Photocatalytic ammonia synthesis from nitrogen in water using iron oxides: Comparative efficiency of goethite, magnetite, and hematite

Photocatalytic ammonia synthesis from nitrogen and water presents a promising pathway for decentralized sustainable ammonia production, leveraging the abundant solar energy. In this study, we explore the efficacy of three iron oxide polymorphs - goethite (alpha-FeO(OH)), magnetite (Fe3O4), and hematite (alpha-Fe2O3) - as photocatalysts for nitrogen reduction under ultraviolet (UV) light. The materials were synthesized using hydrothermal and polymeric precursor methods, characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), UV-Vis spectroscopy, photoluminescence spectroscopy, and thermal analysis to understand their structural, surface, and optoelectronic properties. Among the materials tested, goethite demonstrated the highest ammonia production rate (20.6 mu mol g- 1h- 1), which we attribute to its larger specific surface area and the stability of its surface hydroxyl groups, which play a critical role in facilitating the protonation and electron transfer necessary for nitrogen reduction. Curiously, magnetite also displayed some activity (10.3 mu mol g- 1h- 1), likely due to the formation of a heterojunction with the co-occurring goethite phase. Hematite showed the fastest area-based production rate (1.05 mu mol m- 2h- 1), suggesting it is the polymorph with highest density of active sites for N2 reduction. This work contributes to the ongoing search for greener and lower-cost alternatives to the Haber-Bosch process, with implications for both agriculture and energy storage.