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

The photocatalytic reduction of harmful nitrates (NO 3 - ) in strongly acidic wastewater to ammonia (NH 3 ) 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 3 - -to-NH 3 reduction with water under ambient conditions. The photogenerated conduction band electrons promoted the reduction of NO 3 - -to-NH 3 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 2 , facilitating catalytic reduction of NO 3 - -to-NH 3 with water as the electron donor in the entire system. Simulated sunlight irradiation of the catalyst in a strongly acidic nitric acid (HNO 3 ) solution (pH ∼ 1) containing Cl - stably generated NH 3 with a solar-to-chemical conversion efficiency of ∼0.025%. This strategy paves the way for sustainable NH 3 production from wastewater.