Selective Synthesis of Nitrite and Nitrate by Liquid-Phase Plasma Using a Dual-Cell: Role of Active Species

Plasma-assisted nitrogen fixation has emerged as a promising strategy for sustainable nitrate production. However, the coexistence of multiple interfaces and complex multi-step reaction pathways within the plasma-liquid system often leads to the formation of mixed nitrogen species, posing a significant challenge for achieving high product selectivity. In this study, a dual-cell reactor was introduced in liquid-phase plasma (LPP) system, enabling selective product distribution. Optical emission spectroscopy revealed pronounced signals corresponding to the second positive system (SPS) of N2 and the first negative system (FNS) of N2+, indicative of strong plasma excitation and ionization processes that facilitated the formation of reactive nitrogen oxide intermediates. These species were subsequently converted into aqueous NO2− and further oxidized into NO3− only in the reaction cell where reactive species are generated. The effects of key parameters, including electrode material, treatment time, solution pH, and discharge conditions, were comprehensively evaluated. As a result, the reaction cell achieved a nitrate selectivity of 98.9%, whereas the absorption cell achieved a nitrite selectivity of 100%. Findings from EPR and scavenger analyses collectively provide a detailed mechanistic understanding of LPP-driven nitrogen fixation and highlight the importance of controlling plasma parameters to achieve highly selective production of nitrogen compounds.