We performed density-functional theory calculations using the generalized gradient approximation for the exchange-correlation functional to investigate the unusual catalytic behavior of Ru under elevated gas pressure conditions for the carbon monoxide oxidation reaction, which includes a particularly high CO2 turnover. Our calculations indicate that a full monolayer of adsorbed oxygen actuates the high rate, enabling CO2 formation via both scattering of gas-phase CO molecules as well as by CO molecules adsorbed at oxygen vacancies in the adlayer, where the latter mechanism is expected to be very efficient due to the relatively weak adsorption energy of both CO and O, as well as the close proximity of these reactants. We analyze the bonding and electronic properties associated with the reaction pathway for CO2 production via the scattering reaction. We find that the identified "bent" transition state is due to electron transfer into the unoccupied 2 pi orbitals of the CO molecule which reduces the Pauli repulsion between the impinging 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 region between CO and the adsorbed O atom.