Availability and reactivity of N2(v) for NH3 synthesis by plasma catalysis

Abstract

Production of vibrationally excited N₂ (N₂(v)) in atmospheric pressure nonthermal plasma and loss of N₂(v) by gas-phase reactions and reactions on catalytic surfaces are analyzed to examine the role of N₂(v) in NH₃ formation by plasma catalysis. Vibrational state-to-state kinetic models complemented with molecular beam mass spectrometry (MBMS) measurements demonstrate that N₂(v> 0) is produced with densities 100× greater than the density of N radicals by a radiofrequency atmospheric pressure plasma jet. The experimentally measured loss of N₂(v) corresponds with a state-to-state kinetic model that describes loss of N₂(v) by surface-mediated vibrational relaxation without consideration of reactions that convert N₂(v) to NH₃ over the catalyst surface. Rate constants for vibrational relaxation of N₂(v) on catalyst surfaces exceed upper bounds on proposed rate constants for NH₃ formation reactions from N₂(v) over Fe when v < 9, Ni when v < 18, and Ag when v < 39, which indicates that only higher vibrational levels can possibly contribute to catalytic NH₃ formation faster than they undergo vibrational relaxation on the surface. Densities of N₂(v> 8), vibrational levels that can possibly react over Fe to form NH₃ faster than they undergo vibrational relaxation, are less than or similar to N densities at the inlet of the catalyst bed and measured NH₃ formation for the investigated conditions in this work, while densities of N₂(v> 17) and N₂(v> 38) are orders of magnitude below the N density at the inlet of the catalyst bed and the measured NH₃ formation. The loss of N₂(v) by vibrational relaxation on the surface limits the ability of N₂(v) to contribute to catalytic NH₃ formation and explains why N₂(v) does not produce NH₃ in quantities that are comparable to NH₃ formation from N even though N₂(v > 0) is more abundantly produced by the plasma.