The surface chemistry of atomic oxygen pre-covered gold
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Gold used to be regarded as catalytically inert until about 20 years ago when it was shown that supported gold clusters < 5 nm in diameter exhibited some unique catalytic properties. Based on this revelation, several studies have demonstrated the feasibility of reactions previously thought of as impossible on gold. The ability of gold to oxidize CO below ambient temperatures at rates higher than conventional CO oxidation catalysts (Pd and Pt) has been shown to hold potentials for technological applications. Extensive past and on-going research are geared towards elucidating the mechanistic details of this reaction. The nature of the active sites, the effect of the supports and the effect of moisture are still debated in literature. I therefore present some experimental results supported with density functional theory calculations to shed additional light on some of the issues concerning gold catalysis in general, and low temperature CO oxidation in particular. Previous studies of the effect of moisture on oxide-supported gold reported that although water promotes CO oxidation on this surface by as much as two orders of magnitude, it is only a spectator molecule on the surface. I present here evidence for strong water-oxygen interactions when water is co-adsorbed with atomic oxygen on Au(111). Impinging a CO beam on the surface co-adsorbed with oxygen and water produces water-enhanced CO oxidation. Based on these results, I propose that CO reacts with hydroxyls formed from water-oxygen interactions to form CO₂, similar to a previous observation on Pt(111). Exposing a Au(111) surface pre-covered with ¹⁶O to isotopically labeled carbon dioxide (C¹⁸O₂) showed that ¹⁶O¹⁸O (m/e = 34) was produced from carbonate formation and decomposition. Estimates of reaction probability and activation energy gave ~ 10⁻⁴ - 10⁻⁵ and -0.15 eV respectively. The effect of annealing on the reactivity of oxygen pre-covered Au(111) was investigated using water, carbon monoxide and carbon dioxide as probe molecules. Precovering Au(111) with atomic oxygen followed by annealing resulted in surfaces that were less reactive towards water, CO and CO₂. Annealing is believed to stabilize the reactive metastable oxygen thereby increasing the barrier to reaction similar to what is reported on other surfaces.