Kinetics of Palladium Oxidation in the mbar Pressure Range: Ambient Pressure XPS Study

Palladium oxidation was studied by ambient-pressure X-ray photoelectron spectroscopy in the mbar pressure range on the Pd(111) and Pd(110) surfaces. The oxidation kinetics on both surfaces show an induction period when the oxidation rate was low at the beginning and then accelerated. The slow initial oxidation is governed by (a) the rate of nucleus formation, and (b) the rate of oxide nucleus growth. Depth profiling varying photon energy/kinetic photoelectron energy pointed to a 3D oxidation. It is remarkable that the oxidation of Pd(110) proceeds at ~100 K lower temperatures than on Pd(111). We suggest that at the high temperature required on Pd(111) nucleation is thermodynamically controlled, and therefore, the nucleation rate decreases with temperature. On Pd(110), nucleation is predominantly kinetically controlled and thus the oxidation rate increases with temperature