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.

Abstract

Through synthesizing Ln2Zr2O7 and LnAlO3 (Ln = La, Nd, Sm) catalysts, the origin of active sites for oxidative coupling of methane (OCM) on A2B2O7 fluorite and ABO3 perovskite compounds has been compared and elucidated. Ln2Zr2O7 catalysts show much better reaction performance than the respective LnAlO3 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 La2Zr2O7 > Nd2Zr2O7 > Sm2Zr2O7 > LaAlO3 > NdAlO3 > SmAlO3. It is revealed that the unit cell free volume (Vf) plays an important role in affecting the catalytic activity, and the Ln2Zr2O7 catalysts with a disordered defect fluorite phase have inherent oxygen vacancies, which can directly activate gas-phase O2 molecules to generate OCM reactive O2- anions. However, the oxygen vacancies of LnAlO3 with a perovskite structure can only be generated by lattice distortion/transformation above 600 °C. Moreover, Ln2Zr2O7 fluorites have weaker B-O bonds than LnAlO3 perovskites, thus making it easier to generate surface vacancies as well as active O2- 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 LnAlO3 catalysts show much worse performance than Ln2Zr2O7 catalysts below 600 °C, which will be evidently improved at elevated temperatures due to phase transformation.