Analysis of ammonia synthesis pathways from nitrogen–hydrogen plasma on Ni-based catalysts: a combined experimental and simulation study

Abstract

Ammonia (NH₃), a zero-carbon energy carrier with high hydrogen density, is pivotal for hydrogen storage and sustainable synthesis. Plasma-catalytic NH₃ synthesis offers a promising alternative to energy-intensive Haber–Bosch processes; however, its underlying mechanism remains elusive, particularly regarding the specific roles of the catalyst surface. This study investigates N₂/H₂ for NH₃ synthesis using a dielectric barrier discharge (DBD) plasma-coupled Ni/γ-Al₂O₃ catalyst at 20 kHz, 1 atm, and 373 K under varying voltages. A zero-dimensional kinetic model (ZDPlasKin) incorporating 44 species (atoms, radicals, excited species, ions, and surface adsorbates) predicted NH₃ yields within 15% of experimental values. Simulations reveal that increased input voltage enhances NH₃ synthesis rates. Among N₂(v1–8) vibrational states, N₂(v8) exhibits the highest reactivity, though rates remain substantially lower than H₂(v1–3). The non-stoichiometric ratio between N₂ consumption and NH₃ production suggests that nitrogen undergoes both gas-phase reactions and surface dissociative adsorption. Simultaneously, optical emission spectroscopy confirms electronic excitation and ionization processes through the identification of N₂(C–B) and N₂⁺(B–X) band systems. This plasma-induced excitation drives dual-pathway kinetics, vibrational excitation accelerates N₂ dissociative adsorption and weakens NH₃(s) binding to expedite desorption, while electronic excitation enhances surface reactivity. By mitigating the adsorption–desorption kinetic limitations, this synergistic regulation facilitates an equilibrium shift toward enhanced NH₃ yields. Building upon this mechanism, we propose practical optimization strategies, such as operating at elevated temperatures and implementing efficient discharge systems.