(335b) Pathways and Timescales for N2 Conversion By Reductive or Oxidative Routes in Plasma Catalysis

We report pathways and timescales for reductive and oxidative conversion of plasma-derived N₂ species (N, N₂(v)) using a combination of molecular beam mass spectrometry (MBMS) measurements to enumerate densities of plasma-derived species and short contact time reactors with sub-millisecond residence times. For reduction of N₂ in Ar/N₂/H₂ mixtures, rates and quantities of NH₃ formation correlate with rates and quantities of N consumption, indicating that NH₃ formation occurs from surface-mediated reactions involving N radicals. When densities of H and H₂ are sufficient, conversion of N to NH₃ is 100% selective over Fe, Ni, and Ag. Through MBMS-validated state-to-state vibrational kinetic modeling, we show that N₂(v), though produced in quantities exceeding N by 100×, is not reactive for NH₃ formation at the investigated experimental operating conditions and loss of N₂(v) occurs due to vibrational relaxation on the catalyst surface. For oxidation of N₂ in Ar/N₂/O₂ mixtures, the timescale for N consumption in the gas phase decreases from ~10^-3 to ~10^-4 s as the O₂ concentration increases from 0.1-5%, with gas-phase NO formation increasing as the O₂ concentration increases. When catalyst is present in the reactor, the timescale for N consumption decreases, and MBMS measurements show that NO formation with Fe or Ag catalyst exceeds NO formation from gas-phase reactions alone at equal residence time, although NO formation is always less than N consumption. These findings and timescales for N consumption in the gas phase and with catalyst present in oxidative and reductive environments will be discussed.