(220c) Pathways for NH3 Formation from N, H, and N2(v) in Plasma Catalysis

NH₃ synthesis by plasma catalysis at atmospheric pressure has been previously demonstrated [1], but pathways for NH₃ formation involving plasma-derived radical (N, H) and vibrationally excited species (N₂(v)) over catalytic surfaces have not been clearly identified. We measure densities of N, H, and N₂(v) produced by an atmospheric pressure plasma jet using threshold ionization molecular beam mass spectrometric techniques [2-3] and correlate consumption of these species with NH₃ formation in a packed bed of nonporous metal wools of Fe, Ni, and Ag. Results show that formation of NH₃ and consumption of N, H, and N₂(v) is faster over the catalytic surfaces than in the gas phase. N consumption correlates with NH₃ formation when the ratio between the H density and N density at the inlet is greater than 3 (stoichiometric for NH₃ formation) for all three metals. For Ni and Ag, N consumption is ~100% selective to NH₃ formation. For Fe, selectivity to NH₃ formation is dependent on the H₂ density. When the ratio between H density and N density at the inlet is less than 1, N consumption does not correlate with NH₃ formation for all three metals, indicating that NH₃ formation is limited by H density at these conditions. N₂(v) consumption exhibits no correlation with NH₃ formation for any of the examined metals, indicating that surface-mediated vibrational relaxation of N₂(v) is more significant than dissociative adsorption at these conditions. These correlations between NH₃ formation and consumption of plasma-derived intermediates, preliminary spectroscopic studies probing identity of catalyst-bound surface species formed in N₂/H₂ plasma, and design and operation of a reactor that enables these measurements will be discussed.

References

[1] M. L. Carreon, J. Phys. D: Appl. Phys. 52 (2019) 483001

[2] J. Jiang et al., J. Phys. D: Appl. Phys. 55 (2022) 225206

[3] J. Jiang et al., in preparation.