Supported nanosized gold catalysi the influence of support morphology and reaction mechanism 1

1
Chapter 1 Introduction
1.1 General background
Since the discovery of atoms as the basic building blocks of all matter, scientists have
labored on to find means of controlling these miniscule entities, sensing the power to
create new materials will follow. Indeed, in his president's address to the American
Physical Society in 1959, Richard Feynman predicts:
1,2
“…. I can hardly doubt that
when we have some control of the arrangement of things on a small scale we will get
an enormously greater range of possible properties that substances can have….”
Nanosized materials are generally defined as materials having at least one dimension
less than a hundred nanometers, whether in particle diameter, grain size, layer
thickness, or width. Scientific work on nanomaterials dates back to over a century
ago.
3-6
After British chemist Thomson Graham discovered a solution containing
nanosized particles in suspension (colloid), the likes of Rayleigh, Maxwell and
Einstein began to investigate the colloidal systems. By 1930, the Langmuir-Blodgett
monolayer film was developed. By 1960, electron microscopy and diffraction were
used to study fine particles; while the production of submicron particles was further
enhanced with the utilization of arc, plasma, and chemical flame furnaces. In 1970s,
magnetic alloy particles were used in magnetic tapes. By 1980, clusters that contained
less than 100 atoms were studied. The C60 molecules were discovered by Harold
Kroto, James Heath, Sean O'Brien, Robert Curl, and Richard Smalley in 1985.
7-11
In
1991, Iijima reported the finding of multiwalled carbon nanotubes. Since then
nanoscience and technology have become a hot area of research and development
addressing the control, modification and fabrication of materials, structures and
2

devices with nanometer precision and the integration of such structures into systems
of micro- and macroscopic dimensions.
1.1.1 Development of nanotechnology worldwide
Every month, specialist newsletters journals report bewildering new advances in
sciences and technology in nanoscles. And here we would like to give a brief review
of the global strategies, industry trends and application of these technologies.
In July 2001 the Ministry of science and Technology, China, issued a policy planed
for the general strategy and objective of nanotechnology development in China for the
period 2001 to 2010, i.e. the basic principles of physical and chemical characteristics
at the nanoscale with the purpose of finding new concepts and new theories. Key
Laboratory of Molecule Nanostructure and Nanotechnology was founded in 2001 and
is dedicated to construct new nanostructures, analysis the surface and interface
structures at nanoscale as well as physical and chemical properties of single
molecules, and development and application of apparatus for nanosciences.
The current researches are focused on the fields:
1.Development and application of scanning probe microscopy
2. Characterization of chemical and physical properties of single molecule
3. Construction and characterization of molecular nanostructures
4. Microstructure and properties on the surface and interface
5. Theoretical study on molecular nanostructure and properties
6. Nanoelectrochemistry
3
7. Single biomolecules
8. Cluster materials
And the new focus of application of nanotechnology will be on health, environment,
energy and national security. The government also encourages all participants, creates
environmentally beneficial nanotechnology and implements national nanotechnology
initiatives.
12,13
In Europe, nanotechnology has been receiving around €1.2 billion annually of public,

regional and industry funding each year. Nanotechnology has attracted much
attentions in Europe partially because the influence of US and Japan
14
. It is also
because nanotechnology was viewed as offering tremendous opportunities in
healthcare, energy and environment aspects. One of the problems for Europe now is
the lacking of the scientist in the area; this problem has been reflected from the fact
that Europe holding only 9% patents in advanced technology sector at the US office,
in comparision to the fact that US holds 57% and Japan 22%.
12
In Singapore, nanotechnology has been identified by the Economic Review
Committee as one the key areas for Singapore’s pursuit of competitive advantages.
The agency for science, technology and research (A*STAR) being the main funding
agency for the science and technology research in Singapore, started the A*STAR
Nanotechnology initiative in September 2001. Nanotechnology research programs are
carried out through existing institute of A*STAR as follow:
15
Institute of Materials
Research and Engineering (IMRE)—Photonics, Advanced Materials. Institute of
Microelectronics (IME) and Data Storage Institute (DSI)—Semiconductor,
Electronics, Storage. Institute of Bioengineering and Nanotechnology (IBN)—
4
Bionanotechnology. Institute of Chemical and Engineering Science (ICES)
nanoscience and nanotechnology have also been applied in developing nanocatalysts
for clean energy, nanomaterials for hydrogen storage and drug delivery. There are
also some other nanotechnology institutes in Singapore. The Nanotechnology
Initiative (NUSNNI) in National University of Singapore concentrates on Bio-
nanotechnology, nanoelectronics, nanophotonics, nanomagnetics, self-assembly
molecular devices, nanostructures and nanomaterials.
16

The precision Engineering
and Nanotechnology (PEN) Center, at Nanyang Technological University, focuses on
the nanoscale precision machining, nano-metrology nanodefects detection, in
particular: nanoparticles and nanodefects detection system for unpolished silicon
wafers; next-generation “breathable” contact lenses.
17
The nanoscience &
nanotechnology Cluster from Nanyang Technological University (NNC NTU)
focuses on the area of nanoelectronics, nanomagnetics and nano-optics, organic and
molecular electronics, nanocomposites, energy and catalysis, and
nanobiotechnology.
18
The Singapore Economic Development Board provides active support for technology
and business development in Singapore. In the field of Nanotechnology, EDB is
particularly active and dynamic in bringing foreign technology R&D and industries to
Singapore and facilitate their fusion with the local R&D institutions as well as
industries. EDB is taking the initiative to establish the Nanotechnology Industry
application Center where star-ups can co-develop applications with market leaders in
Singapore.
19
5
Attempts to coordinate US federal work on the nanoscale began in November 1996,
when staff members from several agencies decided to meet regularly to discuss their
plans and programs in nanoscale science and technology.
20-23
This group continued
informally until September 1998, when it was designated as the Interagency Working
Group on Nanotechnology (IWGN) under the National Science and Technology
Council (NSTC).
24

The IWGN sponsored numerous workshops and studies to define
the state of the art in nanoscale science and technology and to forecast possible future
developments. Ever since US President Bill Clinton’s speech in 21 January 2000
25
at
the California Institute of Technology, Clinton, "Some of our research goals may take
twenty or more years to achieve, but that is precisely why there is an important role
for the federal government", nanotechnology has opened an era of scientific
convergence and technological integration. Government’s support advocated
nanotechnology development. President George W. Bush further increased funding
for nanotechnology and has transformed the issue into his own. In 2003 Bush signed
into law the 21st Century Nanotechnology Research and Development Act (Public
Law 108-153),
26
which authorizes expenditures for five of the participating agencies
totaling $3.63 billion over four years.
27.
It should be noted that this law is an
authorization, not an appropriation, and subsequent appropriations for these five
agencies have not met the goals set out in the 2003 Act. However, there are many
agencies involved in the Initiative that are not covered by the Act, and requested
budgets under the Initiative for all participating agencies in Fiscal Years 2006 - 2008
totaled over $1 billion each. The current NNI budget supplement for Fiscal Year 2009
provides $1.5 billion dollars to the NNI, reflecting steady growth in the
nanotechnology investment.
28-33
6
1.1.2 Applications and challenges of nanotechnology
In China, there are three main applications of nanotechnology in industry so far, they
are: material processing; nanochip fabrication and integration and nanochip

processing method.
12
In Singapore nanotechnology were mainly applied in healthcare, cosmetic,
environment, thin film, chemical industry, precision engineering to industry,
biosensor, fuel cells and photovoltaic devices, and also in waste water treatment.
34,35
In US and Europe, the applications of nanotechnology are in wider range of everyday
life products. Manufactures like Johnson & Johnson, L’Oréal already used nanoscale
titanium dioxide and zinc oxide in their sunscreen and anti-wrinkle cosmetics.
Nanosized iron oxide is used for some lipsticks as a pigment.
36,37
Also fabrics used for
clothes, mattresses, upholstery and soft toys are sometimes treated with nanosized
coatings. The fabrics can be made water- or stain- or perspiration-resistant while
retaining breath ability if the porosity of the fabrics can be controlled in nanoscale.
Brands like Nike, Dockers, DKNY, Savane, Benetton and Levi’s have employed the
new nano-coating material in some of their products.
38,39
Nano-sized Titanium
dioxide has been used in self-cleaning windows and suites. The UK manufacture of
glass Pilkington already put their product in market.
40
Titanium dioxide in nanoscale
was also used in Paint producers. Millennium Chemicals is the world’s second-largest
producer of titanium dioxide and a leading producer of titanium chemicals, they
developed a paint that uses the absorbance of UV light of nanosized TiO
2
.
41,42
This

time the energy absorbed by TiO
2
is used to convert nitrogen oxide pollutants in the
air into naturally washed away nitric acid. General Motors was the first to use a
7
nanocomposite material; this lightweight high performance material has been used in
GM’s Hummer H2 series.
43
Nano-sized catalysts has been used in oil refining and
petrochemical industries to improve the yield. By using nanoscale materials in oil
refining, US have been saved $8-16 billion yearly in oil imports.
44
Nanotechnology
might have biggest impact on the data storage of information technology among all
industrials. In 2003, PC with hard drives use nanosized material quadrupled the data
storage capacity. And the computer chips had structures with widths of 130
nanometers used in 2004 were soon to reduce to 90 nanometers due to the improved
lithographic technology. The 65 nanometer technology is widely used in major IT
companies in CMOS semi-conductor fabrication by year 2007.
45
With the rapid development of nanotechnology and the uncertainty in the physical
and chemical properties of nanosized materials, more issues should be paid attention,
like the health and safety concerns, ignorance in the workplace, effects on the food,
environment and the absence of regulatory control.
1.2. Nano-gold Catalysis
The classical age of metal colloidal science can be said to begin with Michael Faraday
when he formed deep red solutions of colloidal gold by reduction of chloroaurate
[AuCl
4
]

-
using phosphorus as reducing agent in the mid-nineteenth century.
46
Although Faraday has no mean of determining the size of the produced gold particles,
he has elucidated the mechanism of their formation and called them divided metals. In
addition, Faraday has noted that these colloidal gold sols are thermodynamically
unstable and hence needed to be stabilized kinetically against aggregation. Once
coagulation occurred, it is irreversible. Remarkably, Faraday has also identified the
8
essence of the nature of these nanoscale particles of gold where he concluded (in
1857).
46-48
A recent reproduction of his work by J.M. Thomson, in Faraday’s original
laboratory at the Royal Institute of London, demonstrated the gold sols contained
particles of 3-30 nm in diameter.
49
Nanomaterials often possess very different
properties from their bulk form. Nano-gold catalysis is such an example. Gold
occupies a position at one extreme of the range of metallic properties, and its
legendary chemical inertness is attributable to the Lanthanide Contraction and the
relativistic effect: which becomes significant when atomic number Z exceeds about
50. When the 1s orbital of gold shrinks, in order to maintain orthogonality, the s
orbitals of higher quantum number have to contract in sympathy. In fact the 6s orbital
shrinks relatively more than the 1s. The same effect also operates to a lesser extent on
the p electrons, but d and f electrons are hardly affected, never coming close to the
nucleus. This energetic stabilization of the 6s and 5d shells because the 4f electrons
do not adequately shield them from the increasing nuclear charge would result in the
disposition of their orbital: 5d and 6s electrons are therefore drawn towards the
nucleus. Hence gold is inert compared to other metals including its neighboring
elements (e.g. Cu, Ag and Pt). Gold (5d

10
6s
1
) chemistry is determined by (i) the easy
activation of the 5d electrons, and (ii) its desire to acquire a further electron to
complete the 6s
2
level and not to lose the one it has, due to the 6s
2
“inert pair effect”.
This latter effect awards it a much greater electron affinity and higher first ionization
potential than those of copper or silver, and accounts for the ready formation of the
Au
-
state. The former effect obviously explains the predominance of the Au
III
state,
which has the 5d
8
configuration. The Au
I
state is of lesser importance and the Au
II
state is unknown except in a few unusual complexes.
9
Interestingly, the properties of the nanoscaled particles can be changed with their
dimension scale. Recent studies by M. Haruta have demonstrated that highly
dispersed gold particles supported on some oxides such as Fe
2
O

3
and TiO
2
are
surprisingly active in low (ambient or less) temperature CO oxidation, more active
than noble metals of Group 8-10.
50
The activity of supported Au catalyst is shown to
be structure-sensitive, remarkably sensitive to the size, shape and morphology of Au
particles. A sharp increase in the CO oxidation turn-over-frequency, the reaction rate
over one single metal atom per second, is observed with a decrease in the diameter
from 5 nm.
50,51
The smaller the particle, the greater will be the fraction of atoms
directly in contact with the support and therefore influenced by it, while at the same
time the fraction of coordinatively unsaturated surface atoms also increases, and this
changes the physical properties of the whole particle.
50
It is therefore virtually
impossible to draw a clear distinction between intrinsic particle size effects and those
that are due to metal-support interactions. M. S. Chen and D.W. Goodman revealed
that on TiO
2
(110) Au particles bound first on the oxygen vacancies. Two-dimentional
Au islands were initially formed up to a critical coverage that depends on the defect
density. Au clusters with sizes ranging from 2 to 4 nm that were specifically two
atoms thick, showed maximum reactivity, optimally active for CO oxidation.
50
Very
recently A. A. Herzing et al. have used aberration-corrected scanning TEM to analyze

the active gold nanoclusters on real Au/Fe
2
O
3
catalysts, revealing that high catalytic
CO oxidation activity is correlated to bilayer Au clusters of ~0.5 nm, whereas the
monolayer cluster containing only 3-4 Au atoms and isolated Au atoms are essentially
inactive.
53
1.3 Oxidation of carbon monoxide over nanosized gold
10
An acceleration of interest in gold as a catalyst proceeded Masatake Haruta et al.
discovery of extremely small (<5 nm) gold particles on oxide support as a catalyst for
CO oxidation below room temperature.
52,53
It is known that Gold demonstrates more
active behavior than noble metals of Group 8-10
54
, as it does not bind to CO easily,
and therefore show higher activity in the oxidation of CO. However, the exact
mechanism in which this could be done proves elusive because most work has not
directly addressed this procedure. In papers measuring orders of reaction and
activation, the avoidance of mass transport limitation is not always specified. The
common practice of measuring catalyst performance by looking at conversion as a
function of temperature also poses an obstacle as it has great limitations. In fact,
reliable kinetic information, such as order of reaction and activation energy, and
thorough kinetic modeling is hard to come by, resulting in a vacuum where testing of
possible mechanism proves to be difficult. Additionally, a combination of sensitive
and interrelated variables (and contradictory literature) makes controlled testing a
highly challenging task. For example, gold compounds are photosensitive. Hence

preparation of compound should be done in darkness to avoid reduction of Au
3
precursors.
11,55-58
Reduction is also minimized through drying at room temperature or
373K in vacuum; and then precautions taken to avoid water vapor by storing in
vacuum or in a freezer.
55-59
Still another variable is the amount of residual chloride
ions left after washing, as it causes agglomeration of gold particles during thermal
treatment, and reduced activity in the oxidation of CO.
59-63
Catalytic activity is also
affected by residual sodium from the usage of hydroxide or carbonate in the
preparation by deposition-precipitation (DP). This is further complicated by studies
which do not agree on the role of sodium as poison
57,64
or promoter,
62,65
possibly
11
dependent upon its concentration and the support used. The optimal size of gold
particles is also in contention depending on the conditions of thermal pretreatment.
Again, literature disagrees on the exact activation procedures. Most papers
inaccurately term 'calcination' when the gold particles are actually thermal treated in
air, causing a reduction of gold. On the other hand, using a vacuum or hydrogen may
result in early deactivation (some papers have claimed that hydrogen treatment is
preferred because unwanted chloride could be eliminated through HCl
formation
61,66,67

We also have to be aware that Au
0
and their precursors are highly
unstable. Precisely speaking, the surface of gold particles below 3 nm will react with
oxygen in ambient air; while Au
3
species in Au/TiO
2
will be reduced in a mixture of
CO and O
2
at room temperature, becoming active in CO oxidation.
68-71
Exceptions
exist in dried samples of Au/Al
2
O
3
,
70
Au/TiO
2
,
72
and Au/Fe
2
O
3.
It is also very
difficult to control the conditions of reaction though most are done at atmospheric

pressure. Precise temperature control is challenging because CO oxidation is highly
exothermic (ΔH
0
= 283kJmol
-1
). Reaction temperature of 2.5wt% Au/Fe
2
O
3
can raise
from 293 to 393K.
73
Although measures such as dilution of catalyst have been
adopted to counter this, local temperature proves difficult to control as supports often
have poor thermal conductivity.
74-77
Another variable often uncontrolled is the amount of water in the reactants, which
may have a significant influence, although studies on its effect are inconsistent and
general characteristics are difficult to map out. Some studies have shown that the
effect of water is dependent upon its concentration and the support used.
78,79
While
the activation energies of Au/TiO
2
and Au/Al
2
O
3
show that they are almost
independent of water concentration, Au/SiO

2
showed marked differences in its
12
conversion-temperature curve in the presence of water. This is suggestive that the
hydroxyl groups at metal-support interface are crucial for reaction efficiency, and that
the catalyst may be sensitive to moisture in its catalytic activity.
1.4 Preparation method of Oxide-supported Nano-gold Catalysts is critical
It has been widely accepted that support surface area, support defects density, average
Au nanoparticles size and their dispersion (hence number of low coordinated Au
atoms) are all the important factors that determine the catalytic activity of metal oxide
supported gold nanoparticles.
80-85
Preparation method could affect all the factors
mentioned above. Thus choosing a suitable method for sample preparation is of the
first concern.
There are generally two preparation methods of nanosized metal particles, the top-
down approach (physical route) or the bottom-up (chemical route) approach. The top-
down method involves reducing macroscopic particles to nanometer size scale.
However this route produces particles of un-uniform shapes, and needs expensive and
complicated equipments. Hence this method is opted out in our study. Conversely, the
bottom-up procedure is able to generate particles of uniform size, shape and structure
with distinct features. This is because the bottom-up approach uses atoms that will
then aggregate to form particles of definite sizes in both solution and gas phase.
Three kinds of methods are widely used in bottom-up approach, i.e. co-precipitation,
deposition-precipitation and colloid-based method. In the co-precipitation process, the
support and the gold particles are formed simultaneously in solution from initially
soluble precursors. The main shortcomings of this method include long-term reaction
process, low efficiency in Au deposition (usually below 20%) and time-consuming
13
washing steps to remove the chloride contamination as trace amount of it can poison

the catalysts. Deposition-precipitation process deposits metal particles from initially
highly soluble precursor to a support of low solubility. The pH value of the solution
is adjusted primarily based on the isoelectric points of the metal oxide support so that
the precipitation takes place only on a support and not in solution. This method is the
easiest to be handled, but may not show as good particle dispersions as that obtained
in the co-precipitation method, and has similar shortcomings to the co-precipitation
method, e.g., the Au-deposition efficiency is usually low, etc.
86-88
Differing from the
above two methods, the colloid-based method has the advantage of small mean
particle size and narrow size distribution under appropriate conditions, because these
metal nanoparicles are protected with stabilizing agent or capping agent in the
solution. In Chapter 3 of this thesis the above three preparation methods are
compared. Among the three methods, the colloid-based method needs shortest
reaction time, has highest Au deposition efficiency and lowest chloride
contamination, thus it is selected and applied in Chapters 4 and 5 for preparing the
Au/TiO
2
and Au/CuO catalysts respectively.
1.5 Support Effects and the Choice of Oxide Support
The importance of the metal oxide support to the nanogold catalysts is the topic of
many papers.
89-95
Firstly, the oxide support may act as a stabilizer of the Au particle
dispersion. Hence the surface area of the support is significantly important. My
experimental data in Chapters 3-5 indicate that nanogold catalysts on higher surface
area oxide always have higher catalytic activity. Recent paper on Au/Fe
2
O
3

reported
that nanogold particles > 1nm, which accounts for 20-40% of total Au atoms are not
very active for low temperature CO oxidation, while the most active gold clusters are
14
two-layer particles of ~10 Au atoms.
89
Based on the volumetric packing density of
~59 Au atoms per nm,
48
the large nanoparticles (5-7nm) would contain large number
of Au atoms (1900 to 5250 atoms if they have hemispherical shape). On the other
hand, the small clusters with high activity (10 atoms cluster with 0.5 nm diameter),
though accounting for only 20% of total Au atoms, would require large support area.
High surface area of the support in this case is beneficial for increasing number of low
coordinated Au atoms since gold particles can be further apart on a higher surface
area support and maintain smaller size better. Nevertheless, some papers reported the
limitation of surface area effect on low-loading Au catalyst, and Au on a lower
surface area oxide support may be more active than those on higher surface area
supports.
90
Secondly, a clear correlation between the reducibility of the support and
the CO oxidation activity has been found. The oxides possess the same crystalline
structure and similar specific surface area but different reducibility may exhibit
different activity. The CO oxidation on Au/TiO
2
was found to be four times faster
than that on Au/Al
2
O
3

.
91,92
It has been shown that on TiO
2
support, Au exclusively
binds on the oxygen vacancies on the oxide surface, and the CO oxidation activity is
related to the number of oxygen vacancies under the Au particles.
93
Au supported on
defects-free MgO surface shows no activity whereas those on MgO with F-center
defects are active for CO oxidation.
94
Metal oxide supports for the CO oxidation
nanogold catalysts have been divided into three categories: easily reducible, less
easily reducible and non-reducible supports. On easily reducible oxide surface oxygen
ions are easily removed during the preparation process or by reducing agent such as
hydrogen or CO, leaving large amount of anion vacancies. These vacancies can serve
as active centers for O
2
adsorption and activation. The use of various methods
15
(coprecipitation, impregnation and deposition-precipitation) confirmed the superiority
of the transition metal oxide as supports,
95
these being more easily reducible than the
ceramic oxide that gave low activities. With mixed Fe
2
O
3
-MgO supports activity

increased with iron content, not withstanding a growth in gold particle size. We may
conclude that while reducible supports perform best, gold on irreducible ceramic
oxide still shows a modicum of activity provided that the particles are small enough;
the reaction then proceeds solely on the gold without any assistance from the support.
The method of preparation plays a dominant role in determining the structure and
composition of the finished catalyst, and in the COPPT method the support is formed
during the preparation. Hematite is an easily reducible support, and hence is one of
the best supports for CO oxidation. Titanium oxide, which is classified as less easily
reducible oxide, is also one of the earliest and most investigated gold nanoparticle
systems. Copper oxide is a very easily reducible oxide, but less investigated compared
with other supports. These three oxides are studied in Chapters 3, 4 and 5 respectively
in this thesis.
Thirdly oxide support may change the electronic structure of gold, resulting in a
metal-insulator transition. Charge transfer to or from the support has been reported by
M.S. Chen and D.W. Goodman
96
and many other researchers. The initial nucleation
of Au occurred on Ti
3+
defect sites, and the Au
3+
between the Au particle and oxide
support interface was a chemical glue to anchor the Au particles strongly.
97
The
presence of reactant gases under realistic conditions could further affect the admetal’s
ability to wet the surface and prevent them from catalyst agglomeration/deactivation.
4
Au
0

and Au
n+
both are found to have a role to play in CO oxidation, and the
simultaneous existence of Au
0
and Au
3+
is essential in various models of reaction
16
mechanism.
90,97,98
For Au on TiO
2
, the Au 5d bands are found to be much closer to
the Fermi level due to charge polarization in the interfacial region.
99
In another oxide
supported Au system, Au/MgO, charge transferred from the support (F center defects)
to Au is found to play key role in promoting their chemical activity. Both theoretical
and experimental data show that low-coordination Au atoms possess a d-band that is
closer to the Fermi level than their close-packed counterparts so that they can adsorb
O
2
more readily. The negative charge transferred from the support would increase the
population of the anti-bonding orbital of the adsorbed O
2
molecule, therefore
weakening the O-O bond and the subsequent O-O bond breaking.
94,51
Fourthly, the

oxide support may directly participate in the reaction pathways. Theoretical DFT
calculation on two reaction paths (with or without the direct participation of TiO
2
) on
Au
10
/TiO
2
(110) shows that the direct involvement of the support can enhance the
bonding of O
2
and reduce the energy barrier.
48
The tendency of the oxide support to retain hydroxyl or water could have significant
implication on the catalyst activity and stability. Careful spectroscopic measurements
often observed the presence of carbonate or formate or acetate as the intermediates of
the CO oxidation. Additionally moisture or small amount of water in the reaction
system is found to be favorable to the catalytic CO oxidation on supported gold
nanoparticle.
18
The enhancement of the activity by two order of magnitude was
reported by Haruta and coworkers.
19
In these cases the participation of surface OH
group is often needed to explain the experimental observations.
49,51,97,100-105
In Chapter
3 of this thesis the support effect, in particular the direct involvement of OH in CO
oxidation, will be discussed carefully.
1.6. Reaction Mechanism of Low Temperature CO Oxidation

17
Although there are many reaction mechanisms proposed by different groups,
44,85,92
in
general there are two categories of mechanism for the CO oxidation over metal oxide
supported gold nanoparticle systems, i.e. whether the metal oxide support is involved
in the reaction. All proposed mechanisms assume adsorption and activation of CO on
Au nanoparticles. The difference occurs in discussing the adsorption and activation
of O
2
. Category one suggests that oxygen adsorption on Au particles and the reaction
between the adsorbed CO and O species proceeds by Langmuir-Hishelwood
mechanism. Category two involves support in the reaction, and anionic vacancies of
the support are the active center for O
2
activation. The oxygen adsorption may result
in formation of O
-
or O
2
-
, but which one: molecular or atomic interacts with CO is
open.
Whether it is Au metal or Au
n+
ions are the active sites for the CO oxidation reaction
is also a controversial issue. The presence and the role of Au
0
are well established.
The existence of Au

n+
was detected by various tools including XPS, FTIR, XANES
and Mossbauer spectroscopy, and reported in many papers. However there is not yet
direct evidence that the reaction rate is linearly correlated to Au
n+
and cationic golds
are necessary in the CO oxidation reaction. Further work is needed.
Another issue, which should be further studies, is to identify the reaction
intermediates and to determine how OH group is involved in the reaction
mechanism.
49,90,97
1.7 Motivation of the Thesis
As mentioned in the above introduction, among many factors that are able to affect
the catalytic activity of nanogold supported on metal oxides, the preparation method
18
is primarily important for high catalytic activity of low temperature CO oxidation.
Thus in Chapter 3, one of the most active supported gold nanoparticle system – iron
oxide supported gold nanoparticles system is selected for the comparison
investigations of three widely used preparation methods: coprecipitaion (CP),
deposition-precipitation (DP) and colloids-based impregnation (CI). CI is found to be
superior over other two methods. Therefore in Chapter 4, Au/TiO
2
samples were
prepared by CI with different surface area, structure and crystalline structures. The
effect of support surface area, support structure, support-retained water/OH groups on
the low temperature CO oxidation on gold/titanium oxide system were discussed. And
then in Chapter 5, a less popular (or relatively new) system—Au/CuO system is
carefully studied. The focus in this chapter is the study of reaction mechanism of CO
oxidation over Au/CuO system. In-situ DRIFT is able to identify the reaction
intermediates and provide evidence for the mechanism investigations. The conclusion

will be summarized in Chapter 6.
19
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