Catalytic Activation of Molecular Nitrogen by Enzyme-Inspired Metal–Organic Catalysts

Photocatalytic nitrogen fixation under ambient conditions has emerged as an eco-friendly and energy-efficient alternative to the traditional Haber–Bosch process. Design of catalysts to this end can be directed with insight gained from nitrogenase enzymes. In this study, drawing from previous computational analyses of electronic and structural factors that prime nitrogenase for N2 activation, we report the synthesis, characterization, and performance evaluation of low-valent iron and cobalt nanoparticles supported on two distinct carbon-based materials─multiwalled carbon nanotubes (CNTs) and graphitic carbon nitride (g-C3N4)─denoted as Fe0@CNTs, Co0@CNTs, Fe0@g-C3N4, and Co0@g-C3N4. The catalysts were synthesized via a two-step process involving impregnation with metal precursors followed by thermal treatment in a hydrogen atmosphere. Comprehensive physicochemical analyses (XRD, SEM–EDS, TGA–DTG, BET, UV–vis, XPS) confirmed the successful reduction of metal ions to their zerovalent state and their effective anchoring on the supports. Among the prepared catalysts, Fe0@CNTs exhibited superior photocatalytic performance with an ammonia yield of 660.31 μmol/gcat at pH 5.5, which further increased to 756.33 μmol/gcat at pH 3. This enhancement was attributed to its high surface area (62.39 m2/g), increased light absorption, favorable band gap, and strong interaction with nitrogen molecules. Linear sweep voltammetry (LSV) measurements indicated that Fe0@CNTs has a lower onset potential for the nitrogen reduction reaction (NRR) and better selectivity over the competing hydrogen evolution reaction (HER). Hydrazine and nitrate byproducts were also monitored, with Fe0@CNTs showing minimal nitrate formation and relatively high hydrazine intermediate yield, supporting the N2-to-NH3 pathway. Furthermore, reusability tests demonstrated that Fe0@CNTs maintained substantial activity over six consecutive cycles with moderate loss in efficiency. These results highlight the potential of Fe0@CNTs as a promising catalyst for sustainable nitrogen fixation.