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 H₂ and N₂ 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 NO₃^–-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 (N₂) via five-electron reduction of NO₃^–. Here, we report that unmodified TiO₂, when photoexcited by ultraviolet (UV) light (λ > 300 nm) with formic acid (HCOOH) as an electron donor, promotes selective NO₃^–-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 (N₂ 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.