BaBO₃ (B = Ti, Sn, Zr) perovskites for oxidative coupling of methane: elucidating the effects of intrinsic oxygen vacancies, high-temperature crystal phase transition on reaction performance

Herein, we synthesized BaBO₃ (B = Ti, Sn, Zr) using the hydrothermal method for the oxidative coupling of methane (OCM). BaTiO₃ undergoes a tetragonal-cubic crystal phase transition at high temperatures, generating oxygen vacancies. Cubic BaSnO₃ possesses intrinsic oxygen vacancies and active surface lattice oxygen, while cubic BaZrO₃ lacks such vacancies and shows no high-temperature phase transitions. Chemisorbed oxygen species O₂^- and O₂^2- are beneficial to C₂ selectivity, and surface lattice oxygen promotes the deep oxidation of methane and C₂ hydrocarbons. At lower temperatures, chemisorbed oxygen activates methane and is C₂ selective. BaTiO₃ and BaSnO₃ can directly activate gas-phase oxygen through oxygen vacancies to produce chemisorbed oxygen species. Increasing the reaction temperature rapidly transforms this electrophilic/electron-deficient oxygen into nucleophilic/electron-rich surface lattice O^2-. However, activation on the BaZrO₃ surface starts from gas-phase oxygen, transforming into electrophilic and nucleophilic oxygen. This reaction rate is slower; thus, at high temperatures, BaZrO₃ displays a higher C₂ selectivity than BaTiO₃ and BaSnO₃ (with the active lattice oxygen of BaSnO₃ further decreasing its C₂ selectivity). Perovskite-type catalysts in low-temperature OCM benefit from intrinsic and induced oxygen vacancies, though lattice oxygen activity should be avoided. In high-temperature OCM reactions, these oxygen vacancies are unfavorable. Additionally, the slow exchange rate between gaseous and surface oxygen is another factor.