Catalytic decomposition of gaseous ozone over manganese dioxides with different crystal structures

Ozone is a ubiquitous pollutant and manganese dioxide (MnO2) has been widely used for ozone decomposition. However, the effect of MnO2 structure on ozone decomposition has never been investigated. Three tunnel-structure polymorphs, i.e., α-, β- and γ-MnO2 were prepared and characterized by BET, TEM, XRD, H2-TPR, O2-TPD, NH3-TPD, TGA-MS and XPS. The activity of three MnO2 polymorphs for ozone decomposition followed the order of α- > γ- > β-MnO2. The α-MnO2 owned the largest specific surface area and lowest average oxidation state of Mn. Furthermore, the adsorbed oxygen species on the surface of α-MnO2 were more easily reduced. In-situ Raman spectroscopy results showed that peroxide species formed during ozone decomposition, and over α-MnO2 they were more easily decomposed by increasing reaction temperature. It was found that the catalytic activity of MnO2 strongly depended on the density of oxygen vacancies. Accordingly, the ozone decomposition mechanism based on the involvement and recycling of oxygen vacancy (VO) is proposed. The decomposition of peroxide species is a rate-limiting step. These findings are helpful for designing more effective catalyst for ozone removal.