Chapter 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 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 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)— 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 Bionanotechnology, 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 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 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 TiO2.41,42 This time the energy absorbed by TiO2 is used to convert nitrogen oxide pollutants in the air into naturally washed away nitric acid. General Motors was the first to use a 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 - 46 [AuCl ] using phosphorus as reducing agent in the mid-nineteenth century. 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 essence of the nature of these nanoscale particles of gold where he concluded (in 46-48 1857). 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 49 particles of 3-30 nm in diameter. 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 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 (5d106s1) chemistry is determined by (i) the easy activation of the 5d electrons, and (ii) its desire to acquire a further electron to complete 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 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 AuIII state, which has the 5d8 configuration. The AuI state is of lesser importance and the Au II state is unknown except in a few unusual complexes. 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 Fe2O3 and TiO2 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 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 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 defect density. Au clusters with sizes ranging from to 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/Fe2O3 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 An acceleration of interest in gold as a catalyst proceeded Masatake Haruta et al. discovery of extremely small ( 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 13 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/TiO2 was found to be four times faster than that on Au/Al2O3.91,92 It has been shown that on TiO2 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 O2 adsorption and activation. The use of various methods 14 (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 Fe2O3-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, and 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. Goodman96 and many other researchers. The initial nucleation of Au occurred on Ti3+ defect sites, and the Au3+ 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 Au0 and Aun+ both are found to have a role to play in CO oxidation, and the simultaneous existence of Au0 and Au3+ is essential in various models of reaction 15 mechanism.90,97,98 For Au on TiO2, 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 O2 more readily. The negative charge transferred from the support would increase the population of the anti-bonding orbital of the adsorbed O2 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 Au10/TiO2(110) shows that the direct involvement of the support can enhance the bonding of O2 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 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 16 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 O2. 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 O2 activation. The oxygen adsorption may result in formation of O- or O2-, but which one: molecular or atomic interacts with CO is open. Whether it is Au metal or Aun+ ions are the active sites for the CO oxidation reaction is also a controversial issue. The presence and the role of Au0 are well established. The existence of Aun+ 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 Aun+ 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 17 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. 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Today 112 (2006) 27 [...]... 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 Au10/TiO2 (11 0) shows that the direct involvement of the support can enhance the bonding of O2 and reduce the energy barrier.48 The tendency of the oxide support to retain hydroxyl or water... 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 ,10 0 -10 5 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 16 Although there are many reaction mechanisms proposed... 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... 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/TiO2 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... 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 O2 Category one suggests that oxygen adsorption on Au particles and the. .. Treffry, P.M Harrison and S Mann, J Mol Biol 2 21 (19 91) 14 43 7 V Ponec, Catal Rev Sci Eng 11 (19 75) 41 8 J.H Sinfelt in Bimetallic Catalysis, Wiley, New York (19 83) 9 C.R Martens (Ed.) in Technology of Paints, Varnishes and Lacquers, Reinhold, New York (19 68) pp 335 10 R.W Siegel, S Ramasamy, H Hahn, R Gronsky, Z.Q Li and T Lu, J.Mater Res 3 (19 88) 13 67 11 T.W Barbee Jr in Synthetic Modulated Structures... 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 17 is primarily important... 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... 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 O2 more readily The negative charge transferred from the support would increase the population of the anti-bonding orbital of the adsorbed O2 molecule, therefore weakening the O-O bond and the. .. Mavrikakis, P Stoltze and J.K Norskov, Catal Lett 64 (2000) 10 1 85 M Valden, X Lai and D.W Goodman, Science 2 81 (19 98) 16 47 86 G.C Bond and D.T Thompson, Catal Rev.-Sci.Eng 41 (19 99) 319 87 M Haruta, Cattech 6 (2002) 10 2 25 88 A.M Venezia, G Pantaleo, A Longo, G.D Carlo, M.P Casaletto, F.L Liotta and G Deganello, J Phys.Chem B 10 9 (2005) 28 21 89 M Haruta, Stud Surf Sci Catal 11 0 (19 97) 12 3 90 S Tsubota, . on two reaction paths (with or without the direct participation of TiO 2 ) on Au 10 /TiO 2 (11 0) shows that the direct involvement of the support can enhance the bonding of O 2 and reduce the energy. 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. 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.