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, ...

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