Tailoring La2Ce2O7 catalysts for low temperature oxidative coupling of methane by optimizing the preparation methods

摘要

With the objective to develop OCM catalysts having high reaction performance at low temperature, a La2Ce2O7 compound has been optimized to tailor its surface reactive sites by using co-precipitation (CP), hydrothermal (HT) and glycine nitrate combustion (GNC) methods. All the catalysts show considerable low temperature reaction performance, following the sequence of La2Ce2O7-GNC > La2Ce2O7-HT > La2Ce2O7-CP. On La2Ce2O7-GNC, the best catalyst in this study, the reaction has already started even at 450 degrees C with a C-2 product yield around 2.5%, indicating its superior low temperature reaction performance. All the La2Ce2O7 catalysts consist of a disordered defective cubic fluorite crystalline phase. It is revealed that superoxide O-2(-) anions with a bond order of 1.5 have been formed on the surfaces of all the catalysts. However, peroxide O-2(2-) anions with a bond order of 1.0, which are more reactive than O-2(-) anions, is uniquely formed on La2Ce2O7-GNC, which explains its superior low temperature reaction performance. A La2Ce2O7 catalyst with higher reaction performance usually possesses a larger amount of surface electrophilic oxygen and base sites. Therefore, it concluded that the interaction between these two kinds of surface active sites in fact controls the OCM reaction performance of the La2Ce2O7 catalysts.