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

Abstract

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.