GOLD NANOPARTICLES FOR CATALYST

 Few people look beyond gold's glitter and rarity, but chemists have found that its chemical properties are just as interesting, making it a unique catalyst for producing unusual organic molecules. UC Berkeley's Dean Toste, a leader in the area of gold catalysis, attributes these properties to relativistic effects in the gold atom, the same effects that give gold its yellow luster. Essentially, gold catalysts are 'hot' because their electrons are heavy, this chemist proposes.

Toste has found a mother lode of new and unique gold-catalyzed reactions by applying Einstein's theory of relativity to the rare and precious metal.Catalysts are metals that speed up chemical reactions, such as when the platinum in a car's catalytic converter instantly converts polluting engine exhaust to oxygen, nitrogen, carbon dioxide and water.Dean Toste, a UC Berkeley associate professor of chemistry, was one of the first chemists to experiment with gold as a catalyst. He opened the door for others interested in gold's versatility and in the potential to generate chemicals of interest for the chemical and pharmaceutical industry more efficiently and using less toxic precursors."This is a really hot area," Toste said. "If you look at the most-cited articles in the Journal of the American Chemical Society, many are about gold catalysis."With this class of gold catalysts, you can develop a number of unprecedented reactions that have never been seen before."Toste discusses the new field and proposes a new theory for why gold has such unusual, and practical, catalytic properties. So far, the hypothesis has successfully predicted the behavior of gold catalysts in new chemical reactions."Our hypothesis really allows us to approach catalysis in a new way, melding the two fields of theoretical chemistry and synthetic chemistry," Toste said.At the heart of his hypothesis is the special theory of relativity, proposed by Albert Einstein 102 years ago and typically thought of as applying only to cosmological questions. But late UC Berkeley chemist Kenneth Pitzer showed some 70 years ago that the theory comes into play in chemistry as well. Other researchers have used so-called relativistic quantum mechanics to explain gold's yellow color and why mercury is a liquid instead of a solid.Toste now takes this explanation a step further, crediting special relativity with making gold - and perhaps the related and widely used catalyst platinum - act as both an acceptor and a donor of electrons in a catalytic reaction. Typical metal catalysts do one or the other, but not both.One of the key tenets of relativity is that nothing can travel faster than the speed of light. The reason for this is that objects become heavier, or more massive, the faster they go, with the mass approaching infinity as the object approaches the speed of light.In an atom, where electrons race around the nucleus like buzzing bees, the velocity of an electron doesn't get anywhere near the speed of light until the atomic nucleus fills up with lots of positively charged protons - the negatively charged electrons have to move faster to keep from being pulled into the highly positive nucleus. This occurs in the transition metals of the periodic table of elements, metals ranging from tantalum and tungsten to platinum and gold. In a gold atom, with 79 protons in the nucleus, the 79 electrons whip around the nucleus at about half the speed of light.The net effect is that gold's electrons are much heavier and are pulled in closer to the nucleus, lowering the energy levels and making the atom more compact. According to this hypothesis,

gold's s shells, which are its lowest energy spherically symmetric electron shells, contract. This shields the electrons in outer, asymmetric p and d orbits from the nuclear charge, allowing them to expand slightly. In gold, the contraction of the outermost (6s) shell and the expansion of the next-inner (5p) shell reduces the energy difference between the two to the equivalent of a photon of blue light. This allows gold to absorb blue light and, thus, look yellow. Silver, because it exhibits a much less dramatic relativistic effect, is unable to absorb any visible light and is totally reflective.Toste proposes that this same shielding effect allows the more tightly bound s shell to easily accept electrons from other molecules, while the partly shielded d shell can easily donate electrons to a reaction.Thus, gold is able to participate in reactions both as a donor and as an acceptor of electrons, which makes it particularly useful in catalyzing reactions at carbon-carbon bonds, the backbone of all organic molecules. According to Toste, a gold atom can attach to carbon loosely, with a single bond or a double bond, allowing flexibility in reactions that can lead to novel organic molecules.Using this model, he has accurately predicted the products in various organic reactions. For example, a gold atom attached to the chemical phosphine and dispersed homogeneously in a liquid can efficiently convert alkynes to pyrroles, which are ring structures found in many drugs. Gold-phosphine catalysis also can create an unusual carbon triangle called cyclopropane that is used in industrial organic synthesis."We can make cyclopropanes without the need for explosive diazo compounds," Toste said.Toste predicts that gold catalysts also will be very useful in producing chemicals with a specific handedness, that is, a left-handed molecule, but not its right-handed or mirror image. Such stereoselective reactions are becoming more important because many drugs come in right- and left-handed forms, but only one form is effective in the body. The most efficient synthesis would produce only the effective form, not its ineffective mirror image. He is tuning the phospine attached to gold to affect this stereoselectivity."The future of gold catalysis still involves a lot of theoretical work, and we need to understand more about how it works," Toste said. "But already, some of these reactions are being used by medicinal chemists, and it's a really exciting field."

Gold Nanomaterials

A nanoparticle consists of a few atoms forming a cluster with size in the nanometer range. A nanometer is a millionth of a millimeter.Considerable attention has been focused duringthe last few decades on developing and optimising methods for the preparation of gold nanoparticles to size and shape e.g. spherical andnon-spherical (triangles and hexagons) particles,as applications of nano structured materials are dependent on these properties. The colour of ultrasmall gold spheres or clusters, also known as nanoparticles, has been known for centuries as the deep red ruby colour of stained glass windows in cathedrals and domestic glassware. The colour results from the plasmon resonances in the metal cluster. Most gold nanoparticles are produced via chemical routes. Biosynthesis, investigated by Project AuTEK, is an alternative route used to create gold nanoparticles with yeast and bacteria resulting in non-spherical particles. These particles have different properties to conventional spherical particles and could further increase the use of gold.

Gold nanoparticles are easily functionalised to further exploit their properties. The term monolayer protected clusters (MPCs) defines surface functionalisation of these nanoparticles by self-assembled monolayers. A cluster of gold atoms or a gold nano-particle can be stabilized by a monolayer of, for example, alkanethiolated or phosphine ligands. Gold MPCs provide commercial interest in materials science, biological science and various chemical platforms. One example where gold nanoproperties are second to none is in the area of medical diagnostics and therapies. Since gold nanoparticles have such good light-scattering properties, as well as easy functionalisation and biocompatibility, they are ideal for a wide rangeof biological and pharmaceutical applications such as bio-labeling and other types of diagnostics. Research conducted at the University of Liverpool in collaboration with Project AuTEK aims to improve these MPC gold nanoparticles even further, by designing stable, water soluble, biocompatible and functionalized nanoparticles.Gold is a critical component in certain therapies, more specifically, in the treatment of cancer by hyperthermia and thermoablation. These twotherapies use heat to kill cancer cells. In the case of hyperthermia, the cancerous tissue is heated to enhance conventional radiation andchemotherapy treatments, while in thermoablation the tissue is heated so that thecancer tissue is destroyed by the localised heat. There are two methods one can use to provide heating, infra-red absorption and the applicationof an oscillating magnetic field to magnetic nanoparticles. At the University of Rice, silica nanoparticles are being developed that are coated with gold nanolayers to form a gold shell around a glass core. By changing the thickness of the gold shell, one can control the wavelength of light that is absorbed by the gold/silicananoparticle. Tuned to capture infrared light and coated with cancer-specific antibodies, the nanoshell becomes a precision-guided cancer treatment and works as follows (see diagram).

Wide application of gold catalyst nanoparticles

(1) To diagnose and treat cancer, thousands of gold-coated nanoshells are injected into the patient’s bloodstream. Each gold-coated nanoshell is about 10,000 times smaller than a white blood cell. Inside the bloodstream, the nanoshells are taken up naturally by the tumour cells via antibodies stuck to their surface.(2) Each tumour is covered by approximately 20 nanoshells and a brief exposure to near-infrared light by a very simple handheld laser, which passes harmlessly through tissue and illuminates the shells. The doctor then delivers a more intense near-infrared dose resulting in heat generated at the tumour.(3) Free-floating electrons on the outer gold shells concentrate the intensified near-infrared energy, heating each individual nanoshell and burning the tumour cells without burning nearby tissue. The tumour tissue heats up to about 55 degrees Celsius. What makes this technique unique is that the wavelength of light required is in the nearinfrared, and body tissue is transparent to light ofthis wavelength. Therefore, this is an uncomplicated non-invasive technique of cancer therapy, and preliminary tests will begin next year on patients with soft-tissue tumours, like breast, brain or prostate cancer. The technique of using magnetic nanoparticles to destroy cancer tissue is already commerciallyavailable. One of the obstacles of using these magnetic nanoparticles is that the nanoparticles are not biocompatible. This means that the magnetic nanoparticles have to be injected directly into the cancerous tissue. Research at Los Alamos National Laboratory is focused on coating magnetic nanoparticles with thin layers of gold, since gold is biocompatible and easily functionalised for biological markers. This will allow easier transportation of the magnetic nanoparticles to the cancerous sites, such as orally.

The color of nanoparticlesThe color known as "Purple of Cassius" in glass and glass enamel is created by incorporating a colloidal suspension of gold nanoparticles, a technology in use since ancient times. Colloidal silver is yellow, and alloys of gold and silver create shades of purple-red and pink.Nanoshells are a recent product from the field of nanotechnology. A dielectric core is coated with metal, and a plasmon resonance mechanism creates color, the wavelength depending on the ratio of coating thickness to core size. For gold, a purple color gives way to greens and blues as the coating shell is made thinner. In the future, jewelry applications may include other precious metals, such as platinum.

Dispersions of discrete gold nanoparticles in transparent media provide a fascinating range of

colors, only recently exploited in the manufacture of paints and coatings. The shape of the

particles and the viewing conditions determine the color we see. The gold particles in the test

tubes on the left are shown in transmitted light, while the image on the right shows the same gold

nanoparticles viewed in reflected light.

The diameter of gold nanoparticles determines the wavelengths of light absorbed. The colors in

this diagram illustrate this effect.

Different sized quantum dot nanoparticles are shown above, first in ultraviolet light and then in

ambient light. The length of the synthesis reaction determines particle size for CdSe, increasing

from left to right. In colloidal suspension, this semiconductor behaves in the same way as a

metal.

GOLD CATALYSIS

Golden nanotechnology + sunlight = greener chemistry

Since the SPR is a local effect, limited to the noble metal particles, the particles – which generally account for only 2-4 wt% of the overall catalyst mass – are heated up quickly to a temperature at which the organic molecules are activated to react with oxygen. Compared to the conventional catalytic oxidation in which both the gold particles and the support material are heated to high temperatures, this leads to significant saving in energy consumption for catalyzing organic

compound oxidation.The scientists found that a number of factors, such as particle size and morphology, and the dielectric constant of the oxide medium, can affect the SPR absorption of gold nanoparticles and thus the activity of the gold catalyst. “Our findings open up a new direction in photocatalysis as the proposed reaction mechanism is distinctly different from that catalyzed by semiconductor photocatalysts" says Zhu. "Based on our results, we propose a tentative reaction mechanism for the light-driven catalytic oxidation. The irradiation of incident light with a wavelength in the range of the SPR band may result in two consequences. The first is that light absorption by the gold nanoparticles could heat these nanoparticles up quickly. The second consequence is that the interaction between the oscillating local electromagnetic fields and polar molecules also assists in activating the molecules. The activated polar organic molecules react with oxygen in close proximity."

Zhu points out that this light-driven reaction can proceed at ambient temperature at reaction rate similar to those of the catalytic oxidation under heating and could, for example, find convenient applications in indoor air purification. The finding also reveal the possibility to drive other reactions with abundant sunlight on gold nanoparticles at ambient temperature.

. Au nanoparticle-supported catalytic organic reactions

. It was found that the catalyst supported on the Au nanoparticle surfaces exhibited almost the same catalytic activity with that of the unbound Grubbs Ru catalyst.

 An Au nanoparticle-supported Ru catalyst for promoting ring-opening metathesispolymerizationofnorborene.

An Au MPC-bound Ru-carbene catalyst employed for catalyzing ring-closing olefin metathesis.

The use of Au MPC-supported chiral catalysts for asymmetric induction reactions was also reported. Mrksich's laboratory has investigated the employment of a dihydroquinidine catalyst immobilized on Au nanoparticles for the sharpless asymmetric dihydroxylation of olefins . Three substrates were examined and an average yield of 80% with an e.e. up to 90% was found. The dihydrolation of trans-stilbene and methyl trans-cinnamate under the Au catalyst was compared favorably in terms of yields and selectivities against those obtained using-polymer supported counterparts. The catalyst could be isolated and recovered by gel permeation chromatography. The recycled catalyst was found to promote the subsequent rounds of the dihydroxylation of trans-stilbene, however, with a modest loss in reaction yields and e.e. values of the reactions.

 A dihydroquinidine catalyst immobilized on the surface of an Au nanocluster for the sharpless asymmetric dihydroxylation of olefins.

Another type of catalytic asymmetric reactions that have been examined under Au nanoparticle-supported chiral catalysts is the asymmetric alkylation of benzaldehyde with Et2Zn .(R)-1,1'-Bi-2-naphthol ((R)-BINOL) moieties were immobilized on the surface of Au clusters and such nano-composites were examined for catalyzing the asymmetric Et2Zn alkylation of benzaldehyde by forming the Ti-BINOLate complex to afford the adduct in up to 98% yield with 86% e.e. Variations of the length of the ether linking the chiral ligand and the surface of an Au MPC host led to slight changes of the reaction yields, but more significant variations of the e.e. values. In addition, Sasai's group has also found that the MPC was readily recovered from the reaction mixture and showed stability under acidic conditions. Similar catalysts immobilized on the surface of magnetic nanoparticles were also synthesized and investigated for catalyzing asymmetric alkylation reactions in our group .Unfortunately, lower conversion yields combined with moderate e.e. values were found. This might be due to the effects from the surface structure differences between the Au and iron oxide nanoparticles.

  CATALYTIC PROPERTIES OF GOLDIn general, the widely used catalysts based on the platinum group metals are produced by impregnating a porous ceramic substrate with a solution of metal salt, followed by drying, and then thermal decomposition toproduce a dispersion of metallic particles loaded onto a meso-porous oxide or other support. In addition, optimum use of such catalysts generally requires some form of chemical reduction of the metallic particle immediately prior to use. As a general observation, attempts to prepare active gold-based catalysts by this route produce indifferent results .For example, Bamwenda et al reported that Au/TiO2 prepared by impregnation had very low CO conversion compared with catalyst prepared by depositionprecipitation. It is this idiosyncrasy of gold-based catalysts that more than any other probably explains why they have been disregarded for so long. However, a modified impregnation method using hydroxide as support and gold phosphine complexes as precursor compounds has resulted in catalysts with activities similar to those prepared by co-precipitation .showing that there is no a priori reason why active gold catalysts cannot be prepared by impregnation. It has become abundantly evident that gold only becomes catalytically active in very particular instances. However, there appear to be more than one means to achieve this special state. The common denominator that links successful preparation routes from the others still not absolutely established, despite some systematic studies, such as the recent one by Wolf and Schüth .and there are still manyinteresting disagreements in the literature .However, one fact that is agreed is that active gold

catalysts may be reliably produced by attaching nanoparticles of gold to the surface of particular transition metal oxides by co-precipitating the transition metal and the gold (possibly in the form of thermally unstable hydroxides or carbonates) followed by a calcination treatment, or by deposition-precipitating the Au(OH)3 onto a previously synthesised oxide substrate, followed by calcination treatment. Details of the preparation techniques for successful catalysts vary widely between laboratories, with the effects of aging, stirring, washing, order in which reactants are added,temperature, concentration of reactants, and calcining conditions, for example, all evidently important, but still with such poor consistency and reproducibility that a completely convincing overarching hypotheses has yet to be formulated .However, even the above paradigm is evidently not completely inclusive of all gold heterogeneous catalysts made by aqueous precipitation, and some workers have found that thebest results are actually obtained for mixtures of Au (OH)3 nanoparticles on transition metal hydroxides

Oxidation of carbon monoxide

Gold-based catalysts have been shown to have the best low-temperature activity for CO oxidation of all catalysts and, at low temperatures, exceed the activity of catalysts based upon platinum group metalsby about a factor of five for this reaction .This reaction is of great interest in terms of the purifying of indoor and in-vehicle air, and a large number of studies have been conducted .In most of these studies the gold nano-particles, generally of between 2 and 10 nm in size, have been supported on metal oxides of various sorts. There is a degree of disagreement on details, but the broad consensus is that reducible oxides of variable stoichiometry such as MnOx, TiOx, FeOx and CoOx somehow contribute significantly to the catalytic activity of the gold, whereas oxides of fixed stoichiometry, for example Al2O3, do not Goldon Fe2O3 is active for this reaction at temperatures as low as 197K (-76°C), a fact which helped to alert the wider scientific community to the unusual properties of gold nano-particles. Excellent and sustained activity is also obtained when catalytic gold nano-particles arecombined with the already catalytic mixed MnOx-CuOy oxide (‘hopcalite’) .The selective oxidation of CO in H2-rich streams is also being studied by several groups, because of this reaction’s probable importance in future PEM fuel cell systems for motor vehicles. In these systems it is expected that the H2 needed for the fuel cell will be obtained by reforming a hydrocarbon fuel in an onboard reformer system. Unfortunately, a small percentage of CO is inevitable in the resulting reformed gas and this species is a potent poison of thePGM anode catalysts of the fuel cell. Therefore, CO must be somehow removed from the gas stream and selective oxidation over a gold-based catalyst is one of the possibilities .

Oxidation or partial oxidation of alkanes and alkenesGold-based catalysts are capable of catalysing the oxidation of methane (CH4), propane (C3H8) and propene (C3H6). For example, Au on Co3O4 is able to match the activity of commercial catalysts based onPd/Al2O3 for alkanes, but is not as effective for alkenes (see Thompson [9] for a review of these phenomena). On the other hand, it has been claimed that gold catalysts are uniquely selective for the partial oxidation of propene to propene oxide [6]. For example, Au/TiO2in a feed consisting of 10% propene, 10% oxygen, 10% hydrogen and 70% Ar, gives 99%

selectivity to propene oxide with 1.1% propene conversion [34]. Curiously, the nature of the reactions catalysed by goldappears to depend on the particle size. For example, it has been reported that particles above 2 nm in size facilitate the oxidation to propene oxide, whereas when they are below 2 nm in size they cause its reduction to propane [35].

INTRINSIC PROPERTIES OF GOLD NANO-PARTICLES

Crystal structure and morphology

In the bulk form gold has the face centred cubic (fcc) structure which is closest-packed. Naturally-occurring macro-crystals of native gold exhibit the highly symmetrical cubic, octahedral or rhombododecahedral crystal forms associated with this crystal structure [31]. However, there is debate and even confusion in the literature regarding the structure and external form of nano-particles of gold, arising no doubt from thedifficulties inherent in trying to physically characterize such tiny objects. As a result, several attempts have been made to predict the structure of gold nanoparticles using molecular dynamics or other calculations. However, there is not yet complete agreement between the results of the calculations and of the experimental measurements, with conflictingclaims being made for icosahedral or decahedral quasi crystal structures, amorphous structures, or octahedra, cuboctahedra and truncated octahedra based on fcc packing. The icosahedral and decahedral clusters have five-fold symmetry, and for nano-scale particles the icosahedron in particular appears to yield an efficient compromise between surface area and packing density, whereas certain decahedra (an example of which is the Marks decahedron) have re-entrant facets, and are said to offer a compromise between surface area and strain energy [32]. Icosahedral clusters have a theoretical packing density of 68.8% in contrast to the 74.0% of the fcc structure [33]. Examples of the three shapes mentionedare given in Figure 1. Perfect icosahedral shapes are attained only for certain special numbers (‘magic numbers’) of atoms, a series, which starts 13, 55, 147, 309, 561, 923 … atoms [33,34]. However, other ‘stable’ non-crystalline configurations besides icosahedra are possible, and they include low symmetry clusters based on 38 to 55 atoms [35,36]and Marks decahedra, based on 75, 77, 101 and 146 atoms [32,37]. Some groups, e.g. Cleveland et al. [32] have grouped all of the above non-crystallographic structures together as ‘decahedral’. However, there is a continuum of possible atomic arrangements thatextends from these ‘icosahedral’ or ‘decahedral’ structures through to ones of such low symmetry as to be effectively amorphous (e.g. refs. [35,36,38]). Some theoretical studies have concluded that it is these,practically amorphous, clusters that are most stable [36]. Most authors agree that the energies of all these configurations are so similar that they become very hard to differentiate [35,36,38]. Of course, the resultsof the calculations depend on assumptions regarding the nature of the inter-atomic interactions, e.g. their range [38] and how they are modified for surface atoms [36], and some use empirical parameters derived fromfitting models to bulk gold , so the results should be treated with some caution. For example, it is apparently the models based on pair potentials that tend to predict that Au38 will be a truncated octahedron,

whereas those using the more ‘metallic’ n-body potentials will predict that this will be amorphous . For 35<n<500 at least, it appears that various truncated octahedra should be more stable than icosahedra andcuboctahedra [32], although their stability would nevertheless not quite match that of Marks decahedra based on Au75, Au101 and Au146, which would tend to dominate any actual experimental particle dispersion [32]. Perfect cuboctahedral clusters with fcc packing can beassembled from the same series of magic numbers as for the icosahedra [33], and there are other perfect fcc clusters with octahedral or truncated octahedral crystal forms but different special numbers. These latterclusters include [35,39] a version of Au38, and Au116, Au140, Au225, Au314 and Au459.

Comparison of (a) truncated octahedron, (b) icosahedron, (c) Marks decahedron and (d) cuboctahedron.Note the re-entrant facets in the truncated octahedron and the Marks decahedron. ((a) and (c) redrawn from Doyeb and d redrawn from Mackay

A predicted atomic configuration of the gold-oxide chains (gold: gray, oxygen: pink) at the boundary of a monolayer-thick gold cluster (gold: yellow), supported by a thin magnesium oxide (magnesium: green, oxygen: red) on silver. The numbers indicate atomic charges in units of electron chargeA predicted atomic configuration of the gold-oxide chains (gold: gray, oxygen: pink) at the boundary of a monolayer-thick gold cluster (gold: yellow), supported by a thin magnesium oxide (magnesium: green, oxygen: red) on silver. The numbers indicate atomic charges in units of electron

charge

MECHANISMS OF CATALYSIS

Active-perimeter models

The pioneering work by M. Haruta showed that activity in gold catalysts could be associated with hemispherical particles of Au0 with diameters of less than about 5 nm, which are attached to oxide supports.He, and several subsequent workers have concluded that it was the perimeter of the hemisphere which was the active site, at least for the CO oxidation reaction. This explanation is currently the mostgenerally accepted one in the literature for catalysis by gold nano-particles, although there are differences on details. For the sake of a simplified analysis let the catalyst particles be represented by a hemisphere of radius r, attached to a substrate. If the conversion of reactant is proportional to the length of the perimeter, then it canbe shown that the conversion, In this model, the activity of the perimeter atoms of the gold cluster may be in a special state controlled by electronic interactions with the oxide substrate. In general, one of the reacting species, e.g. O2 would adsorb either at the perimeter site itself or on the oxide support, and the other, e.g. CO, on the gold nanoparticle,with the reaction between the two species occurring at the perimeter to produce CO2. Either way, the nature of the support would control the reaction of O2 and CO along the perimeter. There is certainlyconsiderable evidence in support of the claim that the chemical identity of the support is important, and for example it has been shown that different activities are obtained for different supports, even if the Au particle size is unchanged [47]. However, for activity to occur,the gold nano-particles must be properly attached to the support too. There is also evidence to support this and, for example, the beneficial effect of calcination in converting inert colloidal Au plus TiO2 composites into active Au/TiO2 catalysts has been interpreted in termsof the interaction of the discrete colloidal particles with the substrate during calcination, and the associated formation of an attachment perimeter [36]. Conversely, Grisel et al. [51], have shown how the addition of a little MnOx to an Au/Al2O3 system was able to stimulate a significant improvement in CO oxidation, allegedly by changing the properties of the perimeterwithout changing the Au particle size or characteristics. As mentioned, there is disagreement amongst the supporters of this model concerning whether the O2 is adsorbed on the perimeter itself or the nearby oxidesupport. Even if the O2 is adsorbed onto the oxide support, the perimeter might still enjoy a special role. Bond &for example, have proposed that the gold nano-particle is attached to the oxide support via an intermediate layer of Aux+ atoms, and the oxidized state of these atoms is essential for catalytic ability.Supporting evidence has come from Mössbauer studies, which have shown that the most active catalysts (in metal hydroxide-supported systems at least) have the highest fraction of Au+, while EXAFS analysis of Au/γ-Al2O3 prepared by depositing AuCl3

onto γ-Al2O3 indicated that activity for CO oxidation requires a polymeric Au(OH)3 precipitate [52]. In this case the presence of Au-(O)-Au was beneficial while that of Au-(O)-Al was not. On the other hand, studies by Haruta’s group found that Au+ played no role in the activity of Au/TiO2, with maximum activity only occurring once all Aux+ had been reduced to Au0 by calcination [56]. In yet anothercomplication, the beneficial role of humidity has been explained [38] as being the result of hydroxyl ion adsorbed along the perimeter, and serving there as an intermediary in the reaction between O2 and CO2. If the active-perimeter model were true then the crystal structure of the catalytic gold particles should not be particularly important. The fact that both icosahedral and cuboctahedral particle are catalytically active (butperhaps on different supports) has been advanced as evidence in support of the active-perimeter model .

Active-particle (‘electronic’) model

The oxidation of CO on a heterogeneous gold catalyst evidently requires that at least one molecule each of carbon monoxide and oxygen come into contact with one another on the surface of the gold nano-particle.Both gases have a negligible affinity for bulk gold surfaces , at east for temperatures above 150K thereby explaining the absence of catalytic activity for this reaction on bulk gold. However, very smallnano-particles are obviously different and direct evidence that both oxygen and carbon monoxide can adsorb on neutral or negatively-charged gold nano-particles is available. In particular, Whetten andco-workers at Georgia Institute of Technology have showed [46] that O2 bonds readily with Aun - (provided n is even and n ≠ 16), with the oxygen attaching as a superoxide (O2 -). No oxide support was involved in these experiments, and catalytic activity for COoxidation was shown by a naked gold particle suspended in a gas stream. This has led to an alternate hypothesis to explain catalysis by gold nano-particles, in terms of which the catalytic activity occurs on thesurface of a cluster, and arises from a special electronic configuration of it. In this mechanism the role of the support is simply to modulate the electronic structure of the cluster.One issue to consider is whether it is, for example, both O2 and CO that are co-adsorbed onto the catalyst system, where they subsequently react (the Langmuir- Hinshelwood reaction pathway [45]) or whether onlyone of the reacting species adsorbs, with the other coming into contact with it by kinetic collision from the gas phase (the Eley-Rideal reaction pathway [45]). However, current opinion is for the former [47]. Interestingly, it has been shown that catalytically activegold results if Au clusters are deposited onto F-centre (oxygen-vacancy) sites on MgO but not if the F-centres were absent [42,45]. This observation may be interpreted as support for either the active-perimeter or the active- particle hypotheses. If we accept the activeparticlemodel then we would suppose that the excess electron density at the F-centre site is transferred to the cluster and onward to the adsorbed O2 to form the superoxide needed for the catalytic reaction. Further indirect evidence in support of this may be drawn from the work of Valden et al [45] who observed that the catalytic activity of the gold was reduced or absent if it was placed onto a stoichiometric (i.e. fully oxidised) surface of TiO2. Such a fully oxidised surface will have

very few F-centres and therefore, the gold clusters on it would not be negatively charged and hence not able to react with O2 to form superoxide. Of course, supporters of the active-perimeter model, who included Valden et al., would say that the difference is that an oxidewithout F-centres is not able to adsorb O2 and therefore unable to supply it to the active perimeter sites. In this view, it is the supply of adsorbed O2 - that is rate limiting, and the reaction can be considerably expedited if O2 - can be supplied from the substrate rather than on the nano-particle itself .In a completely opposite view of the issue, Japanese workers26 have claimed that gold nano-particles of < 2nm diameter become catalytically active by transferring electronic charge to the TiO2 substrate. The Aun δ+ clusters then apparently acquire the ability to dissociate H2, for example. The adsorption of O2 onto clusters can also be correlated to the electronic configuration of the cluster, which was discussed earlier. In this case, charge andcrystal structure is apparently not as important, and what counts is that there should be a single electron in the valence shell of the cluster. This hypothesis has been supported by detailed ab initio calculations [44]. The electronic configurations of Au18 - and Au20 -, both of which react strongly with O2, and which contain 19 and 21 electrons respectively, may be written in termsof the simple shell model introduced earlier as(1s21p61d10)2s1 and (1s21p61d10)2s21f1 respectively [34].By donating an electron to O2 to form O2 the cluster’s electronic configuration assumes a closed shell form with an associated energetic advantage. According to this hypothesis Au19 0 and Au21 0 would also be reactive since they ought to have the same shell electronic configuration with an unpaired electron. Unfortunately the available experimental data and ab initiocalculations for neutral clusters are in conflict, with calculations supporting this contention [44] andmeasurements in conflict with it [42]. However, note that O2 should in principle bond more strongly with negatively charged clusters than to neutral ones with the same number of electrons. The reason for this isthat the ionisation potential of a negatively charged cluster should be lower than that of a neutral one. Heiz et al. [42] observed that Au8 was far more reactive than Au13, an observation ostensibly at oddswith the spherical shell model described previously, until it is noted that they only obtained activity for Au clusters when they were present on defect-rich MgO substrates. Once again, one possible explanation for thediscrepancy is that the extra electron density from the F-centres of such substrates was taken up into the cluster to provide a reactive Au8- cluster with a (1s21p6)2s1 configuration and an unpaired electron. If indeed only one oxygen molecule can be absorbed per cluster at a time, then the acute size sensitivity of the catalytic activity of gold nano-particles can be readily explained. After dividing the number ofparticles with their mass, and simplifying, the activitycan be shown to beC(reaction) is inversely proportional to (R).The issue of whether the adsorption of the O2 on gold is eventually dissociative or not also arises, with some authors7 claiming the adsorbed O2 goes on to form monatomic oxygen while others [45] disagree with this on the basis [32] that the formation of two oxideions necessarily involves four electrons, a process considerably more complex and less probable on goldthan the simple one electron scheme required to produce O2 -. However, the four electron process is known to occur with more active metal clusters, such as those of copper or sodium [45]. Certainly,

calculations show that, the stronger the bond between O2 - and Aun, the greater the O-O distance in the superoxide [44], but this apparently does not occur to a sufficient extent in gold to dissociate the bond. It may be the unique characteristic of gold nano-clusters to form superoxide adsorbates rather than disassociated oxides that defines its unusual catalytic activity .In an alternative but nearly equivalent view, it is the low bond strengths between Au and the reacting species that makes gold such an active catalystn [31].The geometric modelsIf the surface of a material (or some site on the surface of the material) has a particular ability to catalyse a chemical reaction, then it follows that increasing the specific surface area (expressed as m2/g) of the material will increase the activity of the catalyst. In this schemethe chemical identity of the catalyst support is completely unimportant; it exists solely to pin the catalytic particles and prevent them from sintering together. After dividing the surface area of a particle by its mass, it can be shown that C(reaction) is inversely proportional to (r).The efficacy of the catalyst in terms of the three models is plotted inWhen plotted in a linear scale (Figure 4a), it is evident that all three models produce a sharp increase in activity with decrease in particle size. Greater differentiation between the various models is visiblewhen they are plotted on a logarithmic scale (Figure 4b).Much of the increase in catalytic activity of other metals and materials may be explained entirely or substantially by the above geometric argument. However, in the case of gold, the explanation iscontroversial. This is because bulk gold apparently has no catalytic activity at all for reactions such as CO oxidation and, as mentioned, it has scarcely any ability to absorb O2 or CO (although the very activemonatomic O will attach to it [31]). This is quite unlike the case [31] for Pt, upon which CO will readily absorb and which can split the O-O bond in O2.

Dependence of catalyst activity on diameter of hemispherical catalyst particles, as a function of three different mechanisms. The curves have been normalised to a typical activity of 2x10-5 moles CO/second/g Au at a particle size of 5 nm, (a)linear, (b) logarithmic.

DISCUSSION

Future prospects for gold catalysts

Gold has been demonstrated to be the best catalyst for at least three reactions of industrial importance so far. These are the hydrochlorination of ethyne (acetylene) to make vinyl chloride monomer, a possibility that was actively pursued [55] in the late 80s, the low temperature oxidation of carbon monoxide, and the production of vinyl acetate monomer from acetic acid, ethylene and oxygen. Commercial exploitation of the first reaction would in some instances displace a troublesome and toxic catalyst containing mercuric chloride but would of necessity need to compete with other means of producing vinyl chloride. However,world production of vinyl chloride continues to grow at about 5% despite concerns about its toxicity, carcinogenicity and possible effect on the ozone layer, and there appears to be some merit in reconsideringgold for this role. With regard to the third reaction, gold-palladium catalysts were very successfully

applied in 2001 at a new plant to make vinyl acetate monomer in the UK and the result is said to a saving of30% in capital costs [56]. The opportunities to use Au-based catalysts for COremoval appear to arise from three factors. The first is the extraordinary activity of these catalysts at room temperature and below. This offers the prospect of removing low concentrations of CO, e.g. from the air conditioning systems of buildings or vehicles, withoutany need to heat the catalyst bed using external means. This property might also be profitably employed to remove the first CO produced from a motor vehicle engine when it is started. Conventional catalysts require some time to reach their ‘light off temperatures, and the quantity of CO (and hydrocarbon fumes) emitted to the environment during this warm-up period is significant. Use of gold-based catalysts would allow for catalytic removal of CO from the instant the engine is started. The second property of interest is that the activity of some conventional catalyst systems, e.g. those based on hopcalite (MnOx.CuOy), is significantly prolonged by the incorporation of gold nano-particles.Ordinary hopcalite, which is used in respirators such as might for example be used by fire-fighting personal, deactivates after only an hour or so of use, which is obviously a critical safety issue. However, asmentioned previously, Au/MnOx.CuOy catalysts retain their activity for days at a time. Finally, gold-based catalysts appear not to be deleteriously effected by humidity, and in fact appear in most cases to beenhanced by it This is unlike most of the existing competing catalyst systems. However, the low melting point of gold, and especiallyof gold nano-particles, is a matter for concern, given that the exhaust gas streams of petrol engines, for example, can readily reach 800°C. Clearly, any goldbased catalyst used in this application must be protected from being overheated, for example by cooling it with cold air, or by the action of a valve that diverts the hot exhaust away from the catalyst once the normal operating temperature of the exhaust has beenreached. Prospects for gold-based catalysts in remediating the fumes from diesel engines is better, not only because they operate at lower temperatures and generate exhaust gas streams of a nominal 500°C, but also because the legislation and technology for removing pollutants from diesel exhausts is still in its infancy, thereby giving gold-catalysts a chance to get a foothold. In this respect it is the CO-oxidising powersof gold at low temperatures, its ability to catalyse the reduction of NO2, e.g. with entrained propene, and its tolerance to H2O, that commends its use. Another possible growth area for gold-based catalysts is in the production of hydrogen peroxide. Production of H2O2 is enjoying strong growth due the fact that it is now considered a more eco-friendly disinfectant and oxidant than the halogens. It has been claimed thatAu might form the basis for an excellent catalyst in this system due to its reluctance to dissociate O2. However, once again it would have to compete with established processes, for example the one using alkylhydroanthraquinone.

CONCLUSIONS

Gold-based catalysts have some unusual and useful features, in particular their high activity at comparatively low temperatures and their tolerance to humidity. In general, gold’s efficacy as a

catalyst isbelieved to be the result of its very low chemical reactivity which ensures that adsorbed reacting species are for the most part only weakly bound. The special, low reactivity of gold follows from its rather inert bulk properties, so that catalytically useful surfaces are mostreadily achieved only when the gold is exposed as nano-particles or nano-porous surfaces. The explanation for the special activity of the gold in these states is less well established, and the actual mechanism of thecatalysed reactions not known with certainty. It was shown that few of the proposed mechanisms are able to explain all aspects of catalysis by gold, and the only hypothesis which is overarching is that the catalyticactivity is the result of some special site on the gold, the density of which is highly sensitive to material properties, and the effect of which is amplified in a geometric fashion by expression in materials of veryhigh specific surface areas. In this explanation, the role of the oxide support and of gold particle size and structure is indirect, and they would serve mainly to modulate the specific surface area of the gold, and the electronic configuration of its special surface sites.

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