Catalytic CO oxidation over ruthenium - bridging the pressure gap

The CO oxidation over Ru under UHV conditions reveals by far the lowest catalytic activity among the late transition metals, while under strongly oxidizing reaction conditions the catalytic activity of Ru turns out to be superior to Pt, Rh and Pd. This observation was taken as manifestation of the so-called pressure gap. Recent experiments have shown that this transformation from an inactive catalyst towards an active catalyst is attributed to a structural transformation of Ru to RuO2. The autocatalytic oxidation of the Ru(0 0 0 1) surface leads to the formation of an epitaxially grown RuO2(1 1 0) film whose catalytically active sites are identified with the onefold under coordinated Ru atoms (1f-cus-Ru). On the 1f-cus-Ru atoms the CO molecules adsorb strongly (120 kJ/mol), ensuring a high CO concentration on the oxide surface under reaction conditions. Experiments together with density functional theory (DFT) calculations indicate that the most important elementary reaction step during the CO oxidation is the recombination of adsorbed CO molecules with bridging O atoms from the oxide surface. Equally important for being a good oxidation catalyst is the facile replenishment of bridging O atoms on RuO2 by oxygen exposure,. This process is mediated by on-top O atoms above the 1f-cus-Ru atoms. The on-top O atoms are by 1.4 eV less strongly bound than bridging O atoms. However, the activation barriers for CO recombination with on-top O and bridging O are nearly degenerated as determined by DFT calculations. Entropy effects make the oxidation von on-top CO less efficient with on-top O than with bridging O. Recent experiments provide evidence that the pressure gap for the CO oxidation reaction on RuO2 is successfully closed. (C) 2003 Elsevier Science Ltd. All rights reserved.