NH3 Synthesis from N1 Compounds by Photocatalytic Technology: Promotion Mechanism, Reaction Pathways, and Efficiency Evaluation Criteria

Ammonia (NH₃) is one of the most important chemicals in high demand in human society. Given the high-energy consumption and environmental impact associated with the Haber–Bosch process, an environmentally friendly method for NH₃ synthesis under ambient conditions should be developed. The reduction of N₁ compounds, including nitrate (NO₃^–), nitrite (NO₂^–), and nitric oxide (NO_x), are more energetically favorable than that of nitrogen (N₂), avoiding the activation of inert N≡N bonds. Photocatalytic NH₃ synthesis from N₁ compounds’ reduction, which utilizes sunlight to convert contaminants into value-added chemicals, offers an intriguing approach to NH₃ synthesis. This review offers a comprehensive overview of the progress of research in photocatalysis technology for reduction of N₁ compounds to NH₃ synthesis. Insight into the efficiency promotion mechanism is provided, particularly focusing on the optimization of the photocatalyst, the activation and mass transfer of reactants, and the redox synergistic promotion. Moreover, the reaction pathways are summarized. The efficiency evaluation criteria, including accurate quantification of the NH₃ yield, comprehensive performance evaluation indicators, and NH₃ separation and recovery, are discussed to guide systematic and reliable NH₃ synthesis. Finally, the current achievements and future challenges of photocatalytic N₁ compounds to NH₃ synthesis are critically discussed.