As a common monocomponent metal oxide, an a-MnO 2 nanotube was synthesized by a simple hydrothermal method and developed as an ozonation catalyst for the first time. a-MnO 2 nanotubes revealed excellent catalytic activity and stability for degradation of phenol in water. The strong interaction between ozone and a-MnO 2 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 2 À , *O 2 and *O 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 a-MnO 2 , 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 a-MnO 2 .
A novel visible-light-driven AgBr-Ag-BiOBr photocatalyst was synthesized by a facile hydrothermal method. Taking advantage of both p-n heterojunctions and localized surface plasmon resonance, the p-metal-n structure exhibited a superior performance concerning degradation of methyl orange under visible-light irradiation (λ>420 nm). A possible photodegradation mechanism in the presence of AgBr-Ag-BiOBr composites was proposed, and the radical species involved in the degradation reaction were investigated. HO2(⋅)/(⋅)O2(-) played the same important role as (⋅)OH in the AgBr-Ag-BiOBr photocatalytic system, and both the electron and hole were fully used for degradation of organic pollutants. A dual role of metallic Ag in the photocatalysis was proposed, one being surface plasmon resonance and the other being an electron-hole bridge. Due to the distinctive p-metal-n structure, the visible-light absorption, the separation of photogenerated carriers and the photocatalysis efficiency were greatly enhanced.
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