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.