Ca and Sr co-doping induced oxygen vacancies in 3DOM La2−xSrxCe2−yCayO7−δ catalysts for boosting low-temperature oxidative coupling of methane

Abstract

It is urgent to develop catalysts with application potential for oxidative coupling of methane (OCM) at relatively lower temperature. Herein, three-dimensional ordered macroporous (3DOM) La2−xSrxCe2−yCayO7−δ (A2B2O7-type) catalysts with disordered defective cubic fluorite phased structure were successfully prepared by a colloidal crystal template method. 3DOM structure promotes the accessibility of the gaseous reactants (O2 and CH4) to the active sites. The co-doping of Ca and Sr ions in La2−xSrxCe2−yCayO7−δ catalysts improved the formation of oxygen vacancies, thereby leading to increased density of surface-active oxygen species (O2−) for the activation of CH4 and the formation of C2 products (C2H6 and C2H4). 3DOM La2−xSrxCe2−yCayO7−δ catalysts exhibit high catalytic activity for OCM at low temperature. 3DOM La1.7Sr0.3Ce1.7Ca0.3O7−δ catalyst with the highest density of O2− species exhibited the highest catalytic activity for low-temperature OCM, i.e., its CH4 conversion, selectivity and yield of C2 products at 650 °C are 32.2%, 66.1% and 21.3%, respectively. The mechanism was proposed that the increase in surface oxygen vacancies induced by the co-doping of Ca and Sr ions boosts the key step of CH bond breaking and CC bond coupling in catalyzing low-temperature OCM. It is meaningful for the development of the low-temperature and high-efficient catalysts for OCM reaction in practical application.