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†
Abstract
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 O22− 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 O2−. 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.