Mechanistic interpretation of CO oxidation turnover rates on supported Au clusters

► H2O addition is essential in CO/O2 reactions to obtain high rates with Au clusters. ► CO oxidation on Au involves H2O-mediated O2 activation steps to form *OOH species. ► *OOH species are responsible for the high rates measured with CO/O2/H2O mixtures. ► Support identity is inconsequential for CO oxidation in the presence of H2O.