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

Production of vibrationally excited N-2 (N-2(v)) in atmospheric pressure nonthermal plasma and loss of N-2(v) by gas-phase reactions and reactions on catalytic surfaces are analyzed to examine the role of N-2(v) in NH3 formation by plasma catalysis. Vibrational state-to-state kinetic models complemented with molecular beam mass spectrometry (MBMS) measurements demonstrate that N-2(v> 0) is produced with densities 100x greater than the density of N radicals by a radiofrequency atmospheric pressure plasma jet. The experimentally measured loss of N-2(v) corresponds with a state-to-state kinetic model that describes loss of N-2(v) by surface-mediated vibrational relaxation without consideration of reactions that convert N-2(v) to NH3 over the catalyst surface. Rate constants for vibrational relaxation of N-2(v) on catalyst surfaces exceed upper bounds on proposed rate constants for NH3 formation reactions from N-2(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 N-2(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 N-2(v> 17) and N-2(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 N-2(v) by vibrational relaxation on the surface limits the ability of N-2(v) to contribute to catalytic NH3 formation and explains why N-2(v) does not produce NH3 in quantities that are comparable to NH3 formation from N even though N-2(v > 0) is more abundantly produced by the plasma.