Multiscale modeling of radical and vibrational pathways in plasma-assisted ammonia synthesis on Fe(110) and Ni(111)

Low-temperature plasma (LTP)-assisted ammonia synthesis is a promising alternative to the Haber-Bosch process for decentralized, renewable-energy-driven production. However, the roles of vibrationally excited nitrogen (N_2,ν) and plasma-generated N˙/H˙ radicals remain debated. We apply first-principles multiscale modeling, combining density functional theory (DFT) and a packed-bed reactor model, to disentangle these contributions to LTP-assisted NH₃ synthesis over Fe(110) and Ni(111). Vibrational excitation alone cannot explain the observed insensitivity of ammonia production to catalyst identity: it enhances rates on Ni but is deleterious on Fe, where accelerated nitrogen dissociation covers the surface with nitrogen-containing intermediates. Surface quenching of vibrationally excited nitrogen increases the predicted ammonia outlet density on both metals, but does not eliminate the strong catalyst dependence under conditions that consider only vibrational excitations. In contrast, Eley-Rideal reactions involving N˙ and H˙ radicals are the dominant source of plasma-induced ammonia formation on both catalysts, although hydrogenation following a Langmuir-Hinshelwood mechanism remains kinetically competitive on Ni. N˙ radicals supply nitrogen to the otherwise nitrogen-deficient Ni surface, but only when H˙ is present to hydrogenate the NH intermediates they create. N˙ supplied without H˙ instead saturates Ni and gives far less NH₃ than vibrational excitation alone. H˙ radicals in turn provide an alternative hydrogenation pathway that overcomes the slow Langmuir-Hinshelwood surface reactions on Fe. This synergy brings the predicted NH₃ outlet densities on Fe and Ni to within 13% of one another, explaining the observed insensitivity to metal catalyst identity and highlighting the central role of radical chemistry in plasma-assisted ammonia synthesis.