Gold catalysts have superior activity in CO and other oxidations at low temperatures. Both a small (~5nm) particle size and the presence of a partly reducible oxide (ceria or a transition metal oxide) have a beneficial effect on the catalyst performance. The present paper reviews our recent studies focused on understanding the specific role of the Au particle size and that of the oxide (MO). Our personal viewpoint on gold catalysis is outlined. The effects of Au particle size and of the oxidic additive are distinguished by using several alumina-supported gold catalysts having different gold particle sizes and various oxidic additives. The most active catalyst in CO oxidation is the multicomponent catalyst Au/MgO/MnOx/Al2O3 with MgO being a stabilizer for the Au particle size and MnOx being the cocatalyst. This catalyst also exhibits good performance in selective oxidation of CO in a hydrogen atmosphere, a reaction relevant for the development of polymer electrolyte fuel cell technology.In its bulk form, gold has been regarded to be chemically inert towards chemisorption of reactive molecules such as oxygen and hydrogen. Consequently, pure gold was considered to be an uninteresting metal from the point of view of catalysis. The most noticeable exception to this was its use as a 'diluent' for an active metal: the addition of the inert gold to an active metal such as platinum affects to a significant extent the selectivity of the catalyst (1). However, recently, gold catalysts have attracted a dramatic growth of interest, since gold was reported to be extremely active in the oxidation of carbon monoxide if deposited as nanoparticles on partly reducible oxides. It is in particular the pioneering work of Haruta et al, which has stimulated research in this area (2). Early work performed with gold catalysts has been reviewed by Bond (3) and by Hutchings (4). For recent reviews on gold catalysis see references 2 and 5 -7.A large range of chemical reactions are now known to be catalysed by gold catalysts including total and selective oxidation, and reduction of nitrogen oxides. Based on the growth of the number of papers and patents dealing with gold-based catalysts for a range of potential applications in pollution control, chemical processes and development of fuel cells, it can be concluded that there may be a bright future for gold-based catalysis.In the present paper we discuss some of our research on catalysis by gold (8 -14), and in particular, the following topics will be discussed: a) the effect of the gold particle size and the role of the oxidic additive; b) the selective oxidation of CO in the presence of hydrogen, a reaction relevant to hydrogen fuel cell applications. ExperimentalAll the catalysts discussed in this paper are supported on γ-alumina, the gold loading is 5.0 wt%. The gold catalysts were prepared by homogeneous deposition precipitation using urea as precipitating agent. The advantage of the use of alumina as support is the high stability of the catalysts up to relatively high temperatures (10, ...

The interaction of hydrogen with the Pt(110)-(1×2) surface is studied using temperature programmed desorption (TPD) measurements and density functional theory (DFT) calculations. The ridges in this surface resemble edges between micro-facets of Pt nano-particle catalysts used for hydrogen evolution (HER) and hydrogen oxidation reactions (HOR). The binding energy and activation energy for desorption are found to depend strongly on hydrogen coverage. At low coverage, the strongest binding sites are found to be the low coordination bridge sites at the edge and this is shown to agree well with the He-atom interaction and work function change which have been reported previously. At higher hydrogen coverage, the higher coordination sites on the micro-facet and in the trough get populated. The simulated TPD spectra based on the DFT results are in close agreement with our experimental spectra and provide microscopic interpretation of the three measured peaks. The lowest temperature peak obtained from the surface with highest hydrogen coverage does not correspond to desorption directly from the weakest binding sites, the trough sites, but is due to desorption from the ridge sites, followed by subsequent, thermally activated rearrangement of the H-adatoms. The reason is low catalytic activity of the Pt-atoms at the trough sites and large reduction in the binding energy at the ridge sites at high coverage. The intermediate temperature peak corresponds to desorption from the micro-facet. The highest temperature peak again corresponds to desorption from the ridge sites, giving rise to a re-entrant mechanism for the thermal desorption.

In this work, we study the relatively weak H 2 O− Au interaction on the highly stepped and anisotropic (310) surface with temperature-programmed desorption and X-ray photoelectron spectroscopy. Compared to Au(111), we report an enhanced adsorption energy of H 2 O−Au(310) as observed from the (sub)monolayer desorption peak. This peak shows zero-order desorption kinetics, which we do not explain with a typical two-phase coexistence model but rather by desorption from the ends of one-dimensional structures. These could cover both the steps and (part of) the terraces. We do not observe crystallization of ice clusters as observed on Au(111). This leads to the conclusion that this stepped surface forms a hydrophilic template for H 2 O adsorption. We also notice that the precise orientation of the steps determines the H 2 O binding strength. Despite the surface's enhanced H 2 O interaction, we do not observe any significant H 2 O dissociation. This indicates that the presence of low-coordinated Au atoms is not enough to explain the role of H 2 O in Au catalysis.

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The catalytic oxidation of CO is studied at low-coordinated Au atoms using a single-crystal approach. Electron irradiation activates an otherwise unreactive overlayer of undissociated D2O on Au(310). A low-coverage D2O/O mixture is subsequently allowed to react at surface temperatures from 105 K upward, with CO supplied from the gas phase. X-ray photoelectron spectroscopy shows the absence of Au oxides and quantifies various O-containing species during the reaction. The dependency of the reaction rate on the surface temperature yields an activation energy for the Langmuir–Hinshelwood reaction of O­(ads) and CO­(ads) between 26 ± 4 and 42 ± 5 kJ/mol. The presented results provide evidence that O­(ads) and not OH­(ads) is the active reactant on small Au nanoparticles. In addition, the observations suggest that water has a negative effect on the reactivity of O­(ads).

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