Oxalate route for promoting activity of manganese oxide catalysts in total VOCs’ oxidation: effect of calcination temperature and preparation method

Abstract

A novel template-free oxalate route was applied to synthesize mesoporous manganese oxides with high surface area (355 m² g⁻¹) and well-defined mesopores which can be obtained in large quantities. The physicochemical properties of the materials were characterized by means of TG, XRD, SEM, TEM, H₂-TPR and XPS techniques. All catalysts were tested on catalytic deep oxidation of benzene, and the effects of calcination temperature on the features of catalyst structure and catalytic activity were investigated. Manganese oxides prepared by oxalate route exhibited better catalytic activities for complete oxidation of benzene, toluene and o-xylene as compared with related manganese oxides prepared by other different methods (NaOH route, NH₄HCO₃ route and nanocasting strategy), and especially the temperature for benzene conversion of 90% on the oxalate-derived manganese oxide catalysts was 209 °C, which is 132 °C lower than required for the catalyst prepared by NaOH route. The catalytic performance of manganese oxide is correlated with surface area, pore size, low-temperature reducibility and distribution of surface species. The mole ratio of Mn⁴⁺/Mn²⁺ on the samples which performed with better catalytic activity was close to 1.0. This is good for the redox process of Mn⁴⁺ ↔ Mn³⁺ ↔ Mn²⁺ which is the key factor in determining the activity on MnO_x, further indicating that the oxalate route is good for keeping the distribution of manganese oxidation states at an appropriate degree. A possible process of VOCs’ complete oxidation on manganese oxide catalysts is discussed. In addition, the best catalyst was highly stable with prolonged time on stream and was resistant to water vapor.