Vacancy-Mediated Processes in the Oxidation of CO on PdO(101).

Metal oxide films can form on late transition-metal catalysts under sufficiently oxygen-rich conditions, and typically cause significant changes in the catalytic performance of these materials. Several investigations using sensitive in situ surface characterization techniques reveal that the CO oxidation activity of Pd and other late transition-metal catalysts increases abruptly under conditions at which metal oxide structures begin to develop. Findings such as these provide strong motivation for developing atomic-scale descriptions of oxidation catalysis over oxide films of the late transition-metals. Surface oxygen vacancies can play a central role in mediating oxidation catalysis promoted by metal oxides. In general, adsorbed reactants abstract oxygen atoms from the lattice of the oxide surface, thereby creating oxygen vacancies, while gaseous O2 replenishes the reactive surface oxygen atoms and eliminates oxygen vacancies. Oxygen vacancies also represent a distinct type of surface site on which the binding and reactivity of adsorbed species can differ compared with sites on the pristine oxide surface. Detailed characterization of vacancy-mediated rate processes is thus essential for developing reliable mechanistic descriptions of oxidation catalysis over reducible metal oxide films. Careful measurements performed in ultrahigh vacuum (UHV) using well-defined oxide surfaces in combination with molecular simulations afford the capability to isolate and characterize such reaction steps, and thus provide information that is needed for developing mechanistic models of oxidation catalysis over metal oxides. In this Account, we discuss vacancy-mediated processes that are involved in the oxidation of CO on the PdO(101) surface as determined from UHV surface science experiments and density functional theory (DFT) calculations. These studies show that CO binds strongly on Pd atoms that are located next to surface oxygen vacancies, and diffuses rapidly to these sites during reduction of the oxide surface by CO. The enhanced binding also raises the energy barriers for desorption and oxidation of CO, but the difference in these barriers remains nearly identical to that for CO adsorbed on the pristine PdO(101) surface. These recent studies also show that oxygen from the subsurface efficiently eliminates surface oxygen vacancies during CO oxidation at temperatures as low as 400 K, and thereby reveal a facile pathway by which PdO(101) surface domains can be maintained during oxide reduction.