Fabrication of LnZrO Fluorite and LnAlO Perovskite (Ln = La, Nd, Sm) Compounds to Catalyze the OCM Reaction: On the Temperature-Induced Phase Transformation and Oxygen Vacancy.

Through synthesizing LnZrO and LnAlO (Ln = La, Nd, Sm) catalysts, the origin of active sites for oxidative coupling of methane (OCM) on ABO fluorite and ABO perovskite compounds has been compared and elucidated. LnZrO catalysts show much better reaction performance than the respective LnAlO catalysts at low temperatures (500-600 °C), but the difference will be mitigated significantly above 600 °C. The reaction performance ranks in the order of LaZrO > NdZrO > SmZrO > LaAlO > NdAlO > SmAlO. It is revealed that the unit cell free volume () plays an important role in affecting the catalytic activity, and the LnZrO catalysts with a disordered defect fluorite phase have inherent oxygen vacancies, which can directly activate gas-phase O molecules to generate OCM reactive O anions. However, the oxygen vacancies of LnAlO with a perovskite structure can only be generated by lattice distortion/transformation above 600 °C. Moreover, LnZrO fluorites have weaker B-O bonds than LnAlO perovskites, thus making it easier to generate surface vacancies as well as active O sites. The surface alkalinity is intimately relevant to the active oxygen species, which act together to decide the OCM performance on both types of catalysts. Indeed, this explains that LnAlO catalysts show much worse performance than LnZrO catalysts below 600 °C, which will be evidently improved at elevated temperatures due to phase transformation.