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

Detailed reaction mechanisms for the oxidative coupling of methane (OCM) over Ce₂(WO₄)₃ catalysts at low temperatures in an electric field were investigated. The influence of Ce cations in the Ce₂(WO₄)₃ catalyst was evaluated by comparing the OCM activity over various Ln₂(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 Ce₂(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 Ln₂(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 Ce₂(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 Ln₂(WO₄)₃ induced by an electric field brings lattice strain and a high OCM activity in an electric field.