Selective Nitrate-to-Ammonia Transformation on Surface Defects of Titanium Dioxide Photocatalysts

Ammonia (NH3) is an essential chemical in modern society, currently manufactured via the Haber-Bosch process with H-2 and N-2 under extremely high pressure (>200 bar) and high-temperature conditions (>673 K). Toxic nitrate anion (NO3-) contained in wastewater is one potential nitrogen source. Selective NO(3)(-)to-NH3 transformation via eight-electron reduction, if promoted at atmospheric pressure and room temperature, may become a powerful recycling process for NH3 production. Several photocatalytic systems have been proposed, but many of them produce nitrogen gas (N-2) via five-electron reduction of NO3-. Here, we report that unmodified TiO2, when photoexcited by ultraviolet (UV) light (lambda > 300 nm) with formic acid (HCOOH) as an electron donor, promotes selective NO(3)(-)to-NH3 reduction with 97% selectivity. Surface defects and Lewis acid sites of TiO2 behave as reduction sites for NO3-. The surface defect selectively promotes eight-electron reduction (NH3 formation), while the Lewis acid site promotes nonselective reduction (N-2 and NH3 formation). Therefore, the TiO2 with a large number of surface defects and a small number of Lewis acid sites produces NH3 with very high selectivity.