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

The photocatalytic reduction of harmful nitrates (NO) in strongly acidic wastewater to ammonia (NH) 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 NO-to-NH reduction with water under ambient conditions. The photogenerated conduction band electrons promoted the reduction of NO-to-NH 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 O, facilitating catalytic reduction of NO-to-NH with water as the electron donor in the entire system. Simulated sunlight irradiation of the catalyst in a strongly acidic nitric acid (HNO) solution (pH ∼ 1) containing Cl stably generated NH with a solar-to-chemical conversion efficiency of ∼0.025%. This strategy paves the way for sustainable NH production from wastewater.