(337au) Species, Pathways, and Timescales for Plasma-Driven Nitrogen Fixation over Catalytic Surfaces

Plasma-driven N₂ fixation provides an renewable electricity-driven alternative to fossil fuel-based industrial processes. Pathways and timescales for reductive and oxidative conversion of N₂ by plasma catalysis are determined by measuring consumption of plasma-derived N₂ species (N, N₂(v)) and formation of desired products (NH₃, N_xO_y) in reactors with ~10^-5 - ~10^-3 s residence times with or without catalyst. An atmospheric pressure plasma jet is coupled with a packed bed of nonporous, unsupported transition metal wools to study these conversions, with species densities enumeration performed by molecular beam mass spectrometry (MBMS). For reduction of N₂ in Ar/N₂/H₂ mixtures, rates and quantities of NH₃ formation correlate with 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. Timescales for N consumption in the gas phase (> 10^-3 s) are larger than timescales for N consumption with catalyst present (~10^-5 s). 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 magnitude of NO formation is lower than N consumption from gas-phase reactions. With Ag catalyst present, the rate of N consumption increases, and conversion of N to NO is ~100% when the O₂ content in the gas phase is sufficient. By measuring consumption of N and formation of NO with various contact times in the catalyst bed and performing a parameter estimation for a reduced kinetic model of surface and gas-phase reactions, we show that the rate of consumption of N from surface reactions is limited by external mass transfer of N from the gas phase to the catalyst surface. Process conditions exist in which the consumption of N in the gas phase limits the ability N to participate in surface reactions that contribute to NO formation. These findings, along with the design and operation of a reactor that enables these measurements, will be discussed.