Optimizing the Reaction Performance of La 2 Ce 2 O 7 ‐Based Catalysts for Oxidative Coupling of Methane (OCM) at Lower Temperature by Lattice Doping with Ca Cations

To optimize the reaction performance of La 2 Ce 2 O 7 for OCM, catalysts with varied La/Ce molar ratios and doped by Ca additive have been synthesized and characterized by different techniques. It is discovered that a La 2 Ce 2 O 7 compound having a disordered cubic defective fluorite phase is predominantly formed in all the samples. Varying the La/Ce ratio influences the fine crystalline structure of the catalysts. As a result, the abundance of the surface facile oxygen and alkaline sites increase by increasing the La/Ce ratio. Moreover, the Ca additive exists on the surface of Ca 0.5 La 2 Ce 2 O 7 and Ca 0.5 La 1.5 Ce 2 O 7 as dispersed CaCO 3 , which covers the active sites and is harmful to the OCM reaction. However, for La 2 Ce 1.5 Ca 0.5 O 7 , the major part of the Ca additive has entered into the lattice of La 2 Ce 2 O 7 phase as Ca 2+ cations, thus creating more oxygen vacancies and improving the surface alkalinity of the catalyst, which is favorable to the OCM reaction. Both the active surface oxygen and alkaline sites are vital for OCM, and their synergism determines the reactivity. By increasing La/Ce ratio and doping Ca 2+ into La 2 Ce 2 O 7 lattice, the quantities of both kinds of active sites can be significantly improved, thus obtaining catalysts with much improved reaction performance. La 2 Ce 1.5 Ca 0.5 O 7 owns the largest amount of both types of active sites, hence depicting the best reaction performance, over which the highest C 2 yield of 22.5 % can be achieved even at 750 °C.