Surface Oxygen Vacancies Induced by Calcium Substitution in Macroporous La2Ce2-x Ca x O7-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 La2Ce2-xCaxO7-delta (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 CH4) to the active sites. The partial substitution of the B site (Ce) with low-valence calcium (Ca) ions in La2Ce2-xCaxO7-delta 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 CH4 and govern the following step of the oxidative dehydrogenation of C2H6 to C2H4. La2Ce2-xCaxO7-delta catalysts have high catalytic activity for low-temperature OCM, and the La2Ce1.3Ca0.7O7-delta catalyst with the highest density of O-2(-) species exhibits the highest catalytic activity during low-temperature OCM into C2H4 and C2H6 (C-2) products, i.e., its CH4 conversion, selectivity, and yield of C-2 products at 600 degrees 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 La2Ce2-xCaxO7-delta 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.