NO formation by N₂/O₂ plasma catalysis: The impact of surface reactions, gas-phase reactions, and mass transport

Pathways and timescales relevant to facilitate plasma-assisted N₂-O₂ reactions are assessed by measuring the consumption of plasma-derived N and the formation of NO in the gas phase and over Ag catalytic surfaces. These measurements are enabled by a setup that enables N₂ activation in an atmospheric pressure RF plasma jet, enables O₂ addition in the plasma afterglow, facilitates reactions over an Ag wire catalyst, and allows species density quantification by molecular beam mass spectrometry. Gas-phase reactions consume N but do not form NO with high selectivity. The presence of the non-porous Ag wire catalyst increases the rate of N conversion to NO, though mass transfer processes, not surface reactions, dictate the rate of N consumption. When O₂ concentrations and the ratio of the surface area of the catalyst to the void volume of the reactor are high (3–5 mol% O₂, 10900 m⁻¹), N conversion to NO reaches 100 % selectivity. When both N₂ and O₂ are fed through the plasma jet, gas-phase NO production increases 10×, although plasma and gas-phase processes do not exclusively produce NO. Above a threshold NO density, N cannot diffuse to the catalyst surface faster than it is consumed in the gas phase by reactions with NO. Thus, the use of heterogeneous catalysts to enhance plasma-driven N_xO_y formation and control N_xO_y product selectivity is limited to cases where diffusive transport of N from the gas phase to the catalyst surface is faster than consumption of N from gas-phase reactions with NO.