Investigation into the Catalytic Roles of Various Oxygen Species over Different Crystal Phases of MnO2 for C6H6 and HCHO Oxidation

Although manganese oxide (MnO2) exhibits excellent activity in various oxidation reactions, especially for volatile organic compound oxidation, the origin of the catalytic activity remains ambiguous and controversial. In this study, four types of MnO2 catalysts with crystal phases corresponding to α-, β-, γ-, and δ-MnO2 were synthesized, and their catalytic properties in HCHO and C6H6 oxidation were studied. α- and γ-MnO2 were found to possess much better activity for C6H6 oxidation than δ- and β-MnO2 catalysts, whereas δ-MnO2 exhibited the best performance for HCHO oxidation as compared with other types of MnO2. Three kinds of oxygen species were discriminated based on their Mn–O bond strength and reducibility. By quantitatively correlating the amount of specific oxygen species with the reaction rates, the catalytic roles of different oxygen species in HCHO and C6H6 oxidation were clarified. With the assistance of isotopic labeling studies, the participation of oxygen species in C6H6 oxidation—which followed a Mars–van Krevelen mechanism—was illuminated.