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

Abstract

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 (α-FeO(OH)), magnetite (Fe₃O₄), and hematite (α-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 µmol 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 µmol 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 µmol 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.