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

Abstract

Species, pathways, and timescales for NH 3 production by plasma catalysis over transition-metal wools are determined by measuring plasma-derived species densities [N, H, and N 2 ( v )], quantitatively correlating consumption of these species with NH 3 formation, and measuring consumption of plasma-derived species at different residence times. These findings are enabled by a capillary flow through Ar/N 2 /H 2 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 NH 3 formation over Fe, Ni, and Ag surfaces. N reacts to form NH 3 with ∼100% selectivity over Ni and Ag when H/N > 3 and % H 2 ≥ 0.5. The selectivity to ammonia drops as H and H 2 densities decrease for each catalyst. A comparison between amounts of NH 3 formed and N consumed with and without catalysts present shows that surface reactions enable higher and more selective conversion of N to NH 3 than gas-phase reactions alone. The conversion of N to NH 3 is negligible in the absence of H, demonstrating that H is required to produce NH 3 at these operating conditions. The consumption of N occurs on the same timescale as NH 3 formation, further confirming that reactions involving N contribute to NH 3 formation. Though vibrationally excited N 2 [N 2 ( v )] is produced in quantities exceeding N by 100-fold, consumption of N 2 ( v ) on the catalytic surface does not contribute to NH 3 formation. These findings show that for low-temperature atmospheric pressure plasma catalysis, surface-mediated reactions among radical N and H species drive NH 3 formation.

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