The growth of metal-surface oxides and its roles in surface reactions have been continuously studied (and debated) as it provide fundamental knowledge on how the surface reaction occurs in the presence of oxides. Among many of those studies, the surface oxides of Pt and Pd have been intensively studied and two different reaction mechanisms, i.e. Langmuir-Hinshelwood mechanism and Mars-van-Krevelen mechanism, have been employed to explain CO oxidation reaction.
Recently, we revisited the study of CO oxidation on Pt(110) and Pd(100) surface using AP-XPS. When the surface temperature reaches the activation temperature for CO oxidation under elevated pressure, the presence of surface oxides is observed on both surfaces. Due to exothermic nature of CO oxidation, the temperature of both Pt and Pd surfaces increases as CO oxidation takes places. Interestingly, the gas phase peaks of oxygen behave differently from Pt surface to Pd surface under oxygen rich condition, reflecting opposite reaction properties of surface oxides. The origin of the reaction properties of Pt and Pd will be discussed.