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.