Photocatalytic ammonia synthesis: Recent progress and future

Abstract

Ammonia (NH₃) is one of the most important commodity chemicals in today's chemical industry. Industrially, ammonia is synthesized via the Haber-Bosch process at high temperature and pressure (typically 400 °C and 200 atm). In nature, the nitrogenase enzyme can convert N₂ to NH₃ at ambient conditions, motivating the search for similar sustainable technologies for industrial-scale NH₃ production. Over the past few years, photocatalytic ammonia production using sunlight and photocatalysts has attracted much attention, allowing the reduction of N₂ to NH₃ under very mild reaction conditions. Whilst the rates of photocatalytic ammonia synthesis are still a long way off practical requirements, some promising photocatalytic materials have already been identified which encourage wider research in this field. This review aims to capture recent advances in photocatalytic N₂ fixation to NH₃, by encompassing fundamental aspects of photocatalytic ammonia synthesis, as well as effective photocatalyst and reactor design strategies. Further, the review offers some practical guidelines to researchers regarding the appropriate selection of ammonia detection methods and the performance assessment of ammonia synthesis photocatalysts. The overarching aims of this review are i) to support the development of solar-driven ammonia synthesis, and ii) to assist researchers in moving into this exciting new research space.