Constructing La2B2O7 (B = Ti, Zr, Ce) Compounds with Three Typical Crystalline Phases for the Oxidative Coupling of Methane: The Effect of Phase Structures, Superoxide Anions, and Alkalinity on the Reactivity

To probe the phase structure–reactivity relationship of A2B2O7 catalysts for the oxidative coupling of methane (OCM), three model La2B2O7 compounds with Ti4+, Zr4+, or Ce4+ at the B-site have been purposely designed. By decreasing the rA/rB ratios in the order of La2Ti2O7 > La2Zr2O7 > La2Ce2O7, typical monoclinic layered perovskite, cubic ordered pyrochlore, and disordered defective cubic fluorite phase are formed, respectively. The reaction performance of the catalysts based on CH4 conversion and C2 product yield follow the order of La2Ce2O7 > La2Zr2O7 > La2Ti2O7. It has been discovered that superoxide O2– is the active oxygen species detected on all the catalysts and is responsible for the OCM reaction, whose amount follows also the sequence of La2Ce2O7 > La2Zr2O7 > La2Ti2O7. Moreover, the surface alkalinity related to the superoxide anions observes the same order. This testifies that the amount of surface superoxide O2– determines the OCM reaction performance over the La2B2O7 compounds. On the basis of the characterization results, the formation of active O2– species could follow two pathways. For La2Zr2O7 and La2Ce2O7 possessing intrinsic 8a oxygen vacancies, O2– anions are formed by activating the oxygen species entering into the vacancies in the bulk and then migrating to the catalyst surface. For La2Ti2O7 possessing no oxygen vacancies, they are formed directly by transforming the O2 molecules adsorbed on its surface. Usually, the former pathway generates more abundant O2– species than the latter one. La2Ce2O7 displays not only promising reaction performance in the low-temperature region, but also potent sulfur and lead poisoning resistance, thus having the potential for application after further optimization.