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

Abstract

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