MECHANISM OF EFFICIENT CARBON-MONOXIDE OXIDATION AT RU(0001)

We performed density-functional theory calculations using the generali zed gradient approximation for the exchange-correlation functional to investigate the unusual catalytic behavior of Ru under elevated gas pr essure conditions for the carbon monoxide oxidation reaction, which in cludes a particularly high CO2 turnover. Our calculations indicate tha t a full monolayer of adsorbed oxygen actuates the high rate, enabling CO2 formation via both scattering of gas-phase CO molecules as well a s by CO molecules adsorbed at oxygen vacancies in the adlayer, where t he latter mechanism is expected to be very efficient due to the relati vely weak adsorption energy of both CO and O, as well as the close pro ximity of these reactants. We analyze the bonding and electronic prope rties associated with the reaction pathway for CO2 production via the scattering reaction. We find that the identified ''bent'' transition s tate is due to electron transfer into the unoccupied 2 pi orbitals of the CO molecule which reduces the Pauli repulsion between the impingin g CO and the O-covered surface. Bond formation to CO2 then proceeds by electron transfer back from the CO 2 pi orbitals into the bonding reg ion between CO and the adsorbed O atom. (C) 1997 American Vacuum Socie ty.