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.
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