Kinetic Coupling among Metal and Oxide Phases during CO Oxidation on Partially Reduced PdO(101): Influence of Gas-Phase Composition

We used direct-rate measurements and reflection absorption infrared spectroscopy (RAIRS) to investigate CO oxidation on a PdO(101) film at 450 K and focused on characterizing how the gas-phase composition (O-2 + CO) influences the reaction kinetics and the coupling between the reaction and the surface phases that develop. We find that chemisorbed O atoms on Pd(111) are intrinsically more reactive than PdO(101) and that CO oxidation rates can be as much as 2-3 times higher on Pd(111) compared with PdO(101) for the conditions studied. Using RAIRS, we show that reduction of PdO(101) by CO produces metallic Pd(111) domains at 450 K and identified several rate processes that couple the PdO(101) and Pd(111) phases during reaction, including the formation of Pd(111) domains from O vacancies on PdO(101), oxygen transfer from the oxide to the metal to generate chemisorbed O atoms, and the healing of surface oxygen vacancies on PdO(101) via O atom migration from the bulk oxide as well as O-2 dissociative adsorption. We show that the CO oxidation rates increase autocatalytically as the surface fraction of metallic Pd initially increases during reaction because Pd(111) is more reactive than PdO(101) over the range of conditions studied. Our results further demonstrate that the oxide and gaseous O-2 can act cooperatively to supply metallic Pd(111) domains with chemisorbed O atoms, resulting in higher CO oxidation rates achieved on mixtures of PdO(101) and Pd(111) compared with pure Pd(111). The present study clarifies key rate processes and the resulting kinetic couplings among the gas-phase and solid phases during CO oxidation on partially reduced PdO(101). We expect that our findings will provide useful guidance for advancing first-principles kinetic modeling of CO oxidation promoted by transition-metal catalysts.