Gas-Phase Photocatalytic Nitrogen Conversion over Nanostructured Metal-Modified Fe2O3: The Role of Surface Chemistry, Band Alignment, and Hydrogen Activation

Abstract

Photocatalytic nitrogen conversion over iron oxide-based materials remains challenging, particularly under gas-phase conditions, where reaction pathways and product distributions are poorly understood. Herein, nanostructured Fe2O3 and metal-modified Fe2O3 photocatalysts (M = Ni, Cu, Mo, Ru, and Pt) were investigated for gas-phase photocatalytic nitrogen conversion under visible-light irradiation in N2/H2 mixtures. The materials were characterized by BET, X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and UV−vis DRS to examine relationships between phase composition, surface chemistry, electronic structure, and photocatalytic response. Although samples retained hematite as the crystalline phase and displayed similar optical band gaps, metal incorporation produced distinct secondary phases, surface chemical environments, and band-edge positions. Among the catalysts, Mo/Fe2O3 showed the highest NH3 formation after prolonged irradiation, whereas Ni/Fe2O3 displayed a quasi-steady-state behavior. Pt/Fe2O3 exhibited a distinct response, with the detection of N2H4 suggesting hydrogenation-favored pathways, consistent with the presence of surface-accessible Pt0 species. In contrast, Cu/Fe2O3 showed limited NH3 formation, in agreement with its lower H2 consumption and less favorable hydrogenation behavior under the applied conditions. Overall, the results indicate that gas-phase photocatalytic nitrogen conversion over nanostructured metal-modified Fe2O3 cannot be explained by a single unified band-alignment framework but rather by the combined influence of phase composition, surface oxygen chemistry, electronic structure, and the balance between N2 and H2 activation.