Surface Oxygen
Vacancies Induced by Calcium
Substitution in Macroporous
La2Ce2–xCaxO7−δ Catalysts for Boosting
Low-Temperature Oxidative Coupling of Methane
posted on 2024-01-20, 14:08authored byTongtong Wu, Peng Zhang, Yuechang Wei, Jing Xiong, Dawei Han, Tao Li, Yitao Yang, Zhen Zhao, Jian Liu
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−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 La2Ce2–xCaxO7−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. La2Ce2–xCaxO7−δ catalysts have high catalytic activity for low-temperature OCM,
and the La2Ce1.3Ca0.7O7−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 La2Ce2–xCaxO7−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.