Molecular beam studies of low temperature CO oxidation on gold

Gold is considered as noble among other metals because of its resistance to oxidation and corrosion. It is the most electronegative metal and its electron affinity is actually greater than that of oxygen. For this reason gold will not react directly with other electronegative elements such as molecular oxygen. As a result, gold has not been given much attention as a potential active ingredient for heterogeneous catalysis until it was discovered that gold particles that are 2-5 nm in diameter have exceptional catalytic activity towards many reactions. Among these reactions, low temperature CO oxidation is one of the most unique regarding gold catalysts in that it cannot be matched by other metals. Although it is widely accepted that gold particles which are 2-5 nm in diameter exhibit the greatest activity in CO oxidation, there is still much debate on the nature of the active sites for these catalysts and also the details of the reaction mechanism. Using molecular beams in conjunction with a radio frequency generated plasma jet, I have studied CO oxidation with atomically adsorbed oxygen on Au/TiO2 and Au(111). It is shown that CO reacts readily with pre-adsorbed oxygen atoms on a Au/TiO2 planar model catalyst and on Au(111) to produce CO2 even at temperatures as low as 77 K. The results presented show that gold particle size seems to have little effect on CO oxidation when oxygen adatoms are pre-adsorbed. This suggests that if reactive oxygen is primarily supplied through dissociation of oxygen molecules on the surface, the rate-limiting step in CO oxidation over gold is likely to be the dissociation of molecular oxygen. Another notable aspect of low temperature CO oxidation is that the addition of water in the feed stream is believed to enhance the reactivity by as much as two orders of magnitude. Here, evidence is shown that water can participate in CO oxidation on Au(111) surface populated with atomic oxygen by directly supplying its oxygen to CO to form CO2 at low temperatures. The results strongly suggest the direct involvement and promoting role of water in CO oxidation on oxygen covered Au(111).