CO Oxidation on the Pd(111) Surface

Under technologically relevant oxygen-rich conditions, the reaction mechanism of CO oxidation over transition metals can be complicated by the formation of oxides. Questions of whether the active surface for CO oxidation is a pristine metal, a surface oxide, or a bulk oxide is still under active debate. In this study, density functional theory calculations are used to model CO oxidation on the Pd(111) surface. Our results show that a thin layer of Pd5O4 surface oxide is stable under catalytically relevant gas-phase conditions. Three-fold oxygen atoms in the surface are found to react with gas-phase CO molecules following an Eley−Rideal reaction mechanism. Such CO oxidation reduces the surface oxide, but the oxide can be replenished by O2 dissociation. Kinetic analysis shows that experimentally observed reaction conditions, that are uninhibited by CO and limited only by mass transfer, correspond to a surface oxide phase with CO oxidation occurring though the Eley−Rideal mechanism. Under steadystate operating conditions, the continuous formation and decomposition of the surface oxide is expected and is key to the high CO oxidation rate on Pd(111).