursors, which must be balanced by adjusting the reaction
parameters. A modification of the core-shell principle is the preparation of sandwiched
structures in which a 2nd shell is added to the particles. If the initial layer is afterwards
removed, particles with a yolk-shell structure are obtained in which the core can move freely
within the shell.
2.3 Synthetic strategy
For the preparation of nanoparticles with a core-shell structure a great variety of synthetic
approaches are available that allow a high degree of control over the materials properties as
shown in the previous chapter. Yet, most of the developed methods for highly functionalised
materials are only suitable for one specific material.
In the following chapters, a core-shell model system is introduced that combines the distinct
structure of core-shell nanoparticles with a great flexibility of the chemical composition.
As illustrated in the overview shown in Figure 2.21, several modification pathways were
developed in order to prepare a variety of compositions of both the core and the shell
material. As a starting point, Fe2O3@SiO2 nanoparticles are chosen that can be modified to
yield in core-shell nanoparticles with noble metal cores or TiO2, ZrO2, and carbon shells.
Additionally, the preparation parameters are chosen and adjusted in a way that the formation
of the different materials is possible under the same conditions. As already discussed in
Chapter 2.2.2, the synthesis conditions have a strong impact on the properties and catalytic
performance of materials in the nano-range. In order to analyse the influence of different
catalyst components and to compare the catalytic results of different catalysts, it is important
to prepare materials of different chemical composition under similar conditions.
The variation of the core material is achieved via a metal replacement reaction with the
reduced iron core of the initial Fe2O3@SiO2 particles. By simply changing the noble metal
precursor, nanoparticles with a platinum, palladium, rhodium and ruthenium core can be
prepared under the same synthesis conditions.
Also for the TiO2 and ZrO2 encapsulated Fe2O3 nanoparticles the preparation pathway was
developed in a way that it is suitable for both materials. The catalytic performance of the
32
obtained materials can therefore be easily compared to show the true impact of the different
shell materials. Fe2O3@C particles with a yolk-shell structure are prepared by a different
method due to the different chemical nature of the shell material (see Chapter 3.4.1.2). Yet,
important synthesis parameters, like the preparation of Fe2O3 nanoparticles, heat treatments
and leaching steps are similar, allowing a comparison to the materials containing a transition
metal oxide shell in their catalytic activity.
Figure 2.21: Overview of the possible modifications of the core and shell materials based on
Fe2O3@SiO2 nanoparticles.
The metals and metal oxides used in the developed pathways were specifically chosen due to
their importance and general applicability in heterogeneous catalysis. Noble metal
nanoparticles find wide-spread application in heterogeneous catalysis, showing high catalytic
reactivity in various reactions as discussed in Chapter 2.2.1. Transition metal oxide
nanoparticles like Fe2O3 are often less active but also much cheaper than their noble metal
counterparts, making transition metal- based catalysts attractive alternatives nevertheless.
While SiO2 is commonly used as a catalyst support material due to its good chemical and
thermal stability, other oxides can have a stronger support effect enhancing the catalytic
activity of the metal or metal oxide catalyst. Both for TiO2 and ZrO2 these beneficial support
33
effects have been documented. Still, the thermal stability is comparable to SiO2, and the
chemical resistance of TiO2 and ZrO2 is even higher compared to SiO2.
The great flexibility of the chemical constituents together with the distinct core-shell hierarchy
leads to an interesting model system for solid catalysts.
2.4 Results and discussion
2.4.1 Preparation of Fe2O3@SiO2 nanoparticles as starting point of model system
As illustrated above, Fe2O3 nanoparticles encapsulated by a porous SiO2 shell are used as a
platform material based on which a variety of other chemical compositions can be realised.
Two steps lead to the initial Fe2O3@SiO2 nanoparticles as illustrated in Figure 2.22: the Fe2O3
nanoparticles are prepared under hydrothermal conditions based on ferric chloride and llysine. In order to prepare Fe2O3 nanoparticles, 0.54 g of ferric chlorid and 0.29 g of l-lysine are
dissolved in H2O and filled in Teflon lined autoclaves, which were placed in a preheated oven at
100 °C for 190 min or at 175 °C for 75 min. The different temperatures during the
hydrothermal synthesis lead to different particle sizes, which is discussed later in detail. In a
2nd step the nanoparticles are encapsulated by a porous SiO2 shell following the Stöber
method. First, PVP is added to the Fe2O3 nanoparticles (8.33·10-4 mmol PVP per 100 mg Fe2O3)
dispersed in H2O to prevent agglomeration during the Stöber reaction. Additionally,
cetyltrimethylammonium bromide (CTAB) is introduced to the system as porogen: 0.3 g of
CTAB are dissolved in 200 mL isopropanol and added to the stabilised Fe2O3 nanoparticles. The
pH of the reaction solution is adjusted by adding NH4OH (4 mL in 200 mL isopropanol), and
finally TEOS is injected into the reaction mixture using a syringe. The hydrolysis and
condensation reactions are carried out at room temperature until after 24 h the final product
is obtained.
Figure 2.22: Scheme of the preparation of Fe2O3@SiO2 nanoparticles.
Hydrothermal synthesis was chosen for the preparation of Fe2O3 nanoparticles to ensure a
stable and easily reproducible preparation method which leads to single, unagglomerated
nanoparticles. Additionally, the size of the formed nanoparticles can be adjusted by changing
the synthesis parameters. The reaction temperature and time influence the size and
crystallinity of the formed nanoparticles greatly, as illustrated in the TEM images in Figure
2.23. The hydrothermal synthesis of the sample in picture I and II was carried out at a
temperature of 100 °C leading to an average size of 41 nm. The Fe2O3 nanoparticles in the
34
lower TEM images are slightly larger (73 nm) due to the increased reaction temperature of
175 °. The increased temperature leads to an accelerated crystal growth and consequently to
larger particles. Because the nucleation rate as well as the growth rate is increased by higher
temperatures, the size distribution gets wider. If the temperature is increased, nucleation
occurs over a wider time span, leading to the formation of larger as well as smaller particles.
As shown in Figure 2.23 in the sample prepared at 175 °C, some very small particles are found,
resulting from a delayed nucleation. The presence of those small particles as well as few larger
particles lead to a broader size distribution compared to the Fe2O3 nanoparticles prepared at
100 °C (see Figure 2.24).
Figure 2.23: TEM images of Fe2O3 nanoparticles prepared by hydrothermal synthesis choosing different conditions
(I) and II) Fe2O3-100; III) and IV) Fe2O3-175)
While the sizes of the Fe2O3-100 nanoparticles range from 20 to 70 nm, the size distribution
ranges from 30 to 100 nm for the nanoparticles prepared at 175 °C. The dynamic light
scattering (DLS) data presented in Figure 2.24 on the right side confirms the particle size found
in TEM and also proves the absence of any large agglomerations and the successful
preparation of single Fe2O3 nanoparticles.
The size and polydispersity of the nanoparticles is additionally influenced by the reaction time.
The size control that is possible by using hydrothermal conditions is based on an interception
of the crystal growth of the nanoparticles. The particle sizes can be modified over a wide
35
range, yet a minimum reaction time must be exceeded. If the synthesis is stopped before this
critical time, long, rod-like akagenenéite particles are still present in the sample
[80]
. This pre-
nucleation species is slowly turned into the particles seen in Figure 2.23. The presence of
akagenenéite particles must be prevented in order to have homogeneously shaped
nanoparticles. At a synthesis temperature of 175 °C the reaction time must exceed 50 min to
ensure the exclusive formation of the targeted Fe2O3 nanoparticles. At lower temperatures the
transformation of the pre-nucleation species takes longer, and longer reaction times are
necessary.
Figure 2.24: Size distribution of Fe2O3-100 and Fe2O3-175 based on TEM results (I) and DLS measurements (II).
Apart from the size and size distribution, the different reaction conditions influence the shape
and surface properties of the nanoparticles: the shape of the Fe2O3 particles changes from
spheroidal to octahedral at higher reaction temperatures. Additionally, the surface of the
Fe2O3-100 nanoparticles is distinguished by a high surface roughness and the presence of
surface cracks, as visible in the scanning electron microscopy (SEM) images (Figure 2.25, image
I and II). For the Fe2O3-175 nanoparticles the SEM images show a much smoother surface with
sharp crystal faces and edges.
Another factor that is strongly influenced by the reaction temperature and time is the amount
of Fe2O3 nanoparticles obtained per synthesis batch. The yield is much higher at 175 °C
compared to a reaction temperature of 100 °C in which case only sub-milligram amounts can
be obtained per 100 mL of reaction volume. While a narrow product size distribution is
advantageous in many applications of nanoparticles (compare Chapter 2.2.1.1), the obtained
yield is the decisive factor in this synthesis, being the 1st step in a multi-step synthesis
approach. In the following modification steps, only the results based on Fe2O3-175
nanoparticles are shown, yet all developed syntheses can be carried out using the smaller
Fe2O3-100 nanoparticles, as well.
36
Figure 2.25: Comparison of Fe2O3-100 and Fe2O3-175 nanoparticles using SEM.
L-lysine is used to stabilise the nanoparticles under hydrothermal conditions. In addition to
this, l-lysine interferes with the crystal growth and in changing the concentration of the
stabiliser the crystal shape can be varied. Yet, no effect on the size distribution could be
observed by changing the amount of l-lysine.
The prepared Fe2O3 nanoparticles are subjected to the Stöber process in order to form a
porous SiO2 layer around the particles. TEOS is used as silica precursor for the formation of
Fe2O3@SiO2 nanoparticles. Because the Fe2O3 nanoparticles are not stable under the
conditions necessary for the hydrolysis and condensation of TEOS (high pH, solvent), PVP is
used to stabilise the nanoparticles. The polar amide group interacts with the surface of the
Fe2O3 nanoparticles while the long polymer chains form a protective layer around the
nanoparticles. In order to introduce pores in the SiO2 layer, CTAB is added to the reaction
mixture. The long hydrocarbon chain of the molecule is incorporated into the SiO2 shell during
the condensation reactions, and after the Stöber reaction is completed, the product is calcined
in air to remove all organic compounds leaving pores behind.
As shown in the TEM images in Figure 2.26, single Fe2O3 nanoparticles are covered by a porous
SiO2 shell. The stabilisation of the Fe2O3 nanoparticles before the reaction prevents the
formation of large agglomerations. Even after the calcination step at 350 °C the particles can
be dispersed in various solvents, and no agglomeration occurs.
37
Figure 2.26: TEM images of Fe2O3 nanoparticles covered by a porous SiO2 shell.
While usually the Stöber reaction is carried out in ethanol, it was found that the product
quality can even be improved by using isopropanol instead. The Stöber process is known for
the necessity of high solvent amounts to ensure the formation of single particles. In the
introduced system the solvent volume can be reduced by 30 vol% without changing the
formed product. If the solvent volume is decreased beyond this value, particle agglomeration
occurs, lowering the quality of the core-shell material. While single Fe2O3 nanoparticles are still
encapsulated by a SiO2 layer, the encapsulated nanoparticles form large, intergrown networks
that cannot be separated.
Table 2.1: Modification of the SiO2 shell thickness covering Fe2O3-175 nanoparticles (values per 100 mg Fe2O3).
Sample name
Shell thickness
TEOS added
Molecular
(nm)
(µL)
weight of PVP
Fe2O3@SiO2 – 17
17
252
1,300,000
Fe2O3@SiO2 – 25
25
360
1,300,000
Fe2O3@SiO2 – 37
37
600
360,000
Fe2O3@SiO2 – 53
53
1200
360,000
The thickness of the SiO2 layer can be varied over a wide range: in changing the amount of
TEOS added to the reaction mixture, the shell thickness can be tuned between 17 and 53 nm
(Table 2.1 and Figure 2.27). Apart from the amount of TEOS, the type and amount of PVP has
to be varied to ensure the formation of single core-shell nanoparticles. In order to account for
the changes in the ratio of number of nanoparticles to the amount of silica precursor, the
stabilisation of the nanoparticles must be adjusted by choosing PVP polymers with different
chain lengths. If very thin SiO2 layers are targeted (Fe2O3@SiO2–17 and Fe2O3@SiO2–25), PVP
molecules with high molecular weights are necessary to stabilise the Fe2O3 nanoparticles
during the synthesis. While the stabilisation with long PVP molecules is improved, the resulting
SiO2 layer is less homogeneous compared to the use of shorter PVP polymers. If PVP with a
lower molecular weight is used under otherwise the same conditions, only long chains of coreshell nanoparticles can be obtained and the formation of single particles is not possible. When
38
the amount of TEOS is slightly increased (Fe2O3@SiO2–37), it is possible to use PVP with a
lower molecular weight. The shorter chains interfere less strongly with the formation of the
SiO2 layer, leading to a homogeneous shell around the Fe2O3 nanoparticles as shown in Figure
2.27. The SiO2 layer of the Fe2O3@SiO2–37 nanoparticles appears to be much smoother and
more even compared to the core-shell nanoparticles prepared by using PVP with a higher
molecular weight.
At high concentrations of silica precursor (Fe2O3@SiO2–53), free PVP molecules must be
removed before the addition of TEOS. PVP molecules not adsorbed at the Fe2O3 surface form
micelles that lead to the formation of separate SiO2 spheres when high amounts of TEOS are
added to the reaction mixture. To ensure the exclusive formation of core-shell nanoparticles,
free PVP molecules are removed by centrifugation before the Stöber reaction is carried out.
Figure 2.27: Comparison of Fe2O3@SiO2 nanoparticles of different shell thickness using TEM: I) Fe2O3@SiO2–17; II) .
Fe2O3@SiO2–25; III) Fe2O3@SiO2–37; IV) Fe2O3@SiO2–53.
The preparation of Fe2O3@SiO2 nanoparticles with different shell thicknesses is important for
the following modification steps of the overall synthetic concept (see Figure 2.21): while the
formation of TiO2 and ZrO2 shells is carried out more easily with Fe2O3@SiO2–53 nanoparticles
(see Chapter 2.4.2), the metal exchange reaction was found to benefit from thin SiO2 layers
(for more details Chapter 2.4.3).
39
Figure 2.28: Characterisation of the porous SiO2 shell around Fe2O3 nanoparticles using SEM (Fe2O3@SiO2 – 37).
The surface of the core-shell nanoparticles is characterised by SEM. Exemplarily, the SEM
images of the Fe2O3@SiO2–37 sample are shown in Figure 2.28. The overview on the left
proves once again the successful formation of single particles and the absence of large
agglomerations. The porosity of the SiO2 shell is already indicated in the SEM image on the
right side, where small pore openings can be seen on the surface.
Figure 2.29: N2 physisorption isotherm of the Fe2O3-175 nanoparticles encapsulated by a porous SiO2 shell
(Fe2O3@SiO2 – 37).
The N2 physisorption isotherm and the pore size distribution shown in Figure 2.29 confirm the
presence of micropores and small mesopores in the SiO2 shell of the material. The steep incline
of the isotherm at low relative pressures is due to the adsorption of N2 in micropores. The
micropore volume of the material is 0.16 cm3/g, compared to a total pore volume of
0.42 cm3/g. The isotherm continues to incline at slightly higher relative pressures indicating the
presence of large micropores and small mesopores. This is confirmed by the calculated pore
size distribution: apart from micropores, the SiO2 shell contains pores in the range of 2 to
4 nm. The hysteresis at high relative pressures can be assigned to interparticle voids. For the
BET surface area of the Fe2O3@SiO2-37 sample a value of 422 m2/g is calculated based on the
sorption data.
40
If the thickness of the SiO2 shell is changed, the general pore system stays unchanged, yet the
absolute values for pore volume and surface area vary due to the different relative fraction of
the unporous Fe2O3 nanoparticles. The pore size distribution shown for the Fe2O3@SiO2–37
nanoparticles in Figure 2.29 is representative for all Fe2O3@SiO2 nanoparticles.
Finally, the crystalline phases of the core-shell nanoparticles are characterised using X-ray
diffraction (Figure 2.30). The SiO2 shell is amorphous which is expected due to the lowtemperature preparation method. In contrast, the Fe2O3 core of the material shows sharp
reflexes, proving the high degree of crystallinity of the Fe2O3-175 nanoparticles already
indicated in the TEM and SEM images. The amorphous SiO2 shell leads to the slightly increased
background especially at low diffraction angles. If the thickness of the SiO2 shell is increased
the contribution of the amorphous phase is even higher, leading to an increased background.
Figure 2.30: Characterisation of crystalline phases of the Fe2O3@SiO2 – 37 nanoparticles using XRD.
In brief, Fe2O3 nanoparticles were encapsulated by a porous SiO2 shell using the Stöber
reaction. For the preparation of Fe2O3 nanoparticles a synthesis under hydrothermal
conditions was chosen, in order to prepare unagglomerated, crystalline nanoparticles under
defined conditions. The size and size distribution of Fe2O3 nanoparticles can be changed by
tuning the reaction parameters. For the following encapsulation, the Fe2O3 nanoparticles are
stabilised by PVP prior to the Stöber reaction, which is carried out in isopropanol and under
alkaline conditions. The SiO2 shell thickness can be tuned by adding different amounts of silica
precursor: The change in the ratio of Fe2O3 nanoparticles to added amount of TEOS requires
different PVP polymers in order to optimise the stabilisation of the Fe2O3 nanoparticles during
the shell formation. In order to obtain a porous SiO2 shell, CTAB is added to the reaction
41
mixture. The final core-shell nanoparticles consist of a highly crystalline Fe2O3 core
encapsulated by an amorphous, porous SiO2 shell. This material is the basis for the following
modifications of the core and shell composition.
2.4.2 Variation of shell material
SiO2 is a popular support material for nanoparticles in heterogeneous catalysis, and an often
chosen material for the preparation of core-shell nanoparticles due to its properties like
thermal stability, chemical inertness and surface properties. Yet, other materials including
various metal oxides and carbon possess desirable properties in combination with metal or
metal oxide nanoparticles, as well. The core-shell geometry ensures the close proximity of
both components enabling interactions and distinct support effects. Moreover, the interaction
between the nanoparticle in the core and shell material is not affected by nanoparticle growth
and sintering, and therefore, stable over a wide temperature and time range.
Figure 2.31: General preparation scheme for the encapsulation of Fe2O3 nanoparticles in transition metal oxide
shells.
With the introduced core-shell system it is possible to vary the chemical composition of the
shell by adding a TiO2 or ZrO2 shell on top of the preformed SiO2 layer (see Figure 2.31). In
order to successfully coat the previously prepared Fe2O3@SiO2 nanoparticles with a 2nd shell in
step I, the solid must be carefully redispersed in ethanol (100 mg Fe2O3 per 133 mL ethanol).
The pH of the reaction mixture is increased by the addition of 0.6 mL NH4OH, before
0.38 mmol of the respective metal oxide precursor is added dropwise. The reaction mixture is
then heated up to 50 °C and kept at this temperature for 24 h under reflux. After the
hydrolysis reaction is completed, the reaction product is calcined at 400 °C, and the SiO2 layer
is removed by leaching with 1 M NaOH at 60 °C to obtain the final product (step II).
Because of the small size of the particles, the synthesis parameters must be adjusted to meet
the requirements of the formation of nanoparticles with a core-shell geometry. The synthesis
temperature, chosen solvents, pH of the reaction solution, precursors, aging time and
calcination treatment had to be be carefully tuned for the successful preparation of a TiO2 or
ZrO2 shell. On top of that, the preparation parameters are chosen and adjusted in a way that
the formation of a TiO2 and a ZrO2 shell is possible under the same conditions.
42
One of the challenges that has to be met by the synthesis is the stability of the MO2 (M= Ti, Zr)
shell after removal of the SiO2 template. The MO2 shell thickness is an important parameter in
this context: if the shell is too thin, the shell disintegrates after the supporting SiO2 layer is
removed. On the other hand, if the amount of the metal oxide precursor is increased, the
formation of separate MO2 particles can be observed. The formation of separate MO2 spheres
must be prevented to obtain a homogeneous product consisting only of Fe2O3@MO2
nanoparticles. The reactivity of the metal oxide precursor plays a crucial role in controlling the
formation of the MO2 phase. Furthermore, the reaction rate can be adjusted by increasing the
reaction temperature and by changing the pH of the reaction solution. Due to the small size of
the particles, the surface curvature is problematic for the formation of a stable shell: the
crystallites formed during the calcination need to be interconnected in order to form a
coherent network. This is only possible, if the crystallites stay small and do not grow into large
crystals. Finally, the core-shell structure of the material should be maintained and the
intergrowth of particles avoided. Especially in the case of ZrO2 it is often observed that the
particles agglomerate during the growth of the shell. With these factors in mind, a synthesis
concept was developed to prepare Fe2O3@MO2 yolk-shell nanoparticles based on the
Fe2O3@SiO2 nanoparticles.
The prepared Fe2O3@SiO2-53 nanoparticles described above were dispersed in ethanol using
ultrasonication to separate the particles. The core-shell nanoparticles with a thicker SiO2 shell
were chosen because of its beneficial influence on the stability of the final MO2 shell: high
curvatures of very small, spherical nanoparticles prevent the formation of a stable MO2 shell,
while the slightly larger Fe2O3@SiO2 core-shell particles were found to be ideal for the addition
of a MO2 shell. After re-dispersing the core-shell nanoparticles, the pH of the solution was
increased carefully with NH4OH. The base catalyses the hydrolysis of the metal oxide
precursors and influences the reaction rate: if the NH4OH concentration is too high, the
formation of separate MO2 particles is observed while in the absence of NH4OH the formation
of MO2 is kinetically hindered. Then the respective metal oxide precursor is injected dropwise
to the reaction solution using a syringe, and the reaction mixture is heated to 50 °C and kept at
this temperature for 24 h. The metal oxide precursors used in this step of the synthesis are
TBOT and ZBOT because the reactivity of both reactants can be adjusted. The added amount of
both reactants was chosen to balance the stability of the resulting MO2 shell on the one hand,
while preventing the formation of separate MO2 particles on the other. The increase in
temperature after the addition of TBOT and ZBOT is especially important for the formation of
the ZrO2 shell, which is difficult at room temperature. After 24 h at 50 °C the reaction mixture
is cooled down to room temperature and aged in the reaction medium for another 24 h. The
calcination step that follows is important for the stability of the MO2 shell, since crystallites are
formed and continue to grow during this step. The materials are calcined in air at a
temperature of 400 °C for 2 h to remove the organic compounds left in the shell and to create
43
intergrown crystallites on top of the still present SiO2 layer. After 2 h heating is stopped and
the crystal growth is terminated. In the final step, the SiO2 template is removed by leaching
with NaOH at 60 °C, and the final product is obtained.
The obtained Fe2O3@TiO2 yolk-shell nanoparticles were analysed by TEM (Figure 2.32). The
particles have an average diameter of 234 nm with a TiO2 shell thickness of 28 nm. All particles
contain a single Fe2O3 core no longer fixed in the centre of the particle by SiO2. Small TiO2
crystallites are already visible in the TEM images obtained at lower magnifications due to the
irregular contrast of the shell materials. Moreover, single crystallites are clearly visible in the
HR-TEM images on the bottom of Figure 2.32 which can be distinguished by the differently
oriented lattice fringes.
Figure 2.32: TEM images of Fe2O3 nanoparticles encapsulated by a TiO2 shell.
The polycrystalline TiO2 shell was further characterised using SEM (Figure 2.33). The small
crystallites are clearly visible on the surface of the TiO2 shell. The analysis with SEM confirms
the size of the crystallites of 10 nm already distinguishable in the TEM images presented
above, leading to the rough surface of the TiO2 shell observed. The single crystallites are
interconnected, forming a stable shell around the Fe2O3 core. In the SEM image on the right
side (Figure 2.33), pores between the TiO2 crystallites are visible, forming a network of pores
throughout the shell through which the Fe2O3 core is accessible. The Fe2O3 nanoparticles are
completely encapsulated by the TiO2 shells, and only very few broken shells and free Fe2O3
44
nanoparticles can be found throughout the sample using both TEM and SEM. Furthermore,
only single particles are found by using microscopy techniques, hence the formation of larger
agglomerations was prevented by adjusting the reaction parameters.
Figure 2.33: SEM images of the prepared Fe2O3@TiO2 nanoparticles after removal of the SiO2 template.
The crystal phases of the material are characterised by powder diffraction using Mo radiation,
and the diffraction pattern is given in Figure 2.34. Apart from the crystalline Fe2O3
nanoparticles the diffraction pattern identifies the TiO2 shell as anatase phase. The Fe2O3
nanoparticles contain hematite and maghemite domains, both iron oxides which differ in their
crystal structure.
Figure 2.34: Powder diffractogram of Fe2O3 nanoparticles encapsulated by a TiO2 shell using Mo radiation.
The diffraction peaks are slightly broadened by the small size of the crystalline domains in the
sample, yet the intensity of the peaks proves the distinct crystallinity of the components of the
yolk-shell material and the absence of large amounts of amorphous material.
45
The yolk-shell material consisting of an Fe2O3 core and a ZrO2 shell prepared by the same
synthesis are shown in Figure 2.35. The particles size is in the same range as that of the
Fe2O3@TiO2 particles with an average size of 239 nm. The particle size distribution ranges from
218 to 282 nm. The ZrO2 shell has an average thickness of 33 nm and appears to consist of
small crystallites, suggested by the change in contrast visible in the TEM images. Using higher
magnifications, the single crystallites of around 8 nm become visible due to the differently
oriented lattice fringes which can be distinguished in the lower two images shown in Figure
2.35.
Figure 2.35: TEM micrographs of Fe2O3@ZrO2 nanoparticles using different magnification factors.
While the Fe2O3@ZrO2 particles appear to have similar surface properties as the Fe2O3
nanoparticles covered by TiO2 based on the TEM images, the analysis using SEM reveals
differences in the surface roughness: The ZrO2 shell also contains interconnected crystallites
(as shown in the TEM images), yet the outer surface of the shell looks more even compared to
the TiO2 analogue (Figure 2.36, I). While the surface of the ZrO2 shell cannot be described as
smooth, the crystallites are much more connected and smaller compared to the observed TiO2
crystallites and therefore the ZrO2 surface appears more even. As shown in previous studies of
our group, the ZrO2 crystallites are linked by an amorphous siliceous compound formed during
the synthesis that enhances the stability of the ZrO2 shell [81]. Yet, pore openings are visible in
the ZrO2 shells through which the Fe2O3 core is accessible.
46
Figure 2.36: SEM (I) and STEM (II) image of Fe2O3 nanoparticles encapsulated by a ZrO2 shell.
On the right side of Figure 2.36 a STEM image of the same Fe2O3@ZrO2 particle is shown to
illustrate the yolk-shell nature of the material. Due to the different electron densities, the
Fe2O3 core and the ZrO2 shell are visible, and the void left behind by leaching the SiO2 template
appears much lighter. Both types of microscopy techniques used to characterise the material
reveal the exclusive formation of single particles. The addition of the ZrO2 shell does not lead
to the formation of larger agglomerates or the interconnection of ZrO2 shells, which is often
observed for nanoparticles encapsulated in ZrO2. Broken shells are scarcely found in the
sample, yet compared to the TiO2 encapsulated Fe2O3 nanoparticles the ratio of broken shells
is slightly higher.
Figure 2.37: Characterisation of Fe2O3@ZrO2 nanoparticles using powder diffraction (Mo radiation).
Powder diffraction was used to characterise the Fe2O3@ZrO2, as well. Both for the Fe2O3 core
and the ZrO2 shell the presence of crystalline domains is confirmed. The diffraction peaks are
broadened, confirming the small size of the present crystalline domains. In contrast to the
47
Fe2O3@TiO2 particles, for the Fe2O3 nanoparticles encapsulated by ZrO2 only crystalline
domains belonging to the hematite crystal structure can be found.
Apart from the development of a preparation pathway for the encapsulation of Fe2O3
nanoparticles in transition metal oxide shells, the modification of the platform material
Fe2O3@SiO2 was further extended to the preparation of Fe2O3@C nanoparticles (see Figure
2.21). The preparation pathway to the yolk-shell particles shown in Figure 2.38 differs from the
procedure introduced in this chapter due to the different chemical nature of the shell
materials.
Figure 2.38: Fe2O3 nanoparticles encapsulated by a porous carbon shell prepared by nanocasting procedure
introduced in Chapter 3.4.1.2.
The preparation of the porous carbon shell is based on nanocasting: the carbon shell is formed
by carbonisation of a polymer that was formed within a porous SiO2 layer. The developed
synthetic procedure to prepare Fe2O3@C nanoparticles is introduced and described in detail in
Chapters 3.4.1.1 and 3.4.1.2.
In this chapter the variation of the shell material of the initial Fe2O3@SiO2 particles is
described. The preparation pathway was developed in order to meet the challenges arising
from the small size and the core-shell geometry of the particles. Additionally, the synthesis was
adjusted to enable the preparation of a TiO2 and ZrO2 shell under the same reaction
conditions. Since the synthesis conditions have a strong impact on the properties and catalytic
performance, it is important to prepare materials of different chemical composition under
similar conditions in order to be able to truly compare the catalytic performance and other
properties. By tuning the reaction parameters, Fe2O3@TiO2 and Fe2O3@ZrO2 nanoparticles
with a yolk-shell structure could successfully be prepared. The encapsulation of the Fe2O3
nanoparticles with TiO2 or ZrO2 is complete, and broken shells are found only scarcely
throughout the samples. The transition metal oxide shells consist of small crystallites that are
interconnected, forming a stable, yet porous shell around the Fe2O3 nanoparticles.
Apart from the encapsulation of Fe2O3 nanoparticles in transition metal oxides, the
preparation of Fe2O3@C nanoparticles was realised. The preparation is introduced in detail in
48
Chapter 3.4.1.2 in a different context. Yet, the preparation of carbon encapsulated Fe2O3
nanoparticles complements the chemical variability of the model system introduced here.
2.4.3 Variation of core metal via metal exchange
The variation of the core nanoparticle is achieved by a metal replacement reaction. The
preparation pathway to the final material consists of three steps (see Figure 2.39) beginning
with the reduction of the Fe2O3 core to elemental iron at 600 °C using H2. In the 2nd step, 0.2 g
of Fe@SiO2 nanoparticles are then mixed with the noble metal precursors dissolved in H2O
(1.2 mL, 0.1 M) and stirred for 2 h. After a 2nd reduction step the remaining iron nanoparticles
in the core of the SiO2 shells are leached with concentrated HCl at room temperature (step III).
After washing the core-shell nanoparticles thoroughly, the material is dried, and the final coreshell nanoparticles are obtained.
Figure 2.39: Schematic illustration of the preparation of noble metal@SiO2 nanoparticles by metal exchange.
The development of this system began by verifying whether the SiO2 shell thickness of the
reduced core-shell nanoparticles had an influence on the metal exchange reaction. All four
samples introduced above were analysed, and the Fe2O3@SiO2-17 and Fe2O3@SiO2-25
nanoparticles showed the best results for the exchange of the iron cores with small noble
metal nanoparticles within the SiO2 shell. As a consequence, the particles with a SiO2 shell
thickness of 25 nm were used for the metal exchange reaction. The chosen noble metal
precursors are Rh(acac)2, K2PtCl4, RuCl3·H2O and K2PdCl4 that were selected due to their high
solubility in H2O. The concentration of the noble metal salts was kept constant at 0.1 mol/L to
realise the same reaction conditions for each noble metal.
The iron nanoparticles after reduction in H2 are surprisingly stable in an atmosphere containing
O2 despite their small size as confirmed by X-ray diffraction (XRD) (compare Figure 2.41).
Oxidation occurs, though, at slightly elevated temperatures, and due to the exothermic
reaction, once the reaction has started the complete sample is oxidised. The problem of
oxidation in air becomes more pronounced after the 2nd H2 treatment in the presence of both
iron and noble metal nanoparticles in the core of the material. Yet, the particles can be
carefully transferred into flasks for the following leaching step in air.
49
After reduction, the Fe@SiO2 nanoparticles and the noble metal precursor solution are mixed
and sonicated for 1 h. The intensive mixing of both reactants is proven to be very important for
the metal exchange reaction, and in using sonication the best results were observed. The use
of a magnetic stirrer was avoided because the magnetic iron nanoparticles cannot be welldispersed in this reaction environment. The last step of the overall metal exchange is the
removal of the remaining iron core within the SiO2 shells carried out with concentrated HCl. In
this step the complete removal of iron is important, especially with regard to the potential
application in heterogeneous catalysis. To ensure a complete dissolution of the iron
nanoparticles, the leaching step is carried out twice. The particles are washed with H2O until all
traces of HCl are removed.
Figure 2.40: TEM images of the reduced nanoparticles with an iron core and SiO2 shell.
After the 1st step, Fe@SiO2 nanoparticles are obtained as shown in Figure 2.40. Due to the
reduction of the Fe2O3 nanoparticles in the core of the material, the cores shrink, leading to
particles with a yolk-shell structure. Apart from the decreased size, it is observed that the
initially single Fe2O3 nanoparticle disintegrates into several small iron particles during the
reduction step. While the change of the core material is apparent, the SiO2 shell is not
influenced or damaged by the exposure to H2 at elevated temperatures. As visible in the TEM
image on the right side of Figure 2.40 the thin SiO2 shell is unchanged (compare Figure 2.27, I).
The slightly inhomogeneous SiO2 shell is a result of the stabilisation of particles with PVP and
not due to the reduction step (see Chapter 2.4.1).
50
Figure 2.41: Powder X-ray diffraction pattern of the reduced core-shell nanoparticles using Mo radiation.
The reduction of the Fe2O3 core is confirmed by powder diffraction (Figure 2.41). The only
crystalline phase present in the sample is the iron core. Due to the sharp diffraction peaks it
becomes clear that the crystalline domains of the iron particles present in the sample are
rather large. The SiO2 shell is amorphous, contributing only to the slightly elevated background
and the broad diffraction peak at low diffraction angles.
51
Figure 2.42: Core shell nanoparticles with various noble metal cores characterised by TEM. I) and II) Rh@SiO2; III)
and IV) Pt@SiO2; V) and VI) Ru@SiO2; VII) and VIII) Pd@SiO2.
After the reduction the core-shell nanoparticles are contacted with the noble metal precursors
for the metal exchange reaction, and in the last step the iron core is leached out. The resulting
material, consisting of small noble metal nanoparticles in porous SiO2 shells was characterised
by TEM. As shown in Figure 2.42, the SiO2 shells contain no longer Fe2O3 or iron nanoparticles
but small noble metal nanoparticles. The leaching of the iron particles with HCl is proven to be
successful because no remaining iron nanoparticles were found throughout the sample.
The TEM images show that while some SiO2 shells are completely empty after the removal of
the iron core, some noble metal nanoparticles can be found on the surface of the SiO2 shell.
52
Yet, most of the noble metal nanoparticles replace the iron core in the centre of the particles
and are fully encapsulated by the SiO2 shell. The size of the noble metal nanoparticles differs
dramatically from the previous iron and Fe2O3 core: the average particle size found for
rhodium nanoparticles is 3.25 nm, while the platinum nanoparticles shown in Figure 2.42 in
picture III and IV are 3.45 nm in average. The noble metal cores of Ru@SiO2 and Pd@SiO2
nanoparticles are, with values of 3.1 and 2.8 nm respectively, in the same size range.
The TEM images presented in Figure 2.42, moreover, prove the chemical stability of the SiO2
shell: the SiO2 shell is not damaged by the treatment with H2 at elevated temperatures nor the
leaching of the iron nanoparticles with HCl.
Figure 2.43: Powder diffraction pattern of Ru@SiO2 yolk-shell nanoparticles after metal exchange (Mo radiation).
The diffraction pattern in Figure 2.43 confirms the absence of large ruthenium particles which
could have formed during the high temperatures during the 2nd reduction step. Instead, the
small and broad reflections stem from the very small nanoparticles already seen in the TEM
images. The characterisation of the particle size using TEM can be problematic if the sample
contains small and very large noble metal nanoparticles. While the small nanoparticles are
easily seen, the large particles cannot be penetrated by the electron beam and appear as black
areas in the microscope that cannot be characterised using this technique. Additionally, by
preparing the TEM sample grid an involuntary exclusion of large particles can already occur at
an earlier point in the sample preparation. The diffraction pattern for the Ru@SiO2
nanoparticles, though, confirms the findings obtained by TEM.
53
In conclusion, core-shell nanoparticles consisting of a SiO2 shell and various noble metal
nanoparticles can be prepared by metal exchange based on the initial Fe2O3@SiO2
nanoparticles. The Fe2O3 core is reduced in the 1st part, and afterwards the resulting Fe@SiO2
nanoparticles are mixed with the respective noble metal precursor. After another treatment
with H2, the iron nanoparticles are removed with HCl and the final product is obtained. Small
rhodium, platinum, ruthenium and palladium nanoparticles encapsulated by a porous SiO2
shell can be obtained by the metal exchange reaction with iron. The resulting nanoparticles are
much smaller compared to the initial Fe2O3 nanoparticles of an average size of 73 nm: the
particle size ranges from 2.8 nm for ruthenium to 3.45 nm for platinum nanoparticles. Even
though the noble metal nanoparticles are too small to lead to intense diffraction peaks, the
absence of reflexes for iron in the diffraction patterns of the final core-shell nanoparticles
proves the complete removal of the initial iron nanoparticles.
2.4.4 Critical assessment of metal exchange reaction
The introduced metal exchange reaction between the initial iron core and noble metal
nanoparticles is an attractive method to prepare core-shell nanoparticles with various noble
metal cores.
Nevertheless, open questions remain regarding the reaction between the iron nanoparticles in
the core of the material and the noble metal precursor. As described in the previous chapter,
after the impregnation of Fe@SiO2 nanoparticles with noble metal precursor solutions, a 2nd
reduction with H2 is carried out before the iron cores are leached. Yet, judging from the redox
potential found for the bulk counterparts, the oxidation of iron nanoparticles and the
reduction of the noble metal precursor should occur spontaneously (see Table 2.2). However,
this cannot be observed in the introduced system: the formation of noble metal nanoparticles
is not observed without the reduction step in H2. After leaching off the iron particles, only
empty SiO2 shells are found if the additional treatment with H2 is not carried out.
Table 2.2: Comparison of the redox potential of iron and platinum based on the bulk phase.
Redox pair
Potential, pH=0
Potential, pH=14
2+
-0.440
-0.877
2+
+1.188
-
Fe/Fe
Pt/Pt
Since many properties change when decreasing the particle size to the nano-range, the values
based on the bulk counterparts can only be used as an indication because the actual values for
nanoparticles are not known. But usually nanoparticles are considered much more active than
the bulk analogue, so following this general tendency it would be expected to apply to the
activity in redox reactions, as well.
54
Redox reactions in general are strongly dependent on reaction conditions such as the chosen
solvent, reaction temperature, pH and the type of reactants. Not only the ion itself influences
the reaction but also the ligands and counter ions can play important roles in the redox
process. The complexation of ions determines their stability, and therefore, the standard
potential. Also the concentration is a critical parameter since it has an influence on the
dissociation degree of the ionic species. The last aspect that might influence the redox reaction
in the discussed system is the solvent. Usually H2O is the solvent of choice in redox reactions
because of the high solubility and dissociation of metal precursors. H2O has a high dielectric
constant (ε = 78.3) which is important for the ion dissociation and support of the electron
transfer. For other solvents like ethanol (ε = 24.3) or aceton (ε =20.7) much lower values are
observed for the dielectric constant, leading to weaker electrostatic stabilisation of the
involved species and an impaired electron transfer. Due to this interplay of various parameters
the calculated potentials of redox pairs often cannot be observed in experiments which can be
the result of kinetic inhibition or undesired side reactions
[82]
. Yet, a thermodynamic origin is
not probable in this particular case given the potential difference is more than 1.5 V under
standard conditions.
To analyse the network of parameters possibly influencing the redox reaction, various
synthesis parameters were changed. Yet, to be able to draw clear conclusions, only one
parameter was changed at a time.
In order to identify possible causes for the apparent hindrance of the redox reaction in the
metal exchange reaction, the influence of all involved reactants must be studied. Iron
nanoparticles are supposed to be the reducing agent in the system. Due to the high reactivity
of the nanoparticles, the formation of a thin oxide layer on the surface of the particles has to
be assumed, even if it cannot be identified by characterisation techniques (e.g. XRD). In order
to verify whether the reaction is not occurring because of a passivating oxide layer, the metal
exchange reaction was carried out in a N2 atmosphere. The Fe2O3 core is reduced in H2
following the conventional protocol and then immediately transferred into a flask under
protective gas atmosphere, followed by the metal exchange reaction carried out under N2. In
spite of the fact that the formation of an oxide layer is not possible under these conditions, the
redox reaction between the iron nanoparticles and the noble metal precursors was not
observed. Yet, an oxide layer can not only form in air but also by contact with water and so the
chosen solvent was varied, as well. Degassed solvents like ethanol and acetone were used
instead of H2O to analyse the influence of the solvent on the metal exchange reaction under
otherwise same conditions (temperature, concentrations etc). Nevertheless, the formation of
platinum nanoparticles could not be observed.
55
Figure 2.44: Reference experiments with macroscopically sized iron in different solvents.
As shown in Figure 2.44, even with macroscopic iron the redox reaction was only observed in
H2O, visible by the precipitation of zero-valent platinum in the solution. The reaction carried
out in acetylacetone under otherwise the same reaction conditions did not lead to the
formation of platinum. To have a closer look at the iron foils employed in these reference
experiments, the foils were carefully washed, and energy dispersive X-ray spectroscopy (EDX)
analysis and mapping was carried out (Figure 2.45). On the surface of the iron foil that did not
lead to an apparent reaction in acetylacetone, small platinum clusters can be found.
Consequently, the redox reaction is occurring, but much slower than in H2O.
Figure 2.45: EDX analysis of the surface of the iron foil used for the reaction in acetylacetonate.
What has to be taken into account at this point, is the use of different platinum precursors
which was necessary due to the solubilities of the platinum precursors. While generally K2PtCl4
is the chosen platinum precursor, the noble metal salt had to be exchanged due to its low
solubility in any solvent except H2O. Because of the influence of the concentration of the
precursor solution on the redox reaction, this parameter was kept constant.
Nevertheless, the noble metal salt as reactant can also affect the redox reaction: the ligands of
the noble metal ion influence the redox potential as discussed before. Moreover, the redox
reaction can be kinetically hindered if the ligands of the noble metal precursor are not suitable
56
to form an ionic complex with Fe2+/3+ ions. The Fe2+/3+ ions are formed during the redox
reaction, and suitable counter ions have to be present in the reaction solution. The
stabilisation of Fe2+/3+ ions in the reaction solution is crucial because otherwise the reaction will
not occur. In a further experiment, Pt(NH3)2Cl2 (cis-Pt) was used as platinum precursor which
contains Cl– - ligands that are suitable to stabilise iron ions. Additionally, cis-Pt is soluble in
both H2O and DMSO to allow the comparison of different solvents. Yet, only the combination
of iron foil with cis-Pt dissolved in H2O led to an obvious reaction.
While the observations indicate why the redox reaction with Fe@SiO2 nanoparticles does not
occur in other solvents than H2O, it is still not clear why the reaction in H2O can be observed
using a iron foil but not using Fe@SiO2 nanoparticles. In order to check if the metal exchange
reaction shows temperature dependency, the metal exchange reaction was carried out in H2O
at elevated temperatures to increase the reaction rate. The general metal exchange protocol
was left unchanged, only the temperature was increased to 80 °C, 70 °C and 50 °C. Directly
after the heat treatment the material was analysed with TEM (Figure 2.46) and XRD (Figure
2.47 and Figure 2.48).
Figure 2.46: TEM images of Fe@SiO2 nanoparticles before the metal replacement reaction (I), and after the reaction
at 80 °C (II), 70 °C (III) and 50 °C (IV).
For comparison sake, the particles before the metal exchange reaction at elevated
temperatures are shown in the 1st TEM picture. The particles after the redox reaction look
completely different than before: long needle-shaped structures are visible and most SiO2
57
shells seem to be empty, despite the fact that the TEM images were taken before the leaching
step with HCl. Furthermore, a trend is visible in the samples treated at 80 °C, 70 °C and 50 °C:
while long needle-shaped structures clearly dominate in the sample treated at 80 °C, the
formation is less pronounced in the other two samples, yet still visible.
To characterise these newly formed structures, powder diffraction was carried out. In Figure
2.47 and Figure 2.48 the diffraction pattern for the samples treated at 80 °C and 50 °C are
compared. In both samples various iron oxide phases are present, while only in the diffraction
pattern of the latter sample iron is still present in crystalline form. Yet, no diffraction peak for
platinum is visible in the diffraction pattern. Instead of the formation of platinum, H2 is formed
during the redox reaction which was observed during the increased temperature treatment by
gas evolution.
Figure 2.47: Characterisation of the crystalline phases present in the product after the redox reaction in H 2O at
80 °C using powder diffraction.
58
Figure 2.48: Diffraction pattern of the reaction product after the reaction at 50 °C in H2O.
A blank experiment in H2O was carried out to analyse the influence of the enhanced
temperatures irrespective of the presence of platinum precursors. After the reaction, the
original shape of the core-shell particles is still visible in some areas while the formation of
needle-like structures is also observed (Figure 2.49). Yet, the change of the particle shape and
structure is not as pronounced as in the sample heated up to 80 °C in the noble metal
precursor solution.
Figure 2.49: Results of the blank experiment with Fe@SiO2 nanoparticles in H2O at 80 °C.
The diffraction pattern of the sample heated up to 80 °C in H2O looks similar to the ones
obtained for the core-shell nanoparticles subjected to the metal exchange reaction at elevated
temperatures. As already suggested by the TEM pictures, several iron oxide phases are formed
while also zero-valent iron remains in the sample after the heat treatment.
59
Figure 2.50: Identification of the formed crystalline compounds during the blank reaction in H2O.
While the targeted redox reaction was clearly not successful in H2O at elevated temperatures,
the reaction was repeated in ethanol at 50 °C to verify, whether the increased temperature
would lead to platinum particle formation in a different solvent than H2O. The TEM images
shown in Figure 2.51 right after the metal exchange look promising due to the retained
structures, but the analysis of the material with XRD showed that while indeed no iron oxide
phases are formed as before, also no crystalline platinum is found in the sample. The only
crystalline phase in the diffraction pattern is iron.
Figure 2.51: Fe@SiO2 nanoparticles after metal replacement reaction in ethanol at 50 °C.
To summarise, the reaction seems to be kinetically supressed in other solvents than H2O even
at higher temperatures. Though the reaction in H2O can be observed by using a macroscopic
iron foil, no reaction occurs when Fe@SiO2 nanoparticles are used- even if the formation of a
protective oxide layer is prevented, the formation of platinum particles is not observed. When
60
the reaction is carried out at higher temperatures, the iron nanoparticles act as reducing agent
but instead of noble metal nanoparticles, H2 is formed. Accepting the data, the question of the
apparent hindrance of the spontaneous redox-reaction between iron nanoparticles and the
noble metal precursors remains unanswered.
The question remains why noble metal particles are formed after a second reduction step (as
shown in the previous chapter). It is not probable that they result from residues of the noble
metal solution, since the samples were carefully washed after exposure to the noble metal
solution. Thus, reduction of noble metal species by the iron nanoparticles might have occurred
to a very limited extent. The second reduction step might be required in order to agglomerate
the noble metal species into a single nanoparticle. From the size of the noble metal particles
formed, one can estimate less than monolayer coverage of the iron particles by the respective
noble metal. If the noble metal species were indeed so highly dispersed, these species would
most certainly be lost during the leaching step with HCl. The reduction at high temperature
could thus only serve to collect the noble metal species into one particle which would be
retained within the SiO2 shell during leaching of the iron core. However, proving these
hypotheses would require a new series of experiments.
2.5 Summary and conclusion
The development of a synthetic approach of core-shell nanoparticles with flexibility regarding
their chemical composition was in the focus of the last section. The core and shell materials
were chosen to meet the requirements of solid catalysts: while noble metal and transition
metal oxides were chosen as core material due to their superior activity - and in the case of
transition metal oxides low cost - chemically and thermally stable materials like SiO2, TiO2 and
ZrO2 were used as shell to encapsulate the catalytically active metal nanoparticles and prevent
agglomeration. Important for an application in heterogeneous catalysis are also the
preparation conditions of the core-shell nanoparticles. To ensure the comparability of the
catalytic performance of the respective core-shell catalyst, the conditions under which the
core and shell material is prepared remains the same for the different realised chemical
compositions.
The starting point of the various modification pathways are Fe2O3@SiO2 nanoparticles. A
hydrothermal synthesis was used to prepare the Fe2O3 nanoparticles in a controlled and
reproducible manner. L-lysine was added to the aqueous solution of FeCl3·6 H2O to control the
size and shape of the resulting product. The reaction time and duration can be adjusted to
tune the resulting particle size and size distribution from smaller Fe2O3 nanoparticles with an
average size of 41 nm and a narrow size distribution, to nanoparticles with an average size of
75 nm and a broader size distribution. The slightly larger nanoparticles were chosen for the
61
development of the synthetic approach due to the higher yields that could be obtained per
reaction batch. In the following step, the Fe2O3 nanoparticles were encapsulated by a porous
SiO2 shell using the Stöber method. The thickness of the SiO2 shell can be varied from 17 to
53 nm by adjusting the reaction parameters slightly- especially the stabilising agent PVP plays
an important role in the realisation of SiO2 shells of different thickness. The porosity of the SiO2
shell is introduced by using the porogen CTAB which results in micropores as well as small
mesopores in the range of 2 to 4 nm within the SiO2 shell after the calcination step. The
conditions of the Stöber reaction were optimised to prevent the formation of large
agglomerates, the success of which was confirmed by TEM and SEM techniques as well as DLS
measurements, and to yield individual core-shell particles that can be dispersed in various
media.
The latter aspect is highly important for the modification reactions that were developed based
on the Fe2O3@SiO2 system. To prepare composite materials with a different shell material, the
initial core-shell nanoparticles were dispersed in ethanol, and a Stöber-like reaction was used
to prepare a TiO2 and ZrO2 layer on top of the initial SiO2 shell. The challenge of this reaction
step is the adjustment of the reaction parameters that allows the formation of a stable TiO2 as
well as a ZrO2 shell under the same conditions. The reactivity of the titanium and zirconium
precursors was tuned by adjusting the used solvent, pH of the reaction solution and reaction
temperature. After the formation of the 2nd shell, the initial SiO2 shell was removed by
leaching, and the final material with a yolk-shell structure was obtained. The TiO2 shell has an
average thickness of 28 nm and consists of small anatase crystallites. The crystalline layer
completely encapsulated the Fe2O3 nanoparticles, and no agglomeration was observed. Similar
results were found for the ZrO2- encapsulated Fe2O3 nanoparticles: the ZrO2 shell has an
average thickness of 33 nm, which is very close to the value found for TiO2. Small ZrO2
crystallites are visible in the TEM images, yet the surface characterised with SEM seems rather
smooth in comparison to the TiO2 shell.
The chemical composition of the core is changed via a metal exchange reaction with the initial
Fe2O3 core. Therefore, the material is reduced in H2 at elevated temperatures and impregnated
with noble metal precursors dissolved in H2O. The success of the metal exchange reaction was
found to be dependent on the SiO2 shell thickness: the initial Fe2O3@SiO2 nanoparticles with a
thinner SiO2 layer led to better results than when using nanoparticles with a thicker shell. After
the metal exchange reaction, the particles are subjected to a 2nd heat treatment under
reducing atmosphere, and afterwards the remaining iron cores are leached with HCl and the
final particles are obtained containing platinum, rhodium, palladium or ruthenium
nanoparticles. The size of the as prepared noble metal nanoparticles is very small, ranging
from an average diameter of 2.8 nm in the case of palladium and 3.45 nm for platinum
nanoparticles.
62
Yet, open questions regarding the course of the metal exchange reaction remain, as illustrated
in Chapter 2.4.4. In the system consisting of Fe@SiO2 nanoparticles and noble metal precursors
the redox reaction leading to zero-valent noble metal nanoparticles should occur
spontaneously, eliminating the necessity of the 2nd reduction step. Despite the values of the
redox potential for the bulk equivalents, the reaction could not be observed. Since redox
reactions are severely influenced by several reaction parameters such as pH, solvent, chosen
reactants and temperature, the system consisting of Fe@SiO2 nanoparticles and various
platinum precursors was systematically analysed. Yet, no major parameter could be identified
that hinders the redox reaction but most likely a network of several factors influences the
described system and prevents the redox reaction to occur spontaneously.
Notwithstanding, the introduced system based on Fe2O3@SiO2 nanoparticles allows the
preparation of materials with various chemical compositions under similar conditions, making
it to an ideal model system for solid catalysts. In choosing Fe2O3 nanoparticles as core and SiO2
as shell material, a system is created that can be used as a platform for the realisation of
various core and shell materials prepared under similar conditions. This simplifies the
screening of different core-shell materials in heterogeneous catalysis and other applications.
63
3 Modification of Hierarchical Zeolites
3.1 Introduction and motivation
While the synthesis of highly functionalised materials is considered very desirable, it is still a
huge challenge to find material classes which allow a high degree of structuring and ordering;
above all, when searching for a versatile model system in which various parameters can be
easily adjusted.
Zeolites are one of the longest known material class with a high degree of crystallinity and
ordered porosity, resulting in unique properties
[20]
. Several of these make zeolites an ideal
component of bi- or multi-functional materials: due to their crystalline nature, zeolites are
highly stable; they have a high surface area and possess acidic properties. Another important
parameter is that by changing the composition of the zeolite, many properties can be tailored
precisely to the respective application (see Chapter 3.2.1.1 and 3.2.1.3). In adding metal
nanoparticles to the material, a highly versatile multi-functional material can be obtained with
varying chemical composition and properties. In the preparation of multi-functional materials
the right balance between each phase and active site has to be found. A high dispersion of
metal nanoparticles within the zeolite is crucial for a close proximity of the active sites. In using
conventional zeolites the homogeneous distribution of metal nanoparticles can be challenging:
due to the micropores, the deposition of the nanoparticles often occurs only on the crystal
surface and not within the crystals (see Chapter 3.2.3.1). A good alternative are hierarchical
zeolites containing a secondary pore system in the meso-range. Nanoparticles can be
incorporated into the larger holes and voids of the zeolite, leading to an improved dispersion.
Moreover, the additional mesopores reduce the mass transport limitations of purely
microporous zeolites and lead to a better performance in catalysis (see Chapter 3.2.2). In a
system consisting of hierarchical zeolite crystals and metal or metal oxide nanoparticles many
parameters can be synthetically varied: apart from a tuneable composition, the porosity and
structure of the system can be tailored.
In Chapter 3.4 two approaches are introduced for the preparation of bi-functional materials
with hierarchical structure consisting of zeolites and metal or metal oxide nanoparticles. Both
methods follow different strategic pathways and synthetic approaches: whereas the synthesis
introduced in Chapter 3.4.1 describes the growth of individual, hierarchical zeolite crystals
around transition metal oxides, a different synthetic pathway is summarised in Chapter 3.4.2:
hierarchical ZSM-5 crystals are modified with noble metal and transition metal oxide
nanoparticles by impregnation.
Zeolite-based bi- or multi-functional materials can be considered as excellent model systems
for the preparation of materials with tailored properties and structures due to the vast
64
possibilities to tune crucial parameters. Furthermore, many catalytic applications can be
considered for zeolite-based materials. The application of bi-functional materials in catalysis
profits from the intensive studies already available for conventional zeolite catalysts. New
reaction systems can be realised due to the use of bi-functional catalysts based on zeolites. An
exciting field are consecutive reactions that are conventionally carried out in two systems.
With the development of tailored bi-functional catalysts it could become possible to carry out
consecutive reactions in one pass. The direct DME synthesis and the Fischer-Tropsch reaction
(see Chapters 3.2.3.3 and 3.2.3.4) are only two examples for that. The beneficial effects of
carrying out consecutive reactions in one pass are both related to the overall activity of the
catalyst and the selectivity of the reaction [83-84].
3.2 State of the art
3.2.1 Zeolites in industry and catalysis
According to the International Zeolite Association (IZA) zeolites are aluminosilicates with an
open, three dimensional framework structure that is composed of corner-sharing TO4
tetrahedra (T= Si, Al). The definition can be expanded to include other T-atoms such as
phosphorous, titanium or boron. Even though the exact definition of the material class
“zeolites” is still subject to vivid discussion, it is generally agreed upon that zeolites reveal a set
of unique properties that stem from their crystalline framework structure. Ever since their first
scientific analysis by Cronstedt in 1756, zeolites and related materials have been a fast
developing field, and new structures are continuously explored and developed. The term
“zeolite” goes back to Cronstedt who observed that the mineral he analysed began to bubble
upon strong heating
[85]
. These observations are reflected in the name “zeolite” based on the
Greek zeo (to boil) and lithos (stone).
Depending on the structure type the crystals contain regular channels or interlinked voids in
the micropore range. Cations balancing the anionic framework are loosely associated to the
framework oxygen and can be exchanged. Apart from these counter ions, mainly alkali metal
and alkaline earth metal ions, water molecules can be found inside the channels and voids of
the zeolite crystals so that a general elemental formula of zeolites can be described as
[
]
. Because of the large number of framework types and
possible chemical compositions, the nomenclature of zeolites follows generally accepted rules:
if a natural analogue exists, the specific zeolite is named after the mineral. In other cases the
material of distinct chemical compositions is named after the scientific institution developing
the specific zeolite [86].
To implement a general system for classifying the vast number of materials, the IZA introduced
a system based on the framework topology that describes only the 3-dimensional connectivity
of the T-atoms, whereas the exact composition of the materials is not specified. Each
framework type is given a three-letter code (e.g. MEL, MFI, FAU) which is based on the most
65
prominent, specific zeolite of the respective framework type – exemplary, the code of the
framework type LTA is based on the zeolite Linde Type A.
After the first scientific report on zeolites by Cronstedt, research activities gained momentum
in the 1930s with the systematic study of the synthesis under high temperature and pressure.
The companies Union Carbide and Mobil Oil got involved in zeolite research in the 1940s and
1960s, resulting in new synthetic principles and novel applications for zeolite materials, so that
in 1999 around 4200 publications on zeolites were published. Today, 213 structure types are
documented by the IZA. Despite this huge number of materials, only around 17 are of
commercial interest today
[87]
. The application of zeolites in industry began in the 1950s with
the successful use of FAU- type zeolites in oil refineries. Today a wide range of processes are
based on the unique properties of zeolites: zeolites are used as laundry detergents, in gas
separation and agriculture, as pigments and as catalysts for processing crude oil.
8
7
Detergents
13
Catalysts
Natural Zeolites
72
Adsorbents
Figure 3.1: Worldwide annual zeolite consumption (wt% of total 1.8 Mio t) by major application (2005). Natural
zeolite consumption of China and Cuba are not considered (> 2.4 Mio t p.a.)
[87]
.
LTA-type zeolites are used in laundry detergents as water softeners which makes up for
72 wt% of the annual zeolite consumption worldwide (Figure 3.1). Even though only 13 wt% of
the annual zeolite consumption accounts for catalysts, these materials have the highest
market value in the field of zeolites.
Apart from synthetic zeolites, natural zeolites are used in industry as well. Natural zeolites are
used in construction materials e. g. to enhance the strength of cement- China and Cuba use
more than 2.4 million tons of natural zeolite per year only for this purpose. Compared to the
annual zeolite consumption of the rest of the world, 8 wt% are made up of natural zeolites for
the use as nutrient release agent in agriculture, as odour control agent in animal husbandry or
pet litter. The application of zeolites as adsorbents accounts for 7 wt% of the annual zeolite
consumption (see Figure 3.1).
After this short overview over the most important fields of application, the description of the
use of zeolites in industry continues in greater detail in Chapter 3.2.1.3. The reason for the
66
wide-spread application of zeolite materials in such diverse fields is based on their unique
properties, which are described in the following chapter.
3.2.1.1 Properties of zeolite materials
Many properties of zeolites are directly linked to the crystal structure of the material.
Properties like ion exchange and adsorption capacity, shape selectivity and catalytic activity
are determined by the structure of the respective zeolite. This makes the classification of
zeolites according to their framework type sensible and improves the clarity of the field when
taking into account the huge number of different materials that are classified as zeolites. As
already introduced above, each framework type is labelled with a three-letter code. The
framework type is a simple description of the connectivity of the TO4 tetrahedra in the highest
possible symmetry [88]. The framework influences the properties of the final material on several
levels: the size and shape of the pore openings, the dimensionality of the pore system, the
volume and arrangement of the cages, and the type of cation sites are directly connected to
the framework structure. The primary building blocks of the framework are TO4 tetrahedra
where T are tetrahedrally coordinated cations like silicon, aluminium, boron, phosphor, or
titanium. Additionally, zeolite structures can be considered in terms of secondary building
units (SBUs), which are arrangements of linked tetrahedra observed in several framework
structures. In the examples shown in Figure 3.2 only the topology of the tetrahedral cations is
given, which will be linked by oxygen in the actual structures.
Figure 3.2: Typical SBUs in zeolite frameworks including rings (I), double rings (II), cages (III) and chains (IV)
[89]
.
SBUs include rings with different numbers of tetrahedrally coordinated cations, double rings
and cages. Especially the sodalite cage (Figure 3.2, III left), also known as the β-cage, is of
importance because it is the key SBU in several important zeolite types, e.g. zeolite A and X, as
well as sodalite itself. Characteristic chains are observed in several zeolite types, too. The
pentasil chain (Figure 3.3, I) is, for example, observed in both the MEL- (e.g. ZSM-11) and the
MFI- (e.g. ZSM-5) framework structure. The chains are connected by oxygen bridges to form
sheets (Figure 3.3, II). If these sheets are connected to adjacent sheets by centres of inversion,
the MFI- framework is obtained. The final MEL- topology is the result of the connection of
adjacent sheets by mirror planes instead (Figure 3.3, III and IV) [90-91].
67
Figure 3.3: Common features of MEL and MFI framework: pentasil-chains (I) are linked together to form sheets (II),
which can link to adjacent sheets either by centres (III) to give MFI, or by mirrors (IV) to give MEL
[89]
.
Even though both framework structures are closely related, the pore system of both types is
different: zeolites of the MFI- type have straight pores and sinusoidal pores perpendicular to
the straight ones, whereas the MEL- type zeolites contain a system of perpendicular straight
pores.
Channels and pores play a major role for the properties of zeolites, particularly because of the
open framework structure of the material. The size of the pore openings is determined by the
limiting ring size of the channel. Zeolites with pores described by planar eight-membered rings
(8MR) are considered small pore zeolites with pore openings in the range of 4 Å. Slightly larger
pores are obtained with 10MR— the pores have a size around 5.5 Å and the material is
classified as medium pore zeolite. So called large pore zeolites contain pore openings with
12MR, which leads to pores of around 7.5 Å. The rings limiting the pore access can be distorted
as well, so the sizes of the pore openings vary slightly [89]. Apart from the size of the pores, the
geometry of the pores is of great importance. The channels inside the zeolite can be uniform
or non-uniform and can intersect with other channels. A different possibility is the connection
of cages by windows to create a pore system throughout the crystal. Another aspect is the
connectivity of the pore space. Three cases can be distinguished: if the connectivity is onedimensional (1-D), the zeolite contains single channels that are not connected. In the case of a
2-D pore system any point in a plane in the pore system can be accessed from any other point
in that plane. And lastly, the pore space is considered to be 3-D if any part of the pore space is
accessible from any other point within the zeolite crystal. ZSM-5 is an example for a zeolite
with a 3-D pore system, whereas the pore space of mordenite is considered 1-D because of the
limited access from one channel to the other.
As already mentioned above, the channels and pores of a zeolite are not empty: apart from
water molecules, cations occupy definite positions within the pore system surrounded by
partially negatively charged framework oxygen. The preference for certain positions (as
illustrated in Figure 3.4) has energetic, steric and coordinative reasons and in the presence of
adsorptive molecules redistribution can occur [86].
68
Figure 3.4: Typical positions of extra-framework cations in zeolite A (I) and faujasite (II)
[89]
.
Apart from the framework structure, the properties of the zeolite materials are strongly
influenced by the composition of the framework. The ratio of silicon to aluminium atoms in the
framework has a strong influence on the chemical and physical properties. The Si/Al- ratio
cannot be smaller than one, because Al-O-Al groups in tetrahedral coordination are
energetically unfavourable (also known as Loewenstein rule). But nevertheless, a wide range of
Si/Al- ratios can be realised. The general classification discriminates three cases: low silica
zeolites with a Si/Al- ratio smaller than 8, intermediate silica zeolites (8 < Si/Al < 40) and finally
high silica zeolites in which the Si/Al- ratio exceeds the value of 40. For synthetic zeolites a
phase breadth with respect to the Si/Al- ratio exists for each structure (see Figure 3.6).
Zeolites are colourless powders which density varies with the openness of the respective
framework structure, but generally lies in the range between 1.9 and 2.3 g/cm3. Since the
cations inside the pores are mobile, zeolites show ionic conductivity. The ionic conductivity
depends on the water content, the diameter of the pores, and the number of cations present
in the channel system [86].
The cations can be exchanged with different cations or ionic organic moieties which can
strongly influence the properties and catalytic behaviour of the zeolites. A change in the
nature and distribution of cations can lead to different apparent pore diameter and charge
distribution and thus alter the adsorption capacity. The selective adsorption of molecules is a
unique property of zeolites: due to the regular pore system, zeolites can be used to separate
molecules of different sizes. The zeolite pores are not rigid and thermal effects can be
observed. The concept of kinetic molecular diameter and effective pore sizes were introduced
to take into account that the exclusion of molecules which diameters are similar to that of the
pore openings of the zeolite is temperature-dependent
[86]
. The adsorption properties of a
zeolite also depend on the Si/Al-ratio: aluminium-rich zeolites preferably adsorb polar
molecules, however, the hydrophobic character can be enhanced by increasing the siliconcontent of the framework. At a Si/Al-ratio of about 40 a transition from hydrophilic to
hydrophobic characteristics is observed [92].
Another unique property of zeolites is the possibility to host shape-selective reactions inside
their pore system. Those can occur if the pores of the zeolite catalyst are of similar dimensions
to those of the reactants, transition states or products. The shape selectivity can either be
based on the size of the reactants: only molecules that can enter the pore system and diffuse
69
to the reactive sites can partake in the reaction, or shape selectivity is introduced by the size of
the products. That is when the more bulky of two or more products is trapped within the pores
and cannot migrate out of the zeolite. The 3rd possible interference of the zeolite on the
reaction pathway is the restricted transition state shape selectivity based on different spatial
requirements of possible transition states.
Figure 3.5: Brønsted acidity of hydrogen zeolites and their interaction with a base.
In the H-form, zeolites are solid acids with both Brønsted as well as Lewis acid sites. The most
common way to produce acidic zeolites is by ion exchange with ammonium ions. NH3 is then
removed by calcination to give the H-form of the zeolite. Brønsted acidity is based on protons
that are bound to framework-oxygen (Figure 3.5). The number and strength of these acid sites
strongly depend on the Si/Al-ratio of the framework. With decreasing aluminium -content the
number of acid sites decreases, while the acid strength reveals the opposite trend. Yet, at high
Si/Al ratios only isolated acid sites occur, and the acid strength converges to a maximum value.
The calcination of zeolites in the H-form above 400 °C causes decomposition of hydroxylgroups and leads to the formation of Lewis acid sites. As the dehydroxylation takes place under
similar conditions to those used for ammonium decomposition, both types of acidic sites can
be simultaneously formed.
Despite their open structure, zeolites are of high thermal and hydrothermal stability, which
varies with the structure type as well as the Si/Al- ratio [93]. The ability to retain their structure
at high temperatures and in the presence of water vapour is of great importance for the
catalytic applications of zeolites (see also Chapter 3.2.1.3).
Up to date only one example of a zeolite with toxic properties has been found: erionite is
oncogenic due to the fibrous crystal morphology (similar to asbestos)
[86]
. Though, extensive
studies on the effect of zeolites on waterborne organisms and plant growth in rivers and slowmoving waters reveal no harmful effects resulting from the extensive use of zeolites in laundry
detergents. In Germany, the critical workspace concentration for zeolites has the same level as
dust.
70
The characteristics of zeolites are strongly influenced by their crystal structure on the one
hand and by the composition of the framework on the other. Presently, 213 different
framework structures are known, each one leading to specific ion exchange and adsorption
capacities, shape selectivities and catalytic properties of the material. Additionally, the
composition of the framework can be chosen to be tailored to specific properties. Thus, the
material class of zeolites can be rightfully considered as unique in their amplitude of
changeable properties and research interest remains at a very high level.
3.2.1.2 Synthetic principles for the preparation of zeolites
In order to synthesize zeolites, conditions of the natural formation of zeolites are mimicked:
the mineralising effect of water and OH-- ions on reactive sources of silica and alumina is used
in the presence of cations. Only the time frame is shortened by using elevated temperatures
and pressures
[86]
. The chosen synthesis conditions along with the precursor materials and
solvents are of great influence on the obtained zeolite phase, crystal shape and size and
properties of the final product. Therefore, various approaches have been developed in order
to tune the zeolites properties and to meet specific requirements.
Today, the basic technique for preparing zeolites is the hydrothermal synthesis. The first
reported synthesis of a zeolitic material dates back to 1862 when levynite was successfully
prepared under hydrothermal conditions
[94]
. The first systematic study of the hydrothermal
preparation of a zeolite was published in the 1940s by Barrer
[95]
. Classified as hydrothermal
are all reactions occurring under high pressure and temperature in aqueous solution. The
advantages of using hydrothermal conditions for the synthesis of zeolites are, among others,
the high reactivity of the reactants, an appropriate control of the reactions in solution and the
formation of unique condensed phases alongside low energy consumption
[96]
. The chemical
and physical properties of the reactants undergo a significant change under hydrothermal
conditions so that the reaction rate is accelerated and the hydrolysing reactions are
intensified. All hydrothermal reactions have in common that water plays several roles in the
synthesis: apart from being the solvent, water changes the chemical and physical properties of
the reactants, it participates in the reactions and transfers the pressure. Zeolite crystallisation,
which is usually carried out in sealed vessels, consists of several complex reactions including
solution-precipitation, polymerisation-depolymerisation and nucleation-crystallisation
[96]
. The
formation of zeolites under hydrothermal conditions is influenced by various parameters, such
as batch composition, reactant sources, pH, Si/Al- ratio, water content, possible presence of
template molecules, temperature, aging, stirring and seeding.
The composition of the reaction batch has a major influence on the zeolite phase obtained – as
illustrated in Figure 3.6 some zeolite structures allow a certain breadth in the initial
composition of the gel, but by only changing the concentrations of the reactants several zeolite
types can be obtained.
Possibly more surprising is the influence of different silicon and aluminium sources on the
synthesis product. Since the chemical and physical properties of the reactants have a
significant effect on the crystallisation process of zeolites, the right choice of silicon and
71
aluminium source is important
[97]
. For example, silicon sources differ in the distribution and
nature of silicate species which impacts the nucleation and crystallisation of zeolites.
Figure 3.6: Influence of batch composition on the resulting zeolite phase using a reaction temperature of 100 °C
(HS= hydroxy sodalite)
[93]
.
Another important parameter is the crystallisation temperature and time. Both nucleation and
crystal growth are strongly influenced by the synthesis temperature: an increase in
temperature leads to an increase in both the nucleation rate and the crystal growth rate. Since
the latter is more strongly enhanced, larger crystals can be obtained at higher temperatures.
Nevertheless, the crystal size is simultaneously dependent on the heating rate: at slow heating
rates, the number of formed nuclei is increased and the particle size therefore restricted. Apart
from the crystal size, the aspect ratio of the crystals can be varied by different temperatures.
This corresponds to the activation energy for the growth of crystal faces that is different for
each face
[98]
. While the crystallinity of the final product increases with crystallisation time, it
has to be taken into account that zeolites are metastable phases, and initial phases can be
successively replaced by more stable phases.
Other routes for the synthesis of zeolites were developed, as well, to tailor the properties of
zeolites in regards to crystal size and morphology.
In solvothermal syntheses zeolites are prepared in non-aqueous systems and alcohols or
ethereal solvents are used. All solvothermal synthesis procedures have a rather slow reaction
rate in common, leading to large zeolite crystals
liquid acts as both solvent and template
[100]
[99]
. In ionothermal synthetic routes an ionic
. If fluoride ions are used as mineralising agent
instead of OH¯-ions the syntheses often lead to exceptionally large zeolite crystals. The use of
F¯-ions shows also beneficial effects on the incorporation of heteroatoms in the framework.
The limitation based on the precipitation of transition metal ions in alkaline media when using
OH¯-ions can be overcome by using F¯-ions as mineralising agent [101]. The crystallisation from
zeolite precursor gels can be accelerated by exposure of the gel to microwave radiation.
Microwave-assisted hydrothermal syntheses use electromagnetic radiation in the range of 0.3
to 300 GHz for the preparation of a wide range of zeolite types including LTA, MFI and BEA [102].
Another possibility to prepare zeolites is the microemulsion-based hydrothermal route which
enables the use of stable dispersions of oil and water to control the size and morphology of
zeolite crystals
[103]
. The synthesis of AlPO4-5 crystals following a microemulsion-based
72
approach impressively shows the opportunities of the synthetic principle: the crystals obtained
from a conventional synthesis are of irregular hexagonal shape (shown in Figure 3.7, II),
whereas the microemulsion-based synthesis route results in the formation of long fibres
(Figure 3.7, I).
Figure 3.7: AlPO4-5 crystals made by microemulsion-based synthesis (I) and conventional synthesis (II)
[103]
.
The conversion of a dry amorphous aluminosilicate gel to crystalline ZSM-5 can be achieved by
the dry-gel conversion route developed in the 1990s [104]. The crystallisation takes place due to
the contact of the dry gel with vapour in a sealed vessel at elevated temperatures. The method
can be further divided into two different methods: the vapour-phase transport (VPT) method
describes the crystallisation of a dry gel in the presence of vapour of water and a volatile
structure directing agent (SDA). A 2nd possibility is the conversion of a dry amorphous gel
containing a non-volatile SDA in steam (steam-assisted crystallisation, SAC).
In brief, synthetic zeolites are prepared by generally mimicking the natural conditions leading
to zeolite formation, but higher temperatures and pressures lead to a much shorter time
frame for zeolite growth. As in any other crystallisation, the synthesis conditions of zeolites
have a strong influence on the obtained products: the zeolite type and the crystal size and
shape can be tuned by choosing a proper synthesis approach.
3.2.1.3 Industrial applications of zeolites
The rise of zeolites to industrial importance began in the 1950s with the commercially
successful application of FAU-type zeolites in oil refining by Union Carbide. Ever since, zeolites
have been used on an industrial scale due to their unique properties regarding ion exchange
and adsorption capacity as well as their catalytic activity.
The largest application is the use of synthetic zeolites as builders in detergents. Zeolite A
replaced the formerly used sodium triphosphate (STPP) to bind ions that cause water hardness
in laundry water (Figure 3.8, I). The replacement was environmentally driven due to the
negative impact of STPP on the ecological system: STPP lead to overfertilisation and
eutrophication of lakes and slow moving water systems, and restrictions on the phosphate
73
content of detergents were passed by law. The ion exchange capacity of NaA zeolites with
respect to Ca2+-ions is 160 mg CaO/g on dry basis at a temperature of 20 °C which is close to
the value achieved with STPP [105].
Whereas the uptake rate of Ca2+-ions is slightly lower compared to STPP, the rate of reaction
with Mg2+-ions of zeolite A is even lower (Figure 3.8, II). Since tap water contains considerably
lower concentrations of Mg2+- than Ca2+-ions, the preference of zeolite A is not problematic
regarding their use as water softeners [86].
Figure 3.8: I) SEM image showing NaA zeolite crystals used in detergents as water softeners. II) Ion exchange by
zeolite NaA as a function of time
[86]
.
Polycarboxylates are added to the detergent as cobuilders that initially bind and transport the
Ca2+-ions to the zeolite surface where the ion exchange takes place. Thus, the cobuilders act as
carriers for the ions whereas the zeolites store the exchanged ions. A positive side effect of the
use of zeolites in laundry detergents is the removal of trace amounts of Cu2+-ions present in
tap water that impair the bleaching effect of perborates and percarbonates. Additionally
laundry discoloration is prevented by the adsorption of dissolved dyes.
Also based on the ion exchange properties of zeolites is the removal of undesired and/or
radioactive cations from waste water [106].
Another property of zeolites that is industrially exploited is the adsorption capacity and
selectivity of some zeolite types. The selective adsorption of zeolites is the consequence of
three effects that can be tuned in order to meet the requirements of a specific application [86]:
1) Characteristic regular pores with defined pore sizes allow the separation of
molecules with different sizes (molecular sieve effect).
2) The separation of differently sized molecules can be based on the different
diffusion rates in the zeolite pores (kinetic selectivity).
3) The adsorption equilibrium is determined by electrostatic forces that are
dependent on the Si/Al-ratio of the framework.
The application fields of the selective adsorption of zeolites can be divided into the purification
of air and gases and the separation of chemicals. Among the purification processes, the
removal of water from gases and liquids is of great economic importance. Dehydrated zeolite
A and X are used in industry both in closed and open systems as drying agents. In many
74
applications it is of great importance to remove traces of water while preventing larger
molecules from being adsorbed and possibly undergoing catalytic reactions. This can be
achieved by using 3A zeolite whose pore diameter has been reduced to 3 Å by ion exchange.
The selective adsorption of moisture allows the application of zeolites in sealing and coating
agents, for example in the panes of insulating glass [107]. Zeolite A is used as a drying agent for
air, natural gas, alkenes and organic solvents [93]. Beyond the removal of water, zeolite A, X and
Y are established in several other purification processes of gases. Zeolite A and X are used to
remove CO2, H2S and organic sulphur compounds from natural gas. Hydrophobic, high alumina
zeolites are used to purify waste air from solvents, dioxins and odoriferous substances. The use
of zeolites is advantageous due to their high thermal capacity, nonflammability and their low
catalytic activity.
In a number of industrial processes zeolites are used for the separation of mixtures of
compounds (Table 3.1). Alternatively to carbon molecular sieves and activated carbon, zeolite
A is applied in the recovery of hydrogen from reformer, refinery and coke-oven gases [108].
Table 3.1: Selection of industrially important separation processes carried out with zeolite adsorbents
Process
Mixture
H2 production by removal of other
Reformer, refinery and coke-oven
gases (N2, CH4, CO, SO2, O2, C2H6)
gases
O2 production
Air
[86]
.
Zeolites
5A
5A, modified X
zeolites
C6-C10 distillate
Separation of n- from iso-alkanes
C10-C18 kerosene
5A
C5-C6, C10-C15, C10-C23 hydrocarbons
Separation of xylene isomers
C8 aromatics
Sr/BaX
Separation of alkenes from alkanes
C8-C18 hydrocarbons
Ca/SrX
A growing field is the production of oxygen from air. The separation of O2 and N2 is possible
with zeolite A and X because the N2 molecule is stronger bound than the O2 molecule [109]. The
separation of n- from iso-alkanes using 5A zeolites is of importance in the oil industry. While
the n-alkanes enter the pore system of the zeolite, the iso-alkanes are too bulky, resulting in an
immediate break-through
[110]
. The recovery of the zeolite is achieved by displacement of the
adsorbed species by NH3 or short-chain alkanes [86].
The acidity and shape selectivity of zeolites are the basis for their application in catalytic
reactions. The major fields of application are oil refining, conversion of gases and the synthesis
of organic intermediates. An overview of zeolite-catalysed processes in oil-refining is given in
Table 3.2. The most important process is the conversion of vacuum distillates and residues into
gaseous alkenes, gasolines and diesel fuel using ultrastable Y (USY) zeolites. Zeolite catalysts
replaced the prior used amorphous silica-alumina catalysts, leading to higher activities, higher
gasoline yields and a decreased coke formation. The latter is due to the technology change in
75
fluid catalytic cracking (FCC) from bed cracking to short contact risers that became possible
with the introduction of stable zeolite-based catalysts
[111]
. The addition of ZSM-5 to the
catalyst composition contributes to the increase of the octane number of the gasoline product
range due to the shape-selective cracking of unbranched hydrocarbons [112].
Table 3.2: Application of zeolite-based catalysts in oil refining
Oil fraction
Naphtha
Gas oil, lubricating oil
[86]
.
Zeolite- catalysed process
Zeolites
C5/C6 alkane isomerisation
Pt/H-mordenite, H-ZSM-5
Reforming of C6/C7 alkanes
Pt/K-L, Pt/BaK-L
Dewaxing/hydrodewaxing
H-ZSM-5, Pt/H-mordenite
Hydrogenation of aromatics/hydro-
Noble-metal zeolite
decylcisation
Vacuum distillates,
residues
FCC
RE-HY, USY, H-ZSM-5
Hydrocracking
Pd-, Pt-, Ni/Mo-USY
Residue hydroprocessing
Modified USY zeolites
More and more importance gains the cracking of carbon molecules with a high molecular
mass. In order to process these compounds, the catalyst matrix must have large pores and be
catalytically active to pre-crack large carbon molecules. Alternatively to catalytic cracking,
vacuum distillates can be treated by hydrocracking: in the presence of hydrogen and bifunctional catalysts gas oils are converted mainly into kerosene and diesel fuel. The zeolite
content of the catalysts ranges between 5 to 80 %, the noble metal content (platinum and
palladium) is much lower (0.5 %). Another important application of zeolite-based bi-functional
catalysts is the isomerisation of C5/C6 n-alkanes. The catalysts containing platinum on either
mordenite or H-ZSM-5 combine both acidity and hydrogen-transfer capabilities necessary for
the reaction (see also Chapter 3.2.3.3) [113]. Other applications of zeolite-based catalysts in oil
refining include the hydrogenation and hydrodecyclisation of aromatics, dewaxing of n-alkanes
and reforming of C6/C7- alkanes (Table 3.2).
In the field of gas conversion the methanol-to-gasoline (MTG) and methanol-to-olefin (MTO)
process profit from the functionality of zeolite catalysts. The MTG process includes the
conversion of methanol by H-ZSM-5 into the intermediate DME and then into short-chain
alkenes. These undergo oligomerisation followed by the conversion into alkanes and
cycloalkanes
[114-115]
. The MTO process is closely related to the MTG process but the object is
the production of C2- to C4-alkenes. Possible catalysts aside from ZSM-5 are narrow-pore
zeolites of the ERI- or CHA-type and structurally modified silicoaluminophosphates (SAPO-17,
SPAO-44)[116]. In a 2nd step, the short-chain alkenes obtained by MTO can be converted into
middle distillates.
The prospect of using shape selective catalysis for the production of organic intermediates led
to the introduction of zeolites as catalysts in the chemical industry. Industrial processes
catalysed by zeolites include the alkylation of aromatics with alkenes, the isomerisation,
oligomerisation and hydration of alkenes and the formation of amines from alcohols and
76
ammonia [86]. Ethylbenzene is an important intermediate for the polystyrene industry and can
be produced by the alkylation of benzene with ethene using ZSM-5- based catalysts[117]. The
shape selectivity of modified ZSM-5 is also used in the production of para-ethyltoluene from
toluene and ethylene
[118]
. The synthesis of para-xylene, an important intermediate in the
polyester industry, can be catalysed by ZSM-5. The great advantage of the application of ZSM-5
catalysts is their ability to isomerise xylenes with a minimum of side reactions and the shapeselective reaction to para-xylene [117].
The unique properties of zeolites enable numerous applications of industrial relevance in the
field of adsorption and catalysis. Whereas the largest application of zeolites (by weight)
represents the function as water softeners in detergents, zeolites applied as catalysts make up
for the most important use in a value-based perspective. The implementation of zeolite
catalysts in diverse industrial applications continues up to date, and with continued research
efforts in the field of zeolites and related materials novel applications continue to be realised.
3.2.2 Hierarchical zeolites
As illustrated in the section above, zeolites are widely used catalysts or catalyst supports in
industrial applications due to their unique properties regarding crystallinity, surface area,
acidity and ion exchange capacity. Nevertheless, the beneficial effect of the micropores in the
material regarding shape selectivity, the microporosity of the material has certain
disadvantages
[119]
. The micropores decrease the accessibility of the material, consequently
leading to a lower activity due to the inefficient utilisation of the whole catalyst particle.
Figure 3.9: Concentration profiles across a zeolite crystal at different values for the Thiele modulus (left) and
interdependence of Thiele modulus and effectiveness factor (right)
[120]
.
The degree of catalyst utilisation can be described by the effectiveness factor η. If η has a value
close to 1 the complete catalyst particle is used and the observed reaction rate equals the
intrinsic reaction rate and no transport limitations occur. In order to obtain a high value for η
77
the Thiele modulus Ø must be small. The Thiele modulus is defined as square root of the ratio
of the intrinsic reaction rate to the rate of diffusion multiplied with a characteristic transport
distance, and helps to determine whether a reaction is diffusion limited or not. For low values
of Ø the rate of diffusion is much faster compared to the intrinsic reaction rate. If the Thiele
modulus reaches higher values the reaction is diffusion limited. Because of the low diffusion
rate, the reactant concentration is lowered in the centre of the zeolite crystal, as shown in
Figure 3.9 on the left side. This leads to large spaces in which the catalyst is not efficiently
utilised, resulting in a low effectiveness factor η. The interdependence of the Thiele modulus Ø
and the effectiveness factor η is shown in Figure 3.9 on the right: high values of η can only be
achieved with low values of Ø and vice versa. If the Thiele modulus has a value of 10, the
effectiveness factor reaches a value of 0.1 (somewhat dependent on geometry), which means
that only 10% of the catalyst is used effectively [120]. In order to keep the Thiele modulus small,
two strategies can be applied since the intrinsic reaction rate kv is given for each reaction and
zeolite: shortening the diffusion length L or enhancing the effective diffusivity Deff in the zeolite
pores (see formula of Thiele modulus in Figure 3.9).
The development of ordered mesoporous materials (OMMs) like MCM-41 with regular pores
in the range of 3 nm lead to optimistic hopes to overcome the transport limitations of zeolites
in enhancing the effective diffusivity
[121]
. OMMs possess uniform pores just like zeolites, as
shown in the isotherms in Figure 3.10, but the pore sizes are approx. 5 times larger. The
diffusion regime in mesopore catalysts is typically bulk or Knudsen diffusion, leading to
diffusivities several orders of magnitude higher than in micropores. However, OMMs could not
replace zeolites in most applications due to the limited success in mimicking the properties of
zeolites like thermal stability and acidity.
Figure 3.10: Nitrogen isotherms (I) and pore size distributions (II) of characteristic porous solids
[120]
.
In a given zeolite the average diffusion length L can be decreased by decreasing the overall
crystal size to keep the Thiele modulus small. Enhancement of the effective diffusivity can be
realised by the preparation of zeolites with larger micropores or the addition of a 2nd pore
system. Following the 1st approach, new zeolite types were developed with pore diameters up
to 1.25 nm containing 12 or more T-atoms [122]. Most of these low density materials suffer from
low thermal stability, low acidity and unidirectional pore systems just like OMMs.
78
Aiming at shorter diffusion path lengths in micropores of existing zeolites, hierarchical systems
have been developed. Hierarchical porous materials contain multiple levels of porosity with
distinct functions. In the field of zeolites, hierarchical materials can be obtained by decreasing
the crystal size or by introducing an additional mesopore system within the individual zeolite
crystals [123]. The connectivity between both pore systems is of paramount importance in order
to improve the diffusion rate. Figure 3.11 illustrates different degrees and types of hierarchy in
zeolites.
Figure 3.11: Different degrees and types of hierarchy in porous solids.
A purely microporous zeolite is considered non-hierarchical due to the uniformity (in size) of
the pores (I). If the crystal size of this given zeolite is decreased, a network of mesopores (red
sphere in Figure 3.11, II) is generated between the crystals, leading to an interconnected
hierarchical system. Intraconnected hierarchical systems are illustrated in (III) and (IV): The
micropores of the zeolite are complemented by larger mesopores (red channels in Figure 3.11,
III and IV). In the 4th case in Figure 3.11 the mesovoid is entrapped within the microporous
matrix. To increase the catalyst effectiveness the mesopores need to be accessible from the
outer surface of the crystal (III). Systems II and III could lead to similar isotherms and pore size
distributions, resembling the one of mesoporous ZSM-5 shown in Figure 3.10. However, the
type and specific location of the mesopores have a strong influence on whether the
hierarchical system shows an improved catalytic activity compared to the parent zeolite. The
introduced secondary pore system must be carefully designed in order to lead to an
enhancement of catalytic properties
[120]
. The prime aim of the development of hierarchical
zeolites is the combination of the catalytic features of micropores with the improved access
and transport of larger pores in one material.
Various zeolite-based materials with improved accessibility, which have been developed over
the last years, can be divided in four classes (Figure 3.12). In all four classes the origin of the
79
mesoporosity is fundamentally different. Wide-pore zeolites are novel classes of zeolites with
substantially wider micropores compared to regular zeolite structures, as already introduced
earlier. The 2nd class of materials consists of nano-sized zeolites, and due to the small crystal
size interparticle voids occur, leading to an improved accessibility. The pore size distribution is
dependent on the packing of the zeolite crystals. In the case of zeolite composites zeolites are
supported on meso- or macroporous solids. The porosity of the support improves the mass
transport. In this material class the relative size of the zeolite crystals and of the support
particles strongly influence the pore size distribution.
Figure 3.12: Types of zeolite materials with improved mass transport characteristics
[120]
.
Lastly, mesoporous zeolites contain intracrystalline mesopores. The pore size distribution is
determined by the crystallographic micropores for a given zeolite and the additional mesopore
system. The size of the secondary pores depends on the preparation method used.
The preparation concepts for hierarchical zeolites can be divided in template-assisted and
template-free approaches. Template-free methods entail the preferential extraction of
framework atoms, as described in Chapter 3.2.2.1. The field of template-assisted preparation
of hierarchical zeolites is further divided in hard-templating (Chapter 3.2.2.2) and
supramolecular templating (Chapter 3.2.2.3) methods.
3.2.2.1 Removal of framework atoms
The removal of framework atoms to create intracrystalline mesopores is a post-synthetic
modification in which atoms are extracted from the zeolite framework. The most common
demetallation methods are the dealumination and the desilication, although over the last
years various new methods have been reported, such as detitanation[124] and deboration[125].
80
Since framework atoms are removed from the crystal, demetallation leads to a loss of a
significant part of zeolite mass.
Dealumination was originally developed as a post-synthetic method to obtain zeolites with
high Si/Al-ratios that were not feasible by direct synthesis
[126-127]
. The removal of aluminium
from the framework is accompanied by the creation of voids and channels, resulting from a
partial collapse of the zeolite structure. The various methods for dealumination can be divided
in two groups: the extraction is either realised by employing chemical agents or based on
hydrothermal treatments [128].
Mild acid leaching can be used to remove amorphous material from the pores. If the acid is
slightly more concentrated it can hydrolyse Si-O-Al-bonds and remove aluminium from the
zeolite framework. Mineral acids (e.g. nitric, hydrochloric or sulphuric acid) as well as organic
acids like oxalic or tartaric acid can be used
[129]
. The dealumination using chemical agents is
also realised with strong chelating agents, such as ethylendiaminetetraacetic acid (EDTA)
[130]
.
The treatment with EDTA leads to the preferential removal of aluminium atoms located on the
external surface, resulting in zeolite crystals with inhomogeneous aluminium-distribution.
For zeolites synthesized by the use of an organic template, already the calcination in air can
lead to the formation of larger pores in the crystals, depending on the chosen conditions.
During the calcination process, aluminium atoms are removed from the zeolite framework to
form extra-framework aluminium (EFAl). The EFAl can be dissolved with mild acid leaching,
leading to the final micro- and mesoporosity of the zeolite without further treatment. The
dealumination can be improved by not only heating the sample but by heating the sample in
the presence of water under hydrothermal conditions. The use of steam enhances the mobility
of aluminium and silicon species
[131]
. The steaming procedure is carried out at temperatures
above 500 °C with zeolites in the ammonium or hydrogen form. As for calcination, aluminium
atoms are removed from the framework by hydrolysing the Si-O-Al bonds, resulting in silanol
rich domains within the crystal. A mild acid leaching completes the treatment to remove extraframework species. As an example, zeolite Y crystals after hydrothermal dealumination and
following acid treatment are shown in Figure 3.13. The visible channels within the crystals
form during migration of silicon species (structure healing) which is also responsible for the
enhanced stability of the material.
Figure 3.13: Zeolite Y crystal after dealumination by steaming and acid leaching
[132]
.
81
During the steaming treatment voids and cylindrical pores, that partially connect the external
surface with the crystal core, are formed. While it is known that the treatment with steam
causes not only dealumination, but desilication as well, aluminium is more easily removed
from the framework under a steam atmosphere, as shown by DFT calculations [133].
Irrespective of the employed method it has to be taken into account, that by removing
framework aluminium severe changes in the acidic properties of the zeolite are caused.
Disadvantageous are the scarce interconnectivity of the voids and channels and the damage of
the microporous structure, as well.
Whereas the dissolution of silicon from zeolites in alkaline media was known and described in
various publications, its potential for the development of controlled porosity was unrecognised
for a long time[134]. Only in the year 2000 scientists described the formation of mesopores in
ZSM-5 crystals after treatment in alkaline media [135].
Figure 3.14: Comparison of SEM images of zeolite crystals before (I) and after (II) desilication treatment
[135]
.
Commonly, the zeolite is treated with low concentrated, aqueous NaOH at slightly elevated
temperatures in order to partly remove the framework silicon. The alkaline treatment leads to
a decrease in micropore volume while the crystallinity is preserved. The mesopore volume is
increased due to the partial dissolution of the zeolite crystal while the amount of Brønsted acid
sites is maintained after the alkaline treatment. The formation of larger pores is easily visible,
as illustrated in Figure 3.14. Whereas the parent zeolite (a) shows a smooth and even surface,
the crystal contains visible cracks and pores after the desilication treatment (b).
Various parameters influence the pore size and crystallinity of the treated zeolites, such as
concentration of base, temperature and duration of treatment
[136]
. The Si/Al-ratio of the
treated zeolite plays a major role in the desilication process, as the aluminium in the
framework supresses the extraction of neighbouring silicon species [137]. Three different cases
can be distinguished, as summarised in Figure 3.15: for low Si/Al- ratios (< 25), the high
aluminium content of the framework prevents silica from being removed. Consequently, the
mesoporosity generated is rather low. In contrast, at intermediate Si/Al-ratios (25-50) the
silicon is extracted in a controlled manner, resulting in the development of enhanced
mesoporosity. Thirdly, at high Si/Al- ratios (> 50), silicon is dissolved in excess, giving rise to
larger pores with lower mesopore surface area. In order to generate mesopores accessible
from the external surface, zeolite crystals with an isotropic distribution of aluminium in the
crystal are necessary.
82
Figure 3.15: Dependence of desilication process on Si/Al- ratio of parent zeolite
[119]
.
The described procedures allow the formation of mesopores in zeolite crystals, but quite often
at the expense of losing considerable amounts of microporosity due to the loss of material.
Since many of the active sites are located in the micropores, this could lead to a loss in
catalytic activity. To minimise losses in the micropore volume, the desilication process was
further
improved
by
substituting
the
NaOH
with
an
organic
base,
such
tetrapropylammonium hydroxide (TPAOH) or tetrabutylammonium hydroxide (TBAOH)
as
[138]
.
The use of organic base leads to slower desilication rates, thus resulting in lower mesoporosity
values and a minor loss of microporosity. The amount of TPA+ -ions can be varied in order to
tune the pore diameter, whilst preserving the existing microporosity. As a further
development, a partial detemplation-desilication method was proposed
[139]
. Based on the
observations, that the desilication of zeolites still containing template molecules is more
difficult compared to the calcined sample, the template was only partially removed by
controlled calcination. Since the contact of the OH--groups with the Si-O-Si –bonds is restricted
by the template, the extent of silicon removal is decreased. In that manner, a tailoring of high
mesoporosity while maintaining the microporosity and crystallinity is possible.
Even though ZSM-5 is by far the most intensely studied system, various types of zeolites can be
modified by desilication. The treatment has been successfully applied to many other zeolite
topologies, such as MOR
[140]
, BEA
[141]
, FER
[142]
and FAU
[143]
. Nonetheless, not all types are
equally suited. For example, due to the lower stability of aluminium in the framework of beta
zeolite, silicon is removed to a higher degree, generating mesoporosity at the expense of
micropore volume and crystallinity [141].
Numerous studies prove the effectiveness of the demetallation treatment on zeolites
regarding their catalytic activity
[144-145]
. The rate of diffusion is enhanced compared to the
purely microporous precursors resulting from improved accessibility and a distinct shortening
of the micropores. A study using fluorescence microscopy illustrated the improved accessibility
of the micropores in the hierarchical zeolite obtained by desilication
[120,
146]
. The
83
oligomerisation of 4-methoxystyrene was measured with in situ confocal fluorescence
microscopy revealing a more uniform coloration in mesoporous zeolite crystals compared to
the microporous parent material (Figure 3.16). The interplay between enhanced mass
transport resulting from shorter diffusion path lengths and preserved acidity is essential for
the improved performance of demetallated zeolites.
Figure 3.16: Comparison of the oligomerisation of 4-methoxystyrene in non-treated ZSM-5 crystals and desilicated
crystals using confocal fluorescence microscopy
[120]
.
The post-synthetic removal of framework atoms to create mesoporous zeolite crystals is an
often applied and well-developed method. Dealumination is either carried out with chemical
reactants like EDTA and acids or by hydrothermal treatment. In contrast, alkaline solutions like
NaOH are used to remove silicon species from the framework in order to create mesopores
and voids. Several parameters like the concentration of the reactants and the Si/Al ratio of the
treated zeolite have to be carefully adjusted to obtain the desired degree of porosity while still
preserving the microporous regions of the crystals. The application of demetallated zeolites in
various catalytic reactions prove the effectiveness of the treatment and the benefits gained by
the additional pore system.
3.2.2.2 Hard-templating methods
The hard-templating technique has been extensively applied to the preparation of hierarchical
zeolites. In general, hard-templating approaches make use of hollow or porous solids as
matrices for the preparation of zeolites, which are crystallised within the cavities of the solid
template. The preparation of hierarchical zeolite single crystals following a hard-templating
method involves three steps, indifferent of the material used as template: in the 1st step a
solution containing the reagents and zeolite precursors are infiltrated into the pores or cavities
of the solid template. Then crystallisation occurs within the solid template by heating the
reaction mixture under hydrothermal conditions. Thirdly, the template is removed by
combustion or dissolution after the zeolite crystals have formed
[119, 147]
. This final step is
generally considered as the main drawback of hard-templating techniques: the hard template
must be destroyed to set free the additional porosity within the zeolite crystals. Additionally,
the severity of the conditions can be detrimental to the microporous properties of the zeolite.
Compared to the described desilication process, studies show a higher amount of defects and
internal silanol groups for zeolite crystals after the combustion of the template. The stronger
84
damage of the zeolite structure results from the very high local temperatures generated during
the template combustion
[148]
. However, the use of hard templates in the preparation of
hierarchical zeolites poses the opportunity of tuning the mesoporosity to great detail.
Various materials are used for the preparation of hierarchical zeolite single crystals. A major
field is the preparation of hierarchical zeolites using carbonaceous templates. The use of
commercial carbons to obtain hierarchical zeolite crystals was a further development of the
confined space synthesis introduced by Christensen and his group for the preparation of nanosized zeolites [123].
Figure 3.17: Scheme of the confined space synthesis of nano-sized zeolite crystals introduced by Christensen et
[149]
al
.
The crystal size is controlled by hindering the zeolite growth within a carbon matrix, as
illustrated in Figure 3.17, with no or little encapsulation of the carbon template. The pores of
the carbon matrix are impregnated by the incipient wetness technique with a solution
containing the zeolite precursors, and the zeolite crystallisation proceeds under hydrothermal
conditions. After burning off the carbon matrix, nano-sized zeolite crystals are obtained. When
the reactant volume slightly exceeds the pore volume of the carbon matrix, the carbon matrix
is encapsulated during the synthesis. When the embedded template is removed after the
crystallisation, zeolite single crystals with a hierarchical pore system are obtained
[150]
. The
preparation principle is illustrated in Figure 3.18.
Figure 3.18: Preparation concept of using carbonaceous solids as hard templates
[119]
.
Apart from the volume of the zeolite precursor solution, various reaction parameters influence
whether nano-sized zeolite crystals or larger hierarchical zeolite crystals are formed. The
reaction temperature, the heating rate and the concentration of the zeolite precursor gel
influence the rate of nucleation and the rate of growth. If the rate of nucleation is increased
e.g. by rapid heating of the reaction mixture, high reaction temperatures and a high synthesis
gel concentration, the formation of nano-sized zeolite crystals is favoured. Whereas a high
85
growth rate leads to larger zeolite crystals with the carbon template embedded inside. The
rate of growth is enhanced by slow heating rates and low concentrated zeolite precursor
solutions. The resulting zeolite crystals (see Figure 3.19) are highly defective, but they are still
considered as single crystals [151]. The crystals exhibit a bimodal pore size distribution, owing to
the additional intracrystalline mesopore network with a broad size distribution between 10 to
100 nm interconnected to the zeolite micropores.
Figure 3.19: TEM (I)
[152]
and SEM (II)
[150]
image of hierarchical zeolite crystal after combustion of carbon template.
Because crystallisation is much slower in the presence of carbon templates, the crystallisation
time is prolonged compared to non-templating preparation concepts
[153]
. Various zeolite
frameworks have been successfully prepared by using commercial carbons as solid template,
including MFI [151, 154-155], BEA [154], MEL [156], and MWT[153].
Figure 3.20: Preparation of hierarchical zeolites using carbon nanotubes (I) and TEM image of resulting zeolite
crystal (II)
[157]
.
Instead of carbon particles, carbon nanotubes and -fibres can be included in the preparation of
hierarchical zeolites as well. The crystals obtained by this approach are characterised by
straight and uniformly sized mesopores as visible in the TEM image in Figure 3.20. The
preparation concept is illustrated on the left side in Figure 3.20
[158]
. In order to get single
86
crystals with intraparticle mesoporosity, the zeolite precursor gel must nucleate between the
voids of the carbon nanotubes or –fibres, and subsequently grow around the template. This is
achieved by the sequential impregnation of each component of the precursor gel [159].
Ordered mesoporous carbons such as CMK-1, -3 or -5 have also been applied as templates for
the preparation of hierarchical zeolite single crystals
[160-161]
. Because of their highly ordered
pore structure, zeolites obtained by this method contain more regular mesoporous structures
with smaller and narrower sizes compared to the zeolites templated with commercial carbon
particles. Moreover, the synthetic approach allows a certain degree of tailoring of structure
and mesoporosity of the final zeolite crystal by tuning the synthesis conditions of the carbon
template [121].
As an alternative to the costly and time consuming preparation approach using ordered
mesoporous carbons as templates, easier preparation routes using sucrose as carbon source
were introduced
[162-163]
. In order to prepare a porous carbon template, a mixture containing
sucrose, ethanol, water and ammonia were hydrothermally treated and the resulting solid was
calcined in nitrogen. The carbon was then impregnated with the zeolite precursor gel and after
the zeolite crystallisation removed by combustion.
Colloidal silica can be used as well to prepare carbon templates for the synthesis of
hierarchical zeolites. Depending on the particle size of the colloidal silica, the carbon templates
present uniform pore sizes in the range of 10 to 90 nm. The porosity and crystal size of the
final zeolite crystal are therefore determined by the particle size of the initially used silica
colloids [164]. The zeolite crystals presented in Figure 3.21 were prepared using ordered carbon
templates based on silica spheres of different sizes. The pore sizes of the final zeolite crystals
reflect the size of the initially used silica spheres to prepare the ordered carbon template.
Figure 3.21: Zeolite crystals templated with ordered carbon based on silica colloid of sizes of 10 nm (I), 20 nm (II)
and 40 nm (III)
[164]
.
Carbon-based templating techniques generally are characterised by a high degree of versatility
with respect to topology, crystal size and chemical composition of the final zeolites. Unlike
desilication (Chapter 3.2.2.1), carbon-templating allows independent control of acidity and
mesoporosity. Additionally, the size of the additional porosity can be varied over a wide range,
which is a great advantage of using carbons as solid templates.
87
Even though carbonaceous templates are by far the most commonly used solids for the
preparation of hierarchical zeolites, other materials, such as polymers, were successfully
applied as templates as well. Since polymers have only limited thermal stability, the
temperature of the zeolite crystallisation is restricted in this form of templating.
Polystyrene spheres can be used for the preparation of hierarchical zeolites (Figure 3.22). After
impregnation with a silicalite-1 precursor gel, the mixture was hydrothermally heated to
facilitate zeolite crystallisation.
Figure 3.22: Zeolitic walls around spherical voids resulting from the templating with polystyrene spheres
[165]
.
The final product obtained after calcination is highly crystalline, possessing hydrothermally
stable pore walls and a bimodal pore system, consisting of the intrinsic zeolite micropores and
additional macropores [165].
In a similar approach silicalite monoliths were prepared using latex beads as solid template.
The template was impregnated with alternating layers of preformed zeolite nanoparticles and
charged polyionic macromolecules (Figure 3.23, left side). Subsequent calcination resulted in
hierarchically ordered monoliths with controlled wall thickness and pore diameter (Figure
3.23, right side).
Figure 3.23: Latex beads coated with silicalite particles and polyelectrolytes (I) to prepare macroporous silicalite
monoliths (II)
[166]
.
88
A different approach is the preparation of hierarchical zeolites in using polymeric aerogels,
resulting in zeolites with additional mesopores in the range of 15 nm
[167]
. Natural polymers
were introduced as templating materials in an attempt to find inexpensive templates to
generate additional porosity during the preparation of zeolites. Especially starch has captured
attention due to the low cost and the good hydrophilicity, which favours the dispersion and
interaction of the template molecule with the Si-OH groups during the synthesis [168].
Apart from carbonaceous and polymeric templates, a wide variety of materials has been
introduced as templates in the preparation of hierarchical zeolites. Inorganic nanoparticles
were added to a zeolite precursor gel and trapped inside the zeolite crystals, e. g. CaCO3
particles which were removed by acid dissolution to open up the intracrystalline mesopores
[169]
. Interesting architectures can be obtained by using both synthetic and biological macro-
templates, containing up to three levels of porosity (micro-, meso- and macroporosity)
[170]
.
Concerning biological templates, bacterial threads, natural sponges and wood tissue have been
used as solid templates in the preparation of hierarchical zeolites [171-173]. But compared to the
earlier discussed carbon-based templates these attempts still have to be considered as slightly
exotic, isolated cases.
The use of hard templates for the preparation of hierarchical zeolites allows to finely tune the
mesoporosity of the resulting zeolite crystals while leaving other parameters such as the Si/Al
ratio unchanged. The major drawback of the introduced hard templating approaches is the
necessity to destroy the template to obtain the hierarchical product which makes the
techniques rather costly.
3.2.2.3 Supramolecular templating
Hierarchical zeolites can also be obtained by using supramolecular templates to generate
additional meso- or macroporosity within zeolite crystals. This form of templating is based on
an organised assembly of surfactant molecules. After crystallisation, the template is removed
by either calcination or extraction to set the additional mesoporosity free. All supramolecular
templating approaches can be categorised by the species interacting with the template:
methods in which the surfactant assists in the assembly of molecular species are summarised
as primary methods. In contrast, secondary supramolecular templating methods are based on
the assembly of partly crystalline species: initially formed protozeolitic units are templated by
a surfactant assembly to form the final product.
In primary templating methods the zeolite crystallisation takes place on the external surface of
the surfactant assembly or within the assembly. If the crystallisation is carried out within
microemulsions or reverse micelles, the supramolecular assembly acts as a nanoreactor and
nano-sized zeolite crystals are obtained [174-175].
For preparing hierarchical zeolite crystals the crystal growth has to take place on the external
surface of the supramolecular assembly. The idea of using surfactant molecules and their
assembly as template originates in the preparation of ordered mesoporous materials like
MCM-41 and MCM-48 in which the formation of surfactant micelles is used to generate
mesopores in the final material [176]. Scientists tried to mimic a similar method for introducing
89
additional mesoporosity in zeolite crystals: a mixture of a conventional SDA and a surfactant
template for generating mesopores was added to the synthesis gel in order to obtain
hierarchical zeolites
[177]
. Because the supramolecular aggregation and the molecular
templating work in a competitive manner, only bulk zeolite without additional mesoporosity,
amorphous mesoporous material, or a physical mixture of both could be isolated (Figure 3.24).
The temperature of the synthesis is an important parameter. A sequence of high-low- or lowhigh-temperature treatment was developed, because the necessary temperatures for the
formation of the two phases are not compatible. With a two-step heating treatment
mesoporous mordenite crystals could be prepared using ternary organic micelles as template.
The micelles used as template were built by a mixture of the three organic molecules
tetraethylammonium hydroxide (TEAOH), CTAB and n-cetylamine (CA) [178].
Figure 3.24: TEM image of MFI/MCM-41 mixture obtained by dual templating
[177]
.
The challenge in dual templating is preserving the stability of the mesoporous structure while
zeolite crystals grow in the walls of the material. The as-prepared materials tend to lose the
mesoscale order after calcination
[179-180]
. Dual templating approaches suffer from various
disadvantages: It is difficult to develop a working system, and complicated heating programs
are necessary to obtain mesoporous products. In addition to this, the materials’ mesoporosity
cannot be maintained in many cases.
Much attention gained the design of a polyquaternary ammonium molecule (Figure 3.26) as
template for the preparation of a zeolite-like microporous crystalline aluminosilicate
[181]
. The
so-called dual-porogenic molecule acts as the SDA as well as the mesopore generating
template. The template was specifically designed to generate hexagonal mesostructures by
aggregation as well as directing the crystallisation of the microporous framework by
quaternary ammonium groups.
Figure 3.25: Specifically designed 18-N3-18 –surfactant for templating hierarchically aluminosilicates
[182]
.
90
The size of the mesopores can be tuned by variation of the length of the alkyl chain. The
functional group linking the ammonium groups has influence on the zeolite structure: if an
alkyl group connects the ammonium groups (as in Figure 3.25) aluminosilicates with MFI
structure are obtained, whereas a phenyl group leads to the formation of BEA-type structures.
One of the drawbacks of this approach is the complex preparation of the template molecule.
Additionally the synthesis is restricted to very few zeolite types.
A slightly different angle is chosen by the usage of silane-based surfactants. To prevent the
surfactant to be expelled from the aluminosilicate domains during the crystallisation of the
zeolite crystals, surfactants containing a silane-functionality are chosen. The organosilane can
form a covalent bond with either SiO2 or Al2O3 moieties and thus interact strongly with the
growing zeolite crystal. The specifically designed class of organosilane surfactants contain
three key functional groups: apart from the hydrolysable methoxysilyl moiety, the molecules
contain a zeolite structure-directing group such as quaternary ammonium, and hydrophobic
alkyl-chains to enable a surfactant-like assembly of the template [183]. The template is added to
the conventional synthesis gel for the preparation of MFI or LTA zeolite crystals followed by
hydrothermal treatment resulting in mesoporous zeolite single crystals (Figure 3.26). The
organosilane- based template is incorporated into the growing zeolite crystals during the
synthesis. After crystallisation is completed the solid is calcined to remove the alkyl-chains and
to obtain the final, hierarchical product. The pore diameter can be tuned by varying the length
of the alkyl-chain between 2 up to 20 nm [184].
Figure 3.26: Mesoporous LTA zeolite synthesized using a specially designed organosilane template
[183]
.
Since organosilanes show a high affinity for silicate and aluminosilicate species as well as for
protozeolitic units, they find application in secondary supramolecular templating methods,
too. Secondary templating methods include all approaches in which the assembly of
supramolecular molecules is used to order crystalline or partly crystalline species such as
zeolite nanoseeds. Zeolite nanoseeds are small zeolite crystals, usually too small to show Bragg
peaks in XRD. Zeolite nucleation is promoted by these species by admitting AlO4- and SiO4connectivities that resemble the secondary structural units in crystalline zeolites.
91
Organosilane-based templates are used to prepare hierarchical zeolites by anchoring the
template molecules to protozeolitic species via a silanol-group. All preparation approaches
using the crystallisation of silanized zeolite seeds consist of four steps [119, 185]: the 1st step is the
pre-crystallisation of the zeolite gel to form protozeolitic species. The prior formed species are
then functionalised by reaction with organosilanes. Thirdly, the crystallisation is continued to
complete the zeolitisation of the functionalised units, and in the final step the sample is
calcined to remove the organic template and to generate mesopores.
Since the organosilane molecules on the surface of the zeolite seeds hinder the aggregation,
sponge-like aggregates of a size of around 300 nm, consisting of small zeolite crystals (510 nm) are obtained (Figure 3.27). Following this synthetic concept a variety of zeolite
structures was prepared, including ZSM-5, zeolite Beta[185-186] and mordenite[187] as well as
zeolites containing heteroatoms like titanium [188].
Figure 3.27: TEM images of ZSM-5 crystals prepared by silanized protozeolitic nano-units
[185]
.
The great advantage of organosilane- based techniques is the prevented separation between
the growing zeolite crystals and the surfactant. The pore sizes can be tuned to some extent
and various zeolite types can be prepared using this method.
Apart from organosilanes, conventional surfactant molecules are used as template for the
assembly of zeolite seeds. The protozeolitic species are used as building blocks which are
assembled into hexagonal, cubic, or wormhole framework structures. Highly steam-stable
mesoporous materials can be obtained by using CTAB to assemble FAU zeolite seeds [189]. The
well-ordered hexagonal structure of the so-called MSU materials is maintained upon heat
treatment (Figure 3.28). Apart from FAU-based zeolite seeds the method was used for the
controlled assembly of seeds based on various zeolite types, including MFI and BEA
[190]
. The
synthetic procedure allows tuning the acidity of the final material by varying the Si/Al-ratio
(ranging from 1.6:1 to 10:1).
A similar class of materials are the co-called MAS/MTS materials prepared by the assembly of
zeolite seeds using templates like CTAB or Pluronic P123
[191-192]
. The synthetic procedure is
very similar to the preparation of MSU materials: after the preparation of zeolite seeds, the
surfactant is added to the solution and the crystallisation of the zeolite seeds is continued in
presence of the surfactant assembly. To date, various MAS/MTS materials have been prepared
92
using this methodology, including zeolite seeds based on ZSM-5 [193], TS-1 [194], zeolite Beta [191]
and zeolite L [195].
Figure 3.28: Retained hexagonal structure of Al-MSU-S based on ZSM-5 (A) and Beta (B) seeds after steaming
[190]
.
The assembly of nanoseeds using supramolecular templates is a versatile tool to prepare
stable materials with zeolite-like properties, but in many aspects they lack the unique set of
characteristics of conventional zeolite single crystals.
In Figure 3.29 another possibility to prepare hierarchical zeolites using supramolecular
templates is illustrated, called delamination. Zeolites with layered precursors (e.g. MCM-22,
ferrierite) can be prepared with a surfactant intercalated between neighbouring zeolite
layers [196-197]. By swelling the layered structure, the lamellar zeolite is exfoliated layer by layer
while the structure of the layers is preserved.
Figure 3.29: Schematic illustration of the delamination method to prepare ITQ-2
[198]
.
As illustrated in Figure 3.29 the delaminated zeolites possess a sheet-like structure with
micropores in one direction and larger pores in the other two directions
[199-200]
. Thin zeolite
sheets with improved mass transport characteristics can also be prepared using the designed
triammonium surfactant presented in Figure 3.25 in a modified hydrothermal synthesis. The
resulting sheets correspond to a thin slice of MFI crystal with a thickness of only 1.5 nm [181, 201].
Zeolites prepared by these methods are generally characterised by a high thermal stability and
acidity typical for zeolite materials, and the accessibility of the active sites was proven to be
93
exceptional high. Granted that, especially the delamination approach is restricted to very few
zeolites and the method allows only restricted control over the final mesoporosity.
In supramolecular templating approaches for the preparation of hierarchical zeolites the
organised assembly of surfactant molecules is used to impart hierarchy to the crystals. The
combination of traditional SDAs for micropore formation and surfactants in one-pot methods
is challenging due to the competitive manner of molecular templating and supramolecular
aggregation, and no homogeneous product phase can be obtained. Yet, specially designed
surfactants could be developed that contain several functional groups with which the
formation of mesoporous zeolites could be achieved.
With all these techniques at hand- supramolecular templates as well as hard templates and
the demetallation approaches- it is possible to improve the accessibility of zeolite crystals to a
desired extent.
3.2.3 Multi-functional zeolitic materials
In various fields of application the combination of two or more functionalities in one material
is highly desirable. In heterogeneous catalysis, for example, consecutive reactions are often
necessary to obtain the final desired product. If this is the case, several catalysts with single
catalytic functionalities are generally used in separate reactions to isolate the desired product
which is illustrated in the upper part of Figure 3.30. Over the last years, catalysts with two or
more catalytically active sites have been developed to allow consecutive reactions using one
catalyst (Figure 3.30).
Figure 3.30: Illustration of the application of bi-functional catalyst in consecutive reactions.
94
Especially for equilibrium-restricted reactions this can lead to a significant improvement
compared to the use of two separate catalysts [202] . Furthermore, the activity and selectivity of
an existing catalytic process can be tuned by the addition of a 2nd active compound to the
conventional catalyst. In using bi-functional catalysts, the conventional product distribution
can be changed, or completely new reaction pathways can be opened up, leading to novel
processes
[203]
. Investigating the fundamentals of the chemical and physical processes taking
place in bi-functional catalysts is necessary to open up the potential of developing a catalyst
meeting all requirements of a specific reaction and to find the perfect-fit regarding activity and
selectivity. Bi- or multi-functional materials are not only developed in the field of
heterogeneous catalysis, but various fields, like electronics, optoelectronics and energy
storage, benefit from specifically designed materials [204]. Even without a specific application in
mind, existing boundaries in the development of materials are overcome in the design of novel
bi-functional materials.
In bi-functional materials the catalytic performance is dependent on the characteristic of both
active compounds, and the right balance between both active sites must be found
[205]
.
Additionally, the materials design must ensure the dispersion of one or both active
components and the accessibility of both types of active sites
[206-207]
. Whilst a high degree of
dispersion usually leads to an improved activity, the different types of catalytically active sites
should be located in close proximity in bi-functional materials as well, to ensure the migration
of the reactants from on active site to the other [208].
Zeolites are often chosen as one component of bi-functional materials. Various properties,
such as stability, acidity, high surface area, pore structure and ion exchange capability, make
zeolites an ideal starting point for the preparation of bi-functional materials [207]. The zeolite as
acidic matrix can catalyse additional reactions and the interaction between the active sites of
the zeolite and a 2nd compound can lead to novel catalytic properties. The micropores offer the
possibility to limit the size of deposited metal or oxidic nanoparticles. Likewise, the additional
pore system of hierarchical zeolites offers the possibility for the deposition of larger
nanoparticles and the functionalisation with organic moieties.
Because the nature of both components plays a major role in the properties of the final
material, great care is taken to adjust the composition of the bi-functional material. But in
order to use the full potential of the material, the chemical composition has to be
complemented by a well-organized structure [208]. If both active sites are randomly or unevenly
distributed, coupled reactions occur independently and the migration from one active site to
the other is not ensured. The structure of bi-functional materials is of paramount importance
and needs to be specifically designed in order to reach the full potential of bi-functional
materials.
All preparation approaches of bi-functional materials consisting of two phases can be
categorised by the chronology of each phase formation: the zeolite is either formed prior to
the 2nd constituent phase (see Chapter 3.2.3.1) or the 2nd phase is formed before or alongside
the zeolite formation (Chapter 3.2.3.2). Each approach has merits as well as drawbacks as
discussed in the following chapters. The importance of and examples for the application of bi95
functional materials based on zeolites are then introduced in the last chapters to complete this
section (Chapter 3.2.3.3 and 3.2.3.4).
3.2.3.1 Modification of zeolite crystals
The 1st possibility for preparing a material containing additional properties apart from the
zeolite-inherent characteristics is the modification of pre-crystallised zeolite crystals with
metal ions, small nanoparticles or metal oxide species. Various preparation methods have
been developed to achieve this post-synthetic modification of zeolite crystals.
The preparation of a bi-functional material in its most basic form can be attained by simply
mixing conventional catalyst particles with zeolite crystals. Simple bi-functional catalysts for
the Fischer-Tropsch reaction can be prepared by this method. Each compound is prepared
individually and then ground, sieved and mixed together to obtain the final catalyst [207, 209-210].
In other cases, a slurry of both pre-formed phases is prepared, then the slurry is filtered, dried
and calcined to form the final catalyst [211]. Today, physically admixed bi-functional catalysts are
still used as a benchmark for the comparison with structurally more advanced materials. The
great advantage is the simplicity of the preparation concept. On the other hand, the
inhomogeneous and unordered structure leads to an inferior performance compared to other
bi-functional catalysts because of the random distribution of active sites. The possible coupling
of the reactions can be hindered by the distance between both kinds of active sites.
Another pathway to functionalise zeolite crystals is by making use of the ion- exchange
capability of zeolites. The dried zeolite is mixed with the precursor solution containing the
ionic species that are supposed to be incorporated, followed by filtration and washing to
remove non-exchanged ions. In order to achieve a full exchange this sequence is repeated
several times [212]. The ion-exchange of zeolites can be carried out in solid state, as well.
Figure 3.31: Preparation of bi-functional material by solid state ion-exchange.
In order to bring the zeolite and the metal source in intimate contact, a mixture of the
reactants is ball milled at elevated temperatures (Figure 3.31). The heat treatment is necessary
96
to initiate the ion- exchange in the solid state. In order to obtain the final catalyst, the material
is calcined after the ball milling treatment.
The use of ion- exchange for the preparation of bi-functional catalysts allows the easy control
over the degree of exchange and therefore the composition of the final material. However, the
ion-exchange in liquid state can lead to major damage of the zeolite structure, depending on
the metal salt used: the pH of aqueous metal salt solutions can be rather low, leading to acid
catalysed damage in the crystal structure of the zeolite. In addition, the choice of suitable
cations is rather limited.
If the ion-exchanged zeolite is exposed to a reducing atmosphere, the formation of metal
nanoparticles is observed
[213]
. The loadings achieved by ion-exchange are usually rather low.
By using impregnation methods instead, higher contents of metal or metal oxide nanoparticles
can be obtained. A wide-spread method is the incipient wetness impregnation approach in
which the intended amount of precursor is dissolved in the amount of solvent that equals the
pore volume of the zeolite [214-216]. After impregnation the solid appears to be dry because the
liquid is completely absorbed by the pores of the material. In contrast to ion-exchange, as
much metal of metal oxide precursor as soluble in the respective amount of solvent can be
introduced to the pores of the zeolite and not just the amount determined by the ionexchange capacity of the zeolite.
Figure 3.32: palladium nanoparticles on MFI-type hierarchical zeolites prepared by impregnation before (I) and after
(II) catalytic reaction
[217]
.
The impregnation of zeolites can also be conducted in the vapour phase if suitable precursors
are available. Exemplarily, Y zeolite was mixed with Ru3(CO)12, sealed in a vessel and heated to
80 °C. At this temperature the vapour pressure of the ruthenium precursor is high enough to
allow the adsorption on the zeolite while the precursor is still stable. In order to obtain
ruthenium nanoparticles the adsorbed complex was decomposed at 420 °C and reduced in
hydrogen [215]. The sublimation of a precursor into the cavities of zeolite crystals is also used for
the preparation of iron containing zeolites. Different set-ups can be used in order to promote
the incorporation of iron species into the zeolite network by sublimation: apart from the prior
described heat treatment in a closed autoclave after mixing the reactants
[218]
, a U-shaped
97
reactor can be applied in which the zeolite and the iron precursor are filled into the two sides.
A porous frit keeps both compounds separated during the sublimation process. The chemical
vapour deposition is then carried out in an Ar flow by raising the temperature [219].
Apart from the variability of the system, the good interactions between the deposited phase
and the zeolite leading to an effective utilisation of the catalytic properties are benefits of the
preparation of bi-functional catalysts by impregnation
[207]
. Disadvantages of the incipient
wetness impregnation pathway arise from the strictly limited amount of solvent: only
precursors with high solubilities can be used for an efficient impregnation step. If this is not
the case, lengthy repetitions are necessary to obtain the intended loadings. Moreover, a
critical loading exists for most systems for the preparation of small particles in the zeolite
crystal. If this limit is exceeded, large particles on the surface of the zeolite crystals are formed.
Another challenge is illustrated in Figure 3.32: small nanoparticles can sinter and grow under
reaction conditions leading to a decrease in activity over time on stream.
A novel approach using a cold plasma treatment allows the incorporation of TiO2- clusters in
beta zeolite crystals to prepare photoactive catalysts
[220]
. To avoid the formation of large
particles deposited on the surface of the zeolite crystals, the process is divided into two steps:
titanium species at an atomic level were first introduced by direct-decomposition of the
titanium precursor by plasma. This was followed by an O2-plasma treatment in which the
titanium species within the zeolite pores were oxidised to form crystalline TiO2. The rather
complex set-up of this process makes it rather difficult to apply the developed route to other
materials.
Most of the described routes for the modification of prior formed zeolite crystals are highly
versatile approaches, and a variety of bi-functional materials can be prepared. Especially
methods based on ion exchange and impregnation are simple, straightforward pathways that
can be widely applied. Other methods were developed to prepare one specific material with
distinct properties, yet the application for the preparation of other materials is difficult.
Generally, any method that is used to modify prior formed zeolite crystals must be challenged
whether the zeolite structure is damaged by the involved techniques.
3.2.3.2 Incorporation during zeolite crystallisation
In contrast to the techniques described in the previous chapter, nanoparticles and other
components can be incorporated into zeolites already during the zeolite crystallisation. This
field of material preparation is much smaller than the one of post-synthetic modification of
zeolite crystals and mostly restricted to singular preparation methods for one specific material.
An exception to this is the approach in which metal salts are simply added into the zeolite
precursor gel in order to obtain a material with various catalytic active sites because several
metal oxide species can be incorporated following this method.
As illustrated in Figure 3.33 small nanoparticles can be incorporated into zeolite crystals by this
approach. In this explicit case RuCl3 was added to the synthesis gel and heated up under hydrothermal conditions. Since the formation of zeolites is a crystallisation process, the addition of
98
RuCl3 had a major influence on the crystallisation rate, and the time span of the hydrothermal
treatment had to be finely adjusted between two and 12 days [221].
Figure 3.33: Incorporation of RuO2 nanoparticles in ZSM-5 crystals. I) SEM image of the ZSM-5 crystals. II) and III)
HR-TEM images of incorporated RuO2 nanoparticles
[221]
.
The ruthenium species are reported to be oxidised under the hydrothermal conditions and the
following calcination step. The size of the RuO2 particles is very small, ranging from 0.5 to
1.5 nm (Figure 3.33, II and III).
Iron oxide containing ZSM-5 membranes and catalysts can be prepared by a similar approach,
as well [222-223]. Similar to the earlier described procedure, the iron precursor was mixed to the
zeolite gel, and zeolite formation was performed under hydrothermal conditions. At low iron
contents the crystallinity of the sample can be retained at a high level, but with increasing
Si/Fe- ratio the crystallinity decreases to a major extent, and the formation of impurities, such
as FeO and amorphous SiO2, is induced.
The described one-pot syntheses can be applied to non-zeolitic, acidic materials for the
preparation of highly active, bi-functional catalysts, too. A copper precursor was added to the
synthesis mixture for preparing meso-structured γ-alumina resulting in small copper
nanoparticles distributed evenly on the porous γ-alumina after a reduction step (Figure
3.34) [84].
Figure 3.34: SEM images of copper nanoparticles on structured alumina support for direct DME- synthesis
[84]
.
The copper- content can be varied over a wide range before the mesopore order of the
material is impaired by the introduction of copper species.
The incorporation of metal and metal oxide nanoparticles into zeolite crystals by adding
precursor salts to the zeolite synthesis gel is an easy and straight-forward method. The strong
99
alkaline conditions of the synthesis gels, however, limit this approach to precursor salts which
do not precipitate or form oxides under these harsh conditions even before zeolite
crystallisation leading to phase separation. In most described syntheses, the loading of metal
or metal oxide species reaches a limit beyond which the incorporation into the zeolite crystals
or the zeolite formation is not possible. Moreover, it is very difficult to control the properties
of the metal or metal oxide phase regarding size and oxidation state; crucial factors for the
application of the materials in catalysis (Chapter 3.2.3.3).
A slightly different approach is illustrated in Figure 3.35 on the left: pre-formed gold
nanoparticles were immobilised in amorphous SiO2 and then embedded into zeolite crystals
when the SiO2 was crystallised into a zeolite phase. The 1 to 2 nm small gold particles are
dispersed throughout the zeolite crystals, as visible in the TEM images in Figure 3.35 on the
right side. The particles size of the gold does not change upon heating which proves the full
inclusion of the nanoparticles (Figure 3.35, II, lower TEM image).
The great advantage of this approach is the possibility to control the size and shape of the
incorporated nanoparticles due to the synthesis of the particles prior to the incorporation into
zeolite crystals. The chemical state of the gold nanoparticles is not changed, as well.
Nevertheless, if the approach is used for the inclusion of other nanoparticles the chemical
change of the nanoparticles due to the harsh conditions during zeolite crystallisation might
become a problem.
Figure 3.35: Incorporation of gold nanoparticles in zeolite crystals. I) Scheme of preparation pathway. II) TEM
images of zeolite crystals containing small gold nanoparticles
[224]
.
Since the interest in gold nanoparticles is high, other techniques for the incorporation of gold
nanoparticles in zeolite crystals were developed. Laser ablation can be used in order to
prepare small nanoparticles from gold flakes in the zeolite precursor gel without influencing
the latter crystallisation
[225]
. The zeolite precursor gel was added to the gold flakes and
subjected to laser irradiation from a pulsed neodymium-doped Y3Al5O12- garnet. After the laser
100
treatment, larger gold particles were separated by centrifugation and the remaining mixture
was heated up in an autoclave under hydrothermal conditions. The zeolite crystallisation is not
influenced by the laser irradiation and small gold particles are incorporated into the growing
zeolite crystals. The gold particle size distribution is rather broad, and the distribution within
the zeolite crystals is random. Additionally, the technique does not allow any control over the
shape and size of the particles. Another disadvantage is the high technical demand of this
concept.
A very elegant way to incorporate transition metal oxide species in and to impose hierarchy to
zeolite crystals is by using ionic complexes as SDA in the zeolite crystallisation. The
+
((C4H9)4N)2 (M(EDTA))
2-
template can be used to incorporate nickel, cobalt, copper or iron
species into zeolites by using the respective precursor in the preparation of the template. The
following zeolite synthesis is carried out conventionally under hydrothermal conditions. The
final products contain between 10 and 15 wt% of transition metal species and show high
crystallinity. The analysis of the samples showed the different positions of the transition metal
species: whereas iron and cobalt atoms are partly incorporated into the framework of the
zeolite, nickel species were mainly found within the pore channels of the zeolite. The different
configurations of the metal species influence the properties of the material strongly- especially
when applied as catalysts. This makes the preparation of a material with tailored properties
highly problematic. In addition, the choice of metal or transition metal species that can be
incorporated by this approach is rather limited which is - apart from the complex synthesis of
the template - one of the major drawbacks.
A different angle is taken by the approach in which a zeolite layer is grown onto conventional
catalyst pellets (Figure 3.36, II). The core-shell structure is chosen in order to enhance the
migration of the reactants from one catalytically active site to the next (Figure 3.36, I). The
conversion and selectivity of a consecutive reaction are supposed to be increased by improving
the collision possibility between intermediate products formed at the core of the catalyst and
the 2nd set of active sites in the shell of the material [202, 208, 226]. The versatile approach is used
to prepare catalysts of different core compositions covered by a polycrystalline zeolite shell.
Figure 3.36: Scheme (I) and SEM image (II) of capsuled catalyst with FT catalyst core and zeolite shell
[208, 226]
.
101
Independent of the targeted chemical composition of the final material, in the 1st step of the
synthesis a pellet is impregnated with a metal salt using the incipient wetness impregnation
technique, dried and calcined. In the 2nd step the zeolite shell is grown onto the millimetresized pellet using a secondary hydrothermal method. Exemplarily, an Al2O3 pellet was first
impregnated with the cobalt precursor and dried. In the 2nd step β zeolite seeds of 180 nm
were prepared and suspended in ethanol. The prior prepared pellet was then soaked in TEAOH
and added to the reaction gel containing the seeds, TEOS, additional TEAOH and water. The
reaction mixture was heated up under hydrothermal conditions in a rotation oven which
prevents the cementation of the catalyst particles during the crystallisation of the zeolite shell
on the pellet surface. After the hydrothermal synthesis, the solid was washed, filtered, dried
and finally calcined. The composition of the reaction gel has to be chosen carefully in order to
prevent poisoning of the core catalyst. Cobalt-based catalysts for the FT reaction, for example,
are sensitive to the presence of ions often found in SDA-molecules such as Cl-- and Br--ions as
well as Na+-ions. This has to be taken into account, and the reaction gel for the zeolite growth
must be carefully adjusted to meet these requirements. Catalysts of various chemical
compositions can be prepared following the introduced method: because of the simple
impregnation step of the catalyst pellet, metal species can be brought together with acidic
sites of the zeolite in a single material.
The well-designed structure is beneficial for the selectivity of consecutive reactions. But the
large dimensions of the catalysts prevent a close proximity of both kinds of active sites.
Whereas the 1st reaction is catalysed by the active sites of the millimetre sized core pellet, the
acid sites of the zeolite shell are restricted to the outer shell. Furthermore, the micropore
nature of the shell can lead to major mass transport limitations in the application of the
materials as catalysts.
The pathways developed for the incorporation of metal or metal oxide nanoparticles in zeolite
crystals are very diverse concerning strategy and synthetic realisation. Most approaches are
tailored for the preparation of one, highly-functionalised material and not applicable to various
materials. The development of these described syntheses is driven by a multitude of possible
applications in the field of heterogeneous catalysis. The large number of different preparation
techniques extend and complements the versatile toolbox of materials’ preparation and a
great variety of materials can be prepared.
3.2.3.3 Fields of application of bi-functional catalysts
Bi-functional catalysts containing a zeolite constituent are used on an industrial scale in
upgrading reactions of crude oil fractions. But apart from this major, conventional application,
newfound materials are tested in novel reactions, and bi-functional catalysts are more and
more established in catalytic processes.
Zeolites containing metal nanoparticles combine acidic sites with hydrogenationdehydrogenation functions and are used for isomerisation reactions on an industrial scale.
Isomerisation processes are important up-grading reactions applied after the initial processing
of crude oil, for example to enhance the octane number of n-alkanes or to produce valuable
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branched chemicals as starting materials for other reactions. The two functionalities are
involved in different steps of the overall reaction: the n-alkane is dehydrogenated at the metal
site and diffuses through the micropores where acidic sites catalyse the protonation,
rearrangement and deprotonation, before the reactant is hydrogenated at a metal site to form
the final, branched product
[227]
. This network of reactions is only possible when the
intermediates can diffuse easily through the catalyst particle and the respective active sites are
accessible. For this reason mostly wide-pore zeolites find application as catalysts, especially
mordenite is used in isomerisation processes. The activity can be further enhanced by
dealumination due to the creation of additional pores and the consequently enhanced mass
transport. Apart from the porosity of the catalyst, the catalytic performance is dependent on
the balance between both kinds of active sites. If the concentration of metal sites is too low,
the overall reaction is limited by the dehydrogenation/hydrogenation steps and the catalyst
deactivates very quickly due to coke formation. If the metal concentration is increased, the
reactions catalysed by acid sites become rate-limiting and less or no catalyst deactivation is
observed [205].
As already mentioned in the beginning of the chapter, bi-functional catalysts are not only used
in oil refining reactions. The direct synthesis of DME can be catalysed by bi-functional catalysts
containing a copper-based constituent and a zeolite constituent. The direct DME synthesis
describes two consecutive reactions leading from syngas to DME: the syngas is initially
converted to methanol which then undergoes dehydration to form DME. Conventionally, those
two reactions are carried out separately in two systems with two different catalysts. The
development of preparation techniques for bi-functional catalysts makes it possible to link
both reactions together. The combination of a conventional methanol catalyst with a
dehydration catalyst is of great advantage because the initial methanol synthesis is limited by
thermodynamics. Hence, the in-situ conversion of the formed methanol to DME can enhance
the overall syngas conversion. A simple bi-functional catalyst for the direct DME-synthesis can
be prepared by physically mixing a conventional catalyst active in the conversion of syngas to
methanol and a material containing acid sites to catalyse the dehydration. A comparison
between a catalyst containing Cu/ZnO mixed with Al2O3 and a bi-functional material based on
Cu/ZnO and HZSM-5 also prepared by physical admixture shows the impact of the acidic
zeolite on the CO conversion of the overall reaction: whereas the CO conversion of the catalyst
without zeolite component reaches a value of 41%, much higher conversions can be realised
when using the bi-functional catalyst (89%)
[228]
. The selectivity of the zeolite-containing
catalyst towards the desired DME was found to be 87%.
Already the catalyst prepared by simple physical mixing shows the potential of carrying out the
direct synthesis of DME using a bi-functional catalyst. Nevertheless, it is know that apart from
the composition of a catalyst, the structure and organisation of a catalyst is of great
importance. For this reason, catalysts with a well-designed core-shell structure were
developed for the conversion of syngas to DME [202].
The core of the bi-functional catalysts contains copper species to catalyse the conversion of
syngas to methanol, and the shell of the catalyst is made of zeolite crystals containing acid
103
sites to catalyse the dehydration of methanol to form the final product DME. This specially
designed catalyst is compared to a catalyst containing only copper species and no acid sites
and to a mixture containing the same core pellet without the zeolite shell but single zeolite
crystals that were added to the material by mixing (see Figure 3.37).
Figure 3.37: Comparison of the application of a physical mixture and a capsule catalyst in direct-DME reaction
[202]
.
The addition of zeolite crystals by physical mixing enhances the conversion of CO compared to
the conventional catalyst by overcoming the thermodynamic limitation of the methanol
formation from 44% to 58%. The conversion of CO using the catalyst with core-shell structure
is decreased to a value of 30%. This decay can be explained by the hydrothermal preparation
of the zeolite shell: the harsh conditions concerning pH and the presence of template
molecules can damage the core material leading to a decreased CO conversion. The great
potential of the core-shell catalyst becomes clear when looking at the product distribution of
the reaction. While the selectivity of methanol is dominant when using the catalyst without
acid sites, the selectivity towards DME can be already enhanced by adding zeolite crystals to
the catalyst. The physical mixture leads to an improved DME selectivity of 40%, but methanol
is still the main product (selectivity: 57%). The random distribution prevents a coupling of both
methanol formation and dehydration resulting in the formation of both methanol and DME.
On the contrary, the application of the bi-functional catalyst with a core-shell structure leads
to the dominant formation of DME, methanol is still detected but to a much lesser extent. The
DME selectivity reaches 79% while the selectivity towards methanol is only 21%, proving the
impact and importance of the structure of the catalyst. Even though challenges concerning the
possible damage of the core catalyst during the zeolite growth must still be overcome, the
synthetic pathway to prepare a catalyst not just of a defined composition but also a welldesigned structure holds much promise. The hierarchy of the material ensures the high
selectivity towards DME.
Another reaction for which bi-functional catalysts are developed is the Fischer-Tropsch (FT)
reaction. The FT reaction describes the hydrogenation of CO using iron- and cobalt-based
catalysts. The application of bi-functional catalysts in this reaction is selectivity driven. Due to
104
the polymerisation mechanism of the reaction (see Chapter 3.2.3.4) the product spectrum is
broad, ranging from C1 compounds to wax-like products. Conventionally, the initial product
mixture is upgraded in various processes mostly using zeolites as catalysts. Therefore, the
development of bi-functional catalysts for the FT reaction is centred on the combination of FTactive metals such as cobalt and iron with acidic zeolites in order to supress the formation of
wax-like products by catalysing cracking and isomerisation reactions.
The preparation method of the respective bi-functional catalyst strongly influences the
resulting activity and selectivity. A study published in 2013 presented the comparison of three
differently prepared iron -based catalysts for the FT reaction
[207]
: a conventional iron-based
catalyst was characterised alongside a hybrid catalyst made by admixing the conventional
catalyst with ZSM-5 crystals and a bi-functional catalyst prepared by impregnating ZSM-5
crystals with an iron precursor. The CO conversion of the conventional catalyst was with a
value of 97% the highest, followed by the supported catalyst (52%) and the hybrid catalyst
made by physically admixing showing a CO conversion of 47%. Due to different metal loadings
of the catalysts it is difficult to compare these values. Whereas the hybrid catalyst and the
catalyst prepared by impregnation contained 9 wt% of iron, the conventional catalyst
consisted only of iron. Therefore, the hydrocarbon productivity of the catalysts is normalised
to the iron-content and can be compared: whereas the conventional catalysts shows a
hydrocarbon productivity of 0.41 g/h·gFe, values of 0.7 g/h·gFe and 1.2 g/h·gFe are obtained for
the supported catalyst and the hybrid catalyst respectively. The great difference between the
catalysts becomes obvious when taking the selectivity into account. For both the conventional
and hybrid catalyst long hydrocarbon chains are found in the product distribution, whereas the
supported catalyst leads to no formation of C19+ products. The product range is much
narrower, and mainly products in the range of C5 to C11 are formed. This effect is due to the
better dispersion of the iron- particles in the catalyst prepared by impregnation and the
proximity between both kinds of active sites compared to the mixed catalyst.
In a different study a bi-functional, iron-based catalyst containing a zeolite shell was compared
to a non-zeolite containing catalyst in the FT reaction [226]. The zeolite-covered catalyst showed
a lower CO conversion compared to its zeolite-free counterpart probably due to damages to
the iron species during catalyst preparation (hydrothermal conditions, compare Chapter
3.2.3.2) and diffusion limitations. But again the selectivity of the FT reaction is tuned by using a
catalyst containing acidic sites: the formation of long hydrocarbon chains is supressed and an
increased selectivity towards lighter products can be observed. Furthermore, the ratio of
alkenes to alkanes is increased by using the zeolite containing catalysts with a core-shell
structure.
Apart from iron, cobalt can be used as FT-active metal in the preparation and application of bifunctional catalysts, as well. When a catalyst based on cobalt species supported on
mesoporous ZSM-5 is compared to one based on cobalt supported on SiO2, a higher CO
conversion is observed for the bi-functional catalyst (Figure 3.38)
[83, 214, 229]
. The product
distribution is narrowed, and mainly C4 to C6 products are formed when using a catalyst
prepared by impregnation of mesoporous ZSM-5. By using the SiO2-based catalyst a much
105
higher degree of heavier products is formed, whereas the selectivity towards the undesired
CH4 is lower compared to the bi-functional catalyst. The results shown in Figure 3.38 illustrate
the difference between using conventional and mesoporous ZSM-5 as support, too. The CO
conversion is much higher when using the mesoporous analogue and the formation of longchain hydrocarbons is much better supressed.
Figure 3.38: Comparison of the product range obtained from cobalt-based FT catalysts
[83]
.
The dispersion of the cobalt-species is the crucial factor leading to the observed differences:
when using conventional, microporous ZSM-5 the cobalt- crystallites form on the surface of
the zeolite crystals, and their size increases strongly with increasing loadings. By using
mesoporous ZSM-5 the dispersion of the cobalt-particles is increased and the diffusion to the
cobalt-species is improved compared to using conventional zeolites as support. Additionally,
the two kinds of active sites are in close proximity due to the high dispersion, so the formation
of heavier products is supressed.
Other reactions are studied for the application of bi-functional catalysts containing zeolite
constituents, too. The selective reduction of NO using ammonia can be catalysed by iron
containing ZSM-5 crystals
[216]
. While the interplay of both components in the reduction
process is still vividly discussed, the zeolite framework has a beneficial influence on the
adsorption of NH4+ species, necessary for the reaction at the redox sites introduced by the iron
species [230]. Another application of iron-modified zeolites is the oxidation of benzene to phenol
in which the proximity of Brønsted acid sites and iron sites in extra-framework positions is
necessary for the successful hydroxylation of benzene [223].
The application of bi-functional catalysts in various catalytic reactions proves the beneficial
effects that can be gained by preparing highly functionalised materials. While zeolites in
combination with noble metal nanoparticles have already been used for some time in major
industrial processes in crude oil refining, new catalytic reactions and the possible application of
bi-functional catalysts are investigated. The direct synthesis of DME from syngas is an example
which shows the potential of higher conversions and selectivities by using specially designed
catalysts containing two different kinds of active sites. Furthermore, the conversion of syngas
into valuable chemicals and fuels by the FT reaction can be carried out more selectively due to
the development of bi-functional catalysts.
106
Still, continuously new reactions are found that benefit from the use of bi-functional materials.
This development is driven by the need for novel catalysts, improving the activity and
selectivity of existing systems and enabling novel processes.
3.2.3.4 Fischer- Tropsch reaction and its industrial relevance
The FT reaction is the catalytic hydrogenation of CO leading to long hydrocarbon chains. The
reaction pathway was discovered by Prof. Dr. Franz Fischer and Dr. Hans Tropsch (Figure 3.39),
scientists at the Kaiser-Wilhelm-Institut für Kohlenforschung [231].
Figure 3.39: Prof. Dr. Franz Fischer (picture I) and Dr. Hans Tropsch (II). Fischer discussing products of FischerTropsch reaction with Max Planck and Otto Roelen at Kaiser-Wilhelm-Institut in Mülheim, Germany (III).
The initially obtained raw product mixture is converted to fuels and valuable chemicals similar
to the products that can be also produced by processing crude oil.
Figure 3.40: Flow sheet of overall Fischer-Tropsch process realised in industry.
107
The overall Fischer-Tropsch process is divided in three process stages on industrial scale: the
initial preparation of syngas, the precursor of the FT reaction (step I, Figure 3.40), is followed
by the actual Fischer-Tropsch reaction (step II) in which syngas is converted into long
hydrocarbon chains. The overall process is completed by an upgrading step to further process
the initial product mixture (step III).
The great advantage of the FT process is its feedstock variability: with syngas as precursor
various feedstocks become available ranging from natural gas to biomass. With the paradigm
shift away from oil- based products in mind, the Fischer-Tropsch process gained importance
over the last years because of its usage of natural gas and coal as feedstock. The ratio between
reserves and production of natural gas and coal is much higher than the one of oil, improving
the estimated future perspective of the technology (Figure 3.41). A non-fossil feedstock for the
Fischer-Tropsch process is biomass, which can be converted to syngas as well.
Figure 3.41: Overview of the reserves-to-production ratios of the three most important energy sources oil, natural
gas and coal
[232]
.
Already since the first industrial application in 1936, the economic viability of the FT process
has been strongly depending on the price and the availability of crude oil and its products.
After WW II products based on crude oil could be obtained and produced less costly, so the FT
process lost importance in the preparation of fuels and raw materials for the chemical
industry. The costs of FT fuels today depend strongly on the process and the feedstock, and
vary between 20 and 240 $/bbl. With increasing prices of crude oil, the FT process becomes
more and more important. The economic situation is even better for the FT-based preparation
of chemicals. The FT process is estimated to become economically viable when the price per
barrel crude oil reaches about 20 US-$[233] .
The reactions occurring during FT reaction include the formation of carbon-carbon and carbonhydrogen bonds as well as oxygen-hydrogen bonds, and also carbon-oxygen bond rupture
occurs. The resulting hydrocarbon chain distribution is explained by a polymerisation process
as one carbon is added at a time to the growing hydrocarbon chain. At any point the chain
growth can be terminated by hydrogen atom addition or by α-hydrogen abstraction, followed
by desorption from the catalyst surface. This leads to a broad mixture of products obtained
from the FT reaction, ranging from C1-compounds like CH4 to wax-like products with long
108
hydrocarbon chains. Paraffins and olefins with varying chain length are obtained as well as
oxygenated products like alcohols, aldehydes and ketones. Only CH4 can be produced with
100% selectivity, yet for intermediate products only certain maximum values are possible as
shown in Figure 3.42.
Figure 3.42: Calculated product selectivities of FT reaction from Schulz-Flory analysis
[234]
.
The product range can be tuned by adjusting various parameters such as catalyst, reactor type
and temperature. The water-gas-shift (WGS) reaction occurs under FT conditions as well,
which can be used to adjust the CO/H2 ratio in the reactor. Finally, the carbon deposition
during the reaction is explained by the Boudouard reaction.
Catalysts for the FT reaction on an industrial level include iron and cobalt. Ruthenium and
nickel have sufficient FT activity, as well, but are not used in industry. Even though ruthenium
is the most active catalyst for the FT reaction, it is not used in commercial application due to its
very high price. Nickel is very active but the selectivity towards the undesired CH4 is high and
the selectivity to alkenes low, ruling nickel out as an industrial catalyst as well. Even though
highly functionalised catalysts with well-defined nano-scale features are developed and
designed in academia through multi-step syntheses (compare Chapter 3.2.3.1 and 3.2.3.2), less
sophisticated- but most of all less cost-intensive- materials are used in industry to catalyse the
FT reaction.
Because of the long continuous runs of an industrial reactor, deactivation and prevention of
such is studied intensively. Several factors are involved in the decline of activity in the FT
process. If iron catalysts are used, poisoning with sulphur, sintering and oxidation of the active
phase play a major role. Already sulphur levels of 0.03 mgm-3 lead to deactivation of the
catalyst. To which extent the catalyst suffers from poisoning depends strongly on the reactor
type. In tubular reactors only the catalyst at the entrance of the tubes is affected and
deactivated by sulphur, whereas in a slurry reactor the complete batch of catalyst present in
the reactor suffers from deactivation. Small iron carbide crystals grow during the FT reaction
resulting in a lower activity of the catalyst. Structural promoters like SiO2 can prevent such
sintering to some extent. The WGS reaction leads to the formation of water vapour resulting in
109
hydrothermal sintering and oxidation of the catalyst and the subsequent deactivation of the
catalyst. A consequence of the Boudouard reaction is carbon deposition on catalyst particles,
which has not only a negative impact on the activity of the catalyst but also challenges the
reactor set-up. The carbon deposition induces scouring of the carbide outer layer and break-up
of catalyst particles leading to a loss of catalyst from the often used fluidised bed reactors
because smaller particles can pass the cyclones at the outlet of the reactor and are not
retained.
The main factor for the deactivation of cobalt catalysts is sintering occurring due to the
continuous cyclic oxidation and reduction of the surface of the cobalt particles, leading to a
loss in activity. Furthermore carbon deposition on the catalyst particles leads to deactivation of
cobalt-based FT catalysts.
The overall FT process in industry includes first the preparation of syngas, followed by the
catalytic hydrogenation of CO using iron- or cobalt-based catalysts and completed by several
product purification steps. These are necessary due to the polymerisation mechanism of the
reaction leading to a broad product distribution, ranging from CH4 to wax-like products. Tuning
the selectivity of the reaction to a desired product fraction is therefore highly important. Due
to continuing advances both in engineering design and especially catalyst development, the FT
process is a versatile pathway from syngas to fuels, valuable chemicals as well as important
intermediates for the chemical industry.
3.3 Synthetic strategy
3.3.1 Synthetic strategy for the incorporation of transition metal oxides in zeolites
One of the many processes that would benefit from a well-designed bi-functional catalyst is
the Fischer-Tropsch reaction (compare Chapter 3.2.3.3). The product distribution is rather
wide and unselective because of the polymerisation nature of the reaction, so improving the
reactions’ selectivity towards desired products is very important. The necessary work-up of the
initial products is very costly and economically only feasible on a large scale. An improved
selectivity of the process is essential for smaller plants using remotely distributed syngas
resources like offshore flare gas [229]. As for other consecutive reactions, bi-functional catalysts
featuring two types of catalytic active sites were proposed for the FT reaction: syngas is
supposed to be converted to long, linear hydrocarbons on ruthenium, iron, or cobalt
nanoparticles followed by hydrocracking and isomerisation reactions catalysed by acid sites.
Zeolites are used in order to provide acidity and to possibly impart shape selectivity to the
reaction.
Three main categories of bi-functional catalyst have been studied for their application in the FT
process: a physical mixture of conventional FT catalysts and zeolites, FT active metal
110
nanoparticles supported on zeolite crystals and thirdly materials in which a conventional FT
catalyst pellet is covered with a thin zeolite layer [83].
In order for a bi-functional catalyst to exploit its full potential in the FT reaction, several
requirements have to be met. The site proximity of FT active and acid sites plays an important
role for the catalysts’ performance alongside the structure of the catalyst. While the high
dispersion of both sites is essential for activity, the close proximity of both components must
be guaranteed for an enhanced selectivity. The close vicinity of both active components of the
catalysts is necessary to avoid that reactions occur independently. If the initial products desorb
from the catalyst surface and leave the particle without further reaction, the intended
enhanced selectivity cannot be realised.
The aspect of site proximity is difficult to influence by only physically mixing a conventional FT
catalyst and zeolite crystals. A high dispersion can be ensured by supporting FT active metal
nanoparticles on zeolite crystals; nevertheless many studies prove the preferential location of
metal nanoparticles on the surface of zeolite crystals. If this is the case, the initially formed
products can leave the catalyst particle without passing the acid sites, leading only to a limited
enhancement of the selectivity. The products of the FT reaction must pass by acid sites if
conventional FT catalyst pellets are covered with a thin zeolite layer. The selectivity is
enhanced due to the improved collision possibility [208]. However, due to the dimensions of the
material (compare with Figure 3.36) a close vicinity of both active sites cannot be realised.
Only on the interface between the core pellet and the zeolite layer both kinds of active sites
can be found close together.
Another requirement for suitable bi-functional catalysts for the FT reaction is the ease of
migration from one site to the next [208]. This is a distinct disadvantage for the use of zeolites as
acidic catalyst: due to the micropores, mass transport and diffusion from FT site to acid site is
hindered. The slow diffusion in micropores can lead to undesired overcracking and coke
deposition lowering the catalysts activity
[235]
. As introduced in Chapter 3.2.2, mesoporosity
can be introduced to zeolite crystals by various techniques. Studies prove the benefit on the
performance of bi-functional, mesoporous catalysts: the activity of the catalysts is proportional
to the mesoporosity of the zeolite [229].
While it is rather easy to include mesoporous zeolites in bi-functional catalysts based on
physical mixtures and zeolite supported metal nanoparticles, mass transport limitations are a
great disadvantage of the existing approaches with zeolite covered FT pellets. In this case it is
almost impossible to benefit from the development of preparation techniques for hierarchical
zeolites described earlier.
A bi-functional catalyst for the FT reaction would additionally benefit from a tuneable metal
content as well as an adjustable Si/Al-ratio of the zeolite compound in order to have a versatile
system for the catalytic application.
111
The design of a bi-functional material which meets all afore described requirements is highly
desirable, yet highly challenging, as well. The zeolite covered pellet with conventional FT
catalysts in the core is already an advanced approach to prepare a highly functionalised
material, yet some disadvantages remain. Bi-functional catalysts for the FT reaction could be
further improved by reducing the distance between the metal sites and the acid sites as well as
by reducing mass transport limitations arising from the micropores of the zeolite shell.
These aspects are met by the materials introduced here: Fe2O3 nanoparticles as FT active sites
are incorporated in single, mesoporous ZSM-5 crystals. The structure of the material ensures a
high distribution of the metal sites whilst procuring a close proximity of both FT active and
acidic site, as well. Mass transport limitations are minimised by introducing a secondary
system of mesopores into the zeolite crystals. Due to the developed synthetic pathway both
iron content and Si/Al-ratio of the zeolite can be varied and tailored in order to enhance the
selectivity of the FT reaction.
In order to incorporate iron oxide nanoparticles in mesoporous ZSM-5 crystals, it is not
possible to simply add the transition metal oxide nanoparticles to the zeolite precursor gel and
proceed with hydrothermal treatment. The Fe2O3 nanoparticles are not incorporated during
the zeolite growth and a phase separation is observed (Figure 3.43): mesoporous ZSM-5
crystals are found alongside single Fe2O3 nanoparticles, but no incorporation occurs.
Figure 3.43: Phase separation after addition of Fe2O3 nanoparticles to zeolite gel. The Fe2O3 nanoparticles are not
incorporated in zeolite crystals (marked with red circles).
The developed pathway for the successful incorporation of Fe2O3 nanoparticles is based on the
observation that carbon particles are easily incorporated during the zeolite growth under
hydrothermal conditions. Usually, the carbon template is removed after the synthesis by
calcination and nothing remains but mesopores. The ready incorporation of carbon materials
in zeolites can be exploited to “smuggle” iron oxide nanoparticles into the crystals if those are
encapsulated by a carbon shell.
112
Figure 3.44: Developed preparation concept for the incorporation of transition metal oxides in zeolites.
In order to prepare the Fe2O3@C particles, Fe2O3 nanoparticles are covered with a porous SiO2
shell after their hydrothermal preparation. The mesoporous shell is then used in a
nanocasting-step as the template for the impregnation with furfuryl alcohol and oxalic acid.
The polymerisation is started by heating the sample up to 90 °C and the polymer shell is then
carbonised in Ar. After the SiO2 template is leached, the Fe2O3 containing carbon template is
mixed with a conventional carbon template. A zeolite gel of chosen composition is prepared
and the carbon mixture is impregnated. After the hydrothermal synthesis of the zeolite
crystals, the sample is calcined in air in order to remove the carbon templates. The formerly
carbon-encapsulated iron oxide nanoparticles remain in the zeolite crystals, while mesopores
are opened up by removing the conventional carbon template.
The hereby introduced approach for the incorporation of Fe2O3 nanoparticles in zeolite crystals
offers another advantage independent of its application in the FT reaction: the incorporation
based on carbon encapsulated nanoparticles can be further expanded to other transition
metal oxides and metals in general. The illustrated concept can be used to prepare various
carbon covered nanoparticles that act as a transport vehicle into the zeolite crystals.
3.3.2 Impregnation of zeolites as versatile strategy to prepare composite materials
Composite materials based on zeolites are interesting solids with novel properties that go
beyond the properties observed for the involved single phases. An easy and versatile way to
prepare composite materials consisting of zeolites and metal or metal oxide nanoparticles is
the post-synthetic modification of zeolite crystals based on impregnation. Reaction
parameters, such as used precursors, solvents, temperature treatment and possible reduction
conditions, are easily adjusted to each new system. The simplicity of the impregnation method
allows the preparation of various chemical compositions by changing only the used metal
precursors. Additionally, the type of zeolite used for the preparation of composite materials
can be varied, and its properties tailored to meet specific requirements. As a consequence, the
post-synthetic impregnation of zeolites results in numerous materials that can be conveniently
prepared by this method. In contrast to the incorporation of nanoparticles during the zeolite
growth, the metal and metal oxide species introduced by impregnation are not subjected to
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the harsh conditions of the zeolite synthesis (high pH, hydrothermal conditions) and possible
calcination.
As already stated in the introduction, zeolites and especially hierarchical zeolites with an
additional network of mesopores are ideal “support” materials for nanoparticles. Due to the
unique framework structure zeolites are highly stable and porous, and contain catalytically
active sites. Hierarchical zeolite crystals have additional, larger pores and voids in which
nanoparticles can be introduced. The increased pore size is also beneficial for the accessibility
of the nanoparticles within the zeolite.
The impregnation of zeolite crystals can be used to incorporate different kinds of chemical
species in zeolites. Noble metal nanoparticles are extensively studied due to their unique
properties (see Chapter 2.2.1). In combination with zeolites, novel properties and novel fields
of applications can be realised. The general preparation of such composite material includes
impregnation, a possible drying or calcination step, and finally the reduction reaction leading
to zero-valent metal nanoparticles.
Apart from noble metals, other nanoparticles are found to be interesting in combination with
acidic zeolites like oxide or sulphide materials. The impregnation of zeolite crystals with the
precursor solution is then completed by heat treatments in air or other gas mixtures to obtain
the targeted product.
A general difference between noble metals and other components is usually the intended
loading. Due to the high activity but also due to the high cost of noble metals, the loading is
very low (1-5 wt%). Small particle sizes and the homogeneous distribution of the nanoparticles
within the zeolite network ensure, nevertheless, a good catalytic performance. Contrary,
materials like transition metal oxides need to be present in higher concentrations for a
comparable catalytic activity. In most cases, the activity is lower compared to a noble metal
catalyst but also the difference in costs can be substantial, making the application of noble
metal disadvantageous. For the post-synthetic modification of zeolites the different intended
loadings lead to changes in the synthetic parameters such as chosen impregnation method,
precursors and heat treatment.
In Chapter 3.4.2.1 a method for the incorporation of noble metal nanoparticles is introduced
based on incipient wetness impregnation. The intended loading is low, yet the synthesis must
ensure a high distribution of the nanoparticles within the zeolite crystals. Another important
factor is the accessibility of the nanoparticles: due to the low noble metal content the existing
active sites must be easily accessible for reactant molecules. Composite materials consisting of
noble metal nanoparticles within the stable framework of a zeolite are widely used as catalysts
with applications ranging from the processing of crude oil to the synthesis of fine chemicals.
The impregnation of hierarchical zeolite crystals with copper and zinc species in order to
obtain a composite material containing CuO and ZnO nanoparticles is presented in Chapter
3.4.2.2. As discussed above, the loadings that are targeted differ greatly from the one found
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for noble metals. Zeolites containing CuO and ZnO species are interesting catalysts for the
direct-DME synthesis (compare Chapter 3.2.3.3): Copper is a well-known catalysts for
methanol synthesis and in combination with acidic sites the dehydration of methanol to form
DME can take place on the same catalyst particle. To ensure that both reactions occur
simultaneously, both active sites must be in close proximity and easily accessible which can be
achieved by impregnating hierarchical zeolites.
3.4 Results and discussion
3.4.1 Pre-synthetic modification of hierarchical zeolites
3.4.1.1 Synthesis of Fe2O3@mpSiO2 nanoparticles
As illustrated above, the 1st step on the way to iron oxide containing zeolite crystals is the
preparation of Fe2O3 nanoparticles encapsulated by a mesoporous SiO2 shell, which can be
achieved in three steps (Figure 3.45).
Figure 3.45: Synthesis steps leading to Fe2O3 nanoparticles encapsulated in a mesoporous SiO2 shell.
The preparation of hematite nanoparticles (step I in Figure 3.45) is described in Chapter 2.4.1
in detail. The Stöber reaction is then used in the 2nd step to add first a dense SiO2 shell followed
by a mesoporous SiO2 layer in step III. The initial Fe2O3 nanoparticles (100 mg) are stabilised by
0.3 g PVP prior to the Stöber reaction, which is carried out in 320 mL isopropanol mixed with
4 mL of NH4OH. The formation of a dense SiO2 shell is then induced by the addition of 1.2 ml of
TEOS, and the reaction is left to proceed for 24 h at room temperature. In order to prepare the
2nd, porous SiO2 layer, 1.7 mL of a mixture of TEOS and OTMS (TEOS/OTMS= 2.46/1) is added
to the reaction mixture. After 24 h at room temperature, the preparation of
Fe2O3@SiO2@mpSiO2 is completed, and the reaction product is washed and collected by
centrifugation.
Figure 3.46: Structure of OTMS used for preparation of mesoporous SiO2 shell.
115
Both types of used silanes undergo hydrolysis and condensation reactions and form a SiO2
network. Yet, the porosity is different compared to a SiO2 material made by the hydrolysis and
condensation of only TEOS. The structure of OTMS is given in Figure 3.46, and based on the
structure the origin of the mesopores in the 2nd SiO2 shell becomes clear: during the calcination
step that follows after the Stöber reaction, the long hydrocarbon chain is burned off leaving
pores behind in the SiO2 network. The temperature chosen for the calcination step is 350 °C
which ensures the complete removal of organic compounds while the formation of insoluble
iron silicates is still negligible. The initial encapsulation of the Fe2O3 nanoparticles in a dense
SiO2 layer in step II is necessary because it acts as an anchor for the mesoporous SiO2 shell
(step III) and leads to more defined nanoparticles with a core-shell structure.
In order to coat the prior prepared Fe2O3 nanoparticles with a SiO2 shell the nanoparticles
must be stabilised to prevent agglomeration upon changing the chemical environment (change
of solvent, different pH). PVP was chosen as surfactant because of its strong chemical
interaction with the Fe2O3 surface via its amide group and ease of tuning the degree of
stabilisation by varying the chain length of the polymer. The polymer with an average
molecular weight of 360,000 g/mol was used in the synthesis because it stabilised the Fe2O3
nanoparticles sufficiently while still allowing a homogeneous coating of the nanoparticles with
SiO2. The ratio of Fe2O3 nanoparticles and the added amount of silica precursor was rather
high, due to the consecutive addition of first a dense SiO2 shell followed by a mesoporous SiO2
layer, and the formation of separate SiO2 spheres had to be minimised. Therefore, PVP
molecules not adsorbed on the surface of the Fe2O3 nanoparticles were removed by
centrifugation. Free PVP molecules aggregate forming micelles which can induce the formation
of SiO2 spheres if the amount of silica source is high enough. Secondly, the addition of the
TEOS/OTMS mixture is performed dropwise, and the stirring speed is drastically reduced after
completing the addition so that adsorbed silane molecules hydrolyse and condense on top of
the existing, dense SiO2 shell. The formation of separate SiO2 spheres is also influenced by the
amount of TEOS/OTMS mixture that is added in the 2nd step. Here, a compromise must be
found: if the amount of silica source is too high the formation of small SiO2 spheres
additionally to the intended Fe2O3@SiO2@mpSiO2 cannot be prevented. Exemplarily, if the
amount of TEOS/OTMS mixture is doubled, small SiO2 spheres of around 100 nm are found
throughout the sample as well as core-shell particles. If the added amount leads to the
formation of a very thin, mesoporous SiO2 layer due to a reduced amount of TEOS/OTMS
mixture (VTEOS/OTMS<0.4 mL), the nanocasting process in the following step is impaired. Apart
from the amount of TEOS/OTMS used in the synthesis, the ratio of the mixture is of
importance. For the preparation of Fe2O3@SiO2@mpSiO2 a TEOS/OTMS ratio of 2.46 was
chosen in order to obtain a SiO2 shell of sufficient mesoporosity that allows the following
nanocasting step. If the ratio is higher, the pore diameter decreases and the possibility of using
the SiO2 shell as a template for impregnation vanishes. If the amount of OTMS is too high, the
116
stability of the outer SiO2 shell is affected
[236]
. Finally, the common solvent of the Stöber
process (ethanol) was replaced with isopropanol of HPLC quality because it could be shown
that the quality of the core-shell material improved upon changing the solvent.
Figure 3.47: Hematite nanoparticles encapsulated with a thin, dense SiO2 shell.
The dense SiO2 shell has an average thickness of 27 nm and the diameter of the overall
particles varies between 96 and 173 nm as shown in the TEM micrograph above (Figure 3.47).
Each particle contains one single Fe2O3 nanoparticle encapsulated in a SiO2 shell resulting from
the stabilisation with PVP. No agglomerations can be found throughout the sample, which is in
agreement with the DLS data (Figure 3.51). The SiO2 coating is very homogeneous around the
Fe2O3 core, and the surface of the SiO2 layer seems even (see Figure 3.50). In the overlay of the
bright-field SEM and dark-field STEM picture, the Fe2O3 cores in the centre of the particles
become visible.
Number of Particles (%)
25
20
15
Fe2O3@SiO2
Fe2O3@SiO2
Fe2O3@mpSiO2
Fe2O3@mpSiO2
10
5
0
90
120 150 180 210 240 270 300 330 360 390 420
Particle Size (nm)
Figure 3.48: Comparison of the size distribution of encapsulated Fe2O3 nanoparticles based on TEM data (statistics
based on at least 200 particles of each sample).
117
The dense layer of SiO2 was used in the 3rd step as the starting point for the hydrolysis and
condensation of a mixture of TEOS and OTMS to obtain hematite nanoparticles covered by a
mesoporous SiO2 shell. The 2nd, mesoporous SiO2 layer has a thickness of around 62 nm. The
size distribution of the overall particles is shown in the histogram above in comparison to the
particles before adding the 2nd SiO2 layer (Figure 3.48). The size distribution gets slightly
broader after the addition of the mesoporous SiO2 layer compared to the initial Fe2O3@SiO2
particles.
Figure 3.49: TEM micrographs of Fe2O3 nanoparticles encapsulated in a mesoporous SiO2 shell.
The difference in density of the two SiO2 layers becomes visible in the transmission electron
microscope, where both layers can be distinguished from each other easily by the difference in
contrast (Figure 3.49). Compared to the dense SiO2 shell (Figure 3.47) the additional porous
layer appears to be less even and smooth. But the layer of mesoporous SiO2 is still
homogeneously distributed, and the porous shell encapsulates the particles completely. The
formation of the mesoporous layer does not lead to agglomeration and only single particles
are found.
When characterising the material with SEM the pore openings of the outer SiO2 layer become
visible. In comparison to the surface of the dense SiO2 layer the mesoporous shell looks much
rougher and uneven (Figure 3.50). Due to the thicker SiO2 layer that surrounds the Fe2O3 core
after the addition of the mesoporous layer, the core is less clear in the overlay of the SE and
the STEM picture (image IV in Figure 3.50) as compared to the Fe2O3@SiO2 particles in the
upper pictures.
118
st
nd
Figure 3.50: SEM pictures of Fe2O3 nanoparticles covered by the 1 dense SiO2 layer (I and II) and the 2
mesoporous SiO2 shell (III and IV).
The result of the TEM analysis that the addition of the mesoporous SiO2 does not lead to the
formation of any agglomeration of the particles is confirmed by DLS measurements shown
below. Also the sizes measured with DLS support the results of both TEM and SEM. The size
distribution of the obtained particles gets continuously broader starting from the initially
prepared Fe2O3 nanoparticles to the core-shell particles containing both a dense and
mesoporous SiO2 layer.
30
Intensity (%)
25
20
Fe2O3
Fe2O3
15
Fe2O3@SiO2
Fe2O3@SiO2
10
Fe2O3@mpSiO2
Fe2O3@SiO2@mpSiO2
5
0
0
100
200
300
400
Size (nm)
500
600
nd
Figure 3.51: Comparison of DLS data before and after the addition of the 2 , mesoporous SiO2 layer.
119
The porosity of both materials was characterised using N2 physisorption as well. As expected,
the particles containing only the 1st, dense SiO2 layer show no significant porosity, and the
isotherm shows type II behaviour following the IUPAC classification (Figure 3.52).
350
Volume ads. (cm-3/g STP)
300
Fe2O3@SiO2@mpSiO2
250
200
150
100
Fe2O3@SiO2
50
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0
Relative pressure (p/p )
0.8
0.9
1
Figure 3.52: Nitrogen adsorption and desorption branches for Fe2O3@SiO2 and Fe2O3@mpSiO2 after calcination at
350 °C.
The SiO2 shell contains some micropores in the range of 1 to 2 nm and the micropore volume
has a value of 0.06 cm3/g resulting in the small adsorption of nitrogen at low relative
pressures. The adsorption curve continues very flatly showing no significant adsorption in the
range of 0.1 to 0.8 relative pressures. The hysteresis at higher relative pressures is due to interparticle voids which account for much of the calculated pore volume of 0.25 cm3/g of the coreshell material. For the surface area a value of 181 m2/g is calculated. In comparison, the
particles encapsulated with mesoporous SiO2 possess a much higher porosity as shown in
Figure 3.52. The sample reveals an uptake of N2 at low relative pressures indicating the
presence of micropores in both SiO2 layers. Additionally, the particles containing the additional
mesoporous SiO2 layer show significant adsorption in the range of 0.1 to 0.4 relative pressures
which relates to small mesopores and the high mesoporous pore volume of 0.33 cm3/g (total
pore volume: 0.47 cm3/g). Due to the curve progression at these intermediate relative
pressures the isotherm measured for the Fe2O3@SiO2@mpSiO2 nanoparticles should be
considered a type IV isotherm despite no major hysteresis. The adsorption of N2 between
relative pressures of 0.9 to 1.0 is again due to the filling of interparticle voids.
The calculated pore sizes of the 2nd SiO2 shell range between values of 2.8 and 4.4 nm, and the
material is therefore considered to be mesoporous. The overall surface area of the core-shell
nanoparticles containing two SiO2 layers of different porosity is 533 m2/g.
120
To complete the characterisation of the prepared SiO2 encapsulated Fe2O3 nanoparticles, X-ray
powder diffraction was used to analyse the crystalline phases of the material. As suggested by
HR-TEM images, the SiO2 layers are amorphous in contrast to the Fe2O3 nanoparticle core.
Figure 3.53: Powder diffraction pattern of Fe2O3@SiO2and Fe2O3@mpSiO2 samples using Mo radiation.
Both patterns prove the crystalline phase of the iron oxide core to be α-hematite. As a
comparison, the calculated pattern of α-hematite is given in the plot as well (grey lines, Figure
3.53), which is in agreement with the measured patterns of the Fe2O3@SiO2 as well as the
Fe2O3@mpSiO2 sample. While the XRD pattern of the material containing only a thin, dense
SiO2 shell reveals only a slight background resulting from the amorphous shell, the background
especially at lower diffraction angles is higher for the sample containing both SiO2 layers. The
reason for the high background is the increased ratio of amorphous SiO2 of the material
compared to the crystalline Fe2O3 phase.
The 1st intermediate material of the synthetic concept to incorporate Fe2O3 nanoparticles in
zeolite crystals are Fe2O3@SiO2@mpSiO2 nanoparticles. By adjusting the reaction parameters
such as solvent, chain length of stabiliser, ratio of TEOS/OTMS and amount of TEOS/OTMS
added to the reaction solution, single particles with a Fe2O3 core and two layers of SiO2 with
different porosities could be prepared. The 1st dense SiO2 shell has a thickness of around 20 nm
and shows only a limited microporosity. In contrast, the 2nd SiO2 layer prepared by mixing TEOS
and OTMS reveals a high degree of mesoporosity after burning of the long hydrocarbon chain
of OTMS. The mesoporous SiO2 shell homogeneously covers the 1st layer and is of 80 nm
121
thickness. DLS measurements prove the absence of large agglomerations and confirm the final
particles size of around 330 nm.
3.4.1.2 Preparation and characterisation of Fe2O3@C nanoparticles
The mesoporous SiO2 shell is used as a template for the formation of a carbon shell around the
hematite core. Figure 3.54 summarises the steps that are necessary to get from the previously
prepared Fe2O3@SiO2@mpSiO2 to Fe2O3 nanoparticles covered with a carbon shell.
Figure 3.54: Schematic overview of the preparation of Fe2O3@C nanoparticles.
In step I the mesoporous SiO2 shell is impregnated with a mixture of furfuryl alcohol and oxalic
acid (ratio 2 mL/0.021 g) to incipient wetness. The monomer mixture is polymerised in the 2nd
step at 90 °C in air, which is followed by the carbonisation of the polymer within the pores of
the outer SiO2 shell at 650 °C in an argon flow (step II). In the final step, the SiO2 template is
removed by leaching with 1 M NaOH at 60 °C for 48 h resulting in the targeted Fe2O3@C
nanoparticles.
The mesoporous SiO2 exotemplate was impregnated with furfuryl alcohol mixed with oxalic
acid using the incipient wetness approach. Oxalic acid is the catalyst for the polymerisation
reaction within the pores of the SiO2 template. Furfuryl alcohol was chosen as carbon
precursor because it had already been proven as effective reactant in other nanocasting
procedures. The monomer is liquid at room temperature and the acid catalysed reaction can
be conveniently started by increasing the temperature moderately. More importantly, the
carbonisation can be carried out at low temperatures (200-500 °C), which is necessary to
prevent the formation of silicate phases, occuring at higher temperatures in the absence of
oxygen.
The great challenge of the nanocasting step is the complete filling of the pores with the
monomer mixture, because only if the resulting polymer forms an interconnected network
within the porous SiO2 shell, stable carbon shells can be obtained at the end. To ensure that
the monomer mixture infiltrates the complete SiO2 shell, the impregnation step must be
tuned: the porous nanoparticles were placed in a vacuum oven at 50 °C for 24 h to remove
water molecules from the pores. After determining the exact pore volume of the SiO2 shell by
N2 physisorption the respective volume of monomer mixed with oxalic acid was prepared. The
nanoparticles were placed into a stable polypropylene (PP) tube, and the liquid was added in
three parts. In between each addition, the closed tube with the Fe2O3@SiO2@mpSiO2
nanoparticles inside was forcefully crushed on a hard surface so the furfuryl alcohol could
122
penetrate the pore system of the outer SiO2 shell. It could be shown that only following this
impregnation procedure the final carbon shells were intact, and the ratio of broken shells was
minimal. Even the impregnation using mortar and pistil only lead to the formation of broken
carbon shells resulting from an insufficient penetration of the monomer. Another important
synthesis parameter for the successful nanocasting step is the following heat treatment:
before the polymerisation is initiated at 90 °C, the impregnated nanoparticles are kept at 50 °C
for 24 h so the furfuryl precursor can diffuse into the pore system of the SiO2 shell. The
combination of the improved impregnation step and the temperature treatment at 50 °C lead
to the formation of complete carbon shells.
The successful impregnation of the SiO2 template can be proven by N2 physisorption
experiments as shown in Figure 3.55. The comparison of the core-shell nanoparticles with the
impregnated analogue shows that the previous pores of the SiO2 shell are no longer accessible
after the impregnation step. Neither micro- nor mesopores are detectable in the isotherm of
the impregnated sample. The N2 uptake at high relative pressures can be attributed to the
filling of interparticle voids of the impregnated nanoparticles, which are not filled due to the
careful incipient wetness impregnation approach used.
After carbonisation, the SiO2 template is removed by leaching with NaOH. To support the
dissolution reaction, the temperature was raised to 60 °C, and after 24 h the supernatant was
removed by centrifugation. The leaching process was continued for another 24 h at a
temperature of 60 °C to ensure the complete removal of the SiO2 template.
350
Volume ads. [cm3/g STP]
300
Fe2O3@SiO2@mpSiO2
250
200
150
100
Fe2O3@SiO2@mpSiO2 impregnated
50
0
0
0.1
0.2
0.3
0.4
0.5
Relative pressure
0.6
0.7
0.8
0.9
1
[p/p0]
Figure 3.55: Comparison of N2 sorption data of Fe2O3@SiO2@mpSiO2 and impregnated analogue.
The resulting nanoparticles are 340 nm in average size, as shown in the TEM images in Figure
3.56. The shell thickness varies between 53 and 78 nm. The particle size and the carbon shell
thickness mirror the values for the particles containing the mesoporous layer very closely due
123
to the nanocasting-like preparation procedure. The carbon shells encapsulate the Fe2O3
nanoparticles completely, and only very few broken shells can be found throughout the
sample.
Figure 3.56: TEM images of iron oxide nanoparticles covered by carbon shell.
The spherical shape of the carbon shell is maintained upon removal of the SiO2 template and is
an exact replica of the mesoporous SiO2 shell previously prepared. The stability of the carbon
shell is guaranteed by a sufficient thickness which had to be tuned by the amount of silica
precursor added to the reaction solution in the previous synthesis part (see Chapter 3.4.1.1).
The 1st, dense SiO2 layer is not infiltrated with the monomer mixture which can be deduced
from the sharp change in contrast of the inner edge of the carbon shell. Apart from the
stability and completeness of the carbon shell, the developed impregnation process ensures
the formation of single yolk-shell nanoparticles. Agglomeration and intergrowth of the spheres
is not observed using microscopy techniques.
The full encapsulation of the iron oxide core is proven by the combination of several
techniques. In combining SE- and dark-field STEM-technique the shell as well as the core of the
material is made visible, as shown in the overlay in Figure 3.57. To illustrate the different
recording techniques, the picture in the SE-mode is given in green and the STEM-picture is
coloured red (see picture II and IV in Figure 3.57). The carbon shell is visible in the SEM picture
(picture I and III in Figure 3.57) and only using the dark-field STEM mode the iron oxide core of
the material can be recognized. Additionally, cross-sectional cuts of the material were made
and the core-shell nature of the nanoparticles is revealed (picture V and VI in Figure 3.57). In
Figure 3.57 (picture V) an overview of one of the cross-sectional cuts is shown. While the Fe2O3
nanoparticle is still visible in most carbon shells, some Fe2O3 cores are missing as a
consequence of the preparation of the cuts. Yet, the Fe2O3 nanoparticle is still in the core of
the particle as shown in image VI in Figure 3.57. The cross-sectional cutting also illustrates
nicely that the impregnation of the Fe2O3@SiO2@mpSiO2 nanoparticles with the monomer
124
mixture is restricted to the outer SiO2 shell. The 1st, dense SiO2 layer is not penetrated and only
the outer shell acts as a template. This results in the large void around the Fe2O3 core.
Figure 3.57: Characterisation of Fe2O3@C nanoparticles using SEM (detailed description see text).
The conventional SEM images (Figure 3.57, I and III) show the rough surface of the carbon
shell. Partly it can be observed that small amounts of carbon are formed outside of the actual
shell. Most likely this formation is due to small, porous SiO2 particles that formed during the
Stöber reaction and that are impregnated alongside the mesoporous SiO2 shell.
Additionally, the material was characterised using powder diffraction and X-ray photoelectron
spectroscopy (XPS). Whereas the diffraction signals of the iron oxide core are clearly visible in
the diffraction pattern, only weak signals of iron can be found in the surface-sensitive XPS
measurement. The largest signal stems from the carbon shell of the material. On the surface of
125
the carbon shell, heteroatoms such as sodium and oxygen are found as well as traces of
flourine atoms (Figure 3.58, II). The leaching of the SiO2 template with NaOH easily explains the
presence of sodium and oxygen atoms on the carbon shell of the solid. The source of the
fluorine atoms is less clear, but most likely fluorine atoms are either present in trace-amounts
in the NaOH used or result from the use of Teflon-coated stirring bars during the leaching step.
During the carbonisation of poly(furfuryl alcohol) the formation of CO, CO2, H2, CH4 and H2O
occurs, leading to the partial reduction of the iron oxide core, as shown in the powder
diffraction pattern in Figure 3.58 by the presence of FeO and Fe3O4 phases. The amorphous
carbon shell of the material contributes to the high background especially at low diffraction
angles.
I
II
Figure 3.58: I) Powder diffraction pattern of Fe2O3@C nanoparticles using Mo radiation. II) Results of XPS
measurement of the material.
As expected in a nanocasting process, the resulting carbon shell is porous, being the inverse
replica of the porous SiO2 template. The porous structure of the carbon shell can already be
seen in the HR-TEM image (Figure 3.59, I).
126
Figure 3.59: Analysis of pores in carbon shell using HR-TEM (I) and HR-SEM (II).
Moreover, the pore openings in the uneven carbon shell are clearly visible in the HR-SEM
image shown in Figure 3.59. The quality of the SEM image is sufficient to measure the size of
the pore openings, which are around 3 nm.
The porosity of the carbon shell is confirmed by N2 sorption measurements, too. The pore size
distribution in Figure 3.60 is calculated based on the nitrogen sorption data using DFT
calculations. The uptake of N2 at low relative pressures reveals the presence of micropores in
the carbon shell. The isotherm continues to incline in the range between 0.1 and 0.4 relative
pressures, resulting from multilayer adsorption within small mesopores. At high relative
pressures a small hysteresis is observed resulting from interparticle voids.
As shown in the pore size distribution (Figure 3.60, II), the yolk-shell material shows the
presence of micropores alongside a distinct mesoporosity with pore sizes ranging from 2.5 to
4 nm. The mesopore volume of around 0.95 cm3/g accounts for much of the total pore volume
of 1.3 cm3/g. The porosity of the carbon shell reflects the porosity of the mesoporous SiO2
template well when taking into account the different densities of the materials.
900
800
700
600
500
II
400
300
200
100
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Relative pressure (p/p0)
dV(w) (cm3/(Å/g))
Volume ads. (cm3/g STP)
I
0.06
0.05
0.04
0.03
0.02
0.01
0.00
10 20 30 40 50 60
Pore width (Å)
Figure 3.60: Physisorption analysis of carbon encapsulated Fe2O3. I) Nitrogen adsorption and desorption branches.
II) Calculated pore size distribution using DFT calculations.
127
In this chapter the preparation of carbon encapsulated Fe2O3 nanoparticles was introduced.
The previously prepared core-shell particles containing a dense and a mesoporous SiO2 shell
were impregnated with furfuryl alcohol. After polymerisation at elevated temperature, the
polymer was carbonised within the pores of the SiO2 template in an inert atmosphere. Finally,
the SiO2 template was removed by leaching with NaOH and the desired iron oxide
nanoparticles encapsulated by carbon were obtained. The carbon shells are intact and no
agglomeration can be found. The shell is around 62 nm thick and contains pores of a diameter
between 2.5 and 4 nm. The full encapsulation of the core is proven by an overlay of SE and
dark-field STEM images as well as XPS measurements which show only traces of iron species
on the surface of the particles.
3.4.1.3 Iron oxide containing ZSM-5 crystals
In the last part of the developed synthetic procedure the carbon encapsulated Fe2O3
nanoparticles are finally incorporated into ZSM-5 crystals. The preparation follows a modified
procedure based on the established templating approach using carbonaceous templates for
the preparation of hierarchical zeolites
[150]
. Instead of using only commercial carbons as
template, the previously prepared Fe2O3@C particles are used as template in the zeolite
synthesis. The core-shell nanoparticles are either used in pure form or mixed with commercial
carbons (Black Pearls 2000, BP2000) and impregnated with the zeolite precursor gel, which is
prepared by mixing 1.47 mL TPAOH with 4.14 mL ethanol, 1.15 mg NaOH and 12.6 mg sodium
aluminate (per 1 g carbon template). Afterwards, 0.71 mL TEOS are added directly to the
carbon template. The impregnated solid is then placed in Teflon lined autoclaves and heated
up to 180 °C under hydrothermal conditions using a heating rate of 0.05 K min-1(step I, Figure
3.61). Once the desired temperature is reached, the reaction mixture is kept at 180 °C for 72 h.
After completion of the hydrothermal synthesis, the solid is dispersed in water and washed
repeatedly. The dried product is then calcined in a static air atmosphere at 550 °C for 8 h
(step II).
Figure 3.61: Schematic preparation pathway for growth of ZSM-5 crystals around Fe2O3@C nanoparticles.
The ratio of Fe2O3@C nanoparticles and BP2000 of the carbon template can be varied freely,
yet, the absolute amount of the mixed carbon template has to be kept constant throughout
the syntheses for a successful formation of large, hierarchical zeolite crystals. For the zeolite
128
crystals to grow around the carbon templates, the volume of the precursor gel has to exceed
the pore volume of the template mixture. This is very important for the product formation,
and even small deviations from this aspect prevent the formation of the desired ZSM-5 crystals
with embedded Fe2O3 nanoparticles. Apart from the carbon template mixture, the
composition of the precursor gel can be varied as well - especially the Si/Al ratio of the ZSM-5
crystals can be tuned. If ZM-5 crystals with a high aluminium content are desired, the
impregnation procedure is slightly modified: due to the low solubility of the aluminium source,
the 1st impregnation of the carbon template with a mixture containing TPAOH, ethanol, NaOH
and sodium aluminate must be carried out repeatedly before finally adding the TEOS.
All impregnation steps are carried out in a mortar made of Teflon to ensure a complete
penetration of the pores of the carbon template while still preventing any contamination with
additional silicon species from a porcelain object.
The heating rate of the zeolite crystallisation is very low (0.05 K min-1) in order to assist in the
formation of large ZSM-5 crystals around the carbon template in contrast to nano-sized zeolite
crystals (see Chapter 3.2.2.2). Since the zeolite growth is decelerated due to the presence of
carbon template the zeolite crystallisation is carried out for 72 h at 180 °C.
The heating rate of the final calcination step must be carefully adjusted (step II in Figure 3.61):
the chosen heating rate has to be very low (0.5 K min-1) so that the zeolite crystals are not
damaged by the removal of the carbon template. Because of the exothermic reaction, the local
temperatures can rise very suddenly, damaging the crystal structure of the resulting zeolite
and leading to fractures in the crystals. Consequently, the average particle size is drastically
reduced. Moreover, the embedded Fe2O3 particles can be separated from the zeolite matrix
when crystal fragments break off. This is especially problematic if only Fe2O3@C nanoparticles
are used as template. The absence of mesopores in the zeolite crystals inhibit the quick
diffusion of the combustion gases out of the crystals leading to a higher degree of
fragmentation in the final product.
The successful removal of the carbon template at a temperature of 550 °C is proven by
thermogravimetric (TG) analysis in which the calcination conditions are mimicked (Figure
3.62): the temperature, heating rate and time are kept at the same values to prove the
complete removal of carbon template in the analoguous calcination step. After 8 h at 550 °C
the temperature is increased to 900 °C to see whether combustible compounds are left in the
material. Since the increase in temperature does not lead to any additional weight loss of the
composite material, the calcination can be considered complete under the chosen conditions.
129
100
1000
90
900
80
800
70
700
60
600
50
500
40
400
30
300
20
200
10
100
0
Temperature (°C)
Mass (%)
0
0
50
100 150 200 250 300 350 400 450 500 550
Time (min)
Figure 3.62: TG analysis of zeolite crystals containing the carbon template to prove the complete removal by
calcination in air.
The obtained solid is bright orange due to the incorporated Fe2O3 nanoparticles; in Figure 3.63
the appearance of the composite material is compared to the regular white colour of
conventional zeolites. The colour of the composite material varies between light ochre to
bright orange (as shown in the photo) depending on the iron content of the material.
Figure 3.63: Comparison of conventional and Fe2O3 containing ZSM-5 by visual appearance.
The crystal structure of the zeolite containing material was analysed by powder diffraction
using Mo radiation. The resulting diffractogram (Figure 3.64) shows both peaks of the ZSM-5
crystals as well as diffraction peaks of the embedded Fe2O3 nanoparticles. The partial
reduction of the iron oxide cores during the carbonisation step introduced earlier (Chapter
3.4.1.2) is reversed by the calcination step in air, resulting in only one iron oxide phase present
in the zeolite crystals.
The measured data is compared to the calculated diffractogram which is in close agreement.
The broad background between 8 and 14 °(2Θ) in the measured pattern stems from the glass
capillary in which the sample was measured. Apart from that, the diffraction pattern proves
130
the high crystallinity of the zeolite crystals as well as the presence of Fe2O3 nanoparticles in the
sample.
Figure 3.64: Diffraction pattern of the product using Mo radiation and comparison to calculated pattern of ZSM-5.
The hierarchical structure of the zeolite crystals is made visible by SEM: the surface of the
zeolite is uneven, and the pore openings are easily distinguishable (Figure 3.65). The Fe2O3
nanoparticles embedded in the zeolite crystals are only detectable by using dark-field STEM
techniques, and by creating an overlay with the conventional SEM image both the outer
surface of the zeolite crystal and the Fe2O3 nanoparticles within the zeolite is made visible
(Figure 3.65).
Figure 3.65: Fe2O3 containing ZSM-5 crystals in an overlay of an SE- and dark-field STEM-picture.
That the iron oxide nanoparticles are truly incorporated into the zeolite crystals is not only
visible in the overlay shown in Figure 3.65, it also becomes clear after heating the sample up to
131
800 °C and comparing it to a sample in which the Fe2O3 nanoparticles were simply added to
zeolite crystals by admixing. Whereas the particle size of the incorporated Fe2O3 nanoparticles
stays constant after the heat treatment, large agglomerations of iron oxide are clearly visible in
the sample prepared by admixing.
Figure 3.66: Comparison of the TEM images of ZSM-5 with embedded Fe2O3 nanoparticles (I) and ZSM-5 crystals
mixed with Fe2O3 nanoparticles (II) after heat treatment.
The iron content of the final product can be easily varied over a wide range in tuning the ratio
of Fe2O3 containing carbon particles to the amount of BP2000 used in the preparation. What
has to be kept in mind is that by changing the iron content of the zeolite, the porosity of the
resulting crystals is changed, as well. If only Fe2O3@C yolk-shell nanoparticles are used, the
degree of mesoporosity of the final crystals is drastically reduced. In contrast, if the mixture of
carbon template for the preparation of the material contains only a small amount of carbon
covered Fe2O3 nanoparticles- and consequently a much higher amount of BP2000- the
mesoporosity of the zeolite is much more pronounced. In order to illustrate the different
properties of the final material depending on the composition of the carbon template used,
three samples are compared in the following. In Table 3.3 the weight-based ratio of the initially
used carbon templates is given, together with the abbreviation used for the respective
samples. The ratios were chosen to cover a wide range of possible compositions: the 1st
sample was prepared by using only Fe2O3@C nanoparticles as template, leading to iron
contents of up to 40 wt% in the final material.
Table 3.3: Zeolite crystals prepared by using carbon templates with different percentages of Fe2O3@C and BP2000
of total template amount.
Material label
Fe2O3@C/
BP2000
Fe2O3/ZSM-5 - 100
100/0
Fe2O3/ZSM-5 - 67
67/33
Fe2O3/ZSM-5 - 05
5/95
132
The ratio of Fe2O3@C nanoparticles and BP2000 of the carbon mixture that were impregnated
with the zeolite precursor gel to prepare the 2nd sample was 2/1. Finally, only small amounts of
carbon encapsulated Fe2O3 nanoparticles (5 %) were used to prepare composite ZSM-5 crystals
for the 3rd sample. Due to the different particles sizes (BP2000: 20 nm, Fe2O3@C: 300 nm) and
densities of the materials, the given ratios of used template mixtures do not reflect the
number of template particles of the respective template type: Exemplarily, while the template
mixture of the Fe2O3/ZSM-5-67 sample contained 0.67 g Fe2O3@C and 0.33 g of BP2000, still
per each Fe2O3@C particle, the overall mixture contained more than 2000 BP2000 particles
due to the smaller density.
Apart from the different iron content of the finally obtained zeolite crystals, the porosity is
mostly affected by changing the composition of the carbon template. The N2 isotherms of the
three samples are given in Figure 3.67, and the influence of the template mixture on the
porosity becomes clear. All three samples show adsorption at low relative pressures as
expected due to the microporous zeolite crystals. Yet, the initially obtained values for the
micropore volume differ quite substantially: for the material with the highest Fe2O3 loading a
micropore volume of only 0.08 cm3/g is found, while the sorption data of the Fe2O3/ZSM-5–67
and Fe2O3/ZSM-5– 05 sample indicate values of 0.07 and 0.14 cm3/g respectively. The isotherm
of the Fe2O3/ZSM-5–100 sample continues to incline at intermediate relative pressures only
slightly. In the range of high relative pressures a steep incline is visible alongside a small
hysteresis, which is not a result of mesopores but due to intercrystalline voids.
350
300
250
200
150
100
50
300
250
200
150
100
50
0
350
II
I
350
300
250
200
150
100
50
0
0
0.2 0.4 0.6 0.8 1
Relative pressure (p/pO)
III
0
0
0.2 0.4 0.6 0.8 1
0
0.2 0.4 0.6 0.8 1
Figure 3.67: Physisorption isotherms of ZSM-5 crystals prepared with different amount of Fe2O3@C template. I)
Fe2O3/ZSM-5-100; II) Fe2O3/ZSM-5-67; III) Fe2O3/ZSM-5-05.
In contrast, the materials with lower iron contents show adsorption also in the range of 0.2 to
0.8 relative pressures and especially approaching values of 0.9 and higher. This curve
133
progression is due to the mesopores present in the ZSM-5 crystals after burning off the BP2000
template. When comparing the isotherm of the Fe2O3/ZSM-5–67 sample with the one of the
Fe2O3/ZSM-5–05 sample, the higher degree of mesoporosity in the latter sample is indicated
by the more pronounced hysteresis visible at high relative pressures. Noteworthy is also the
small step that is visible in all three isotherms at a relative pressure of 0.15 which is due to the
rearrangement of the adsorbate N2 and usually observed in zeolite crystals with a welldeveloped crystallinity [237]. The surface area of all three samples was calculated using DFT,
leading to a value of 199 m2/g for the Fe2O3/ZSM-5–100 sample, 207 m2/g in the case of
Fe2O3/ZSM-5–67, and for the Fe2O3/ZSM-5–05 sample a surface area of 372 m2/g was
calculated.
The different apparent values that are obtained for the micropore volume of the Fe2O3/ZSM-5
composite materials can be explained by the different amounts of Fe2O3 nanoparticles in the
material. While the nanoparticles are completely unporous and do not add to the porosity of
the zeolite crystals, the weight influence of the Fe2O3 nanocrystals is pronounced due to their
higher density. If the measured physisorption data is normalised with the specific ratio of
Fe2O3 nanoparticles used for the synthesis of each composite material, the isotherms of the
different materials reflect similar adsorption properties, especially at lower relative pressure
ranges.
350
300
250
200
Fe2Series1
O3/ZSM-5 -100
Fe2Series3
O3/ZSM-5 -66
Fe2Series5
O3/ZSM-5 -05
150
100
50
0
0
0.2
0.4
0.6
0.8
1
Figure 3.68: Comparison of the isotherms after factoring the different Fe2O3 contents into the sorption data.
The corrected micropore volumes for the composite materials range from 0.17 cm3/g for the
Fe2O3/ZSM-5-100 sample to 0.14 cm3/g for the Fe2O3/ZSM-5-67 sample and a value of
0.15 cm3/g calculated for the Fe2O3/ZSM-5-05 material, which are in close agreement to the
value of 0.18 cm3/g usually found for ZSM-5. The differences of the isotherms of the composite
materials at higher relative pressures remain due to the different degree of mesoporosity
formed within the zeolite crystals.
134
The pore size distribution was calculated for all samples using DFT calculations (see Figure
3.69). A similarity of all three plots is the presence of a pronounced peak at a pore width of
around 3 nm. This is not due to the presence of pores in this size range, but rather an artefact
resulting from the rearrangement of N2 molecules in the pores of the zeolite crystals. Apart
from small micropores, no pronounced mesoporosity is found for the Fe2O3/ZSM-5–100
sample, which is expected due to the absence of BP2000 in the carbon template. The observed
larger pores are of intercrystalline nature, resulting from a smaller crystal size. The voids left
behind from the carbon shells of the Fe2O3@C particles, which are around 300 nm in diameter,
are much larger compared to the pores introduced by the conventional carbon template, and
therefore not included in the sorption data. Based on the pore size distribution of the other
two samples larger pores of sizes ranging from 15 to 30 nm are present both in the Fe2O3/ZSM5–67 and Fe2O3/ZSM-5–05 sample. Yet, the larger mesopores contribute much more to the
porosity of the sample containing the lowest ratio of Fe2O3 nanoparticles, which is expected
dV(w) (cm3/Å/g)
due to the higher amount of BP2000 used for the preparation.
0.006
6.00E-03
0.006
0.005
5.00E-03
0.005
0.004
4.00E-03
0.004
0.003
3.00E-03
0.003
0.002
2.00E-03
0.002
0.001
1.00E-03
0.001
0.000
20
0.00E+00
120 220 320 420
20
Pore width (Å)
0
120 220 320 420
Pore width (Å)
20
120
220
320
420
Pore width (Å)
Figure 3.69: Calculated pore size distribution of Fe2O3/ZSM-5 samples using DFT.
The discussed interdependence of the iron content and the porosity of the final product is
illustrated in the graph in Figure 3.70. With increasing Fe/Fe2O3 loading the pore volume is
decreased which is attributed to the reduced amount of BP2000 in the carbon template
mixture.
135
Pore volume (cm3/g)
0.45
0.4
212
Fe
O /ZSM-5 - 05
2 3
0.35
210
Fe
O /ZSM-5 - 66
2 3
0.3
201
Fe
O /ZSM-5 - 100
2 3
0.25
0.2
0.15
0.1
0.05
0
0
5 10 15 20 25 30 35 40 45 50 55 60
Fe2O3 content (wt%)
Figure 3.70: Interplay of pore volume and Fe2O3 content of the final product.
Only materials with properties described by a point on the trendline drawn in the plot can be
prepared by the developed method due to the preparation based on the impregnation of a
carbon template mixture. Yet, a wide range of Fe2O3 loadings can be realised alongside
beneficial mesopores.
The results of the analysis of the samples using TEM are shown in Figure 3.71. In all three cases
the distinct crystal shape of the ZSM-5 phase is visible, yet the different degree of
mesoporosity becomes clear in the TEM images, as well. Fe2O3 nanoparticles are embedded
within the zeolite crystals of all three samples. The material prepared by using only Fe2O3@C
as template is more difficult to characterise with TEM: due to the high density of the ZSM-5
crystals, the incorporated Fe2O3 nanoparticles can only be seen at thin crystal edges or within
smaller, less thick crystals. The crystals are relatively small with an average crystal size of
0.79 µm. While a larger crystal is shown in the TEM image (Figure 3.71, I) the smaller average
size is due to many small crystal fragments present in the sample that result from the breaking
of larger crystals. This occurs most likely during the calcination process when the carbon
template is burned off. Due to the micropores of the material, the resulting combustion gases
cannot leave the crystal easily, and in combination with high local temperatures larger zeolite
crystals are damaged and break into smaller fragments. While smaller crystals are not
necessarily per se disadvantageous, the fracturing of the zeolite crystals also leads to the
separation of initially incorporated Fe2O3 nanoparticles.
136
Figure 3.71: Comparison of ZSM-5 crystals resulting from different mixtures of the carbon templates using TEM: I
and II) Fe2O3/ZSM-5–100; III and IV) Fe2O3/ZSM-5–67; V and VI) Fe2O3/ZSM-5–05.
The presence of large mesopores within the zeolite crystals is clearly visible in the TEM images
of both the Fe2O3/ZSM-5–67 and Fe2O3/ZSM-5–05 sample (Figure 3.71; III and IV, V and VI).
The Fe2O3 nanoparticles are clearly visible within the zeolite crystals due to the improved
contrast between both compounds. The average crystals size of the material prepared by
mixing Fe2O3@C nanoparticles with BP2000 in the ratio of 67/33 is 1.01 µm. The last sample
contains Fe2O3 nanoparticles embedded in ZSM-5 crystals with an average size of 1.55 µm. The
high concentration of mesopores in the zeolite crystals is visible in the TEM images shown in
Figure 3.71, indicated by the seemingly heterogeneous zeolite phase. The proof, that the dark
spots within the zeolite crystals in the TEM images are truly the incorporated Fe2O3
nanoparticles, is provided by EDX analysis (Figure 3.72). The spot analysis reveals the presence
of large amounts of iron in this area alongside silicon and oxygen from the zeolite crystal.
137
Figure 3.72: TEM image and local EDX analysis of Fe2O3 nanoparticles in ZSM-5 crystals.
Apart from the Fe2O3 content of the resulting material, the Si/Al- ratio of the zeolite crystals
can be varied to adjust the acidity of the zeolite. As already mentioned above, the zeolite
crystals are prepared by impregnating the carbon template with the zeolite precursor gel. The
chemical composition of the gel can be adjusted with respect to the aluminium amount used.
In the following data, zeolite crystals with different Si/Al- ratios, prepared with conventional
carbon templates following the same procedure introduced above, are compared to show the
versatility of the system.
The SEM images in Figure 3.73 reveal no great differences in crystal size and structure when
varying the Si/Al- ratio, whereas the results of the EDX prove the different aluminium content
of the crystals observed, ranging from no aluminium in the 1st sample to Si/Al ratios of
appropriately 100/1 and 50/1 in the other two samples.
Figure 3.73: SEM and EDX analysis of zeolite crystals with different Si/Al- ratios.
138
The rough and uneven surface of the zeolite crystals is created by the templating procedure,
and the pore openings of the hierarchical crystals can be seen clearly. The size of the silicalite
and ZSM-5 crystals is unaffected by the change in chemical composition.
The N2 physisorption isotherms of the samples show that the micro- and mesoporosity is only
very little influenced by changing the Si/Al- ratio.
350
350
I
250
200
150
100
250
200
150
100
250
200
150
100
50
50
0
50
0
0
0.2
0.4
0.6
Relative pressure
0.8
(p/po)
1
III
300
300
300
350
II
0
0
0.2 0.4 0.6 0.8
Relative pressure
1
(p/po)
0
0.2 0.4 0.6 0.8
Relative pressure
1
(p/po)
Figure 3.74: Comparison of the physisorption isotherms of zeolite crystals with different Si/Al- ratios: I) without Al,
II) Si/Al- ratio of 100/1, III) Si/Al- ratio of 50/1.
The isotherms of all three samples show adsorption of N2 at small relative pressures due to the
micropores of the zeolite. The isotherms continue to incline slightly at relative pressures of 0.2
to 0.8 and the rearrangement of N2 molecules can be seen based on the small hysteresis visible
at 0.15 relative pressure. The presence of large mesopores in the zeolite crystals lead to the
adsorption at high relative pressures and the observed hysteresis. The evaluation of the
sorption data using the BET method gives a micropore volume close to the conventionally
found value for ZSM-5 of 0.18 cm3/g for all samples, independent of the Si/Al ratio (see Table
3.4). Also, the total pore volume determined by DFT calculations is very similar for all samples.
The surface area of the prepared hierarchical zeolites lies in the range of 400 m2/g.
Table 3.4: Comparison of the pore volume and surface area of ZSM-5 samples with different Si/Al ratios.
Si/Al ratio
Micropore
Total pore
Surface
volume
volume
area
3
3
no Al
0.15 cm /g
0.43 cm /g
387 m2/g
100/1
0.17 cm3/g
0.37 cm3/g
415 m2/g
50/1
0.17 cm3/g
0.4 cm3/g
422 m2/g
139
The reason for varying the Si/Al ratio is the possibility to tune the acidity of the zeolite. Apart
from changing the iron content of the bi-functional material, it is important to adjust the
properties of the 2nd component at will, too. The possibility to change the Si/Al ratio over a
wide range allows fine tuning of the acidity of the final material. The number and strength of
acid sites of the ZSM-5 crystals were characterised by temperature programmed desorption of
NH3 (NH3-TPD).
0.025
TCD signal (a.u.)
0.02
0.015
0.01
0.005
0
120 160 200 240 280 320 360 400 440 480 520 560 600
Temperature (°C)
Figure 3.75: NH3-TPD curve of hierarchical zeolite with a Si/Al ratio of 100/1.
The curve progression shown in Figure 3.75 is typical for a ZSM-5 zeolite containing acid sites.
The two peaks with their respective maxima at 224 °C and 430 °C correspond to desorption of
NH3 molecules which are differently strongly bound to the zeolite surface. The detailed
interpretation of the curve progression is still vividly discussed among zeolite scientists: while
in some studies the 1st peak is associated with Lewis acid sites within the zeolite crystals and
only the 2nd peak is considered the result of Brønsted acid sites, other scientist prefer to
include both peaks for the calculation of Brønsted acid site concentration
[238]
. Despite the
controversy, it is generally agreed upon, that desorption of NH3 at the observed temperatures
is a valid confirmation of the acidity of zeolites. To calculate the acid site concentration only
the area of the 2nd peak is taken into account leading to a value of 0.13 mmol/g, which is in
good agreement with the calculated maximum concentration of 0.16 mmol/g based on the
Si/Al ratio.
This proves exemplarily that the aluminium atoms are incorporated into the zeolite
framework, leading to negatively charged structures, as opposed to the formation of EFAl
which is located in the pores of the zeolite framework. EFAl can lead to an enhanced Lewis
acidity but does not contribute to Brønsted acidity. The latter is strongly influenced by the Si/Al
140
ratio and can be tuned by changing the chemical composition of the zeolite precursor gel as
described before.
In summary, Fe2O3 nanoparticles covered by a carbon shell were successfully incorporated in
ZSM-5 crystals by the developed synthesis. A mixture containing different ratios of Fe2O3@C
nanoparticles and BP2000 was impregnated with the zeolite precursor gel and subjected to
hydrothermal conditions. The heating rate was chosen to be very low to support the growth of
hierarchical zeolite crystals. After the formation of ZSM-5 crystals, the carbon template
mixture is removed by calcination, and the composite product is obtained. The zeolite crystals
are several µm in size with Fe2O3 nanoparticles embedded inside the crystals. The Fe2O3
loading of the composite material can be adjusted over a wide range by changing the ratio of
Fe2O3@C nanoparticles in the template mixture. By varying the Fe2O3 content, the degree of
mesoporosity is changed as well, due to the different amount of BP2000, which create
mesopores in the ZSM-5 crystals after the combustion. Depending on the composition of the
initial carbon template mixtures, mesopores of around 20 nm are formed, proven by the pore
size distribution based on DFT calculations. The high degree of crystallinity of the zeolite phase
is proven both by XRD and sorption measurements.
Apart from the Fe2O3 content of the final product, the Si/Al ratio of the zeolite phase can be
adjusted. With the developed method silicalite-1 and ZSM-5 crystals with Si/Al ratios of 100/1
and 50/1 were prepared without changing the porosity or crystallinity of the material. The
different Si/Al ratios lead to a change in the acid properties of the zeolite phase.
The possibility to change both the Fe2O3 content and the properties of the zeolite phase leads
to versatile composite materials which can be prepared by the introduced templating
procedure by tuning the reactant concentrations slightly.
3.4.1.4 Deactivation of iron-based FT catalysts with core-shell geometry
The fast deactivation of conventional FT catalysts due to sintering, iron phase change, and
carbon deposition is still a great challenge, impairing the cost effectiveness of FT products in
industry. Due to this huge economic impact, the deactivation of FT catalysts and the involved
reactions are intensely studied, and alternative options regarding the composition and
structure of the used catalysts are tested.
As shown in Chapter 2.2.2.1, a core-shell geometry has proven to be beneficial in many
catalytic processes regarding the stability of the catalytically active material. The core-shell
particles introduced earlier in this section, containing a Fe2O3 core covered by a porous carbon
shell, can be used to study the influence of the core-shell geometry on the activity and
deactivation of FT catalysts. In order to investigate the influence of the encapsulation of the
Fe2O3 nanoparticles in a carbon shell, their performance is compared to that of a physical
mixture of Fe2O3 nanoparticles and empty carbon shells (Fe2O3 + @C), regarding the catalytic
activity and carbon formation.
141
For the catalytic testing, that was carried out by Dr. Marianna Casavola in the group of Prof.
Dr. Krjn de Jong, the catalyst particles were mixed with SiC (18 mg of catalyst and 150 mg SiC)
and filled into a plug flow reactor. Prior to the FT reaction, the catalysts were reduced in
hydrogen, before the gas flow was switched to syngas (CO:H2 = 1:1, 6 mL/min) and the
catalytic activity of the materials was measured. The temperature during reduction and during
the FT reaction was varied to study its influence on activity, selectivity and carbon formation.
In Figure 3.76 and Table 3.5 the results of the catalytic measurements are summarised. The
activity of the materials is given in iron time yield (FTY) at different temperatures and times
during the measurement. The temporal behaviour of the catalysts during the reaction is
studied by comparing the activity of the samples after 1 h and 15 h on stream (Table 3.5).
Figure 3.76: Comparison of FT activity at different temperatures between Fe2O3@C (I) and Fe2O3 + @C (II) sample.
-6
Table 3.5: Comparison of activity in FT reaction over time (values given in 10 molCO/gFe·s).
Temperature
Fe2O3@C
Fe2O3 + @C
1h
15 h
1h
15 h
300 °C
1.08
0.31
1.57
0.21
350 °C
0.81
0.54
0.48
0.19
400 °C
0.92
1.36
0.84
2.04
For the core-shell catalyst a decreasing activity over time is found both at a temperature of
300 °C and 350 °C. The activity at both temperatures reaches a plateau after 7 h on stream, yet
at 350 °C the activity is higher with a FTY of around 0.54·10-6 molCO/gFe·s. In comparison, the
activity decreases from 1.08·10-6 molCO/gFe·s to 0.31·10-6 molCO/gFe·s at 300 °C. Similar results
are found for the Fe2O3 + @C sample: after an initially very high activity in the first hours, the
activity decreases and reaches a plateau after 5 h if the reaction temperature is set at 350 °C,
and after 10 h in the case of 300 °C. Compared to the core-shell catalyst the decrease in
activity is more pronounced and a much lower activity is measured after 15 h on stream
(compare Table 3.5). The selectivity of the reaction is not influenced by the catalyst’s
geometry: at 300 °C no change in the selectivity towards CH4 and the desired C2-C4-products is
142
observed for both the core-shell and the mixed catalyst over time. If the temperature is
increased to 350 °C the CH4 selectivity increases with time for both catalysts (see Figure 3.77).
Figure 3.77: Selectivity of tested catalysts to CH4 and C2-C4-products at different temperatures.
Different results are obtained for both catalysts, if the temperature is increased to 400 °C:
after a slight decrease in the first hours of the reaction, the measured activity progressively
increases over time (Figure 3.76, Table 3.5). The selectivity towards CH4 increases with time at
the expense of the C2-C4-fraction, independent of the chosen catalyst (Figure 3.77).
The spent catalysts were analysed by TEM in order to determine carbon filament formation
and the fragmentation of the Fe2O3 nanoparticles which can lead to deactivation and changes
in the selectivity of the reaction. As shown in Figure 3.78, small iron nanoparticles are formed
within the carbon shells during the reaction at 300 °C (image I and II) and 350 °C (image III and
IV). Nevertheless, the carbon shells are mostly still intact, and only very few iron nanoparticles
can be found outside of the shells. Carbon filament formation is not observed in the spent
catalyst if the reaction is carried out at 300 °C. In contrast, the catalyst treated at 350 °C
reveals extensive carbon formation, and the breakage of carbon shells is observed occasionally
(Figure 3.78).
143
Figure 3.78: TEM images of Fe2O3@C particles after catalytic testing at 300 °C (I and II) and 350 °C (III and IV).
During the reaction at 400 °C the catalyst undergoes a significant phase change: both the
formation of carbon filaments in the spent catalyst and the Fe2O3 particles fragmentation is
extensive. The carbon shells are not stable under these conditions and break (Figure 3.79).
Figure 3.79: TEM images of the spent Fe2O3@C catalyst after FT reaction at 400 °C.
The extent of iron particle fragmentation during the FT reaction at different temperatures is
shown in Figure 3.80. With increasing temperature the ratio of small particles resulting from
particle fragmentation due to carbon deposition is increasing. While at a reaction temperature
of 300 °C only few particles are smaller than 40 nm, more and more particles disintegrate at
350 °C and 400 °C (compare initial size distribution in Figure 2.24).
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Figure 3.80: Particle size distribution of Fe2O3@C catalysts after reaction at different temperatures.
The analysis of the catalyst prepared by mixing Fe2O3 nanoparticles and hollow carbon spheres
after the reaction shows that already at low temperatures the shape and structure of the
particles changes severely (Figure 3.81). Large iron agglomerations are found throughout the
sample already at a reaction temperature of 300 °C. The catalyst treated at 300 °C shows no
major carbon deposition, and the particle fragmentation is limited (image I and II, Figure 3.81).
If the temperature is increased to 350 °C the iron particles start to break into smaller particles
and the deposition of carbon is more pronounced compared to the sample treated at 300 °C.
As for the core-shell catalyst discussed before, the formation of carbon filaments is extensive
at a reaction temperature of 400 °C (image V and VI, Figure 3.81). Apart from large iron
agglomerates, a high ratio of small particles is found throughout the sample. As for the
catalysts with core-shell geometry, the fragmentation of the iron phase of the mixed material
gets more pronounced with increasing temperature, as shown in Figure 3.82. At low
temperatures the majority of the particles retains their original size, and at a temperature of
350 °C the fragmentation of the iron particles leads to a broader size distribution and
decreasing particle size. Important to note is that the large agglomerations found in all
Fe2O3 + @C samples are not considered in the size distributions shown in Figure 3.82.
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Figure 3.81: TEM images of the Fe2O3 + @C particles after catalytic testing at 300 ° (I and II), 350 °C (III and IV) and
400 ° (V and VI).
Figure 3.82: Observed particle size distribution of Fe2O3 + @C sample after catalytic testing at different
temperatures.
146
As shown in Figure 3.76 for both catalysts an initial decrease in activity is observed at 300 °C
which can be explained by the carbon deposition visible in the TEM images of the spent
catalysts. The core-shell catalyst is less active than the mixed catalyst in the first hours of the
reaction, which could be due to the hindrance of adsorption of reactants and desorption of
products by the shell. In contrast, the activity of the Fe2O3@C catalyst after 15 h on stream is
much higher than the activity found for the mixed catalyst (see Table 3.5). As shown in the
TEM images of the spent catalysts, the core-shell structure prevents the formation of large iron
agglomerations, which are found already at 300 °C in the mixed sample. The higher stability of
the iron nanoparticles within the carbon shell leads to a less pronounced deactivation of the
catalyst over time. The core-shell catalyst shows high selectivity towards the C2-C4-range and
only low selectivity to CH4, and the selectivity is stable over time due to the limited particle
fragmentation found in the sample. The selectivity of the mixed catalysts towards C2-C4products is stable over time, too. However, with increasing time the selectivity towards the
undesired CH4 is increased which is probably due to the formation of smaller iron particles in
the course of the reaction (see particle size distribution of spent catalyst in Figure 3.82) [239]. At
350 °C the stabilising effect of the core-shell structure of the catalyst is even more pronounced
than at lower temperature: the deactivation of the Fe2O3 + @C catalyst is much faster and the
activity after 15 h on stream is much lower compared to the core-shell catalyst (see Table 3.5).
The encapsulation of the Fe2O3 nanoparticles in a carbon shell allows a more stable catalytic
performance and a moderate activity loss because the formation of large iron agglomerates is
prevented. The selectivity of both catalysts towards CH4 increases over time due to the
progressive formation of smaller iron particles that is observed in TEM. At 400 °C the structure
of the catalysts is not retained and increasing particle fragmentation leads to the observed
increase in activity over time. Due to the small particle size, the selectivity towards the
undesired CH4 is rather high at a reaction temperature of 400 °C.
In brief, Fe2O3@C nanoparticles were studied in the conversion of syngas to lower olefins and
compared to a simple mixture of Fe2O3 nanoparticles and hollow carbon spheres to investigate
the influence of the core-shell structure on the activity and deactivation of FT catalysts. The
core-shell structure of the catalyst leads to a less pronounced deactivation over time if the
reaction is carried out at a temperature of 300 °C. Even though the initial activity of the mixed
catalyst is higher, agglomeration of iron particles leads to fast deactivation. At low
temperatures, for both catalysts only limited particle fragmentation is observed, and the initial
particle size distribution is not strongly changed. If the temperature is increased to 350 °C,
particle fragmentation is more pronounced in both catalysts, leading to an increased selectivity
to CH4 at the expense of the C2-C4- range. The fast deactivation of the mixed catalyst over time
can be prevented by the core-shell geometry: the catalytic performance of the Fe2O3@C
catalyst is more stable, and only moderate activity loss is observed. At a reaction temperature
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of 400 °C the shape and structure of both catalysts cannot be retained and extensive particle
fragmentation leads to the observed increase in activity.
The core-shell structure of FT catalysts is beneficial for the stability of the catalytically active
iron phase, and leads to an improved long-term activity. Yet, severe reaction conditions can
lead to damages in the structure and the functionality of the material is impaired.
Nevertheless, materials with a core-shell structure are promising candidates for overcoming
the limitations of conventional catalysts, as shown in this chapter.
3.4.2 Post-synthetic modification of hierarchical zeolites
3.4.2.1 Modification with noble metal nanoparticles
For the preparation of composite materials consisting of a zeolite phase and noble metal
nanoparticles, a post-synthetic modification method of previously prepared hierarchical zeolite
crystals was chosen. The hierarchical ZSM-5 crystals were prepared by a hard-templating
method using a commercial carbon template. Afterwards, the porous crystals were
impregnated with the noble metal precursor solution and dried in a vacuum oven at 50 °C. In
the final step, the dried solid was reduced in a H2 flow at 300 °C for 2 h.
The impregnation of the zeolite crystals was carried out using the incipient wetness method, in
which the volume of the precursor solution does not exceed the pore volume of the support
material. This method was chosen to support the formation of noble metal nanoparticles
within the zeolite crystals and not on their surface in order to obtain composite materials in
which both phases are closely spaced. As solvent for the precursor solution, H2O was used, due
to the high solubility of the noble metal precursors. The drying step is crucial for the
incorporation of the nanoparticles inside the crystals: the solvent must be carefully removed
before the following heat treatment to prevent the formation of nanoparticles predominantly
on the crystal surface. The complete removal of H2O is achieved by drying the impregnated
solid under vacuum at elevated temperatures. The reduction of the noble metal precursor is
carried out in a constant H2 flow at 300 °C. The reduction temperature has to be carefully
adjusted to balance the formation of zero-valent noble metal nanoparticles while also
preventing the formation of large agglomerates. If the reduction temperature is increased to
450 °C the formation of larger particles within the zeolite crystals and especially on the surface
can be observed. Nevertheless, the chosen temperature of 300 °C is sufficiently high because it
already ensures the formation of zero-valent noble metal nanoparticles within the zeolite
crystals.
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Figure 3.83: SEM images of hierarchical zeolite crystals with platinum nanoparticles.
The size and surface structure of the product is analysed with SEM (Figure 3.83). The
hierarchical zeolite crystals prepared by carbon templating have the typical crystal form
observed for ZSM-5, yet, the surface appears rough and uneven. Pore openings and voids of 10
to 50 nm are visible on the surface of the crystal left behind after removing the carbon
template by calcination. The ZSM-5 crystals have an average size of 2.1 µm based on the
evaluation of SEM data. The formation and incorporation of noble metal nanoparticles cannot
be confirmed by conventional SEM analysis, even though local electric charging on the surface
of the ZSM-5 crystals suggests the presence of metal species in the material (see Figure 3.83,
right picture).
Yet, to fully prove the presence of noble metal nanoparticles within the hierarchical zeolite
crystals the material was embedded in an epoxy resin and cut into thin slices.
Figure 3.84: Analysis of cross-sectional cuts of hierarchical ZSM-5 crystals with ruthenium nanoparticles using SEM.
149
These slices were then characterised with SEM and STEM techniques in both bright- and darkfield mode. Pictures I and II in Figure 3.84 show the cross sectional cutting of a complete
hierarchical zeolite crystal impregnated with ruthenium. Especially in the 1st micrograph, the
size and form of the zeolite crystal is nicely visible, with pores and voids in the range of several
10 nanometres. In using the dark-field mode the finely distributed nanoparticles are made
visible (picture II and III). The average particle size is found to be 3.50 nm, and the STEM
images of the cross sectional cuts suggest that the nanoparticles are deposited close to the
voids of the zeolite crystals introduced by the carbon template- assisted preparation. In an
application in heterogeneous catalysis this would lead to an excellent accessibility of the noble
metal nanoparticles.
Apart from this local concentration of ruthenium nanoparticles, the distribution within the
zeolite crystals is rather homogeneous. More importantly, nanoparticles are not formed on the
outer surface of the zeolite crystals but only within the pore system of the crystals.
Figure 3.85: SEM and EDX analysis of cross sectional cuts of platinum nanoparticles containing ZSM-5.
The results of the cross sectional cutting for the sample containing platinum nanoparticles also
prove the fine distribution of small nanoparticles within the zeolite crystals. Yet, only
fragments of the zeolite crystals can be found when analysing the thin cuts by SEM and STEM
techniques while the analysis with conventional SEM of the initial sample (Figure 3.83)
indicated the large ZSM-5 crystals present in the sample. The reason for this apparent
differences can be found in the preparation method of the cross sectional cuts: the epoxy resin
150
did not penetrate the complete pore system of the crystals and consequently did not stabilise
the complete cross section of the crystals during the preparation of thin slices. Therefore, only
fragments of the initial crystals can be found in the analysis of the cuts using SEM and STEM
(Figure 3.85, picture I and II). Nevertheless, the micrographs prove the presence of small
nanoparticles throughout the sample, and EDX analysis was carried out to confirm the
chemical nature of the nanoparticles as platinum (Figure 3.85, picture III). No large platinum
agglomerates can be found in the fragmental cross sectional cuts, but only small nanoparticles
with an average size of 3.16 nm are determined.
The results found for the platinum containing sample suggest, as in the prior described
Ru/ZSM-5 system, that the platinum nanoparticles are deposited predominately around the
pores and voids left behind by the carbon template.
As in the case of the composite materials consisting of ZSM-5 and platinum nanoparticles, only
zeolite fragments are found in the analysis of the cross sectional cuts of the Pd/ZSM-5 sample.
The epoxy resin used for the stabilisation of the material during the preparation of the thin
slices did not completely penetrate the zeolite crystals leading to smaller fragments (Figure
3.86, pictures I and II). Granted that, the small size of the palladium nanoparticles is clearly
visible leading to an average size of 3.00 nm based on the evaluation of STEM images. The
nanoparticles embedded in the zeolite matrix were also characterised with EDX, as shown in
Figure 3.86. palladium nanoparticles are homogeneously distributed throughout the fragments
of the cross sectional cuts and larger agglomerations cannot be found.
Figure 3.86: palladium nanoparticles within hierarchical zeolite crystals analysed with SEM and EDX.
151
The particle size distribution of the respective nanoparticles is compared in the plot given in
Figure 3.87. It shows that while the sizes of the ruthenium and palladium nanoparticles vary
only slightly, a broader size distribution is found for platinum nanoparticles.
25
20
Ru
Number (%)
15
Pt
Pd
10
5
0
0
1
2
3
4
5
6
7
8
9
10
11
12
Diameter (nm)
Figure 3.87: Size distribution of noble metal nanoparticles incorporated in ZSM-5 crystals.
The average size of platinum nanoparticles lies in the range of the values obtained for both
ruthenium and palladium nanoparticles; the analysis of the platinum/ZSM-5 sample shows also
larger nanoparticles. The particle size distribution found for the Pt nanoparticles ranges from
1 nm to 7.4 nm. While the other samples also contain some larger noble metal nanoparticles,
the ratio found in the platinum containing material is much higher.
The general noble metal content was determined by EDX analysis of the material, taking into
account a much larger area compared to the spot analysis shown in Figure 3.85 and Figure
3.86. For the samples containing platinum and rhodium nanoparticles a noble metal content of
1.75 wt% and 1.73 wt%, respectively, was found, while the ruthenium loading was determined
to have a value of 1.17 wt%. Especially the values found for platinum and palladium represent
the intended loading of 1.5 wt% very closely according to the data obtained by EDX.
To characterise the crystalline phases of the composite materials containing noble metal
nanoparticles X-ray diffraction was carried out (Figure 3.88).
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Figure 3.88: Characterisation of noble metal containing zeolites using XRD.
The powder pattern measured with Cu radiation shows reflections of the ZSM-5 crystals
present in the samples, but no other reflexes belonging to different crystalline phases. The
presence of noble metal nanoparticles proven both by SEM and EDX cannot be confirmed with
XRD. This is most likely due to the small size of the noble metals and their low concentration.
Yet, the comparison to a calculated pattern of calcined ZSM-5 confirmed the high degree of
crystallinity of the hierarchical zeolite crystals prepared prior to the impregnation with noble
metals. The preparation method using commercial carbon templates does not impair the
crystallinity of the ZSM-5 phase.
In summary, composite materials consisting of hierarchical ZSM-5 crystals modified with noble
metal nanoparticles were prepared by an impregnation method. The dried zeolite was
impregnated to incipient wetness with the noble metal precursors dissolved in H2O. After the
solvent was removed by drying in a vacuum oven the sample was reduced at elevated
temperatures to obtain zero-valent noble metal nanoparticles. The analysis of the samples
using SEM proved the formation of small nanoparticles with an average size of 3.5 nm for
ruthenium nanoparticles, 3.16 nm for platinum and finally 3.0 nm for the sample containing
palladium nanoparticles. Cross sectional cuts were prepared for all samples to analyse the
location of the nanoparticles within the zeolite matrix. The characterisation using SEM and
STEM showed the homogeneous distribution of the nanoparticles within the zeolite crystals.
Moreover, the nanoparticles are formed predominantly around the voids and channels of the
mesopore network within the zeolite crystals.
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3.4.2.2 Post-synthetic addition of transition metal oxides
For the preparation of a composite material consisting of hierarchical ZSM-5 crystals modified
with CuO and ZnO species a post-synthetic method based on incipient wetness impregnation
was chosen. The additional system of mesopores within zeolite crystals prepared by carbon
templating is ideal for the deposition of additional phases and nanoparticles.
In order to obtain the modified ZSM-5 crystals, a synthesis was developed that includes four
steps: the hierarchical zeolite crystals are prepared by the impregnation of a commercial
carbon template as already described in the previous chapter. After removal of the carbon
template by calcination, the zeolite is dried and impregnated with transition metal oxide
precursors in the 2nd step. The solvent is removed by freeze drying, and finally the oxides are
formed during calcination in air in the final part of the synthesis.
The drying of the zeolite prior to the impregnation is necessary to remove all traces of H2O
from the pores and to ensure a successful incipient wetness impregnation. Therefore, the
zeolite is placed in a vacuum oven at 50 °C for 48 h. Then the zeolite is placed in an agate
mortar and impregnated to incipient wetness with an aqueous Cu(NO3)2 solution. In order to
remove the solvent from the pores of the zeolite, the sample is freeze dried and afterwards
placed in a vacuum oven at a temperature of 50 °C. Depending on the intended CuO loading of
the final material, the cycle consisting of impregnation and freeze drying is repeated several
times to reach higher loadings. If the impregnation is carried out once, a theoretical loading of
11 wt% of CuO is obtained, while a CuO content of 28 wt% can be realised with 3 cycles of
impregnation and freeze drying. The sample is finally calcined to form CuO species within the
zeolite crystals, before the material is impregnated with an aqueous Zn(NO3)2 solution. The
sample is carefully mixed in an agate mortar with the zinc precursor and freeze dried. After
24 h under vacuum at 50 °C and a 2nd calcination step, the final product is obtained. The final
composite material consisting of ZSM-5, CuO and ZnO contains a calculated loading of
10.8 wt% of CuO and 4.2 wt% of ZnO if the impregnation with the Cu precursor is carried out
once. If the CuO content is increased to 26.7 wt% by the repetition of the impregnation and
freeze drying step, the relative amount of ZnO is decreased accordingly to a value of 3.4 wt%.
The chosen precursors Cu(NO3)2 and Zn(NO3)2 have a very high solubility which is crucial for
syntheses based on incipient wetness impregnation. Due to the limited volume of precursor
solution used in one impregnation step, it is very important to incorporate as much metal
species as possible. Otherwise the process gets very time-consuming due to the necessity of
repeated impregnation steps. Additionally, the zeolite framework can be damaged by the
repeated contact to the transition metal salt solution.
The removal of solvent of the transition metal precursor solution and the following calcination
are crucial steps with which the distribution of the CuO and ZnO species is strongly influenced.
The solvent must be carefully, yet completely removed before the calcination step. If solvent
molecules are still present within the pores of the zeolite crystals, copper and zinc species are
154
transported out of the zeolite alongside solvent molecules, due to the rapid temperature
increase. The formation of CuO and ZnO occurs then outside of the zeolite crystals and two
separated phases are obtained. To prevent the formation of CuO and ZnO particles outside the
zeolite crystals the heating rate of the calcination process is very low, too.
The obtained solid was characterised with TEM, and the obtained images are given in Figure
3.89. The additional mesopore network is clearly visible within the zeolite crystals, yet the
crystals have sharp edges due to their crystalline nature. The crystallinity of the zeolite is nicely
illustrated in the HR-TEM micrograph on the right side. Still, no CuO or ZnO nanoparticles can
be distinguished from the zeolite crystals due to the low difference in contrast in the TEM
images.
Figure 3.89: TEM images of hierarchical ZSM-5 crystals with low loading of CuO and ZnO.
To have a better understanding on the location of the CuO and ZnO species in the composite
material, SEM images were combined with EDX. In Figure 3.90 the sample containing a
calculated amount of 10.8 wt% of CuO and 4.2 wt% of ZnO is analysed. The zeolite crystal
shows no apparent changes, the porous structure and rough surface is maintained during the
post-synthetic modification. No major agglomerations of CuO or ZnO separate from zeolite
crystals can be found throughout the sample. Only the analysis with EDX proves the presence
of copper and zinc species inside the zeolite crystal. The spot analysis using EDX measurements
of the shown crystals reveal copper loadings in the range of 2 atom% which corresponds to a
CuO content of 7.45 wt%, and a ZnO loading 0.5 wt%. While the spot analysis of the sample
using EDX gives a CuO loading of only around 7.5 wt%, the calculated amount of CuO was
confirmed by EDX when analysing the entire sample.
155
Figure 3.90: SEM and EDX results for the composite material containing low loading of CuO and ZnO.
In the case of higher CuO loadings different results are found by using SEM and EDX to
characterise the composite material (see Figure 3.91). Apart from the hierarchical zeolite
crystals large particles are found in the product. The higher loading leads to the formation of a
separate phase of CuO, and large particles agglomerate on the surface of the zeolite crystals.
This is proven by the EDX measurement at different positions in the sample: the 1st spot is
centred on the hierarchical zeolite crystal and shows the presence of 3 atom% of Cu. However,
if the large particle on the surface of the zeolite crystals is analysed, a copper ratio of
20 atom% is found.
Figure 3.91: Analysis of ZSM-5 crystals containing a higher loading of CuO.
In addition, a mapping based on EDX data shows the inhomogeneous distribution of copper
and zinc species in the sample containing higher amounts of CuO. As already shown in the SEM
image above, a separate phase is formed outside of the zeolite crystals. The atom mapping
shown in Figure 3.92 on the right side proves the high concentration of copper atoms in the
particle formed outside of the hierarchical zeolite crystals, while the zinc distribution is rather
homogeneous throughout the sample.
156
Figure 3.92: Atom mapping based on EDX data showing the formation of a separate CuO phase.
X-ray powder diffraction was used to characterise the crystalline constituents of the sample.
The diffraction pattern of the zeolite component of the composite material is in good
agreement with the calculated pattern of silicalite-1, which is plotted in grey for comparison
(see Figure 3.93). Additionally, sharp diffraction peaks are visible resulting from the formed
CuO particles. The intensity and the line width of the reflections indicate the presence of large
crystalline CuO domains in the product.
Figure 3.93: Characterisation of the composite material containing 26.7 wt% CuO using X-ray diffraction.
In contrast to the pattern shown above, the diffractogram of the sample containing a lower
amount of CuO shows no strong signals of CuO, leading to the conclusion that the crystalline
domains of the CuO particles in this sample are much smaller.
157
In conclusion, the incipient wetness impregnation of hierarchical zeolite crystals with transition
metal species is an easy and straight-forward method for the preparation of composite
materials. Due to the high solubility of transition metal salts, the impregnation using very small
solvent volumes still leads to substantial metal oxide loadings. It is very important to remove
the solvent prior to the calcination step to inhibit the formation of a separate transition metal
oxide phase. Even if the solvent is completely removed, and the calcination is carried out at
low heating rates, only at low targeted CuO loadings the formation of separate transition
metal oxide particles can be prevented. If the amount of CuO is increased, large particles are
formed on the surface of the zeolite crystals and an inhomogeneous product is obtained. Yet,
for lower loadings of CuO and ZnO the incipient wetness impregnation of hierarchical zeolites
is an easy and uncomplicated preparation method for the synthesis of composite materials.
3.5 Summary and conclusion
Zeolites are ideal components of bi- or multifunctional materials due to their high stability and
catalytic activity. Additionally, many properties of zeolites depend on their Si/Al ratio, which
can be easily tuned by the reactants. The addition and incorporation of metal or metal oxide
nanoparticles open up new possibilities to prepare versatile materials with multiple
functionalities. In the last chapters two general approaches for the preparation of bi-functional
materials were introduced: metal and metal oxide nanoparticles can be incorporated into
zeolite crystals after the zeolite formation is completed or nanoparticles can be encapsulated
by zeolite crystals during their growth under hydrothermal conditions.
The latter approach was developed for the incorporation of transition metal oxide
nanoparticles. If Fe2O3 nanoparticles are simply added to the zeolite precursor gel, phase
separation occurs during the zeolite formation so that the nanoparticles are not incorporated
into the zeolite crystals. Therefore, a novel synthetic approach was developed in which the
Fe2O3 nanoparticles are encapsulated in a carbon shell during the zeolite formation. The
encapsulation of the Fe2O3 nanoparticles in carbon shells is the key step of this concept which
is achieved by nanocasting. First, the Fe2O3 nanoparticles prepared by hydrothermal synthesis
were encapsulated by a dense SiO2 shell which was covered with a 2nd, porous SiO2 layer. The
outer SiO2 layer has an average thickness of 62 nm and contains mesopores in the range of 2 to
4 nm. The synthetic parameters were adjusted in a way that single core-shell particles were
obtained and the agglomeration of particles was prevented. This is highly important for the
following nanocasting step: the mesoporous SiO2 shell was impregnated with furfuryl alcohol
and heated up. The polymer was formed within the pores of the outer SiO2 layer and
carbonised in inert atmosphere at 650 °C. After the SiO2 template was removed by leaching,
carbon covered Fe2O3 nanoparticles were obtained. The carbon shell is an exact replica of the
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porous SiO2 layer- both for the size and porosity similar values are found. The complete
encapsulation of Fe2O3 nanoparticles in single carbon shells is important for the following
incorporation of the particles in zeolite crystals. The zeolite growth around the carbon shells is
achieved by an impregnation technique followed by hydrothermal treatment. Mixtures
containing different ratios of Fe2O3@C nanoparticles and commercial carbon template are
impregnated with the zeolite precursor gel to incipient wetness. The solid is then placed in an
autoclave and heated up to 180 °C under hydrothermal conditions using a very slow heating
rate to promote the formation of large crystals around the carbon templates. After the zeolite
formation is completed, the carbon template and shells are removed by calcination. The
product consists of hierarchical zeolite crystals with Fe2O3 nanoparticles embedded within the
crystals. The Fe2O3 content of the composite material can be varied over a wide range. Yet, by
varying the Fe2O3 content, also the degree of mesoporosity is changed due to the preparation
method based on the impregnation of a carbon template mixture: at high Fe2O3 contents the
mesoporosity of the crystals is low because the initial carbon mixture contained no or only a
low ratio of commercial carbon template that leads to mesopore formation in the ZSM-5
crystals. Despite the templating approach, the crystallinity of the ZSM-5 crystals is high- both
visible in microscopy techniques and XRD pattern. The crystals are between 0.79 and 1.55 µm
and contain several Fe2O3 nanoparticles. Apart from the Fe2O3 content, the properties of the
zeolite phase can be tuned. The Si/Al ratio can be changed over a wide range by changing the
chemical composition of the zeolite precursor gel used to impregnate the carbon template
mixture. While the porosity and crystallinity is not affected by the varying Si/Al ratio, the
acidity of the zeolite phase can be easily adjusted. By both changing the Fe2O3 content and the
acidity of the zeolite phase, the developed approach allows the preparation of a wide range of
materials with different characteristics.
Moreover, the idea of the developed synthetic approach can be applied to the preparation of
various composite materials. It should be possible to encapsulate nanoparticles of different
chemical composition by a carbon shell in order to accomplish the incorporation in zeolite
crystals.
As a 2nd approach for the preparation of bi-functional materials, the impregnation of previously
prepared zeolite crystals, was introduced. To support the formation of nanoparticles within
the zeolite crystals in order to form a composite material in which both phases are in close
proximity, hierarchical ZSM-5 was chosen for the impregnation. The zeolite crystals were
prepared by a carbon templating method leading to large crystals with a system of mesopores
within the crystals. The incipient wetness impregnation technique was then used to prepare
noble metal nanoparticle and transition metal oxide containing composite materials.
Noble metal nanoparticles in combination with zeolites form interesting composite materials
due to the superior chemical reactivity of both components. The hierarchical zeolites were
dried in a vacuum oven prior to the impregnation step to remove H2O from the pores. The
159
noble metal precursors RuCl3·H2O, K2PtCl4, and K2PdCl4 were dissolved in small amounts of H2O
that correspond to the pore volume of the zeolite. During the impregnation the zeolite was
carefully, yet thoroughly mixed with the precursor solutions and dried. In the final step, the
material is heated up in a H2 flow to form zero-valent nanoparticles. Due to the preparation
method a high dispersion of nanoparticles within the zeolite crystals could be achieved. The
analysis of cross sectional cuts led to the conclusion that the nanoparticles were mostly
deposited within the voids and pores of the additional mesopore system which leads to an
excellent accessibility of the noble metal surface. The average size of all noble metals
(ruthenium, platinum, palladium) was in the range of 3 nm. The noble metal content of the
composite material ranges from 1.17 to 1.75 wt%.
In contrast, much higher loadings were targeted at the incorporation of transition metal oxide
species in hierarchical zeolites. While transition metal oxide nanoparticles are attractive
alternatives to expensive noble metal catalysts, the activity is often lower and higher loadings
of the metal oxide species are necessary to obtain comparable catalytic activities. The different
loadings of CuO and ZnO species were achieved by repeating the impregnation step several
times. As described before, the dried hierarchical zeolites were impregnated to incipient
wetness first with the copper precursor. To fully remove the H2O, the impregnated zeolite was
freeze dried and depending on the intended CuO loading the impregnation process was
repeated. The formation of CuO was then carried out by calcination in air. Afterwards, the
impregnation was repeated with the zinc precursor solution in the same way. The CuO and
ZnO loading with one impregnation cycle each led to values of 10.8 wt% of CuO and 4.2 wt% of
ZnO in the final composite material. The analysis of the material with SEM and EDX showed
that the copper and zinc species were incorporated in the hierarchical zeolite crystal. While no
external particles could be observed, EDX analysis proved the high CuO and ZnO content of the
zeolite crystals. At higher CuO loadings the formation of separate CuO particles could be
observed. If the impregnation cycle is repeated 3 times, the CuO content is increased to a
calculated value of 26.7 wt%, which could be confirmed by EDX analysis of the overall sample.
Yet, only low amounts of CuO and ZnO could be detected within zeolite crystals by EDX spot
analysis.
Despite the ease of the preparation of composite materials by impregnation, the limited
loading that can be achieved within the zeolite crystals is a major drawback. While at lower
noble metal as well as transition metal oxide loadings the formation of the 2nd phase is
restricted to the pore system of the hierarchical zeolites, at higher loadings the formation of
separate particles cannot be prevented.
On the contrary, high Fe2O3 loadings of up to 50 wt% can be realised in the templating-based
synthesis that was developed and is introduced above. Even though the reaction pathway is
complex, various material properties can be easily tuned, leading to a promising preparation
approach for transition metal oxide containing zeolites.
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4 Final Remarks
4 Final Remarks
Today’s society faces manifold challenges that need to be overcome to ensure the livelihood
and welfare of humankind. Various chemical fields play a major role in meeting these
challenges: The development and preparation of novel drugs, sustainable techniques for the
production of energy and establishing novel energy storage devices can only be mastered by
advances in fields ranging from pharmaceutical chemistry to chemical engineering. In the
report of the United Nations “21 challenges for the 21st century” more than half of the
described issues are related to technological problems that depend on the application of
functional materials
[240]
. Especially issues concerning energy, technology and waste but also
challenges humankind faces due to climate change and ensuring food safety are connected to
the development of suitable materials. Due to their promising and unique properties,
functional nanoparticles and nanocomposites are considered an integral part of solving the
challenges humankind is facing today. To meet these expectations, targeted preparation
pathways to functionalised nanomaterials and nanocomposites are essential.
The two general preparation concepts introduced in this thesis differ greatly in their idea and
synthetic implementation, yet both contribute to the tool box of material chemistry and the
preparation of highly functionalised composite materials with a distinct hierarchy. The
developed syntheses lead to functional nanocomposites whose structure is designed on a
nanometer level in order to combine the properties of the constituents.
In the 1st part, a core-shell system was introduced which allows the preparation of core-shell
nanoparticles with multiple chemical compositions. On the basis of Fe2O3@SiO2 nanoparticles
several modification reactions were developed to change both the metal core and the shell
material. Both the modification of the core nanoparticle as well as of the formation of different
shell materials were carried out under the same reaction conditions. Fe2O3 nanoparticles
covered with a porous SiO2 shell were chosen as the starting point of the modification
reactions because the SiO2 shell is an ideal anchor point for other shell materials. Hence,
Fe2O3@TiO2 and Fe2O3@ZrO2 nanoparticles with a yolk-shell structure could be prepared. The
preparation method led to the formation of single particles of around 240 nm in diameter with
a porous transition metal oxide shell that encapsulates the Fe2O3 core completely. Both TiO2
and ZrO2 shells are distinguished by their high chemical and thermal stability and a potentially
beneficial support effect in catalytic conversions. The Fe2O3 core of the initial core-shell
nanoparticles can be replaced by noble metal nanoparticles in a metal exchange reaction. A
wide range of noble metal nanoparticles can be placed inside porous SiO2 shells following this
method, including platinum, ruthenium, palladium and rhodium. The average size of the
resulting noble metal nanoparticles is very small, ranging from 2.8 nm to 3.45 nm. By changing
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4 Final Remarks
the chemical composition of both the core and shell material, a versatile model system with a
distinct structure is obtained.
The preparation method introduced in the 2nd part takes a different approach for preparing
highly functionalised nanocomposites with a pronounced organisation and structure.
Hierarchical zeolite crystals are modified with noble metal and transition metal oxide
nanoparticles to combine the properties of the components and to generate novel properties.
The idea of preparing hierarchical zeolites with carbon templates was picked up and modified
for the incorporation of Fe2O3 nanoparticles in zeolite crystals. The developed preparation
concept can be used to prepare a highly versatile, bi-functional material in which both metal
oxide content, porosity and Si/Al ratio can be modified over a wide range. The successful
incorporation of metal oxide nanoparticles in zeolite crystals was achieved by preparing
Fe2O3@C nanoparticles and using these as template in the preparation of hierarchical zeolite
crystals. The core-shell particles are incorporated during the growth of zeolite crystals and the
carbon shell is later removed by calcination. By adding BP2000 to the Fe2O3@C nanoparticles,
the resulting zeolites contain a network of mesopores that increases the accessibility of the
zeolite crystals. Furthermore, the acidity of the zeolite phase can be tuned by changing the
Si/Al ratio. Moreover, the developed preparation concept has the potential to be further
applied for the incorporation of various metal and metal oxide nanoparticles. The idea of
encapsulating nanoparticles with a carbon shell in order to incorporate metal and metal oxide
species in zeolite crystals can be possibly applied to various systems.
Metal and metal oxide species were also introduced in zeolite crystals by impregnation.
Hierarchical zeolite crystals with small noble metal nanoparticles were prepared by incipient
wetness impregnation and reduction in H2. The nanoparticles are homogeneously distributed
throughout the crystals. While the loading of noble metal nanoparticles was rather low
(1.5 wt%), higher loadings were targeted at the preparation of CuO and ZnO containing
zeolites. At CuO loadings of 11 wt the impregnation technique led to a successful incorporation
of metal oxide species in zeolite crystals. If the loading was increased to 27 wt% the formation
of separate CuO particles could not be prevented. Impregnation is a versatile and straightforward method to prepare bi-functional materials; however, the incorporation of metal or
metal oxide nanoparticles especially at higher loadings is limited.
While the approaches for the preparation of functional nanocomposites introduced in this
thesis are based on very different strategies, they are all distinguished by their flexibility and
ease of adjusting the materials properties. In all introduced systems not only the chemical
composition is in the focus, but the structure and organisation of the materials are of great
importance for their properties. The influence of the materials structure on the observed
properties is yet another challenge for the preparation of nanocomposites: not only the
chemical composition must be carefully tuned; also the structure and organisation of the
nanoparticles must be controlled by using suitable preparation methods.
The struggles of synthetic chemists were already described by Richard P. Feynman in his
visionary speech “There’s plenty of room at the bottom” given in 1959: “The chemist does a
mysterious thing when he wants to make a molecule. He sees that it has got that ring, so he
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4 Final Remarks
mixes this and that, and he shakes it, and he fiddles around. And, at the end of a difficult
process, he usually does succeed in synthesizing what he wants.”[1]
The preparation of functional composite materials with a distinct hierarchy in the nano-range
remains a challenge and mystery to some extent, but- in the words of Richard P. Feynman- the
“fiddling around” of synthetic chemists is getting more and more precise in preparing highly
functionalised materials with the help of the ever growing tool box of targeted synthetic
pathways to nano-structured materials.
163
5 Experimental
5 Experimental
5.1 Chemicals
Name
Formula
Abbreviation
Supplier
Purity
2-Propanol
C3H7O
-
Sigma Aldrich
≥99.9%
Aluminium isopropoxide
Al(OC3H7)3
Al-iPro
Sigmal Aldrich
Ammoniumhydroxid solution
NH4OH
-
Sigma Aldrich
28 wt% in water
Black Pearls 2000
C
BP2000
Cabot Inc.
-
Cetyltrimethylammonium bromide
C19H42NBr
CTAB
Sigma Aldrich
98%
cis-Diamminedichloroplatinum
H6Cl2N2Pt
Cis-Pt
Sigma Aldrich
≥99.9%
Ethanol
C2H6O
EtOH
JT Baker
99.8%
Furfuryl alcohol
C5H6O2
FA
Sigma Aldrich
98%
Hydrochlorid acid
HCl
-
JT Baker
37 wt% in water
Iron(III)chlorid hexahydrate
FeCl3·6 H2O
-
Sigma Aldrich
≥99%
l-Lysine
C6H14N2O2
-
Sigma Aldrich
≥99%
Octadecyltrimethoxysilane
C21H46O3Si
OTMS
Sigma Aldrich
90%
Oxalic acid
C2H2O4
OA
Sigma Aldrich
≥99%
Poly(vinylpyrrolidon), MW= 1.3 Mio
-
PVP-1.3Mio
Sigma Aldrich
-
Poly(vinylpyrrolidone)-K90
-
PVP-K90
Sigma Aldrich
-
Potassium tetrachloropalladate
K2PdCl4
-
Sigma Aldrich
98%
Potassium tetrachloroplatinate
K2PtCl4
-
Sigma Aldrich
98%
Rhodium(III) acetylacetonate
Rh(acac)3
-
Sigma Aldrich
98%
Ruthenium(III)chloride hydrate
RuCl3·H2O
-
Sigma Aldrich
98%
Sodium hydroxide
NaOH
-
JT Baker
97%
Tetraethylorthosilicate
Si(OC2H5)4
TEOS
Sigma Aldrich
98%
Tetrapropylammonium hydroxide
C12H29NO
TPAOH
Fluka
20 wt% in water
Titanium butoxide
Ti(OC4H9)4
TBOT
Sigma Aldrich
≥97%
Zirconium tert-butoxide
Zr(OC4H9)4
ZBOT
Sigma Aldrich
≥97%
5.2 Syntheses
5.2.1 Preparation of Fe2O3 nanoparticles
Fe2O3 nanoparticles were prepared using hydrothermal conditions as described by Feyen et
al [58]. Per 100 mL of millipore (mQ) water, 0.54 g of FeCl3 ·6 H2O and 0.29 g of l-Lysine were
dissolved using ultrasonication. The solution was filled in a Teflon inlet with a volume of
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5 Experimental
110 cm3. The Teflon inlet was then inserted in a steel autoclave and placed in a preheated
oven. The size and morphology of the resulting Fe2O3 nanoparticles can be determined by the
temperature and the reaction time. The individual parameters are given in Table 5.1.
Table 5.1: Synthesis parameters for the preparation of Fe2O3 nanoparticles.
Sample
Synthesis
Reaction time
Size
name
Temperature
(min)
(nm)
(°C)
Fe2O3- 100
100
190
40
Fe2O3-175
175
75
75
After the hydrothermal treatment at the given temperatures, the autoclaves were removed
from the oven and left to cool down to room temperature. The resulting Fe2O3 nanoparticles
were collected via centrifugation (16,500 rpm, 12 min) and washed with mQ water. In between
washing and centrifugation, the colloids were redispersed using ultrasonication. The
nanoparticles are stable in aqueous solution, and the formation of agglomerates could not be
observed even after long storage times of 2 years.
5.2.2 Preparation of Fe2O3@SiO2 nanoparticles
In order to achieve a homogeneous dispersion of the prepared Fe2O3 during the following
Stöber process, the amphiphilic polymer PVP was added. Depending on the intended SiO2 shell
thickness, surfactant molecules of different molecular weight were used (s. Table 5.2).
100 mg Fe2O3 nanoparticles were dispersed in 50 mL mQ water and 8.33·10-4 mmol of the
respective PVP-type were added. The mixture remained stirring at room temperature for 24 h
followed by the addition of CTAB dissolved in isopropanol (0.3 g in 200 mL). The addition had
to be carried out very slowly to prevent agglomeration of the nanoparticles due to a rapid
change in the chemical environment. After stirring at room temperature for 1 hr 200 mL
isopropanol and 4 mL ammonium hydroxide solution were added to the dispersion. To initiate
the hydrolysis, TEOS was injected into the mixture using a syringe. To vary the thickness of the
SiO2 shell, different amounts of TEOS were added (see Table 5.2). The hydrolysis and
condensation reactions were left to proceed for 24 h at room temperature. After the reaction
time was completed the solid is collected using centrifugation. The centrifugation speed was
adjusted to the particle size (see Table 5.2) while the time was kept constant (12 min). The
samples were centrifuged and washed with EtOH three times and dried. The solid was finally
calcined in air at 350 °C for 1 hr to remove all organic compounds using a heating rate of
10 K min-1.
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5 Experimental
Table 5.2: Preparation of Fe2O3 nanoparticles with varying shell thickness.
Sample name
TEOS added
Shell thickness
Molecular
Centrifugation
(µL)
(nm)
weight of PVP
speed
(rpm)
Fe2O3@SiO2 – 17
252
17
1,300,000
14,000
Fe2O3@SiO2 – 25
360
25
1,300,000
14,000
Fe2O3@SiO2 – 37
600
37
360,000
12,000
Fe2O3@SiO2 – 53
1200
53
360,000
10,000
5.2.3 Preparation of Fe2O3@TiO2 nanoparticles
For the encapsulation of Fe2O3 nanoparticles in TiO2, first Fe2O3@SiO2 nanoparticles with a
shell thickness of 53 nm were prepared as described in 5.2.2 without the final calcination step.
100 mg of the obtained product were then dispersed in 133 mL EtOH using ultrasonication for
1 hr. In a 2nd step, 0.6 mL NH4OH solution were added to the dispersion and 0.13 mL of TBOT
were injected dropwise within 5 Min. The reaction mixture was then heated up to 50 °C and
kept at this temperature for 24 h. After stirring at room temperature for another 24 h, the
solid was separated using centrifugation (9,000 rpm, 12 min). The product was washed first
with water and then with EtOH and finally dried at 50 °C. Lastly, the sample was calcined in air
at a temperature of 400 °C for 2 h to obtain a stable TiO2 shell.
The inner SiO2 shell was then removed by leaching using 1 M NaOH at a temperature of 60 °C.
After 24 h the leached sample was centrifuged (8,000, 12 min) and washed with deionized
water three times and finally dried.
5.2.4 Preparation of Fe2O3@ZrO2 nanoparticles
For the addition of a ZrO2 shell the synthesis route introduced in chapter 5.2.3 was slightly
varied. Instead of TBOT, 0.13 mL of ZBOT were added dropwise to the reaction mixture. The
mixture was then heated to a temperature of 45 °C for 12 h. After cooling down to room
temperature the product was directly separated using centrifugation. Further purification and
the final leaching step were carried out as described in chapter 5.2.3.
5.2.5 Metal exchange
The Fe2O3 core can be replaced with noble metal nanoparticles in a metal exchange reaction.
Therefore, the iron oxide core was reduced in a H2 flow as a 1st step. The solid was heated up
(heating rate 4.8 K min-1) to 600 °C and kept at this temperature for 2 h in a constant and
undiluted H2 flow. After the sample was cooled down to room temperature, 0.2 g of Fe@SiO2
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5 Experimental
nanoparticles were placed in a glass vial and mixed with 1.2 mL of the noble metal salt
solution. The concentration of the precursor solution was kept constant at a value of
0.1 mol/L. The noble metal precursors and the respective amounts that were used in the metal
exchange are summarised in the following table.
Table 5.3: Type and amount of noble metal precursors used for metal exchange.
Precursor
Amount of
Volume of
Concentration
precursor (mg)
H2O (mL)
(mol/L)
K2PdCl4
0.039
1.2
0.1
K2PtCl4
0.05
1.2
0.1
Rh(acac)2
0.048
1.2
0.1
RuCl3·H2O
0.025
1.2
0.1
After the addition of the noble metal precursor, the reaction mixture was placed in an
ultrasonication bath and kept there for 60 min. Then, the solid was washed with H2O and
dried. At last, the core-shell particles were treated again in an H2 flow at 600 °C, and the
remaining iron nanoparticles were leached with concentrated HCl for 48 h at room
temperature. The core-shell nanoparticles were repeatedly washed with H2O and dried.
5.2.6 Preparation of Fe2O3@SiO2@mpSiO2
Fe2O3 nanoparticles covered by 2 layers of SiO2 of different porosity were prepared by the
Stöber method. First, 100 mg of Fe2O3-175 nanoparticles dispersed in water were stabilised by
the addition of 0.3 g PVP-K90 dissolved in 50 mL of milipore water. The dispersion was stirred
overnight, and afterwards free PVP molecules were removed by centrifugation (16.500 rpm,
12 min). After centrifugation, the nanoparticles stabilised by PVP-K90 were redispersed in
50 mL of milipore water. The chemical environment of the nanoparticles was stepwise
changed by the cautious addition of 160 mL of isopropanol. The pH of the reaction medium
was increased by the addition of 4 ml of NH4OH mixed with 160 mL of isopropanol. After
vigorous stirring at room temperature for 1 hr, 0.6 mL of TEOS were injected rapidly to the
reaction mixture which is stirred continuously for another 60 min. Then, 0.6 mL of TEOS were
added again. The Stöber reaction was allowed to proceed over night at room temperature. For
the 2nd SiO2 layer a mixture of 1.2 mL TEOS and 0.488 mL OTMS was prepared and added
dropwise to the reactions mixture. After the addition was completed, the mixture was stirred
for another 2 min, and then the stirring was stopped while the reaction proceeded at room
temperature. The solid was separated from the liquid by centrifugation (8.000 rpm, 12 min)
and washed with ethanol repeatedly. The reaction product was dried and finally calcined to
remove the organic components at a temperature of 350 °C.
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5 Experimental
5.2.7 Preparation of Fe2O3@C nanoparticles
In order to obtain a carbon shell covering the Fe2O3 nanoparticles, the mesoporous SiO2 shell
was used as a template for the impregnation with a mixture of FA and OA. To determine the
pore volume that needs to be filled without including the inter-particle voids, sorption
measurements were made. The required amount of FA and OA (ratio 2 mL/0.021 g) was added
dropwise in three parts
[61]
. In between the impregnation steps, the solid in a PP tube was
forcefully crushed on the bench surface to achieve an even distribution of the liquid in the
pores. After the impregnation step, the solid was placed in a closed glass vial, heated up to
50 °C and kept at that temperature for 24 h followed by another 24 h at 90 °C in which the
polymerisation proceeded. The following carbonisation was performed under a constant Ar
flow. The sample was heated to 650 °C using a heating rate of 1.7 K min-1 and held at that
temperature for 3 h. In the final step, the inner SiO2 shell was removed using 1 M NaOH as
described earlier in Chapter 5.2.3.
5.2.8 Preparation of Fe2O3 containing ZSM-5
Fe2O3 containing zeolites were prepared following a modified version of the established carbon
template approaches known in literature [150].
The iron content of final product can be modified by changing the chemical composition of the
carbon template: for high iron contents only Fe2O3@C nanoparticles are used as template
whereas lower iron contents can be realised by mixing the core-shell nanoparticles with a
commercial carbon template (Black Pearls 2000).
In order to grow the zeolite crystals around the carbon template, an impregnation method
followed by a hydrothermal synthesis was carried out. The carbon template (1 g) – indifferent
of its chemical composition- was impregnated with a mixture consisting of 4.14 mL ethanol,
0.0855 mL H2O, 0.0015 g NaOH and 0.0126 g Al-iPro that was prepared prior to the
impregnation step. The impregnated solid was placed in a preheated oven at a temperature of
30 °C for 120 min so the ethanol can evaporate. Afterwards, 0.71 mL TEOS was added to the
carbon template. All impregnation steps were carried out in a Teflon mortar to avoid any
contamination with silicon species while still ensuring a through impregnation of the carbon
template. The impregnated solid was then placed in a Teflon inlet which itself was put in a
steel autoclave for the following hydrothermal synthesis. The autoclave was placed in an oven
at room temperature and slowly heated up to 180 °C (heating rate 0.05 K min-1) where it was
kept for 72 h. After the temperature had reached room temperature the solid was removed
from the autoclave and washed repeatedly with water followed by drying. Finally, the carbon
template was removed by calcination in air (0.5 K min-1, 550 °C, 8 h) and the final product was
obtained.
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5 Experimental
5.2.9 Synthesis of hierarchical zeolites using carbon templates
For the preparation of hierarchical ZSM-5 crystals a commercial carbon (Black Pearls 2000) was
used. The carbon material was dried at 90 °C for 48 h prior to use to remove water from the
pores and to improve the following impregnation step.
In a typical reaction, 1 g BP2000 were impregnated with a mixture consisting of 0.085 mL
water, 0.0012 g NaOH, 0.0063 g Al(OC3H7)3 and 4 mL EtOH. After the EtOH evaporated,
0.71 mL of TEOS were added. The impregnated solid was then transferred in a Teflon inlet,
which itself was then placed in an autoclave and heated up to 180 °C with a heating rate of
0.05 K min-1. After 72 h the heating stopped and the autoclave was left to cool to room
temperature. The product was calcined in air to remove the carbon template (0.5 K min-1,
550 °C, 8 h).
5.2.10 Post-synthetic modification of hierarchical zeolites
The previously prepared hierarchical zeolite crystals were modified with noble metal and
transition metal oxide nanoparticles using incipient wetness impregnation. Prior to the
impregnation step, the zeolites were dried in a vacuum oven at 50 °C for 48 h.
Noble metal nanoparticles were incorporated into hierarchical zeolite crystals by impregnation
and following reduction. 0.1 g of the dried zeolite were placed in an agate mortar and 0.05 mL
of aqueous solution of the noble metal precursor were added. In order to obtain zeolites with
1.5 wt% noble metal nanoparticles, the concentrations of the noble metal salt solution varies
with the precursor type. The exact amounts and concentrations are summarised in Table 5.4.
Table 5.4: Used noble metal precursors and concentration of added solution.
Precursor
Amount of
Volume of
Concentration
precursor (mg)
H2O (mL)
(mol/L)
K2PdCl4
5
0.05
0.306
K2PtCl4
3.563
0.05
0.172
RuCl3·H2O
3.525
0.05
0.34
The impregnated samples were dried at 50 °C in air and then under vacuum at the same
temperature for another 48 h. The reduction was carried out under a continuous H2 flow at
300 °C for 2 h.
In order to prepare a composite material consisting of ZSM-5, CuO and ZnO, firstly 0.2 g of
prior prepared hierarchical ZSM-5 was impregnated to incipient wetness with a 4 M, aqueous
Cu(NO3)2 solution. The impregnation step was carried out in an agate mortar, and the zeolite
was thoroughly combined with the liquid. Afterwards the water was removed by freeze drying.
The cycle consisting of addition of Cu(NO3)2 solution, impregnation and freeze drying was
repeated several times depending on the intended CuO loading of the final product. If one
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5 Experimental
cycle was carried out, a calculated loading of 11 wt% of CuO could be realised, while the
repetition of the procedure for three times led to a CuO ratio of 28 wt% in the composite
material. The impregnated and dried zeolite was then calcined in air at 400 °C for 2 h with a
heating rate of 1 K min-1. Thereafter, the intermediate product was impregnated with 0.04 mL
of an aqueous Zn(NO3)2 solution (3 M) and freeze dried. The following calcination step was
carried out under the same conditions as before.
5.2.11 Activity and deactivation processes of iron oxide-based FT catalysts
5.2.11.1 Preparation of catalysts
The preparation of the Fe2O3@C catalyst is described in Chapters 5.2.1, 5.2.6 and 5.2.7.
For the reference catalyst (Fe2O3 + @C) Fe2O3 nanoparticles were prepared according to
Chapter 5.2.1 and mixed with hollow carbon spheres, which were prepared by a nanocasting
procedure based in SiO2 spheres. For the SiO2 particles 600 mL of ethanol were mixed with
114 mL H2O and 11 mL NH4OH. Then, 23.8 mL of TEOS were injected quickly and reaction
mixture was stirred for 12 h at room temperature. After this, a mixture containing of 17.86 mL
TEOS and 7.04 mL OTMS was added to the dispersion. The mixture is slowly stirred for another
12 h at room temperature. The formed SiO2 particles were washed with ethanol and collected
by centrifugation. A calcination step was carried out at 550 °C for 6 h in air. Then the SiO2
particles were impregnated to incipient wetness with a mixture consisting of FA and OA (2 mL
per 0.021 g). The impregnated solid was placed in a glass vial and heated up to 50 °C for 24 h.
Afterwards, the polymerisation was started in heating the material up to 90 °C. The polymer
was carbonised in a tube oven at 650 °C (1.7 K min-1, 3 h)in an argon flow. In the final step, the
SiO2 template was removed using 1 M NaOH as described earlier in Chapter 5.2.3.
5.2.11.2 Catalytic testing
To analyse the influence of the catalyst’s geometry, Fe2O3@C core-shell particles are compared
to a physical mixture of Fe2O3 nanoparticles and empty carbon shells. For the activity
measurements 18 mg of the respective catalyst were mixed with 150 mg of SiC and loaded in a
plug flow reactor. Afterwards the reactor is heated up to the desired temperature (300 °,
350 °C, 400 °C) under argon atmosphere. When the temperature is reached a pre-reduction
step is carried out in diluted H2for 2 h. Then the gas flow is switched to syngas (CO:H2 = 1:1)
and the catalytic activity of the materials is measured at a flow of 6 mL/min for 19 h.
5.3 Characterisation techniques
Dynamic Light Scattering (DLS): The hydrodynamic diameter of dispersed nanoparticles were
characterised using DLS. The measurements were carried out with a Malvern Instruments
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5 Experimental
Zetasizer Nano-ZS using a wavelength of 633 nm and a power of 4 mW. The scattered light was
detected by a photomultiplier at a fixed angle of 90°.
Energy Dispersive X-ray spectroscopy (EDX): EDX measurements were used to complement
both data from the scanning and transmission electron microscope.
In the 1st case, EDX measurements were performed with a Hitachi S-3500N SEM instrument
equipped with an X-ray detector Oxford Inca. A cooled Si(Li)-detector was used to obtain the
data. The maximum acceleration voltage of this instrument was 25 kV with a working distance
of 5 mm. Secondly, a HF-2000 TEM with ThermoNoran X-Ray detector was used to analyse the
materials with EDX and to complement the data obtained from TEM.
Nitrogen Sorption: The N2 sorption experiments were carried out on a Micromeritics ASAP
2010 instrument. The measurements were performed at 77 K using a static-volumetric
method. The calculation of the surface area was carried out according to the BET concept
taking into account values obtained between 0.05 and 0.2 relative pressures. The pore volume
and pore size distribution was calculated based on non-local density functional theory (NLDFT). For the calculation suitable kernels for the respective material were used provided by the
Autosorb software package.
Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM):
SEM micrographs were obtained from a Hitachi S-3500N at an acceleration voltage of 25 kV
with a working distance of 5 mm. STEM analysis was carried out on a Hitachi S-5500
microscope with a Duo-STEM Bright Field/Dark Field detector. The acceleration voltage was set
to 30 kV.
Temperature programmed desorption (TPD): TPD of NH3 was carried out on a Micromeritics
AutoChem II set-up. The desorption was analysed at a heating rate of 10 k/min until the final
temperature of 600 °C was reached with He as carrier gas (flow: 25 mL/min). The zeolites
analysed were in acidic form.
Transmission electron microscopy (TEM): The TEM analysis of the samples was carried out
using either a Hitachi 7110 microscope at an acceleration voltage of 100 kV or for highresolution TEM images a Hitachi HF2000 at an acceleration voltage of 200 kV.
Thermogravimetric Analysis (TG): All materials characterised with TG were measured on a
Netzsch STA 449C thermobalance using air and a heating rate of 10 K min-1.
Ultrasonification Bath: Redispersion in liquid phase was achieved with a Bandelin Sonorex
Digitec DT 102 H with a maximum ultrasonic power of 480 W.
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5 Experimental
X-ray diffraction (XRD): The crystalline phases of all iron containing samples were characterised
using a Stoe STADI P transmission diffractometer in Deby-Scherrer geometry and using Mo Kα1
radiation. All other samples were measured also on a Stoe STADI P transmission diffractometer
in Deby-Scherrer geometry but using Cu Kα1 radiation.
X-ray Photoelectron Spectroscopy (XPS): All XPS measurements were carried out with a Kratos
His spectrometer with a hemispherical analyser. The monochromatised Al Kα X-ray source (E =
1486.6 eV) was operated at 15 kV and 15 mA. As lens mode the hybrid mode was used. During
the measurements the base pressure in the analysis chamber was 4 x 10-7 Pa. All Spectra have
been set to a C 1-value of 284.5 eV in order to account for charging effects.
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6 Appendix
6 Appendix
6.1 List of Figures
Figure 1.1: Structured crystals and minerals found in nature: Magnetotactic bacteria (I)
synthesise chains of magnetite nanoparticles of various shapes (II) that function as a compass
needle [4-5]. Calcite structures prepared by marine alga (III and IV) [6-7]. SEM image of nacre (V)
[8]
. ................................................................................................................................................... 2
Figure 1.2: Examples for the application of bio-mimetic materials: surface structures inspired
by lotus leaves (I) are used in ceramics; the structure of nacre is mimicked to improve the
strength of materials (II). .............................................................................................................. 3
Figure 1.3: Comparison of the endo- and exotemplating procedure for the preparation of
porous solids: I) Scheme illustrating the endotemplating approach (a)[15] and SEM picture of
thusly prepared, porous TiO2[16]. II) Preparation of porous materials using exotemplates (a) and
carbon replica of SBA-15 with two pore systems (b)[15]. ............................................................... 5
Figure 2.1: Comparative size scale over several orders of magnitude [21-22]. ................................ 7
Figure 2.2: Gold and silver nanoparticles embedded in the glass of the so called Lycurgus cup
lead to the different colouring in reflected (I) and transmitted (II) light [25]. ................................ 9
Figure 2.3: Illustration of the classification of nanomaterials according to their dimensionality
[26]
................................................................................................................................................. 10
Figure 2.4: I) Plot illustrating the dispersion as a function of the number of atoms along the
edge of a particle n. II) Possible positions of atoms on the surface of a particle with numbers
indicating the 1st coordination shell [29]. ...................................................................................... 12
Figure 2.5: Silver nanoparticles with cubic (I), cuboctahedral (II), and octahedral (III) shapes [22].
..................................................................................................................................................... 13
Figure 2.6: Lattice contraction ε of metal clusters as a function of particle size [29, 31]. .............. 13
Figure 2.7: Tuning of the emitting colour of quantum dots by changing the particle size [32]. ... 14
Figure 2.8: Comparison of top-down and bottom-up approaches for preparing nanoparticles.16
Figure 2.9: TEM images of gold nanoparticles prepared by the well-known Turkevich method
[21]
................................................................................................................................................. 17
Figure 2.10: Illustration of the general preparation method of metal nanoparticles using
microemulsions. .......................................................................................................................... 18
Figure 2.11: Nanoparticles with different crystal shapes prepared by microemulsion-based
preparation techniques: I) BaCrO4 nanoparticles
elongated and spherical shape
[47]
[46]
, II) and III) copper nanoparticles with
. ............................................................................................. 19
Figure 2.12: TEM images of metal oxide nanoparticles prepared by flame spray pyrolysis [51].. 20
Figure 2.13: Possibilities to tailor properties of core-shell nanoparticles [55]. ............................ 22
173
6 Appendix
Figure 2.14: Comparison of encapsulated (I, II) and non-encapsulated Fe2O3 nanoparticles as
catalysts for the NH3 decomposition [58]. .................................................................................... 24
Figure 2.15: Improved photo-active TiO2 catalyst due to noble metal core [67]. ......................... 26
Figure 2.2.16: SEM images (I and II) and TEM image (III) of TiO2 covered platinum nanoparticles
[69]
................................................................................................................................................. 27
Figure 2.17: I) Structure of PVP. II) Stabilisation of nanoparticles with amphiphilic polymer PVP.
..................................................................................................................................................... 28
Figure 2.18: Encapsulation of TTAB-stabilised Platinum nanoparticles in porous SiO2 shells
[64]
.
..................................................................................................................................................... 29
Figure 2.19: Versatile PVP-based method to encapsulate gold nanoparticles with metal oxides
[76]
................................................................................................................................................. 30
Figure 2.20: Illustration of the preparation of a carbon shell using a mesoporous SiO2 layer as
template and TEM images of Au@mpSiO2, C (I) and the final product Au@C [61]. ..................... 31
Figure 2.21: Overview of the possible modifications of the core and shell materials based on
Fe2O3@SiO2 nanoparticles........................................................................................................... 33
Figure 2.22: Scheme of the preparation of Fe2O3@SiO2 nanoparticles. ..................................... 34
Figure 2.23: TEM images of Fe2O3 nanoparticles prepared by hydrothermal synthesis choosing
different conditions (I) and II) Fe2O3-100; III) and IV) Fe2O3-175) ............................................... 35
Figure 2.24: Size distribution of Fe2O3-100 and Fe2O3-175 based on TEM results (I) and DLS
measurements (II). ...................................................................................................................... 36
Figure 2.25: Comparison of Fe2O3-100 and Fe2O3-175 nanoparticles using SEM. ...................... 37
Figure 2.26: TEM images of Fe2O3 nanoparticles covered by a porous SiO2 shell. ..................... 38
Figure 2.27: Comparison of Fe2O3@SiO2 nanoparticles of different shell thickness using TEM: I)
Fe2O3@SiO2–17; II) . Fe2O3@SiO2–25; III) Fe2O3@SiO2–37; IV) Fe2O3@SiO2–53. ........................ 39
Figure 2.28: Characterisation of the porous SiO2 shell around Fe2O3 nanoparticles using SEM
(Fe2O3@SiO2 – 37). ...................................................................................................................... 40
Figure 2.29: N2 physisorption isotherm of the Fe2O3-175 nanoparticles encapsulated by a
porous SiO2 shell (Fe2O3@SiO2 – 37). .......................................................................................... 40
Figure 2.30: Characterisation of crystalline phases of the Fe2O3@SiO2 – 37 nanoparticles using
XRD. ............................................................................................................................................. 41
Figure 2.31: General preparation scheme for the encapsulation of Fe2O3 nanoparticles in
transition metal oxide shells. ...................................................................................................... 42
Figure 2.32: TEM images of Fe2O3 nanoparticles encapsulated by a TiO2 shell. ......................... 44
Figure 2.33: SEM images of the prepared Fe2O3@TiO2 nanoparticles after removal of the SiO2
template. ..................................................................................................................................... 45
Figure 2.34: Powder diffractogram of Fe2O3 nanoparticles encapsulated by a TiO2 shell using
Mo radiation................................................................................................................................ 45
Figure 2.35: TEM micrographs of Fe2O3@ZrO2 nanoparticles using different magnification
factors. ........................................................................................................................................ 46
Figure 2.36: SEM (I) and STEM (II) image of Fe2O3 nanoparticles encapsulated by a ZrO2 shell. 47
174
6 Appendix
Figure 2.37: Characterisation of Fe2O3@ZrO2 nanoparticles using powder diffraction (Mo
radiation). .................................................................................................................................... 47
Figure 2.38: Fe2O3 nanoparticles encapsulated by a porous carbon shell prepared by
nanocasting procedure introduced in Chapter 3.4.1.2. .............................................................. 48
Figure 2.39: Schematic illustration of the preparation of noble metal@SiO2 nanoparticles by
metal exchange. .......................................................................................................................... 49
Figure 2.40: TEM images of the reduced nanoparticles with an iron core and SiO2 shell. ......... 50
Figure 2.41: Powder X-ray diffraction pattern of the reduced core-shell nanoparticles using Mo
radiation. ..................................................................................................................................... 51
Figure 2.42: Core shell nanoparticles with various noble metal cores characterised by TEM. I)
and II) Rh@SiO2; III) and IV) Pt@SiO2; V) and VI) Ru@SiO2; VII) and VIII) Pd@SiO2. ................... 52
Figure 2.43: Powder diffraction pattern of Ru@SiO2 yolk-shell nanoparticles after metal
exchange (Mo radiation). ............................................................................................................ 53
Figure 2.44: Reference experiments with macroscopically sized iron in different solvents. ..... 56
Figure 2.45: EDX analysis of the surface of the iron foil used for the reaction in acetylacetonate.
..................................................................................................................................................... 56
Figure 2.46: TEM images of Fe@SiO2 nanoparticles before the metal replacement reaction (I),
and after the reaction at 80 °C (II), 70 °C (III) and 50 °C (IV). ...................................................... 57
Figure 2.47: Characterisation of the crystalline phases present in the product after the redox
reaction in H2O at 80 °C using powder diffraction. ..................................................................... 58
Figure 2.48: Diffraction pattern of the reaction product after the reaction at 50 °C in H2O. ..... 59
Figure 2.49: Results of the blank experiment with Fe@SiO2 nanoparticles in H2O at 80 °C....... 59
Figure 2.50: Identification of the formed crystalline compounds during the blank reaction in
H2O. ............................................................................................................................................. 60
Figure 2.51: Fe@SiO2 nanoparticles after metal replacement reaction in ethanol at 50 °C. ..... 60
Figure 3.1: Worldwide annual zeolite consumption (wt% of total 1.8 Mio t) by major
application (2005). Natural zeolite consumption of China and Cuba are not considered (> 2.4
Mio t p.a.) [87]. .............................................................................................................................. 66
Figure 3.2: Typical SBUs in zeolite frameworks including rings (I), double rings (II), cages (III)
and chains (IV) [89]. ....................................................................................................................... 67
Figure 3.3: Common features of MEL and MFI framework: pentasil-chains (I) are linked
together to form sheets (II), which can link to adjacent sheets either by centres (III) to give MFI,
or by mirrors (IV) to give MEL [89]. ............................................................................................... 68
Figure 3.4: Typical positions of extra-framework cations in zeolite A (I) and faujasite (II) [89]. .. 69
Figure 3.5: Brønsted acidity of hydrogen zeolites and their interaction with a base. ................ 70
Figure 3.6: Influence of batch composition on the resulting zeolite phase using a reaction
temperature of 100 °C (HS= hydroxy sodalite) [93]. ..................................................................... 72
Figure 3.7: AlPO4-5 crystals made by microemulsion-based synthesis (I) and conventional
synthesis (II) [103]. ......................................................................................................................... 73
Figure 3.8: I) SEM image showing NaA zeolite crystals used in detergents as water softeners. II)
Ion exchange by zeolite NaA as a function of time [86]. ............................................................... 74
175
6 Appendix
Figure 3.9: Concentration profiles across a zeolite crystal at different values for the Thiele
modulus (left) and interdependence of Thiele modulus and effectiveness factor (right) [120]. .. 77
Figure 3.10: Nitrogen isotherms (I) and pore size distributions (II) of characteristic porous solids
[120]
. .............................................................................................................................................. 78
Figure 3.11: Different degrees and types of hierarchy in porous solids. .................................... 79
Figure 3.12: Types of zeolite materials with improved mass transport characteristics [120]. ...... 80
Figure 3.13: Zeolite Y crystal after dealumination by steaming and acid leaching [132]. ............. 81
Figure 3.14: Comparison of SEM images of zeolite crystals before (I) and after (II) desilication
treatment [135]. ............................................................................................................................. 82
Figure 3.15: Dependence of desilication process on Si/Al- ratio of parent zeolite [119]. ............. 83
Figure 3.16: Comparison of the oligomerisation of 4-methoxystyrene in non-treated ZSM-5
crystals and desilicated crystals using confocal fluorescence microscopy [120]. .......................... 84
Figure 3.17: Scheme of the confined space synthesis of nano-sized zeolite crystals introduced
by Christensen et al[149]. ............................................................................................................... 85
Figure 3.18: Preparation concept of using carbonaceous solids as hard templates [119]. ........... 85
Figure 3.19: TEM (I) [152] and SEM (II) [150] image of hierarchical zeolite crystal after combustion
of carbon template...................................................................................................................... 86
Figure 3.20: Preparation of hierarchical zeolites using carbon nanotubes (I) and TEM image of
resulting zeolite crystal (II) [157]. ................................................................................................... 86
Figure 3.21: Zeolite crystals templated with ordered carbon based on silica colloid of sizes of
10 nm (I), 20 nm (II) and 40 nm (III) [164]. ..................................................................................... 87
Figure 3.22: Zeolitic walls around spherical voids resulting from the templating with
polystyrene spheres [165]. ............................................................................................................. 88
Figure 3.23: Latex beads coated with silicalite particles and polyelectrolytes (I) to prepare
macroporous silicalite monoliths (II) [166]. ................................................................................... 88
Figure 3.24: TEM image of MFI/MCM-41 mixture obtained by dual templating [177]. ............... 90
Figure 3.25: Specifically designed 18-N3-18 –surfactant for templating hierarchically
aluminosilicates [182]. ................................................................................................................... 90
Figure 3.26: Mesoporous LTA zeolite synthesized using a specially designed organosilane
template [183]. ............................................................................................................................... 91
Figure 3.27: TEM images of ZSM-5 crystals prepared by silanized protozeolitic nano-units [185].
..................................................................................................................................................... 92
Figure 3.28: Retained hexagonal structure of Al-MSU-S based on ZSM-5 (A) and Beta (B) seeds
after steaming [190]. ...................................................................................................................... 93
Figure 3.29: Schematic illustration of the delamination method to prepare ITQ-2 [198]. ............ 93
Figure 3.30: Illustration of the application of bi-functional catalyst in consecutive reactions. .. 94
Figure 3.31: Preparation of bi-functional material by solid state ion-exchange. ....................... 96
Figure 3.32: palladium nanoparticles on MFI-type hierarchical zeolites prepared by
impregnation before (I) and after (II) catalytic reaction [217]. ...................................................... 97
Figure 3.33: Incorporation of RuO2 nanoparticles in ZSM-5 crystals. I) SEM image of the ZSM-5
crystals. II) and III) HR-TEM images of incorporated RuO2 nanoparticles [221]. ........................... 99
176
6 Appendix
Figure 3.34: SEM images of copper nanoparticles on structured alumina support for direct
DME- synthesis [84]. ...................................................................................................................... 99
Figure 3.35: Incorporation of gold nanoparticles in zeolite crystals. I) Scheme of preparation
pathway. II) TEM images of zeolite crystals containing small gold nanoparticles [224]. ............. 100
Figure 3.36: Scheme (I) and SEM image (II) of capsuled catalyst with FT catalyst core and
zeolite shell [208, 226]. ................................................................................................................... 101
Figure 3.37: Comparison of the application of a physical mixture and a capsule catalyst in
direct-DME reaction [202]. ........................................................................................................... 104
Figure 3.38: Comparison of the product range obtained from cobalt-based FT catalysts [83]. . 106
Figure 3.39: Prof. Dr. Franz Fischer (picture I) and Dr. Hans Tropsch (II). Fischer discussing
products of Fischer-Tropsch reaction with Max Planck and Otto Roelen at Kaiser-WilhelmInstitut in Mülheim, Germany (III)............................................................................................. 107
Figure 3.40: Flow sheet of overall Fischer-Tropsch process realised in industry...................... 107
Figure 3.41: Overview of the reserves-to-production ratios of the three most important energy
sources oil, natural gas and coal [232]. ........................................................................................ 108
Figure 3.42: Calculated product selectivities of FT reaction from Schulz-Flory analysis[234]. .... 109
Figure 3.43: Phase separation after addition of Fe2O3 nanoparticles to zeolite gel. The Fe2O3
nanoparticles are not incorporated in zeolite crystals (marked with red circles). ................... 112
Figure 3.44: Developed preparation concept for the incorporation of transition metal oxides in
zeolites. ..................................................................................................................................... 113
Figure 3.45: Synthesis steps leading to Fe2O3 nanoparticles encapsulated in a mesoporous SiO2
shell. .......................................................................................................................................... 115
Figure 3.46: Structure of OTMS used for preparation of mesoporous SiO2 shell. .................... 115
Figure 3.47: Hematite nanoparticles encapsulated with a thin, dense SiO2 shell. ................... 117
Figure 3.48: Comparison of the size distribution of encapsulated Fe2O3 nanoparticles based on
TEM data (statistics based on at least 200 particles of each sample). ..................................... 117
Figure 3.49: TEM micrographs of Fe2O3 nanoparticles encapsulated in a mesoporous SiO2 shell.
................................................................................................................................................... 118
Figure 3.50: SEM pictures of Fe2O3 nanoparticles covered by the 1st dense SiO2 layer (I and II)
and the 2nd mesoporous SiO2 shell (III and IV). ......................................................................... 119
Figure 3.51: Comparison of DLS data before and after the addition of the 2nd, mesoporous SiO2
layer. .......................................................................................................................................... 119
Figure 3.52: Nitrogen adsorption and desorption branches for Fe2O3@SiO2 and Fe2O3@mpSiO2
after calcination at 350 °C. ........................................................................................................ 120
Figure 3.53: Powder diffraction pattern of Fe2O3@SiO2and Fe2O3@mpSiO2 samples using Mo
radiation. ................................................................................................................................... 121
Figure 3.54: Schematic overview of the preparation of Fe2O3@C nanoparticles. .................... 122
Figure 3.55: Comparison of N2 sorption data of Fe2O3@SiO2@mpSiO2 and impregnated
analogue. ................................................................................................................................... 123
Figure 3.56: TEM images of iron oxide nanoparticles covered by carbon shell........................ 124
177
6 Appendix
Figure 3.57: Characterisation of Fe2O3@C nanoparticles using SEM (detailed description see
text). .......................................................................................................................................... 125
Figure 3.58: I) Powder diffraction pattern of Fe2O3@C nanoparticles using Mo radiation. II)
Results of XPS measurement of the material. .......................................................................... 126
Figure 3.59: Analysis of pores in carbon shell using HR-TEM (I) and HR-SEM (II). .................... 127
Figure 3.60: Physisorption analysis of carbon encapsulated Fe2O3. I) Nitrogen adsorption and
desorption branches. II) Calculated pore size distribution using DFT calculations................... 127
Figure 3.61: Schematic preparation pathway for growth of ZSM-5 crystals around Fe2O3@C
nanoparticles. ............................................................................................................................ 128
Figure 3.62: TG analysis of zeolite crystals containing the carbon template to prove the
complete removal by calcination in air. .................................................................................... 130
Figure 3.63: Comparison of conventional and Fe2O3 containing ZSM-5 by visual appearance. 130
Figure 3.64: Diffraction pattern of the product using Mo radiation and comparison to
calculated pattern of ZSM-5. ..................................................................................................... 131
Figure 3.65: Fe2O3 containing ZSM-5 crystals in an overlay of an SE- and dark-field STEMpicture. ...................................................................................................................................... 131
Figure 3.66: Comparison of the TEM images of ZSM-5 with embedded Fe2O3 nanoparticles (I)
and ZSM-5 crystals mixed with Fe2O3 nanoparticles (II) after heat treatment. ........................ 132
Figure 3.67: Physisorption isotherms of ZSM-5 crystals prepared with different amount of
Fe2O3@C template. I) Fe2O3/ZSM-5-100; II) Fe2O3/ZSM-5-67; III) Fe2O3/ZSM-5-05.................. 133
Figure 3.68: Comparison of the isotherms after factoring the different Fe2O3 contents into the
sorption data. ............................................................................................................................ 134
Figure 3.69: Calculated pore size distribution of Fe2O3/ZSM-5 samples using DFT. ................. 135
Figure 3.70: Interplay of pore volume and Fe2O3 content of the final product. ....................... 136
Figure 3.71: Comparison of ZSM-5 crystals resulting from different mixtures of the carbon
templates using TEM: I and II) Fe2O3/ZSM-5–100; III and IV) Fe2O3/ZSM-5–67; V and VI)
Fe2O3/ZSM-5–05. ....................................................................................................................... 137
Figure 3.72: TEM image and local EDX analysis of Fe2O3 nanoparticles in ZSM-5 crystals. ...... 138
Figure 3.73: SEM and EDX analysis of zeolite crystals with different Si/Al- ratios.................... 138
Figure 3.74: Comparison of the physisorption isotherms of zeolite crystals with different Si/Alratios: I) without Al, II) Si/Al- ratio of 100/1, III) Si/Al- ratio of 50/1. ....................................... 139
Figure 3.75: NH3-TPD curve of hierarchical zeolite with a Si/Al ratio of 100/1. ....................... 140
Figure 3.76: Comparison of FT activity at different temperatures between Fe2O3@C (I) and
Fe2O3 + @C (II) sample. ............................................................................................................. 142
Figure 3.77: Selectivity of tested catalysts to CH4 and C2-C4-products at different temperatures.
................................................................................................................................................... 143
Figure 3.78: TEM images of Fe2O3@C particles after catalytic testing at 300 °C (I and II) and
350 °C (III and IV). ...................................................................................................................... 144
Figure 3.79: TEM images of the spent Fe2O3@C catalyst after FT reaction at 400 °C. ............. 144
Figure 3.80: Particle size distribution of Fe2O3@C catalysts after reaction at different
temperatures. ........................................................................................................................... 145
178
6 Appendix
Figure 3.81: TEM images of the Fe2O3 + @C particles after catalytic testing at 300 ° (I and II),
350 °C (III and IV) and 400 ° (V and VI). ..................................................................................... 146
Figure 3.82: Observed particle size distribution of Fe2O3 + @C sample after catalytic testing at
different temperatures. ............................................................................................................ 146
Figure 3.83: SEM images of hierarchical zeolite crystals with platinum nanoparticles. ........... 149
Figure 3.84: Analysis of cross-sectional cuts of hierarchical ZSM-5 crystals with ruthenium
nanoparticles using SEM. .......................................................................................................... 149
Figure 3.85: SEM and EDX analysis of cross sectional cuts of platinum nanoparticles containing
ZSM-5. ....................................................................................................................................... 150
Figure 3.86: palladium nanoparticles within hierarchical zeolite crystals analysed with SEM and
EDX. ........................................................................................................................................... 151
Figure 3.87: Size distribution of noble metal nanoparticles incorporated in ZSM-5 crystals. .. 152
Figure 3.88: Characterisation of noble metal containing zeolites using XRD. .......................... 153
Figure 3.89: TEM images of hierarchical ZSM-5 crystals with low loading of CuO and ZnO..... 155
Figure 3.90: SEM and EDX results for the composite material containing low loading of CuO and
ZnO. ........................................................................................................................................... 156
Figure 3.91: Analysis of ZSM-5 crystals containing a higher loading of CuO. ........................... 156
Figure 3.92: Atom mapping based on EDX data showing the formation of a separate CuO
phase. ........................................................................................................................................ 157
Figure 3.93: Characterisation of the composite material containing 26.7 wt% CuO using X-ray
diffraction. ................................................................................................................................. 157
6.2 Sample references
Figure
2.23
2.25
Sample abbreviation
I,II
NED-NA-195-01
III, IV
NED-NA-209-01
I,II
NED-NA-195-01
III, IV
NED-NA-209-01
2.26
2.27
NED-NA-089-13
I
NED-NA-072-03
II
NED-NA-072-23
III
NED-NA-072-33
IV
NED-NA-090-03
2.28
NED-NA-003-02
2.32
NED-NA-177-08
2.33
NED-NA-177-08
2.35
NED-NA-177-26
2.36
NED-NA-177-23
2.38
NED-NA-166-06
2.40
NED-NA-199-06
179
6 Appendix
2.42
2.46
I, II
NED-NA-052-09
III, IV
NED-NA-052-10
V, VI
NED-NA-052-15
VII, VIII
NED-NA-052-16
I
NED-NA-199-06
II
NED-NA-199-10
III
NED-NA-199-41
IV
NED-NA-199-31
2.49
NED-NA-199-51
2.51
NED-NA-199-61
3.43
NED-NA-077-03
3.47
NED-NA-208-04
3.49
NED-NA-166-04
3.50
I, II
NED-NA-208-04
III, IV
NED-NA-166-04
3.56
NED-NA-166-06
3.57
NED-NA-166-06
3.59
NED-NA-166-06
3.65
NED-NA-150-02
3.66
3.71
I
NED-NA-212-14
II
NED-NA-120-12
I, II
NED-NA-201-02
III, IV
NED-NA-210-03
V, VI
NED-NA-212-03
3.72
3.73
3.76
NED-NA-155-03
I
NED-NA-213-02
II
NED-NA-145-04
III
NED-NA-146-02
I, II
NED-NA-166-17
III, IV
NED-NA-166-27
3.77
NED-NA-166-37
3.81
NED-NA-194-17
3.83
NED-NA-105-22
3.84
NED-NA-105-12
3.85
NED-NA-105-22
3.86
NED-NA-105-32
3.89
NED-NA-125-34
3.90
NED-NA-125-34
3.91
NED-NA-125-24
3.92
NED-NA-125-24
180
6 Appendix
6.3 List of Tables
Table 2.1: Modification of the SiO2 shell thickness covering Fe2O3-175 nanoparticles (values per
100 mg Fe2O3).............................................................................................................................. 38
Table 2.2: Comparison of the redox potential of iron and platinum based on the bulk phase. . 54
Table 3.1: Selection of industrially important separation processes carried out with zeolite
adsorbents [86].............................................................................................................................. 75
Table 3.2: Application of zeolite based catalysts in oil refining [86]. ............................................ 76
Table 3.3: Zeolite crystals prepared by using carbon templates with different percentages of
Fe2O3@C and BP2000 of total template amount. ..................................................................... 132
Table 3.4: Comparison of the pore volume and surface area of ZSM-5 samples with different
Si/Al ratios. ................................................................................................................................ 139
Table 3.5: Comparison of activity in FT reaction over time (values given in 10-6 molCO/gFe·s). . 142
Table 5.1: Synthesis parameters for the preparation of Fe2O3 nanoparticles. ......................... 165
Table 5.2: Preparation of Fe2O3 nanoparticles with varying shell thickness............................. 166
Table 5.3: Type and amount of noble metal precursors used for metal exchange. ................. 167
Table 5.4: Used noble metal precursors and concentration of added solution. ...................... 169
6.4 Publications and contributions to conferences
 J. H. Ahn, R. Kolvenbach, C. Neudeck, S. S. Al-Khattaf, A. Jentys, J. A. Lercher, J. Catal. 2014,
311, 271-280
 Carolina Neudeck, Sandra Kestermann, Wolfgang Schmidt, Ferdi Schüth
Novel preparation concept for the incorporation of transition metal oxides in zeolites
Poster presentation at the 4. Junges Chemie Symposium Ruhr, 2013, Mülheim a.d.R.,
Germany
Incorporation of transition metal oxides in zeolites
Oral presentation at the 17th International Zeolite Conference, 2013, Moscow, Russia
Bi-functional catalysts based on transition-metal oxides in zeolites with hierarchical pore
system
Poster presentation at 25. Deutsche Zeolith Tagung, 2013, Hambug, Germany
 Robin Kolvenbach, John H. Ahn, Carolina Neudeck, Suleiman S. Al-Khattaf, Andreas Jentys,
Johannes A. Lercher
Diffusion in nano-sized, hierarchical ZSM-5 zeolites
Oral presentation at 24. Deutsche Zeolith Tagung, 2012, Magdeburg, Germany
181
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