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

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KeyLaboratory of Molecule Nanostructure and Nanotechnology was founded in 2001 and is dedicated to construct new nanostructures, analysis the surface and interfacestructures at nanoscale

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Chapter 1 Introduction

1.1 General background

Since the discovery of atoms as the basic building blocks of all matter, scientists havelabored on to find means of controlling these miniscule entities, sensing the power tocreate new materials will follow Indeed, in his president's address to the AmericanPhysical 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 dimensionless than a hundred nanometers, whether in particle diameter, grain size, layerthickness, or width Scientific work on nanomaterials dates back to over a centuryago 3-6 After British chemist Thomson Graham discovered a solution containingnanosized particles in suspension (colloid), the likes of Rayleigh, Maxwell andEinstein began to investigate the colloidal systems By 1930, the Langmuir-Blodgettmonolayer film was developed By 1960, electron microscopy and diffraction wereused 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 containedless than 100 atoms were studied The C60 molecules were discovered by HaroldKroto, 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 thennanoscience and technology have become a hot area of research and developmentaddressing the control, modification and fabrication of materials, structures and

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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 insciences 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 planedfor the general strategy and objective of nanotechnology development in China for theperiod 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 KeyLaboratory of Molecule Nanostructure and Nanotechnology was founded in 2001 and

is dedicated to construct new nanostructures, analysis the surface and interfacestructures at nanoscale as well as physical and chemical properties of singlemolecules, 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

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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, createsenvironmentally beneficial nanotechnology and implements national nanotechnologyinitiatives.12,13

In Europe, nanotechnology has been receiving around €1.2 billion annually of public,regional and industry funding each year Nanotechnology has attracted muchattentions in Europe partially because the influence of US and Japan 14 It is alsobecause nanotechnology was viewed as offering tremendous opportunities inhealthcare, energy and environment aspects One of the problems for Europe now isthe lacking of the scientist in the area; this problem has been reflected from the factthat 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 fundingagency for the science and technology research in Singapore, started the A*STARNanotechnology initiative in September 2001 Nanotechnology research programs arecarried out through existing institute of A*STAR as follow:15 Institute of MaterialsResearch and Engineering (IMRE)—Photonics, Advanced Materials Institute ofMicroelectronics (IME) and Data Storage Institute (DSI)—Semiconductor,Electronics, Storage Institute of Bioengineering and Nanotechnology (IBN)—

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Bionanotechnology Institute of Chemical and Engineering Science nanoscience and nanotechnology have also been applied in developing nanocatalystsfor clean energy, nanomaterials for hydrogen storage and drug delivery There arealso some other nanotechnology institutes in Singapore The NanotechnologyInitiative (NUSNNI) in National University of Singapore concentrates on Bio-nanotechnology, nanoelectronics, nanophotonics, nanomagnetics, self-assemblymolecular devices, nanostructures and nanomaterials.16 The precision Engineeringand Nanotechnology (PEN) Center, at Nanyang Technological University, focuses onthe nanoscale precision machining, nano-metrology nanodefects detection, inparticular: nanoparticles and nanodefects detection system for unpolished siliconwafers; 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 andmolecular electronics, nanocomposites, energy and catalysis, andnanobiotechnology.18

(ICES) The Singapore Economic Development Board provides active support for technologyand business development in Singapore In the field of Nanotechnology, EDB isparticularly active and dynamic in bringing foreign technology R&D and industries toSingapore and facilitate their fusion with the local R&D institutions as well asindustries EDB is taking the initiative to establish the Nanotechnology Industryapplication Center where star-ups can co-develop applications with market leaders inSingapore.19

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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 theirplans and programs in nanoscale science and technology 20-23 This group continuedinformally until September 1998, when it was designated as the Interagency WorkingGroup on Nanotechnology (IWGN) under the National Science and TechnologyCouncil (NSTC).24The IWGN sponsored numerous workshops and studies to definethe state of the art in nanoscale science and technology and to forecast possible futuredevelopments Ever since US President Bill Clinton’s speech in 21 January 200025 atthe California Institute of Technology, Clinton, "Some of our research goals may taketwenty or more years to achieve, but that is precisely why there is an important rolefor 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 fundingfor 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 agenciestotaling $3.63 billion over four years.27. It should be noted that this law is anauthorization, not an appropriation, and subsequent appropriations for these fiveagencies have not met the goals set out in the 2003 Act However, there are manyagencies involved in the Initiative that are not covered by the Act, and requestedbudgets under the Initiative for all participating agencies in Fiscal Years 2006 - 2008totaled over $1 billion each The current NNI budget supplement for Fiscal Year 2009provides $1.5 billion dollars to the NNI, reflecting steady growth in thenanotechnology investment.28-33

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1.1.2 Applications and challenges of nanotechnology

In China, there are three main applications of nanotechnology in industry so far, theyare: material processing; nanochip fabrication and integration and nanochipprocessing 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 everydaylife products Manufactures like Johnson & Johnson, L’Oréal already used nanoscaletitanium dioxide and zinc oxide in their sunscreen and anti-wrinkle cosmetics.Nanosized iron oxide is used for some lipsticks as a pigment.36,37Also fabrics used forclothes, mattresses, upholstery and soft toys are sometimes treated with nanosizedcoatings The fabrics can be made water- or stain- or perspiration-resistant whileretaining 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 Titaniumdioxide has been used in self-cleaning windows and suites The UK manufacture ofglass Pilkington already put their product in market.40 Titanium dioxide in nanoscalewas also used in Paint producers Millennium Chemicals is the world’s second-largestproducer of titanium dioxide and a leading producer of titanium chemicals, theydeveloped a paint that uses the absorbance of UV light of nanosized TiO2.41,42 Thistime the energy absorbed by TiO2 is used to convert nitrogen oxide pollutants in theair into naturally washed away nitric acid General Motors was the first to use a

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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 andpetrochemical industries to improve the yield By using nanoscale materials in oilrefining, US have been saved $8-16 billion yearly in oil imports.44Nanotechnologymight have biggest impact on the data storage of information technology among allindustrials In 2003, PC with hard drives use nanosized material quadrupled the datastorage capacity And the computer chips had structures with widths of 130nanometers used in 2004 were soon to reduce to 90 nanometers due to the improvedlithographic technology The 65 nanometer technology is widely used in major ITcompanies in CMOS semi-conductor fabrication by year 2007.45

With the rapid development of nanotechnology and the uncertainty in the physicaland 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

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 Inaddition, Faraday has noted that these colloidal gold sols are thermodynamicallyunstable and hence needed to be stabilized kinetically against aggregation Oncecoagulation occurred, it is irreversible Remarkably, Faraday has also identified the

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essence of the nature of these nanoscale particles of gold where he concluded (in1857).

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 containedparticles of 3-30 nm in diameter

49Nanomaterials often possess very differentproperties from their bulk form Nano-gold catalysis is such an example Goldoccupies a position at one extreme of the range of metallic properties, and itslegendary chemical inertness is attributable to the Lanthanide Contraction and therelativistic 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 onthe p electrons, but d and f electrons are hardly affected, never coming close to thenucleus 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 thedisposition of their orbital: 5d and 6s electrons are therefore drawn towards thenucleus Hence gold is inert compared to other metals including its neighboringelements (e.g Cu, Ag and Pt) Gold (5d106s1) chemistry is determined by (i) the easyactivation of the 5d electrons, and (ii) its desire to acquire a further electron tocomplete the 6s2 level and not to lose the one it has, due to the 6s2“inert pair effect”

This latter effect awards it a much greater electron affinity and higher first ionizationpotential 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 AuIII state,which has the 5d8 configuration The AuI state is of lesser importance and the Au IIstate is unknown except in a few unusual complexes

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Interestingly, the properties of the nanoscaled particles can be changed with theirdimension scale Recent studies by M Haruta have demonstrated that highlydispersed gold particles supported on some oxides such as Fe2O3 and TiO2 aresurprisingly active in low (ambient or less) temperature CO oxidation, more activethan noble metals of Group 8-10.50The activity of supported Au catalyst is shown to

be structure-sensitive, remarkably sensitive to the size, shape and morphology of Auparticles A sharp increase in the CO oxidation turn-over-frequency, the reaction rateover one single metal atom per second, is observed with a decrease in the diameterfrom 5 nm.50,51 The smaller the particle, the greater will be the fraction of atomsdirectly in contact with the support and therefore influenced by it, while at the sametime the fraction of coordinatively unsaturated surface atoms also increases, and thischanges the physical properties of the whole particle.50 It is therefore virtuallyimpossible to draw a clear distinction between intrinsic particle size effects and thosethat are due to metal-support interactions M S Chen and D.W Goodman revealedthat on TiO2(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 defectdensity Au clusters with sizes ranging from 2 to 4 nm that were specifically twoatoms thick, showed maximum reactivity, optimally active for CO oxidation.50 Veryrecently A A Herzing et al have used aberration-corrected scanning TEM to analyzethe active gold nanoclusters on real Au/Fe2O3 catalysts, revealing that high catalytic

CO oxidation activity is correlated to bilayer Au clusters of ~0.5 nm, whereas themonolayer cluster containing only 3-4 Au atoms and isolated Au atoms are essentiallyinactive.53

1.3 Oxidation of carbon monoxide over nanosized gold

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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 moreactive 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 exactmechanism in which this could be done proves elusive because most work has notdirectly addressed this procedure In papers measuring orders of reaction andactivation, the avoidance of mass transport limitation is not always specified Thecommon practice of measuring catalyst performance by looking at conversion as afunction of temperature also poses an obstacle as it has great limitations In fact,reliable kinetic information, such as order of reaction and activation energy, andthorough kinetic modeling is hard to come by, resulting in a vacuum where testing ofpossible mechanism proves to be difficult Additionally, a combination of sensitiveand interrelated variables (and contradictory literature) makes controlled testing ahighly challenging task For example, gold compounds are photosensitive Hencepreparation of compound should be done in darkness to avoid reduction of Au3precursors.11,55-58Reduction is also minimized through drying at room temperature or373K in vacuum; and then precautions taken to avoid water vapor by storing invacuum or in a freezer.55-59 Still another variable is the amount of residual chlorideions left after washing, as it causes agglomeration of gold particles during thermaltreatment, and reduced activity in the oxidation of CO.59-63 Catalytic activity is alsoaffected by residual sodium from the usage of hydroxide or carbonate in thepreparation by deposition-precipitation (DP) This is further complicated by studieswhich do not agree on the role of sodium as poison57,64or promoter,62,65 possibly

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dependent upon its concentration and the support used The optimal size of goldparticles is also in contention depending on the conditions of thermal pretreatment.Again, literature disagrees on the exact activation procedures Most papersinaccurately term 'calcination' when the gold particles are actually thermal treated inair, causing a reduction of gold On the other hand, using a vacuum or hydrogen mayresult in early deactivation (some papers have claimed that hydrogen treatment ispreferred because unwanted chloride could be eliminated through HClformation61,66,67 We also have to be aware that Au0 and their precursors are highlyunstable Precisely speaking, the surface of gold particles below 3 nm will react withoxygen in ambient air; while Au3 species in Au/TiO2 will be reduced in a mixture of

CO and O2 at room temperature, becoming active in CO oxidation.68-71 Exceptionsexist in dried samples of Au/Al2O3, 70 Au/TiO2, 72 and Au/Fe2O3. It is also verydifficult to control the conditions of reaction though most are done at atmosphericpressure Precise temperature control is challenging because CO oxidation is highly

exothermic (ΔH0= 283kJmol-1) Reaction temperature of 2.5wt% Au/Fe2O3can raisefrom 293 to 393K.73 Although measures such as dilution of catalyst have beenadopted to counter this, local temperature proves difficult to control as supports oftenhave poor thermal conductivity.74-77

Another variable often uncontrolled is the amount of water in the reactants, whichmay have a significant influence, although studies on its effect are inconsistent andgeneral characteristics are difficult to map out Some studies have shown that theeffect of water is dependent upon its concentration and the support used.78,79 Whilethe activation energies of Au/TiO2 and Au/Al2O3 show that they are almostindependent of water concentration, Au/SiO2 showed marked differences in its

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conversion-temperature curve in the presence of water This is suggestive that thehydroxyl groups at metal-support interface are crucial for reaction efficiency, and thatthe 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 Auatoms) are all the important factors that determine the catalytic activity of metal oxidesupported gold nanoparticles.80-85 Preparation method could affect all the factorsmentioned above Thus choosing a suitable method for sample preparation is of thefirst concern

There are generally two preparation methods of nanosized metal particles, the 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 andcomplicated equipments Hence this method is opted out in our study Conversely, thebottom-up procedure is able to generate particles of uniform size, shape and structurewith distinct features This is because the bottom-up approach uses atoms that willthen 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

top-support and the gold particles are formed simultaneously in solution from initiallysoluble precursors The main shortcomings of this method include long-term reactionprocess, low efficiency in Au deposition (usually below 20%) and time-consuming

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washing steps to remove the chloride contamination as trace amount of it can poisonthe catalysts Deposition-precipitation process deposits metal particles from initiallyhighly 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 thatthe precipitation takes place only on a support and not in solution This method is theeasiest 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-precipitationmethod, e.g., the Au-deposition efficiency is usually low, etc.86-88 Differing from theabove two methods, the colloid-based method has the advantage of small meanparticle size and narrow size distribution under appropriate conditions, because thesemetal nanoparicles are protected with stabilizing agent or capping agent in thesolution In Chapter 3 of this thesis the above three preparation methods arecompared Among the three methods, the colloid-based method needs shortestreaction time, has highest Au deposition efficiency and lowest chloridecontamination, thus it is selected and applied in Chapters 4 and 5 for preparing theAu/TiO2and 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 ofmany papers.89-95 Firstly, the oxide support may act as a stabilizer of the Au particledispersion Hence the surface area of the support is significantly important Myexperimental data in Chapters 3-5 indicate that nanogold catalysts on higher surfacearea oxide always have higher catalytic activity Recent paper on Au/Fe2O3 reportedthat nanogold particles > 1nm, which accounts for 20-40% of total Au atoms are notvery active for low temperature CO oxidation, while the most active gold clusters are

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