Surface Oxygen Vacancies Induced by Calcium Substitution in Macroporous La<sub>2</sub>Ce<sub>2–<i>x</i></sub>Ca<sub><i>x</i></sub>O<sub>7−δ</sub> Catalysts for Boosting Low-Temperature Oxidative Coupling of Methane
Tong Tong Wu, Peng Zhang, Yuechang Wei, Jing Xiong, Dawei Han, Tao Li, Yitao Yang, Zhen Zhao, Jian Liu
Abstract
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.