Surface Oxygen Vacancies Induced by Calcium Substitution in Macroporous La₂Ce_2–xCa_xO_7−δ Catalysts for Boosting Low-Temperature Oxidative Coupling of Methane

Surface oxygen vacancies in the catalysts play a key role in improving the catalytic performances for low-temperature oxidative coupling of methane (OCM). Herein, macroporous La₂Ce_2–xCa_xO_7−δ (A₂B₂O₇-type) catalysts with a disordered defective cubic fluorite phased structure were prepared by a citric acid sol–gel method. The macroporous structure improved the accessibility of the reactants (O₂ and CH₄) to the active sites. The partial substitution of the B site (Ce) with low-valence calcium (Ca) ions in La₂Ce_2–xCa_xO_7−δ catalysts induced the formation of surface oxygen vacancies, which facilitated the adsorption and activation of O₂ molecules to generate the active oxygen species (O₂^– species). The O₂^– species can boost the activation of CH₄ and govern the following step of the oxidative dehydrogenation of C₂H₆ to C₂H₄. La₂Ce_2–xCa_xO_7−δ catalysts have high catalytic activity for low-temperature OCM, and the La₂Ce_1.3Ca_0.7O_7−δ catalyst with the highest density of O₂^– species exhibits the highest catalytic activity during low-temperature OCM into C₂H₄ and C₂H₆ (C₂) products, i.e., its CH₄ conversion, selectivity, and yield of C₂ products at 600 °C are 31.0, 65.6, and 20.3%, respectively. Based on the results of multiple experimental characterizations and density functional theory calculations, the mechanism of La₂Ce_2–xCa_xO_7−δ catalysts for the OCM reaction is proposed: surface oxygen vacancies induced by the substitution of the Ce site with Ca ions significantly promote the critical steps of C–H bond breaking and C–C bond coupling during the low-temperature OCM reaction. It is important for the design of low-temperature and high-efficiency catalysts for practical applications.