Engineering Oxygen Vacancies via Crystal-Phase Modulation in Mn-Ce Oxides for Toluene and Benzene Oxidation

Generally, binary metal oxides often exhibit superior catalytic performance to their single-metal counterparts, with crystal-phase structure being a critical factor alongside the metallic composition. However, systematic studies elucidating the role of crystal phase remain scarce. Herein, we synthesize a series of Mn-Ce oxide catalysts (MnCeO_x) with distinct crystal phases─bixbyite (Mn₂O₃-type) and fluorite (CeO₂-type)─by a sol-gel method and evaluate their performance in the catalytic oxidation of benzene, toluene, and their mixture. MnCeO_x outperformed individual Mn₂O₃ and CeO₂ in both single- and mixed-pollutant scenarios. Noticeably, the fluorite-phase catalysts displayed markedly higher activity than their bixbyite-phase analogues. In single-pollutant oxidation, benzene is oxidized at a lower temperature (T₉₀) than that of toluene. Conversely, in co-oxidation mixtures, toluene exhibits a lower T₉₀ than benzene, indicating a reversal of reactivity under competitive conditions. Comprehensive characterization and theoretical calculations reveal that the high activity of the fluorite phase originates from its lower-valent Mn centers and oxygen vacancies. In mixed benzene/toluene streams, toluene significantly inhibited benzene oxidation because of competitive adsorption. This work highlights the decisive role of crystal-phase engineering in designing efficient multicomponent oxide catalysts for VOC removal.