A simple strategy to tune α-MnO2 and enhance VOC oxidation via precipitation rate control

Abstract Manganese dioxide (MnO2) has been widely used in environmental catalysis. The precipitation method is commonly used for preparing MnO2 in industrial applications, but always leads to unsatisfactory removal efficiency due to the intense precipitation rates of the metal precursors. Herein, a simple strategy was designed to control the precipitation rate. On the basis of 2MnO4- (aq) + 2NH4＋ (aq) = N2 (g) + 2MnO2 (s) + 4H2O (l), the precipitation rate was well regulated by the release of NH4+ in solution. Different precipitants with various release rates of NH4+ (i.e. urea, ammonium carbonate, and sodium bicarbonate) were investigated. The lower precipitate rate obtained by urea is favorable for the formation of nanoparticle-like MnO2 with larger surface area and stronger catalytic activity than the nanorods and bulk MnO2 obtained with ammonium carbonate and sodium bicarbonate, respectively. The precipitate rate could be further optimized by changing the amount of urea to obtain the optimal MnO2 catalysts, which were efficiently applied in the complete catalytic oxidation of VOC, mineralizing 100% of 1000 ppm of ethyl acetate into CO2 at 200 °C under a high GHSV of 78,000 h-1. Various characterization methods combined to reveal that the morphology, surface Mn4+ /Mn3+ ratio, surface oxygen species and low-temperature reducibility determined the excellent catalytic performance. This work provides a new strategy for tuning MnO2 and enhancing VOC elimination via improved precipitation methods.