Photocatalytic Platforms for Removal of Ammonia from Gaseous and Aqueous Matrixes: Status and Challenges

The unbridled expansion of advanced agricultural and industrial facilities has led to the release of large amounts of ammonia (NH3), one of the most pernicious malodorants. Photocatalytic approaches have attracted widespread attention as promising options to turn pollutants into environmentally benign end products under favorable operating conditions. Although most approaches rely on the titanium dioxide (TiO2) structures, many other engineered photocatalytic materials can also be used to enhance overall efficiency and practicality of such systems. This Review provides a comprehensive overview of the available options for discrete mitigation of NH3 in gaseous and aqueous matrixes. The performances of photocatalytic materials and systems are compared with respect to quantum and space-time yields. Special attention has been paid to the reaction mechanisms prevalent during photocatalytic removal of NH3 in each medium coupled with the production of end products (e.g., hydrogen and nitrogen) through NH3 splitting. The effects of process and operational variables (e.g., irradiation time, relative humidity, mode of operation, and environmental matrix type) on performance are also discussed along with the intrinsic properties of the applied materials (e.g., surface functional sites and structure). Existing obstacles, such as the formation of hazardous byproducts through complicated reaction pathways, are explored along with future challenges.