Sunlight-Driven Nitrate-to-Ammonia Reduction with Water by Iron Oxyhydroxide Photocatalysts

The photocatalytic reduction of harmful nitrates (NO₃^-) in strongly acidic wastewater to ammonia (NH₃) under sunlight is crucial for the recycling of limited nitrogen resources. This study reports that a naturally occurring Cl^--containing iron oxyhydroxide (akaganeite) powder with surface oxygen vacancies (β-FeOOH(Cl)-OVs) facilitates this transformation. Ultraviolet light irradiation of the catalyst suspended in a Cl^--containing solution promoted quantitative NO₃^--to-NH₃ reduction with water under ambient conditions. The photogenerated conduction band electrons promoted the reduction of NO₃^--to-NH₃ over the OVs. The valence band holes promoted self-oxidation of Cl^- as the direct electron donor and eliminated Cl^- was compensated from the solution. Photodecomposition of the generated hypochlorous acid (HClO) produced O₂, facilitating catalytic reduction of NO₃^--to-NH₃ with water as the electron donor in the entire system. Simulated sunlight irradiation of the catalyst in a strongly acidic nitric acid (HNO₃) solution (pH ∼ 1) containing Cl^- stably generated NH₃ with a solar-to-chemical conversion efficiency of ∼0.025%. This strategy paves the way for sustainable NH₃ production from wastewater.