Comparing BaZrO3 Perovskite and La2Zr2O7 Pyrochlore for Oxidative Coupling of Methane (OCM): The Importance of Basic Sites vs Oxygen Vacancies.

La2Zr2O7 pyrochlore with intrinsic oxygen vacancies and BaZrO3 perovskite without intrinsic oxygen vacancies were synthesized for the OCM reaction. It has been revealed that the OCM performance and surface selective oxygen species of BaZrO3 are higher than that of La2Zr2O7 under the reaction condition. This is because BaZrO3 possesses more basic sites than La2Zr2O7, and thus it can stabilize the OCM reactive oxygen species better at elevated temperature. In the structure, the A-O bond lattice oxygen of the two compounds mainly provides basic sites, but the B-O bond lattice oxygen mainly promotes deep oxidation of methane and the generated hydrocarbons. The types of the OCM reactive oxygen species are prone to be associated with the properties of the A-site metal oxides.

Preparation of La2Zr2O7 composite oxides with fluorite/pyrochlore phases by excluding element influences for catalyzing oxidative coupling of methane.

To elucidate the effect of the A2B2O7 phase on the oxidative coupling of methane (OCM) while excluding elemental influences, La2Zr2O7 compounds with a disordered defect fluorite (La2Zr2O7-F) structure and an ordered pyrochlore phase (La2Zr2O7-P) have been synthesized. Irrespective of their element composition, the catalytic performance of La2Zr2O7-F exceeds that of La2Zr2O7-P. Furthermore, the La2Zr2O7-F surface has more oxygen vacancies/defects than the La2Zr2O7 surface because La2Zr2O7-F exhibits a higher lattice disorder degree and lower B-O bond strength, which leads to the formation of more reactive oxygen anions (O2- and O22-) and basic sites for OCM. Isotopic exchange results have testified that surface-active oxygen sites are generated due to the gaseous O2 adsorption/activation occurring on the surface vacancies via both simple and multiple hetero-exchange mechanisms. In conclusion, crystal structure is the primary factor that governs the catalytic performance of A2B2O7 compounds, with the disordered defect fluorite phase being the most optimal structure for OCM.

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 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.