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Abstract

The formation of CO₂, by exposing oxygen precovered Ru(0001) surfaces to CO, was investigated as a function of the oxygen coverage for sample temperatures up to 900 K. It turned out that the reaction probability per incident CO molecule is below 5×10^-4 for O coverages up to 3 monolayers (ML); oxygen in excess of 1 ML is located in the subsurface region. The reaction probability for the (1×1)-1O phase is in agreement with the data derived from high-pressure experiments by Peden and Goodman [J. Phys. Chem.1986, 90, 1360]. Even for CO molecules with a translational energy of 1.2 eV (supersonic molecular beam experiments), the reaction probability is less than 5×10^-2. This value is consistent with the activation barrier derived from DFT calculations for a reaction by direct collision from the gas phase (Eley−Rideal mechanism). Beyond an oxygen load of 3 ML, however, the reaction probability increases by 2 orders of magnitude. It is suggested that this enhancement is due to a further destabilization of the surface oxygen by the onset of oxide formation.