Water inhibits CO oxidation on gold cations in the gas phase. Structures and binding energies of the sequential addition of CO, H 2 O, O 2 , and N 2 onto Au .

We report a detailed experimental and theoretical study of the gas phase reactivity of Au+ with CO, O2 , N2 and their mixtures in the presence of a trace amount of water impurity. The gold cation is found to strongly interact with CO and H2 O molecules via successive addition reactions until reaching saturation. The stoichiometry of the formed complex is determined by the strength of the binding energy of the neutral molecule to the gold cation. CO binds the strongest to Au+ , followed by H2 O, N2 and then O2 . We found that the gold cation (Au+ ) can activate the O2 molecule within the Au+ (CO)2 (O2 ) complex which could react with another CO molecule to form Au+ (CO)(CO2 ) + CO2 . The product Au+ (CO)(CO2 ) is observed experimentally with a small intensity at room temperature. However, the presence of water leads to the formation of Au+ (CO)(H2 O)(O2 ) instead of Au+ (CO)2 (O2 ) due to the strong interaction between Au+ and water. The current experiments and calculations might lead to a molecular level understanding of the interactions between the active sites, reactants and impurities which could pave the way for the design of efficient nanocatalysts.