Small gold clusters adsorbed on transition metal oxides, such as TiO2, exhibit unusually high activity as oxidation catalysts. One possible explanation for this enhanced reactivity compared to bulk gold surfaces is that these small clusters more readily dissociate molecular O2, but theoretical calculations and photoemission studies on gas-phase gold clusters suggest that O2 dissociation is a highly activated process. Stiehl et al. studied the oxidation of CO to CO2 over gold clusters that ranged in size from 2 to 5 nm adsorbed on a TiO2(110) surface. A molecular beam source and collision-induced desorption were used to create surfaces covered only with dissociated oxygen and surfaces covered with a mixture of atomic and molecular oxygen. CO2 production increased by about 40% for a surface with both types of oxygen species, as opposed to one with only atomic oxygen. Thus, there are oxidative reaction channels over supported gold clusters that do not require the direct participation of dissociated oxygen. — PDS
J. Am. Chem. Soc. 10.1021/ja046390x (2004).