CO oxidation on Ag(111): The catalytic role of H2O

The reaction mechanism for the oxidation of CO on Ag(111) in the presence of trace amounts of water is investigated via density-functional-theory calculations. A four-step cycle for the reaction is proposed: (1) H2O + O2 → HO + HO2; (2) HO2 + CO → OH + CO2; (3) CO + OH → cis-OCOH; and (4) cis-OCOH + OH → CO2 + H2O. In the mechanism, water is found to directly participate in the reaction as a catalyst, in addition to the previously proposed role of stabilizing the weakly adsorbed oxygen molecules on Ag(111). Moreover, HO2 is an important reaction intermediate, which is produced by transferring a hydrogen atom from water to an oxygen molecule. Because the overall reaction barrier is as low as 0.20 eV, the mechanism is expected to be operative at low temperatures.