Dissecting La2Ce2O7 catalyst to unravel the origin of the surface active sites devoting to its performance for oxidative coupling of methane (OCM)

As an endeavor to trace the origin of the active surface sites on La2Ce2O7 compound for OCM reaction, a dissection strategy is adopted in this work. In detail, the reaction performance of pure La2O3, CeO2, the physical mixture of La2O3 and CeO2 (La2O3 +CeO2), and the combination of pure La2O3 and CeO2 with different orders in the reactor (La2O3-CeO2 or CeO2-La2O3) has been tested for OCM reaction under the same condition. Individual La2O3, CeO2 and their simple mixing/combination display much lower OCM performance than La2Ce2O7 compound, especially below 700 °C. Therefore, the reaction performance of La2Ce2O7 is not due to a simple accumulation of the two pure oxides. To achieve favorable OCM performance, La and Ce cations must be combined into a stable La2Ce2O7 compound possessing a disordered defective cubic fluorite structure with intrinsic oxygen vacancies. In situ Raman, CO2-TPD, EPR and XPS results have demonstrated on the surface of La2Ce2O7, much larger amount of both O2- species and moderate basic sites have been formed in comparison with the individual metal oxides or their simple mixing/combination, thus it exhibits the optimum reaction performance among all the catalysts.