Preparation of La 2 Zr 2 O 7 composite oxides with fluorite/pyrochlore phases by excluding element influences for catalyzing oxidative coupling of methane.

To elucidate the effect of the A 2 B 2 O 7 phase on the oxidative coupling of methane (OCM) while excluding elemental influences, La 2 Zr 2 O 7 compounds with a disordered defect fluorite (La 2 Zr 2 O 7 -F) structure and an ordered pyrochlore phase (La 2 Zr 2 O 7 -P) have been synthesized. Irrespective of their element composition, the catalytic performance of La 2 Zr 2 O 7 -F exceeds that of La 2 Zr 2 O 7 -P. Furthermore, the La 2 Zr 2 O 7 -F surface has more oxygen vacancies/defects than the La 2 Zr 2 O 7 surface because La 2 Zr 2 O 7 -F exhibits a higher lattice disorder degree and lower B-O bond strength, which leads to the formation of more reactive oxygen anions (O 2 - and O 2 2- ) and basic sites for OCM. Isotopic exchange results have testified that surface-active oxygen sites are generated due to the gaseous O 2 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 A 2 B 2 O 7 compounds, with the disordered defect fluorite phase being the most optimal structure for OCM.