Photocatalytic Conversion of Nitrogen to Ammonia with Water on Surface Oxygen Vacancies of Titanium Dioxide

TLDR

Ammonia is essential for modern society, yet its current Haber‑Bosch production requires extreme pressure and temperature, and existing semiconductor photocatalysts achieve only low efficiencies. The study aims to achieve photocatalytic ammonia synthesis from water and nitrogen at ambient pressure and room temperature. Ti³⁺ species at surface oxygen vacancies serve as N₂ adsorption and electron‑trapping sites, enabling efficient N₂ reduction to NH₃ under UV irradiation. Under UV irradiation in water with N₂, the TiO₂ system generates ammonia with a 0.02 % solar‑to‑chemical efficiency, the highest reported for early photocatalytic systems, indicating promise for green ammonia production.

Abstract

Ammonia (NH3) is an essential chemical in modern society. It is currently manufactured by the Haber-Bosch process using H2 and N2 under extremely high-pressure (>200 bar) and high-temperature (>673 K) conditions. Photocatalytic NH3 production from water and N2 at atmospheric pressure and room temperature is ideal. Several semiconductor photocatalysts have been proposed, but all suffer from low efficiency. Here we report that a commercially available TiO2 with a large number of surface oxygen vacancies, when photoirradiated by UV light in pure water with N2, successfully produces NH3. The active sites for N2 reduction are the Ti3+ species on the oxygen vacancies. These species act as adsorption sites for N2 and trapping sites for the photoformed conduction band electrons. These properties therefore promote efficient reduction of N2 to NH3. The solar-to-chemical energy conversion efficiency is 0.02%, which is the highest efficiency among the early reported photocatalytic systems. This noble-metal-free TiO2 system therefore shows a potential as a new artificial photosynthesis for green NH3 production.