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 O-2 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 steady-state 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).