Surface Oxygen Vacancies Induced by Calcium Substitution in Macroporous La 2 Ce 2– x Ca x O 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 La 2 Ce 2– x Ca x O 7−δ (A 2 B 2 O 7 -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 (O 2 and CH 4 ) to the active sites. The partial substitution of the B site (Ce) with low-valence calcium (Ca) ions in La 2 Ce 2– x Ca x O 7−δ catalysts induced the formation of surface oxygen vacancies, which facilitated the adsorption and activation of O 2 molecules to generate the active oxygen species (O 2 – species). The O 2 – species can boost the activation of CH 4 and govern the following step of the oxidative dehydrogenation of C 2 H 6 to C 2 H 4 . La 2 Ce 2– x Ca x O 7−δ catalysts have high catalytic activity for low-temperature OCM, and the La 2 Ce 1.3 Ca 0.7 O 7−δ catalyst with the highest density of O 2 – species exhibits the highest catalytic activity during low-temperature OCM into C 2 H 4 and C 2 H 6 (C 2 ) products, i.e., its CH 4 conversion, selectivity, and yield of C 2 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 La 2 Ce 2– x Ca x O 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.