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

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