Advances in photo-catalytic oxidation of NH3 over modified TiO2 catalysts: Reaction pathways, improvement strategy and promotion mechanism

Photo-catalytic oxidation of NH3 (NH3-PCO) by TiO2 is a promising strategy for the removal of unorganized ammonia waste gas with low concentration. However, pure TiO2 photo-catalyst with band gap of 3.2 eV has the weaknesses of low utilization rate of visible light, fewer acid sites and easy recombination of electron-hole pairs, all which limit its application in practical treatment process. In recent years, researchers have conducted extensive research on improving photo-catalytic performance via the optimization of TiO2 for NH3 oxidation. At present, the proposed modification methods mainly include noble metal deposition, element doping, semiconductor coupling, dye sensitization and multi-element co-doping. Focusing on the problems of TiO2 applied in NH3-PCO, modification methods are classified and summarized in terms of improving the activity, stability, N2 selectivity of NH3-PCO and enhancing the adsorption capacity of NH3. In addition, the promotion mechanism of each modification method and the materials (Pt, porphyrin, WO3, Cu, C, N, F and C-N co-doping, etc.) doped with TiO2 that have better NH3 removal effect are introduced, in which doping of non-metallic elements C, N can not only reduce the band gap of TiO2 to 2.9 eV and 2.5 eV respectively, but also introduce defect structure to inhibit the recombination of carrier. Otherwise, the reaction pathways of NH3-PCO by TiO2 are summarized and the influence of pH, optimized elements (Pt/TiO2, TPP/TiO2, Cu/TiO2) and initial concentration on the reaction pathways are analyzed. It is hoped that the research in this work can provide valuable information for industrial application of NH3-PCO catalyst.