A novel redox-precipitation method for the preparation of α-MnO2 with a high surface Mn4+ concentration and its activity toward complete catalytic oxidation of o-xylene

A novel redox-precipitation method was developed for the preparation of α-MnO2, where Mn(NO3)2 and KOH were titrated into excess KMnO4 solution at pH 8. For comparison, MnOx was also prepared using a conventional precipitation method. These materials were characterized by XRD, BET, SEM/EDS, XPS, and H2-TPR techniques, and their catalytic activities were evaluated for the complete catalytic oxidation of a typical volatile organic compound (VOCs), o-xylene. The novel method produced open porous hierarchically structured microcrystalline α-MnO2 containing almost 100% Mn4+ ion on its surface. Whereas, the conventional precipitation method produced a mixture of MnO2 and Mn3O4 with a closely packed spherical morphology containing only 31% Mn4+ ion on its surface. It was found that α-MnO2 exhibited good low-temperature reducibility, and that it could convert 100% o-xylene into CO2 at 220 °C at a space velocity of 8000 h−1, 50 °C lower than the MnOx prepared by the conventional method. The surface concentration of Mn4+ ion in α-MnO2 played a key role for its high catalytic activity for the complete oxidation of o-xylene. In addition, the open porous structure and the presence of small amount of potassium ion in the microcrystalline α-MnO2 channel may also be responsible for its excellent catalytic performance. Effects of pH and calcination temperature on its catalytic activity were investigated, and optimal preparation conditions were found. Durability of the α-MnO2 was also studied.