Experiments performed at high gas partial pressures have demonstrated that the kinetics of the CO oxidation reaction at Ru (0001) is different and somewhat anomalous compared to that over other transition metal surfaces and, in particular, the turnover rate is exceptionally high. In order to gain insight into the underlying reasons for this behavior, we performed density functional theory calculations using the generalized gradient approximation for the exchange-correlation functional. We find that the high rate is due to a weakly, but nevertheless well bound, (1 × 1) oxygen adsorbate layer which may form for high O2 pressures but not under usual ultra high vacuum conditions. The calculations indicate that reaction to CO2 occurs both via scattering of gas-phase CO molecules as well as by CO molecules weakly adsorbed at vacancies in the oxygen adlayer, where the latter mechanism dominates the rate. I
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