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-degrees), but the difference will be mitigated significantly above 600(degrees)C. The reaction performance ranks in the order of La(2)Zr(2)O7 > Nd2Zr2O7 > Sm2Zr2O7 > LaAlO3 > NdAlO3 > SmAlO3. 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 LnAlO3 with a perovskite structure can only be generated by lattice distortion/transformation above 600 C-degrees. Moreover, Ln(2)Zr(2)O(7) fluorites have weaker B-O bonds than LnAlO3 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-degrees, which will be evidently improved at elevated temperatures due to phase transformation.