An alpha-MnO2 nanotube used as a novel catalyst in ozonation: performance and the mechanism

As a common monocomponent metal oxide, an alpha-MnO2 nanotube was synthesized by a simple hydrothermal method and developed as an ozonation catalyst for the first time. alpha-MnO2 nanotubes revealed excellent catalytic activity and stability for degradation of phenol in water. The strong interaction between ozone and alpha-MnO2 in water was observed and confirmed as a critical step of catalysis. The IR analysis and the influence of phosphate showed that the surface hydroxyl groups and chemisorbed water acted as the active sites in promoting active oxygen species, while Lewis acid sites were confirmed as reactive centers for catalytic ozonation in the aqueous phase. The (OH)-O-center dot, O-2(center dot-), O-star(2) and O-star were not mainly involved in the catalytic ozonation of phenol based on the corresponding experimental results. According to the X-ray photoelectron spectroscopy (XPS) results, in the presence of alpha-MnO2, electrons from the surface Mn(III) were responsible for the catalysis and the oxidation of lattice oxygen enhanced the reversion of Mn(IV) to Mn(III). The balance between Mn(III)/Mn(IV) and O(-II)/O(0) was the primary factor for the catalytic performance of alpha-MnO2.