Methane combustion reactivity during the metal→metallic oxide transformation of Pd-Pt catalysts: Effect of oxygen pressure

Density functional theory combined with kinetic models were used to probe different kinetics consequences by which methane activation on different oxygen chemical potential surfaces as oxygen pressure increased. The metallic oxide -> metal transformation temperature of Pd-Pt catalysts increased with the increase of the Pd content or/and O-2 pressure. The methane conversion rate on Pt catalyst increased and then decreased to a constant value when increasing the O-2 pressure, and Pd catalyst showed a poor activity performance in the case of low O-2 pressure. Moreover, its activity increased as the oxygen chemical potential for O-2 pressure increased in the range of 2.5-10 KPa. For metal clusters, the C-H bond and O=O bond activation steps occurred predominantly on *-* site pairs. The methane conversion rate was determined by O-2 pressure because the adsorbed O atoms were rapidly consumed by other adsorbed species in this kinetic regime. As the O-2 pressure increased, the metallic active sites for methane activation were decreased and there was no longer lack of adsorbed O atoms, resulting in the decrease of the methane conversion rate. Furthermore, when the metallic surfaces were completely covered by adsorbed oxygen atoms at higher oxygen chemical potentials, Pt catalyst showed a poor activity due to a high C-H bond activation barrier on O*-O*. In the case of high O-2 pressure, Pd atoms preferred to segregate to the active surface of Pd-Pt catalysts, leading to the formation of PdO surfaces. The increase of Pd segregation promoted a subsequent increase in active sites and methane conversion rate. The PdO was much more active than metallic and O* saturated surfaces for methane activation, inferred from the theory and experimental study. Pd-rich bimetallic catalyst (75% molar Pd) showed a dual high methane combustion activity on O-2-poor and O-2-rich conditions. (C) 2017 Elsevier B.V. All rights reserved.