Fabrication of Ln2Zr2O7 Fluorite and LnAlO3 Perovskite (Ln = La, Nd, Sm) Compounds to Catalyze the OCM Reaction: On the Temperature-Induced Phase Transformation and Oxygen Vacancy

Through synthesizing Ln 2 Zr 2 O 7 and LnAlO 3 (Ln = La, Nd, Sm) catalysts, the origin of active sites for oxidative coupling of methane (OCM) on A 2 B 2 O 7 fluorite and ABO 3 perovskite compounds has been compared and elucidated. Ln 2 Zr 2 O 7 catalysts show much better reaction performance than the respective LnAlO 3 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 La 2 Zr 2 O 7 > Nd 2 Zr 2 O 7 > Sm 2 Zr 2 O 7 > LaAlO 3 > NdAlO 3 > SmAlO 3 . It is revealed that the unit cell free volume ( V f ) plays an important role in affecting the catalytic activity, and the Ln 2 Zr 2 O 7 catalysts with a disordered defect fluorite phase have inherent oxygen vacancies, which can directly activate gas-phase O 2 molecules to generate OCM reactive O 2 – anions. However, the oxygen vacancies of LnAlO 3 with a perovskite structure can only be generated by lattice distortion/transformation above 600 °C. Moreover, Ln 2 Zr 2 O 7 fluorites have weaker B–O bonds than LnAlO 3 perovskites, thus making it easier to generate surface vacancies as well as active O 2 – 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 3 catalysts show much worse performance than Ln 2 Zr 2 O 7 catalysts below 600 °C, which will be evidently improved at elevated temperatures due to phase transformation.