Species, Pathways, and Timescales for NH₃ Formation by Low-Temperature Atmospheric Pressure Plasma Catalysis

Species, pathways, and timescales for NH₃ production by plasma catalysis over transition-metal wools are determined by measuring plasma-derived species densities [N, H, and N₂(v)], quantitatively correlating consumption of these species with NH₃ formation, and measuring consumption of plasma-derived species at different residence times. These findings are enabled by a capillary flow through Ar/N₂/H₂ plasma jet reactor setup that allows for the measurement of gas-phase species densities by molecular beam mass spectrometry. Surface-mediated reactions involving N radicals are responsible for NH₃ formation over Fe, Ni, and Ag surfaces. N reacts to form NH₃ with ∼100% selectivity over Ni and Ag when H/N > 3 and % H₂ ≥ 0.5. The selectivity to ammonia drops as H and H₂ densities decrease for each catalyst. A comparison between amounts of NH₃ formed and N consumed with and without catalysts present shows that surface reactions enable higher and more selective conversion of N to NH₃ than gas-phase reactions alone. The conversion of N to NH₃ is negligible in the absence of H, demonstrating that H is required to produce NH₃ at these operating conditions. The consumption of N occurs on the same timescale as NH₃ formation, further confirming that reactions involving N contribute to NH₃ formation. Though vibrationally excited N₂ [N₂(v)] is produced in quantities exceeding N by 100-fold, consumption of N₂(v) on the catalytic surface does not contribute to NH₃ formation. These findings show that for low-temperature atmospheric pressure plasma catalysis, surface-mediated reactions among radical N and H species drive NH₃ formation.