catalysis by gold

Chemistry of catalytic

systems II

Catalysis by Gold

6KM11 Eindhoven University of Technology

March 2004 Advisor: Prof. Ben Nieuwenhuys

Group 1: Astrid Frehen

Diana Henning Michel Ligthart Sanne Wijnans

Catalysis by Gold

Summary Gold as noble metal was long considered useless as a catalyst, but small gold particles appear to be extremely active. Since the second half of the last century, experiments on gold proved this. However for the use of gold as a catalyst some requirements must be obtained. The particle size, support materials, structure, and preparation methods are important properties for creating a good catalyst. Kinetics and mechanism for reactions on the catalyst, and its involvement in the reaction, are still not known for sure. A lot of research is done in this area. By showing several mechanisms we hope to give an overview of the catalytic processes that involve small gold particles. One prominent example is that gold catalysts are able to selectively oxidise carbon monoxide in a hydrogen stream at low temperatures, which is important for producing clean hydrogen fuel for fuel cells.

Chemistry of catalytic systems II of 39

Catalysis by Gold

Table of contents Summary....................................................................................................................................................... 2 Table of contents....................................................................................................................................... 3 Chapter 1: Introduction ............................................................................................................................ 4

§ 1.1 Introduction.................................................................................................................................. 4 § 1.2 Catalysis ........................................................................................................................................ 4 § 1.3 Catalysis by Gold ......................................................................................................................... 5

Chapter 2: History of Gold ...................................................................................................................... 7 § 2.1 Gold through time........................................................................................................................ 7

§ 2.1.1 Global gold market: availability......................................................................................... 7 § 2.1.2 Global gold market: use...................................................................................................... 8

§ 2.2 History of gold catalysis........................................................................................................... 9 Chapter 3: Gold as a catalyst ................................................................................................................ 14

§ 3.1 Particles....................................................................................................................................... 14 § 3.1.1 Particle size......................................................................................................................... 14 § 3.1.2 Support and promoter type ............................................................................................ 15 § 3.1.3 Contact structure ............................................................................................................. 16

§ 3.2 Preparation................................................................................................................................. 17 § 3.3 Active sites ................................................................................................................................ 18 § 3.4 Sulphur Poisoning ...................................................................................................................... 19

Chapter 4: Carbon Monoxide Oxidation ..............................................................................................20 § 4.1 General Carbon Monoxide Oxidation.....................................................................................20 § 4.2 Carbon Monoxide Oxidation on Gold.....................................................................................20

§ 4.2.1 Reaction Surface............................................................................................................... 21 § 4.2.2 Kinetics ...............................................................................................................................25 § 4.2.3 Mechanisms........................................................................................................................26

§ 4.3 Catalytic contribution ..............................................................................................................28 § 4.4 NO2 reduction............................................................................................................................30

Chapter 5: Applications........................................................................................................................... 31 § 5.1 Pollution control ......................................................................................................................... 31 § 5.2 Chemical processing .................................................................................................................32 § 5.3 Fuel cells .....................................................................................................................................32

Chapter 6: Conclusion ..............................................................................................................................35 Literature...................................................................................................................................................36


Catalysis by Gold

Chapter 1: Introduction § 1.1 Introduction Gold: • Symbol = Au • Atom number = 79 • [ Xe ] 4f14 5d10 6s1 • Density = 196,9655 g/mol • Metallic radius = 144,20 pm • Melting point = 1337 K • Boiling point = 3081 K Bulk gold has been regarded chemically inert, however small gold particles appear to be extremely active. An example of known catalytic application of gold is the oxidation of carbon monoxide at low temperatures, for producing clean H2 for fuel cells. Interested in understanding more about the complexity of heterogeneous catalysts, we started a literature study into this subject to find out more about the structure of the catalytic surface, the reaction mechanism and the interaction of the adsorbates at a molecular level. This report includes a short history of gold, and gold catalysis and some applications in industry. Furthermore properties of the catalyst are discussed and possible mechanisms are shown, especially focused on the oxidation of carbon monoxide to carbon dioxide at low temperatures. § 1.2 Catalysis 1For a chemical reaction to occur, an input of energy is required, when an activation energy barrier has to be overcome. The higher this energy barrier is, the slower the reaction will proceed at a certain temperature. A catalyst can provide an alternative, easier pathway for this reaction, avoiding the slow rate-determining step of the non-catalysed reaction. As a result the rate at which the products are formed at ambient temperature increases. The rate of the backward reaction will be enhanced as effectively as the forward reaction. In other words, a catalyst enhances the reaction rate, but does not alter the chemical equilibrium for the reaction.


Catalysis by Gold

Catalysts provide easier pathways for chemical reactions often by a lowering of the activation energy. The lower the activation energy of the catalysed reaction, as compared to the non-catalysed reaction, the more active the catalyst is. An important characteristic of catalysis is the ability to selectively remove reactants, or to produce mainly a desired product. A catalyst has a higher selectivity when the relative amount of desired product in the product stream is higher. Catalysts certainly take part in reactions, but they are not spent during the reaction. The catalytic activity of a catalyst may change during the reaction. The ‘inertness’ of the catalyst with regard to changes in activity, or selectivity during operation, is denoted as the catalyst stability. Addition of another component, such as certain metal oxides, can increase the catalytic activity, shift the selectivity to the desired products, or stabilize the active sites on the catalyst. There are mainly two types of additives. The first type is called a promoter and does not itself participate in the reaction. The second type is called a co-catalyst and is involved directly in the catalytic reaction. § 1.3 Catalysis by Gold Gold2 is generally regarded as being the least useful of the noble metals for catalytic purposes. For bulk gold the formation enthalpy of Au2O3 is positive (∆Hf = +19.3 kJ/mol), and this oxide is therefore instable. Assuming that initial enthalpies of chemisorption are linearly related to the formation enthalpies of stable metal oxides (the Tanaka-Tamaru rule), chemisorption of oxygen on bulk gold is impossible. This low chemical activity of gold is due to the filled 5d shell (i.e. [Xe] 4f14 5d10 6s1) and the relatively high value of its first ionisation potential (i.e. 888 kJ/mol).3 Therefore, the surprise was enormous when suitable prepared gold supported on metal oxides could oxidise CO to CO2 at temperatures as low as 200 K. However, any practical application of a catalyst will require a reliable method of manufacture, long-term stability, good response to operating conditions, and more information on the kinetics and mechanism of the reaction.


Catalysis by Gold

Generally, gold catalysts are considered to be superior to platinum catalysts in CO removal for fuel cell applications, because of their ability to oxidize CO selectively at high rates at temperatures corresponding to the operating temperature of the hydrogen fuel cell, without the loss of large quantities of hydrogen.


Catalysis by Gold

Chapter 2: History of Gold § 2.1 Gold through time § 2.1.1 Global gold market: availability South Africa is considered to be the largest gold mining country in the world, followed by the United States, Australia, Canada, and China. Up until now over 47000 metric tons (i.e. 47 x 106 kg) of gold were found in South Africa, accounting for one-third of the world’s find. Today, the country still produces a fifth of global gold, and hosts six of the ten biggest gold mines. Global gold mining covered 2555 tons in 1998 to 2595 in 2001.4,5

Gold has a relatively low and stable price compared to the platinum group metals (see fig. 1). By 1998, global gold production costs fell to an average of US$ 193/ounce, and nowadays the purchase price of gold is around US$ 300/ounce. Since 1998 the price for palladium and platinum exceeds this value.6 Presently, gold is worth about one-quarter of its price during the peak in 1980, and about the same as it was worth in 1973. Two things can explain this low and stable prise:5 Gold is not particularly scarce, and nearly all the gold ever mined, is still available for use. Today, at least 15 % of annual gold consumption is recycled each year.7 Currently, no major nation ties the value of its currency to gold, and it has been against International Monetary Fund (IMF) rules to do so since 1978. Therefore large gold deposits, previously used as bank stock, became available to the market again.


Catalysis by Gold

Fig 1: Precious metal prices 1996 - 2003 http://www.gold.org/discover/sci_indu/gold2003/pdf/s36a1057p1297.pdf § 2.1.2 Global gold market: use About 80 % of all gold produced, is used in the jewellery industry in which both India and the United States are large-scale consumers. Some 15 % (i.e. 350 to 400 metric tons) of the annual demand for gold descends from industry. Compared to other precious metals, this percentage is extremely low (see table 1). The electronics industry accounts for the largest part (e.g. 5.6 % in 1997, 8 % in 20028) of industrial gold consumption. Here, gold is primarily used for the production of plating salts (i.e. 5.6 %), and fine wire or strips used to connect transistors and integrated circuits.9 The dental industry forms the second largest application (i.e. 2.3 % or 60 metric tons annually)10. Because of its biocompatibility, gold alloys are used for dental prosthetic applications11. These two industries have a relative steady annual demand for gold, and little increase is to be expected.12,13,14 Table 1: Annual industrial demand for precious metals as a percentage of total demand Editorial – ‘Perspective on industrial and scientific aspects of gold catalysis’, Applied Catalysis A: General 243 (2003) 201–205 Application Gold Silver Platinum Palladium Industrial [%] 12 66 49 94 Jewellery [%] 80 27 51 3 Other [%] 8 7 - 3


Catalysis by Gold

Some exotic uses for gold are known. The Royal Bank Plaza Building in Toronto, for example, has 14000 of its windows coated with pure gold, because the optical properties of the gold will reduce heating and ventilation costs inside the building. 70 000 g of gold were used, with a total value of 3 to 4 million US$.15 Furthermore, several medical applications can be found, amongst which are gold-based drugs. Since the early 20th century AuI-thiolates are used as a last resort to treat rheumatoid arthritis. Gold complexes have been evaluated for activity against HIV, acute forms of asthma or parasitic diseases such as malaria. Furthermore, several AuI and AuIII complexes are developed and screened for potential against cancer16. Gold also plays a role in targeted drug delivery through the ‘pharmacy on a chip’ concept, where gold-coated microcapsules of drugs are controllably released intravenously into the body. The limited use of gold in industry, and in particular within the chemical industry, can be contributed to a number of factors. First of all, use of gold is considered to be economically unattractive, because of its perceived high price. This contradicts with the great availability of gold and its relatively low and stable price, compared to the widely used platinum group metals. Furthermore, gold, for a long time, has been considered incorruptible and chemically inert.17,18 However, the climate is (gradually) changing… § 2.2 History of gold catalysis Throughout the 20th century sporadic research was conducted into the existence of catalytic activity of gold19. In 1906 W.A. Bone and R.V. Wheeler observed hydrogen oxidation on gold gauze. Together with G.W. Andrew, Bone was the first to report gold catalysed CO oxidation in 1925. During the 50s and the 60s the activity of gold as a hydrogenation catalyst was reported. A landmark was the work of B.J. Wood & H. Wise, who demonstrated that an Au film was active for the hydrogenation of cyclohexene and 1-butene if dissociated H-atoms were supplied through a Pd-Ag alloy timble.20 In the 70s some exciting research was done. G.C. Bond and P.A. Sermon, and also G. Paravano prepared small-dispersed Au particles on different supports


Catalysis by Gold

(MgO, Al2O3 and SiO2). They found that some of the samples exhibited catalytic activity for, for example, the hydrogenation of linear alkenes at low temperatures (e.g. 373 K). For the SiO2 support a low gold content (0.01 – 0.05 %) was sufficient to give catalytic activity. However, the activity found, was inferior compared to those of other noble metals.21,22 In the mid 70s D. McIntosh and G.A. Ozin wanted to study unstable metal-dioxygen complexes and the factors influencing the binding mode (‘side-on’ or ‘end-on’) of the dioxygen. Au vapour was co-condensed with O2 and argon at very low temperatures (i.e. 10 K), subsequently IR and UV-VIS spectra were evaluated. The resulting yellowish-green matrix was attributed to a gold complex containing a single dioxygen ligand. A ‘side-on’ binding mode was proposed for this complex, resembling the Dewar description of metal-olefin bonding of, for instance, (C2H4)Au or (C2H4)Ag, instead of ion-pair bonding like in the case of Ag+(O2

-). When studies revealed that, at cryogenic temperatures, group 1B metal atoms were highly reactive towards carbon monoxide, McIntosh and Ozin conducted similar experiments with Au. Gold complexes containing a maximum of two carbon monoxide ligands were found, and a linear, symmetrical, binding mode was proposed. Furthermore, when gold vapour was surrounded by an O2 / CO matrix, CO2 was formed and released when raising the temperature to 30 – 40 K.23 These results were used to develop a model for the mechanism of heterogeneously catalysed CO oxidation (see fig. 2).24,25

AuOO

AuO

O

O

O O Au O Au CO2CO2CO

+

Fig 2: Proposed mechanism for CO oxidation on a gold atom G.C. Bond, ‘Gold: a relatively new catalyst’, Catalysis Today, 72, 2002, 5-9 At the time, the importance of these observations was not recognised. Renewed interest in gold catalysed reactions developed because of research that was conducted in the second half of the 80s. In 1985 G.J. Hutchings predicted and confirmed Au the most active catalyst for hydrochlorination of acetylene to vinyl chloride26. K. Shinoda conducted extensive studies into various metal chloride catalysts for the hydrochlorination of acetylene to form vinyl chloride. Based on this work G.J. Hutchings plotted the standard electrode potential of the metals


Catalysis by Gold

used against acetylene conversion. The resulting smooth curve was used to predict gold as the most active catalyst for this reaction.27 This prediction was subsequently proved, and the carbon supported gold catalyst was found to be approximately three times more active than the commercial mercuric chloride catalysts, used in those days. At the time, the high gold price prevented further development of gold catalysts for commercial vinyl chloride production.28 MCln + HCΞCH + HCl → MCln. HCΞCH.HCl

Fig 3: Correlation of initial hydrochlorination activity (mol HCl/mol M/hr) of metalchlorides vs standard electrode potential. ‘Perspective on industrial and scientific aspects of gold catalysis’, editorial, Appl. Cat. A: General 243, (2003), 201-205 Around 1987 M. Haruta was the first to show that CO oxidation could be performed at ambient temperatures by using gold dispersed on oxide surfaces29. Co-precipitation or deposition precipitation procedures resulted in very active catalyst systems. These findings attracted interest in the scientific world, and formed the basis of research conducted since.


Catalysis by Gold

During the last 15 to 20 years the interest in, and the research on, gold catalysts and gold catalysed reactions has seen a huge growth. The amount of publications on gold catalysis grew exponentially (see fig. 4) and worldwide numerous groups are working on the subject.

0

100

200

300

400

500

600

2004 1994 1984 1974 1964 1954 1944 1934 1924 1914year

num

ber o

f pub

licat

ions

Fig 4: Number of publications found in CAPLUS containing the concept ‘gold catalysis’. In total 5392 references were found. Search date was 05-02-2004 The World Gold Council developed reference catalysts that enable correlation between different research fields in which gold-based catalysts will be used. Registration of gold catalyst research can be seen in figure 5. This figure gives a nice overview of the efforts made within the gold catalysis research at the moment.

Fig 5: Recipients of the gold reference catalysts have indicated that they will be used for studying a number of reactions, shown above as a proportion of the total number supplied. ‘CatGold’, World Gold Council, issue 5, autumn 2003


Catalysis by Gold

In industry a similar trend can be seen. For the number of patents relating to gold catalysis, the last 10 years show a tenfold increase to approximately 30 new patents in 2001. Around 50 industrial companies worldwide have successfully applied for patents in this area.30 At the moment some gold-based catalysts are even used in commercial production (e.g. BP Chemical uses a gold palladium alloy for the so-called ‘Leap Process’ within its vinyl acetate monomer plant).31


Catalysis by Gold

Chapter 3: Gold as a catalyst § 3.1 Particles 32The catalytic performance of gold can be defined by three major factors: contact structure, support and promoter selection and particle size. Through appropriate selection of support materials, gold can exhibit high selectivity in hydrogenation and partial oxidation, and may contribute to the development of highly selective and green chemical processes. § 3.1.1 Particle size

It is generally accepted that the presence of small gold particles (2-15 nm), stabilised by metal oxides, is indispensable to obtain high catalytic performance.1 The critical diameter of gold particles is 2 nm. When Au particles are smaller than 2 nm they behave different than bulk gold, and more like Pt or Pd. This critical diameter corresponds with a layer of 3 or 4 atoms thick on the support if the gold clusters are hemispherical in shape.32

This different behaviour implies that the surface properties of the gold are changed. It is assumed that during a reaction with oxygen an electron is abstracted from the Au cluster, forming a negative charged oxygen species. For this reaction the perimeter interfaces are important, so loss of support around a small gold particle means loss of activity.32 The catalyst pre-treatment controls the gold particle size, and thus the potential catalytic activity. A low calcination temperature is required to convert the gold precursor into the catalytically active species. Small gold particles (2 nm) have a melting point of about 673 K, compared to a melting temperature of 1346 K for bulk gold.1 At elevated temperatures sintering of the small gold particles is inevitable. The optimum pre-treatment temperature depends on the nature of the support, due to difference in the strength of interaction between gold (precursor) and the support.1 Studies revealed that besides CO, also the hydrogen oxidation activity of supported gold catalysts increases with decreasing gold particle size. Hydrogen dissociation may already occur at room temperature, although the CO oxidation is still the preferred reaction.1


Catalysis by Gold

§ 3.1.2 Support and promoter type

The presence of small gold particles alone cannot explain the catalytic behaviour of gold catalysts; the identity of the support and promoter should also be considered important. The selection of the support material depends on the reaction that is going to be performed. Many (not too strong acidic) metal oxides can be used for CO-oxidation reactions, especially semi-conductive metal oxides like TiO2 and Fe2O3. For the selective oxidation of hydrocarbons only TiO2 or Ti-silicates form a suitable support. The most active noble metal catalyst for low temperature CO oxidation is Au on Mg(OH)2. However the catalytic activity suddenly dies after three months, due to a change in structure of the clusters. Unsupported gold was found to be more active in hydrogen oxidation than in CO oxidation. In contrast with unsupported gold, highly dispersed gold on support exhibits an extraordinary high activity in the low-temperature CO oxidation, whereas the hydrogen oxidation of Au/MOx was found to be only slightly better than that of unsupported Au2O3.32

Highly dispersed gold particles supported on reducible and catalytically active supports (e.g. FeOx, CoOx, TiOx and MnOx) exhibit synergetic behaviour. For example, addition of various MOx to an Au/Al2O3 catalyst influences the conversion in the low-temperature CO oxidation. A Au/MnOx/Al2O3 catalyst was found to have a temperature, needed for 95% CO conversion, 100 degrees lower than a Au/Al2O3 catalyst with similar particle size (see fig. 6).33 A major subject in the study of gold catalysis is the origin of these effects.


Catalysis by Gold

Fig 6: Temperature needed for 95% CO conversion versus the average particle size. Au/Al2O3 was calcinated at 300˚C, 600˚C and 900˚C to change the average particle size (triangle, squire and rhombus, respectively). For the Au/Mox/Al2O3 catalysts several MOx were used (circles). R. Grisel, K. Weststrate, A. Gluhoi, B.E. Nieuwenhuys ‘Catalysis by gold nanoparticles’, Gold bulletin 2002 § 3.1.3 Contact structure The gold particles are definitely not uniform in size, and the gold crystallites are most probably not hemispherical. All kinds of crystallite morphologies are thinkable, in which the correlation between surface area, or perimeter length, and particle diameter are completely different. It was assumed that gold did not migrate into the oxide lattice of the support. This is likely, but it does not exclude that a part of the support, first dissolved and subsequently re-crystallised during deposition of the gold, may decorate the gold particles, and so reduces the number of active sites.1 Contact of gold particles with the support is very important because the perimeter interfaces around the Au particles act as a site for reaction. Depending on the technique used to deposit Au nanoparticles, the contact angle with the support will differ. The deposition-precipitation method yields hemispherical particles with their flat planes strongly attached to the TiO2 support. In contrast, the impregnation method gives spherical particles simply on top of the support (see fig. 7). This difference in contact angle is responsible for more activity of the catalyst because of the interaction with the support material (see fig. 8).32


Catalysis by Gold

Fig 7: Turnover frequencies for CO oxidation over spherical and hemispherical particles of Au and Pt supported on TiO2 from Haruta.

Fig 8: Product yields of the reaction among propylene, O2 and H2 over Au/TiO2 catalysts, prepared by desposition-presipitation (left) and impregnation methods (right). - Haruta

§ 3.2 Preparation The support and the method of preparation highly affect the activity. The nature of the Au precursors, the method to introduce Au to the support (e.g. co-precipitation, deposition, precipitation, impregnation) and the pH are variables worth mentioning. These variables will affect particle size, the particle shape and the amount of residual chloride (which will poison a Au/alumina catalyst) in the catalyst. On the other hand, the stability of the catalytic activity depends on the reaction conditions (e.g. water pressure which is needed to sustain activity for Au/alumina, and hydrogen presence which prevents deactivation of the catalyst). 34

Conventional preparation techniques, such as wet impregnation and pore volume impregnation of the support, result in much larger gold particles than co-precipitation or deposition precipitation. Carefully prepared Au catalysts have a relatively narrow particle size distribution, giving mean diameters in the range of 2 to 10 nm with standard deviation of about 30 %. During preparation the calcination temperature is important for the particle size. At too high temperatures Au particles coagulate with each other.1


Catalysis by Gold

Different preparation methods can be used to deposit gold nanoparticles on a metal oxide support:32

• Impregnation method In an aqueous solution of HAuCl4 a metal oxide support is immersed, after which the water is evaporated. After calcination particles of 30 nm are left behind on the support.

• Well-mixed precursors (co-precipitation, co-sputtering and alloying) A metal mixture of Au and the metal component of the support together with e.g. hydroxide, oxide is used. Metallic gold is formed during calcination of these precursor mixtures, strongly attached to the metal oxide support.

• Deposition or adsorption of Au-compounds onto the support material (deposition-precipitation, gas-phase grafting and liquid-phase grafting) After adsorption of the gold-containing compound to the support, washing and drying off the other chemicals create metallic Au.

§ 3.3 Active sites It is clear that a small particle size is essential for catalytic activity of gold. In literature several theories were proposed to explain this phenomenon, but an unambiguous explanation is still not available. Some of these theories will be discussed in chapter 4 using examples or results obtained in CO oxidation research. The role of metal oxide support is also still under discussion. Whereas the support undoubtedly prevents small gold crystallites from sintering under mild reaction conditions, it has been proposed that the Au/MOx interface may also play a crucial part in the activation of oxygen. One beneficial effect of MOx can be ascribed to stabilisation of small gold particles, so it can be qualified as a promoter. The origin of the stabilising effect of MOx on the gold particle size is not clear. However, it is likely that surface defect sites play a crucial role in the nucleation of the gold particles.1 Furthermore, metal oxide is thought to take part in the reaction by supplying active oxygen, and must therefore be qualified as a co-catalyst. Addition of MOx has significant effects on both the carbon monoxide oxidation activity and selectivity to carbon dioxide.


Catalysis by Gold

§ 3.4 Sulphur Poisoning Due to the recent interest for gold in catalysis, not much is known about sulphur tolerance of gold catalysts. Adsorption of sulphur containing compounds on single crystal surfaces has been studied. Most of these studies were performed to demonstrate the ability of scanning tunnelling microscopy (STM) to resolve atomically sized features of (organic) adsorbates on gold, and other surfaces, rather than to concentrate on the chemical behaviour of gold towards these adsorbates. However, it is clear that sulphate species do adsorb on gold surfaces, and that gold is capable of breaking S-S bonds and splits asymmetric disulfide species even at room temperature.1

K. Ruth and M. Haruta have compared the catalytic activity of Pt and Au supported on TiO2. A strong de-activation of the CO-oxidation activity by SO2 was only observed for the TiO2 supported Au catalyst. Other gold catalysed reactions, like the H2 oxidation, exhibit less deactivation. They show that Au catalysts are much more sensitive for sulphation than Pt. They conclude that the catalytic sites that activate O2 (during CO oxidation) are blocked by SO2. SO2 was tested since it promotes the propane oxidation in supported Pt and Au catalysts through the build up of a sulphate-species at the metal-support surface.35


Catalysis by Gold

Chapter 4: Carbon Monoxide Oxidation § 4.1 General Carbon Monoxide Oxidation The catalytic oxidation of CO on (transition) metal surfaces has gained increasing importance in recent years in industrial chemistry and, in particular, in automotive emission control. Catalysed CO oxidation is one of the possible future prospects for gold. Carbon monoxide is a molecule that can coordinate to two or more metal atoms. Usually, the more metal atoms are involved, the stronger CO is adsorbed on the metal surface, and the weaker the C-O bond becomes. When the metal is positively charged the σ-bond should become stronger, which must lead to enhanced metal-CO bond strength.

Fig 9: Metal-CO complex formation. The highest occupied molecular orbital (HOMO) of CO donates electrons in the empty d-orbital of the metal atom, and forms a s-bond with it (a). A filled d-orbital of the metal can donate electron back into the 2π* antibonding orbital (LUMO) of CO, when it has the right symmetry for overlap (b). R.J.H. Grisel, Supported gold catalysts for environmental applications, thesis, Leiden University, 2002, p 17 § 4.2 Carbon Monoxide Oxidation on Gold As mentioned before, gold is considered to be a very inert metal. Consequently, the bonds between gold surfaces and adsorbates, such as CO and CO2 are very weak. Unlike the considerably stronger CO adsorption on palladium and platinum, it is reported that the bond between CO and the gold surface is almost entirely due to electron donation from molecular CO to the metal. The σ-bond between CO and gold weakens when the coverage increases. As a consequence, a lower ‘direct donation’ contribution to the Au-CO bond strength causes a stronger C-O bond.


Catalysis by Gold

In the low temperature range (i.e. 200 K) CO oxidation catalysts with a highly dispersed gold phase are superior (both with respect to activity and aging characteristic) to the most active palladium, platinum or silver catalysts. A comparison of carbon monoxide oxidation activities on different catalyst supports and with preparation methods is given in appendix A. § 4.2.1 Reaction Surface The reaction between CO and activated oxygen may take place on the support, at the Au/MOx interface, or on the gold particles. Many authors assume that CO is adsorbed on metallic gold sites and, mainly at higher CO partial pressure, also at the border between metallic gold particles and the support (see fig 10).

Fig 10: Bottom part: the different sites at which species may adsorb and react. Top part: proposed reaction mechanism for CO oxidation over Au/TiO2. Masatake Haruta, Catalysis of Gold nanoparticles deposited on metal oxides, Cattech, 2002, volume 6, no.3 1The behaviour of small gold particles and the nature of its active sites yet remain unclear. In literature several theories were proposed to explain the oxidation activity. The first two theories discussed below hold for small gold particles in itself. The other two theories are based on gold particle – support interactions.


Catalysis by Gold

• Electronic transition structure

One could imagine that there should exist a transition state between the electronic structure and behavior of bulk gold, and that of discrete gold atoms. This transitional state could exhibit activity towards other species, whereas the bulk does not.

In order to investigate a possible correlation between the electronic properties and the specific activity of the Au clusters in more detail, scanning tunneling spectroscopy (STS) measurements were carried out on a Au/TiO2(001)/Mo(100) catalyst. Data were acquired by recording topographic images of individual nanometer sized Au clusters, and simultaneously investigating its electronic properties. A metal to non-metal transition is apparent when the cluster size approaches approximately 4 nm in diameter (i.e. 400 atoms/cluster). This corresponds to the size at which onset of catalytic activity is observed for CO oxidation.

These data strongly suggest that the activity of the tested catalyst is determined by the unique electronic structure of its Au clusters. This structure influences the adsorption energy of CO on gold. The clusters exhibit a d-band gap of 0,4 V and have an optimum activity by a thickness of two atomic layers of Au. Specific activities of Au/SiO2, Au/Al2O3 and Au/TiO2 were found to be independent of the support material used, provided the Au cluster sizes were sufficiently small.36

• Specific surface sites

Specific sites may be present at the surface. It is known that CO binds considerably stronger to stepped surfaces than on flat (111) terraces. Adsorption and desorption of several small molecules on different types of Au surfaces were mathematically modelled (see table 2). It was found that dissociative adsorption of oxygen is endothermic on Au(111) facets, whereas the change in enthalpy is nearly zero at the stepped Au(211) surface (see fig. 11). The need for a stepped surface to facilitate chemical activity is in accordance with behaviour seen for other catalytically active metal surfaces. Since the relative concentration of steps and other surface defects increases with decreasing particle size,


Catalysis by Gold

one would expect a higher catalytic activity for small particles. Similar behaviour holds for strain effects due to the mismatch at the Au/support interface.

Fig 11: The two surfaces used for the mathematical modelling of O2 and CO adsorption and O dissorption: the Au(111) surface (a) and the Au(211) surface (b) which models a stepped surface. M. Mavrikakis, P. Stoltze and J.K. Nørskov, ‘Making gold less noble’, Catalysis Letters, 64, (2000), 101–106 Table 2: Calculated binding energies for CO, O2 and O on Au(111) and Au(211) surfaces and a small Au cluster. The Au(211) models a stepped surface. The lowest energy site is indicated in each case. Negative values correspond to stable configurations. M. Mavrikakis, P. Stoltze and J.K. Nørskov, ‘Making gold less noble’, Catalysis Letters, 64, (2000), 101–106

CO/Au (eV) O/Au (eV) O2/Au (eV) Au(111) - 0.30 + 0.18 No adsorption Au(211) - 0.66 + 0.02 - 0.12 Au cluster - 0.07 + 1.54 No adsorption

37Furthermore, like other late 5d metals, Au(100) does not have a simple cut-off structure. The top surface may reconstruct, induced by exposure to gases like CO and NO, in order to minimize its surface energy. It is assumed that the surface atoms are rearranged into a closed-packed, hexagonal-like, arrangement instead of a square arrangement. Because this quasi-hexagonal layer does not match with the unreconstructed underlying layers buckling occurs (see fig 12). Surface properties are greatly affected by this change. Probably, the surface-atoms rearrange to this hexagonal-like structure because they experience a loss in co-ordination. Introducing an adsorbent, like NO or CO, can compensate for this loss (providing it is more strongly


Catalysis by Gold

bound to the unreconstructed phase), and therefore undo the reconstruction (adsorbate induced phase transition).

a b c Fig 12: Schematic representations of an unreconstructed fcc(100) surface (a), and top and side views of the quasi-hexagonally reconstructed surface, (b) and (c), respectively E.D.L.Rienks, G.P.v.Berkel, J.W.Bakker, B.E.Nieuwenhuys, The hidden properties of Au(100) surface, CD-Rom Gold 2003 Conference • Presence of ionic gold species

In some papers the presence of ionic gold species is suggested (see fig 13). It is assumed that the Au1 cation can remain sufficiently stable in a reducing environment and in the neighbourhood of metallic gold. When studying Au/Mg(OH)2 catalysts, Mossbauer spectra found Au1 atoms located at the interface between the Mg(OH)2 and the Au particle. These Au1 atoms were strongly perturbed by the Mg(OH)2 which may play an important role in their assumed catalytic activity. However, for TiO2 catalysts these Au1 atoms were not found. Formation of active Au1-OH species have been proposed, but conclusive proof of their existence and participation in CO oxidation is lacking. In literature it is stressed that presence of ionic gold species alone is not sufficient for activity. Metallic gold particles are needed at all times.38


Catalysis by Gold

Fig 13: Model of the active Au1-OH site for CO oxidation H.H. Kung, M.C. Kung and C.K. Costello, ‘Supported Au catalysts for low temperature CO oxidation’, J. Cat., Vol. 216, Is. 1-2 , (2003), 425-432 • Electronic interaction between gold and its support

An electronic interaction between the gold particle and the support may occur. Oxidation treatment of an Au/Fe2O3 catalyst results in transformation of FeO(OH) species (present at the gold/iron oxide interface) into FeO species (1). This results in oxygen vacancy formation at the surface. When this vacancy is filled by O2(g) adsorption (2), electrons get delocalized, resulting in an increase in metallic character of the small gold particles, which normally are electron deficient. This increased electron density results in enhanced CO adsorption, because back-donation from the Au d-bands to the 2π* CO orbital increases. Furthermore, the C-O bond strength is weakened, and therefore, the CO reactivity is increased.39

4 FeO(OH) → 4 FeO + * + 2 H2O(g) + O2(g) (1) Fe2+ + * + O2(g) ↔ Fe3+ + O2(s) (2) * = oxygen vacancy

§ 4.2.2 Kinetics For low temperature CO oxidation on supported gold catalysts, the reported reaction order in CO ranges between 0.05 and 0.5 and that of O2 between 0 and 0.24, with an Ea value ranging from 2.1 to 54 kJ/mol.1 This means that the values strongly vary with catalyst composition and the temperature range used. Possibly dissimilar reaction pathways exist for CO oxidation on different catalyst types and temperature regions.


Catalysis by Gold

The kinetics of these catalysts are independent of the CO concentration and only slightly dependent of the O2 concentration. This suggests that nearly the entire surface is saturated with adsorbed CO and O2, and that the reaction is the rate-limiting step.32 The O2 needed for the reaction is thought to originate from either the oxide lattice or from the gas phase. Molecular oxygen adsorbed on gold can easily oxidise near the gold particles, preferentially along the gold/oxide interface (see reaction above). The oxygen adsorbed on gold can easily oxidise adsorbed CO to CO2. Unfortunately no experimental data about O2 adsorption on the catalytic surface is known. § 4.2.3 Mechanisms In some cases CO oxidation results can be explained by a simple Langmuir-Hinshelwood mechanism for non-competitive adsorption, combined with an additional mechanism of oxygen activation.1 This reaction mechanism comprises the following elementary steps: CO + * CO* O2 + * O2* O2* + * 2 O* O* + CO* CO2* + * CO2* CO2 + * In other cases however, results do not correspond with a Langmuir-Hinshelwood mechanism according to Tanielyan. Several uncertainties remain and various mechanistic models were developed. It is uncertain whether the oxygen molecule is dissociatively or non- dissociatively adsorbed, but most likely molecular oxygen is adsorbed at the perimeter interface. (see fig 10) Boccuzzi proposes two different pathways for the oxidation of carbon monoxide. First, oxygen (activated on the surface of metallic gold particles) reacts directly and rapidly with CO (also present on the gold particles) leading to the formation of CO2. Boccuzzi considers this mechanism to be the most important for low-temperature CO oxidation.


Catalysis by Gold

Second, oxygen activates CO, or it enhances the reaction rate of CO species (which are adsorbed at the border of the gold particles) with the lattice oxygen. This slow, induced oxidation by surface lattice oxygen of the support results mainly in carbonates. The three mechanisms below all proceed via bidentate carbonate formation, but the reaction and adsorption sites differ. Futhermore, some of the mechanisms use ionic species. • Haruta’s mechanism Haruta showed that three temperature regions exist where different kinetics (e.g. different rates and apparent activation energies) apply. He proposes potential pathways for the oxidation of CO (see reaction mechanism below). Haruta postulated a model in which bidentate carbonates adsorbed on the support are important intermediates for CO2 formation: CO + *Au CO-*Au O2 + *sup + e- O2

--*sup CO-*Au + O2

--*sup CO3-*sup + *Au + e- CO3-*sup + *sup CO2-*sup + O-*sup CO2-*sup CO2 + *sup 2 O-*sup O2 + 2*sup

• Kung’s mechanism H. H. Kung, M. C. Kung and C. K. Costello proposed a model based on the assumption that reactive ionic gold species (i.e. Au1-OH) are present:

Fig 14: The hydroxycarbonyl is oxidized to a bicarbonate, which is then decarboxylated to Au1-OH and CO2. The active site is an ensemble of metallic Au atoms and Au-OH. Metallic Au atoms are responsible for activation of oxygen, perhaps at steps or corner sites. H.H. Kung, M.C. Kung and C.K. Costello, ‘Supported Au catalysts for low temperature CO oxidation’, J. Cat., Vol. 216, Is. 1-2 , (2003), 425-432


Catalysis by Gold

• Bond’s mechanism G.C. Bond and D.T. Thompson proposed the following mechanism, involving gold-hydroxide groups.40 *Au0 + CO CO-*Au0 Au3 + OH-*sup OH-*Au2 CO-*Au0 + OH-*Au2 COOH-*Au2 + *Au0 O2 + *sup + e- O2

--*sup

COOH-*Au2 + O2--*sup *Au2 + CO2 + HO2

-*sup COOH-*Au2 + HO2

-*sup Au2 + CO2 + 2 OH-*sup + *sup *Au2 + *sup + e- *Au3 + *sup § 4.3 Catalytic contribution J.M.C. Soares2 et al. have done a study particularly focused on the catalytic and non-catalytic CO oxidation on Au/TiO2 catalysts. They point out that during this reaction, the formation of oxidic states on the surface of these Au/TiO2 catalysts occurs. This results in stoichiometric, non-catalytic reaction with CO to produce CO2. On the one hand this can be a problem for the measurement of catalytic activity for some systems, on the other hand these processes may give us new clues about the state if the catalyst surface after various types of preparation procedures. Tests performed by Soares on CO oxidation using an Au/TiO2 catalyst (prepared by impregnation method) give data showing non-catalytic and catalytic behaviour. In the first experiment stages (at low temperature), there was 100 % conversion of CO into CO2 but no oxygen was consumed. It was proved that the oxygen provided came from the catalyst sample itself. In the final stage of the reaction (at 573/673 K) the real catalytic oxidation began. They concluded, after further experiments, that it might be, at low loadings, that all the Au is strongly associated to the support layer, even resulting in Au oxidation. At increased loading, it may be that Au particles are formed, in which the top layer is not strongly bound to the surface and so show different reaction characteristics with CO. In the next experiments Au/TiO2 prepared by deposition precipitation was used. It is known that catalysts prepared by this method are more active


Catalysis by Gold

than catalysts prepared by impregnation. They show catalytic activity for CO oxidation even at room temperature. The conversion of CO appeared to be very high, but it was difficult to distinguish whether it was catalytic or non-catalytic reacted. Perhaps the active types of oxygen can be reabsorbed for the deposition precipitation catalyst, making it active at room temperature. Active oxygen cannot apparently be reabsorbed on incipient wetness catalysts in a significant amount, probably due to loss of surface area and poisoning by Cl (which is not used in deposition precipitation preparation).

Fig 15: Effect of preparation and heating on the Au catalysts. J.M.C. Soares, P.Morrall et al., Catalytic and non-catalytic CO oxidation on Au/TiO2 catalysts, Journal of catalysis 219 (2003) 17-24 Determining what species of (oxidised) Au are adsorbed on the surface is incredibly difficult. Using many analysing techniques Soares still was not able to clearly point out the nature of the active Au particle responsible for the catalytic reaction. Concluding, CO can be oxidised by Au/TiO2, prepared by both incipient wetness and deposition precipitation. The latter catalyst however, is much more active for the steady state catalysis. Results have shown at least two ways in with CO can be oxidised by gold:

a)

b)

through reaction of surface active oxygen species activated by gold during the preparation of the catalysts; through catalytic reaction mediated by gold.


Catalysis by Gold

§ 4.4 NO reduction 41As mentioned two factors (i.e. the gold particle size (nanometer-range) and the nature of the support and additives), have significant influence on the activity of gold. Not only has this been proved by the oxidation of CO, but also by the reduction of NO by hydrogen and CO through use of gold particles as catalyst, because of the recent legislation of NOx emission. Supported gold catalysts convert nitric oxide in the presence of hydrogen to the products N2O (at low T), N2 (at intermediate T) and NH3 (at high T). The catalytic activity of Au/ Al2O3 in the N2O/H2 reaction is drastically increased by the addition of metal oxides, showing that Au/ Rb2O/ CeOx/ Al2O3 and Au/ Li2O/ CeOx/ Al2O3 are the most active. This is an important result since nitrous oxide is a harmful gas in automotive exhaust gas. Of course its functionality depends on the preparation method – the gold particles have to be uniformly distributed on the oxide support. In general, TiO2-supported gold is more active towards the oxidation of CO: CO + O2 and CO + N2O, compared to Al2O3-supports (H2 + O2 reaction). The N2O + H2 doesn’t have a preference between TiO2 and Al2O3-supported catalysts. So, the role of the partly reducible metal oxide additive contributes to the formation of new active sites and increase N2O dissociation into N2, and H2O or CO2. And the process benefits from the metal oxides because they stabilize gold-catalyst from sintering. All is in agreement to the CO oxidation.


Catalysis by Gold

Chapter 5: Applications 42In future gold-based catalysts may find various applications. Several of these applications involve the energy and automotive industry. Furthermore, promising results are found within the field of pollution control. Finally gold might open new routes in the production of bulk chemicals, offering better selectivity and/or reactivity, and safer production conditions. Some major applications are discussed below. § 5.1 Pollution control Perhaps the greatest commercial opportunity for gold-based catalysts lies in a number of potential uses within the automotive industry. With over 50 million light vehicles produced annually, this industry forms a huge potential market. The price of gold compares very favourably with platinum. However, when used as a component within a three-way catalyst (TWC), the ability of gold to withstand the elevated temperatures required is in doubt.43 There is the dramatic growth in popularity in diesel engines. Catalytic converter systems for diesel engines tend to operate at lower temperatures than petrol engines. This lower temperature could be critical in providing conditions of stability for gold nanoparticles. For gasoline engines there is an increasing focus on improving the low cold start performance of catalytic systems (i.e. controlling the exhaust composition, before the standard catalyst is warmed up and performing at an optimum). Neither of these applications will be achieved in the short term and there are significant engineering challenges to be overcome. Other potential commercial applications of pollution control comprise:

• Control of mercury, which has been linked to Alzheimer’s disease and autism.

• Decomposition of ozone, which can be emitted by equipment such as photocopiers and laser printers, which contributes to the formation of smog.

• Dioxine decomposition for air quality improvement and control of malodours. For instance, since 1992 Au/Fe2O3 on a zeolite honeycomb has been commercially used as an odour eater in Japanese toilets.


Catalysis by Gold

§ 5.2 Chemical processing The use of gold-palladium catalysts in the production of vinyl acetate monomer is well established, and a number of other reactions are now the focus of intense study, with gold-based catalysts potentially able to improve the economics of existing processes. One of these reactions is the local production of hydrogen peroxide. Global sales of hydrogen peroxide are rising at a rate of about 10 % per year, due in part, to it being viewed as an environmentally friendly alternative to chlorine. Currently, hydrogen peroxide is produced on a large scale and involves several hazardous steps. Transportation from point of manufacture to point of use is expensive. Therefore, a definite market need exists to develop a safe, modular process that can be operated cost effectively at the point of use. There is evidence emerging in the (patent) literature that gold-based catalysts might be applied effectively for this purpose. There is an interesting link in this potential application to the gold mining industry where peroxide is used to destroy cyanide waste. Transport of peroxide to remote mining sites is expensive and local on-site production would be of major benefit. § 5.3 Fuel cells 44,45,46The use of hydrogen based fuel cells, as an alternative source of relatively clean energy, seems very promising. One way to produce the supply of hydrogen needed for operation is from methanol, ethanol or similar fuels through a reforming process (see fig 15). The fuel reacts with steam at high temperature and pressure, and three reactions occur simultaneously: the steam reforming reaction (endothermic), the (direct) partial oxidation reaction and water gas shift reaction (both endothermic). In the ideal situation the product stream from these reactions consists only of CO2 and H2. Subsequently, the H2 is electrochemically converted to H2O whereby energy is generated. The reactions involved are discussed below:


Catalysis by Gold

Fig 15: Schematic representation of fuel cell Fuellcells.si.edu/_basics.htm • Partial oxidation:

CnHm + ½n O2 → ½m H2 + n CO • Steam reforming reaction:

Late transition metals (e.g. Pd, Pt, Ru and Rh) are used for the steam-reforming step (1), but nickel-based catalysts (e.g. Ni particles dispersed on an Al2O3 or an AlMgO4 spinel) are, economically, the most feasible. Even though other metals, like Ru and Rh, are more active. CnHm + 2n H2O → (2n + ½m) H2 + n CO2 (1)

The reaction has to be carried out at high temperatures in order to reduce related reactions. This causes the challenge to avoid carbon filament formation and growth, which can lead to hot spots in the reactor that will destruct the Ni catalyst. Alloying has proved to be a powerful tool to affect the selectivity of metal catalysts. In 1993, F. Besenbacher reported that gold on nickel forms a random two-dimensional surface alloy in the submonolayer regime. Although the gold has no activity for


Catalysis by Gold

the steam reforming reaction, it contributes to the reactivity by breaking up the large Ni ensembles that favour carbon deposition. It appeared that carbon is less strongly bound to nickel atoms, when in close contact with gold atoms. The Ni/Au is a high selectivity catalyst for steam reforming with low selectivity towards carbon deposition. The alloy also maintained a constant level of reforming activity.

• Water gas shift reaction:

Platinum is generally the choice for the catalytic electrode material. It is very efficient in hydrogen activation and has a high ‘oxidative power’. However, the feed stream usually contains several vol% H2O and CO, and especially the presence of CO causes major problems since platinum is effectively poisoned by adsorbed CO at the operating temperatures of the fuel cell (333 to 373K). In addition, over conventional oxidation catalysts, hydrogen oxidation will compete with CO oxidation in gas streams comprising both compounds. Hence, there was an urgent need to find new ways to remove CO selectively from the feed stream. The water gas shift reaction (2) is widely used when CO/H2 ratios need to be adjusted. Commercially available CuO/ZnO catalysts are used in industry, but their sensitivity to air poses limitations for smaller applications. Different Au catalyst types are active for this reaction, and they allow operation at low temperatures (e.g. 433 to 633K), which is favourable for the equilibrium.47

CO + H2O → CO2 + H2 (2)


Catalysis by Gold

Chapter 6: Conclusion It has become clear to us that in recent years the interest in gold catalysis is booming. Gold catalysts have enormous potential applications in many reactions of both industrial and environmental importance. The results on various topics therefore are encouraging but there is a great need for further survey in order to successfully implement gold in chemical industry. There exist a lot of uncertainties considering mechanisms and nature of active sites. Many mechanisms have been proposed in literature but most are suggestions, which include a special role of sites at the metal-support interface, the availability of low coordination surface atoms and quantum size effects. For instance the role of special reaction geometries available at nano-range gold particles in combination with an enhanced ability of low coordinated gold atoms to interact with molecules from the surroundings. Then there is the opinion that the mechanism of the beneficial effects of the additives is still debatable, even though many researchers agree that the interface between gold-particles and transition metal oxides proved to play a significant role. We believe that the mechanism of Haruta (see fig 10) is the most likely pathway for the catalytic CO oxidation by gold nano-particles. By adsorbing O2 molecular on gold one oxygen atom can directly react with adsorbed CO and form CO2. The other oxygen atom can immediately bind to an incoming CO molecule. The energy needed for dissociation of O2 on the gold surface, the rate-determining step, is in this mechanism lowest according to us. So, the number of reaction steps is small and therefore the mechanism is efficient. After discussing and reading about gold we think that it will be an economical favourite catalyst above other noble metals. The recent reversal of the market prices of Au (US$ 255/ounce) with respect to Pd (US$ 397/ounce) and Pt (US$ 397/ounce) will drive Au catalysts to commercialisation with an economical advantage. We have experienced this project to be highly interesting. There were a lot of unknowns, not only for us but also for science. The fact that not everything is certain, made it challenging.


Catalysis by Gold

Literature 1 R.J.H. Grisel, Supported gold catalysts for environmental applications, thesis, Leiden University, 2002 2 J.M.C. Soares, P.Morrall et al., Catalytic and non-catalytic CO oxidation on Au/TiO2 catalysts, Journal of catalysis 219 (2003) 17-24 3 G.C. Bond, ‘Gold: an relatively new catalyst’, Catalysis Today, 72, (2002), 5-9 4 ‘Preface’, Catalysis Today 72 (2002) 1–3 5 www.gold.org 6 Don Cameron, Richard Holliday, David Thompson, ‘Gold’s future role in fuel cell systems’, Journal of Power Sources 118 (2003) 298–303 7 http://www.gold.org/discover/knowledge/amazingfacts/index.html as seen on 3-03-2004 8 CatGold iss 5 9 http://www.gold.org/discover/knowledge/aboutgold/industrial_uses/index.html as seen on 3-03-2004 10 http://www.gold.org/discover/knowledge/aboutgold/industrial_uses/index.html as seen on 3-03-2004 11 P.J. Cascone, ‘A unique gold casting alloy for dental applications’, the Argen Corporation 12 GFMS (Gold Fields Mineral Services) released Gold Survey 2001 - Update 2, London, 17 th January 2002 13 www.gold.org 14 Dr. Christopher W. Corti and Dr Richard J. Holliday, ‘Commercial Aspects of Gold Applications : From Materials Science to Chemical Science’, International Technology World Gold Council, http://www.gold.org/discover/sci_indu/gold2003/pdf/s36a1057p1297.pdf as seen on 3-03-2004 15 Dr. Christopher W. Corti and Dr Richard J. Holliday, ‘Commercial Aspects of Gold Applications : From Materials Science to Chemical Science’, International Technology World Gold Council, http://www.gold.org/discover/sci_indu/gold2003/pdf/s36a1057p1297.pdf as seen on 3-03-2004 16 E.R.T. Tiekink, ‘Gold derivatives for cancer treatment’, Department of chemistry NUS, Singapore 17 C.W. Corti and R.J. Holliday, ‘Commercial aspects of gold applications: from materials science to chemical science’, International Technology, World Gold Councel 18 Don Cameron, Richard Holliday, David Thompson, ‘Gold’s future role in fuel cell systems’, Journal of Power Sources 118 (2003) 298–303


Catalysis by Gold

19 G.C. Bond, ‘Gold: a relatively new catalyst’, Catalysis Today, 72, 2002, 5-9 20 G.C. Bond, gold bull 5, 1971, 11 R.S. Yolles, B.J. Wood, H. Wise, J. Catal. 21 (1971), 66 Haruta, Catalysis of gold nanoparticles deposited on metal oxides 21 Haruta, Catalysis of gold nanoparticles deposited on metal oxides 22 G.C. Bond, ‘Gold: a relatively new catalyst”, Catalysis Today, 72, (2002), 5-9 23 Haruta, Catalysis of gold nanoparticles deposited on metal oxides 24 D. McIntosh and G.A. Ozin, ‘Direct synthesis using gold atoms, monodioxigen gold, Au(O2)’, Inorg. Chem., 15-11, 1976, 2869 25 D. McIntosh and G.A. Ozin, ‘Synthesis of binary gold carbonyls, Au(CO)n (n=1 or 2). Spectroscopic evidence for isocarbonyl(carbonyl)gold, a linkage isomer of bis(carbonyl)gold’, Inorg. Chem., 16-1, 1977, 51 26 G.J. Hutchings, Gold Bull 5 (1971) 11 & J. Catal 96 (1985), 292 & J. Catal 128 (1991), 378 27 G.J. Hutchings, ‘Gold catalysis in chemical processing’, Catalysis Today, 72, (2002), 11-17 28 ‘Perspective on industrial and scientific aspects of gold catalysis’, editorial, Appl. Cat. A: General 243, (2003), 201-205’ 29 Haruta, J. Catal. 115 (1989) 301 30 CatGold, issue 2, spring 2002, World Gold Councel 31 CatGold, iss 4 32 Masatake Haruta, Catalysis of Gold nanoparticles deposited on metal oxides, Cattech, 2002, volume 6, no.3 33 R. Grisel, K. Weststrate, A. Gluhoi, B.E. Nieuwenhuys ‘Catalysis by gold nanoparticles’, Gold bulletin 2002 34 Harold H. Kung, Mayfair C. Kung, C.K. Costello, J.H. Yang, “What makes gold an active catalyst?”, CD-Rom Gold 2003 Conference. 35 K. Ruth, M. Hayes, R. Burch, S. Tsubota, M. Haruta, ‘The effects of SO2 on the oxidation of CO2 and propane on supported Pt and Au catalysts ‘,Applied Catalysis B: Environmental 24 (2000) 36 M. Valden, S. Pak, X. Lai, and D.W. Goodman, ‘Structure sensitivity of CO oxidation over model Au/TiO2 catalysts’, catal. Lett. 56, 1998, p.7-10 37 E.D.L.Rienks, G.P.v.Berkel, J.W.Bakker, B.E.Nieuwenhuys, ‘The hidden properties of Au(100) surface’, CD-Rom Gold 2003 Conference 38 Y. Kobayashi ,S.Nasu , S. Tsubota and M. Haruta, ‘Au Mossbauer study of nano-sized gold catalysts supported on Mg(OH)2 and TiO2’, Hyperfine Interactions 126 (2000) 95–99


39 D. Horvath , L.Toth and L. Guczi, ‘Gold nanoparticles: effect of treatment on structure and catalytic activity of Au/Fe2O3 catalyst prepared by co-precipitation’, Catalysis Letters 67 (2000) 117–128

Catalysis by Gold

40 G. C. Bond and D. T. Thompson, Kinetics for CO oxidation on Au catalyst, Gold Bull., 2000, 33, 41 41 A.C. Gluhoi, M.A.P. Dekkers, B.E. Nieuwenhuys, “Comparative studies of the N2O/H2, N2O/CO, H2/O2 and CO/O2 reactions on supported gold catalysts: effect of the addition of various oxides”, Journal of Catalysis 219 (2003), 197-205 42 Dr Christopher, W. Corti, Dr Richard J. Holliday, international technology, world gold council: ‘Commercial aspects of gold applications: from materials science to chemical science’ 43 C.W. Corti and R.J. Holliday, ‘Commercial aspects of gold applications: from materials science to chemical science’, 44 Jens R. Rostrup-Nielsen, Ib Alstrup, "Innovation and science in the process industry: steam reforming and hydrogenolysis", Catalysis Today 53 (1999) 311-316 45 I. Chorkendorff, J.W. Niemantsverdriet, “Concepts of Modern Catalysis and Kinetics”, 1st edition (2003), 301-310 46 D. Vogt, H.C.L. Abbenhuis, C. Müller, “Chemistry of Catalytic Systems 1”, 1st edition (2003). 47 D.S. Cameron, ‘Fuel cell systems: new industrial applications for gold’, the Interact Consultancy, reading, England


Catalysis by Gold


catalysis by gold

Documents