Mini Review on the Photocatalytic Removal of Gaseous Ammonia: Current Status and Challenges

Ammonia gas (NH₃) is a notorious malodorous pollutant released mainly in agriculture and industry. With the increasing demand for ammonia, environmental pollution caused by ammonia discharge has seriously threatened human health and safety. Due to the discrete emission and low concentration of NH₃, photocatalytic oxidation is an economical and efficient treatment strategy. TiO₂, as a common photocatalyst, has been widely used by researchers for the photocatalytic removal of NH₃. In addition, surface modification, element doping, semiconductor recombination and metal loading are used to improve the utilization rate of solar energy and carrier of TiO₂ so as to find a catalyst with high efficiency and high N₂ selectivity. Further, at present, there are three main removal mechanisms of NH₃ photocatalytic oxidation: ·NH₂ mechanism, iSCR mechanism and N₂H₄ mechanism. Among them, N₂H₄ mechanism is expected to be the main removal path of NH₃ photocatalytic oxidation in the future because the removal process does not involve NO_x and nitrate. This review summarizes recent studies on the photocatalytic oxidation of NH₃, focusing primarily on NH₃ removal efficiency, N₂ selectivity, and the underlying removal mechanisms. Additionally, the potential future applications of NH₃ photocatalytic oxidation are discussed.