Electron-Hopping Brings Lattice Strain and High Catalytic Activity in the Low-Temperature Oxidative Coupling of Methane in an Electric Field

摘要

Detailed reaction mechanisms for the oxidative coupling of methane (OCM) over Ce-2(WO4)(3) catalysts at low temperatures in an electric field were investigated. The influence of Ce cations in the Ce-2(WO4)(3) catalyst was evaluated by comparing the OCM activity over various Ln(2)(WO4)(3) (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) catalysts in an electric field. The electronic states of Ln and W cations and the relationship between the distorted Ce-2(WO4)(3) structure and methane activation were examined using X-ray absorption fine structure (XAFS) measurements and first-principles calculations. The results reveal that the Ln(2)(WO4)(3) catalysts with redox-active Ln cations (Ce, Pr, Sm, Eu, and Tb) show OCM activity. First-principles calculations indicate that Ce3+ species in the Ce-2(WO4)(3) structure are oxidized to Ce4+ species in an electric field by extracting electrons from the Ce 4f orbitals near the Fermi level; as a result, its structure is distorted. The results indicate that the redox reaction of Ln cations in Ln(2)(WO4)(3) induced by an electric field brings lattice strain and a high OCM activity in an electric field.