Resolving Formation, Conversion, and Capture of NOx in Nitrogen Fixation by Atmospheric-Pressure Plasmas

Abstract Atmospheric-pressure plasmas enable electrified, distributed nitrogen fixation from air and water under mild conditions, offering a promising alternative to conventional thermochemical, centralized approaches. However, the complex multiphase mechanisms in these processes, specifically the formation of NOx and subsequent transformation to aqueous NOx–, remain poorly resolved. In this study, we combine simultaneous quantitative gas- and liquid-phase diagnostics across dry gas-only, humid gas-only, and plasma-liquid configurations, supported by NO/NO2 control experiments and systematic variation of the N2/O2 ratio and solution pH, to shed further mechanistic insight. We show that NO2 is not only trapped far more efficiently than NO, but also enhances the overall uptake of NO when both are present. Solution pH strongly influences both capture efficiency and product selectivity, with basic pH increasing overall NOx capture as NO2–, as compared to acidic pH which leads to NO3– as the primary product. In addition, water vapor negatively impacts fixed nitrogen product yields, while direct plasma-liquid contact improves trapping efficiency compared with remote plasma setups. The study identifies key challenges and reveals opportunities to optimize production in future plasma technologies for nitrogen fixation.