Oxidation processes of NO for production of reactive nitrogen species in plasma activated water

Abstract

To understand the mechanisms for selective productions of NO2− and NO3− as reactive nitrogen species (RNS) in plasma activated water, we studied the oxidation reactions of NO as the primary species supposing O, O2, O3, OH, and H2O2 as possible oxidants. A coaxial-type dielectric barrier discharge reactor was employed in cascade connection to a reservoir and/or a water bubbler. First, the densities of NO and NO2 in the gas phase were measured with gas sensors in two gas systems: N2 admixed with water vapor and N2 admixed with O2, for comparison. It was found that while the density of NO in the latter case was one third of that in the former case owing to the difference in the first-step oxidation to NO2 or HNO2, the total yields of RNS in both cases were comparable. To investigate the oxidation pathways, admixing NO and NO2 in the downflow of the reactor was attempted to estimate their reactions with those oxidants and accomodation behaviours on the water surface. Injecting the effluent gas from discharge plasma into solutions of O2, O3, and H2O2, and injecting O3 into PAW, vice versa, were attempted to inspect the reactions in the liquid phase or at the gas–water interface. Based on the obtained results, it was found that, in the N2-H2O gas system, OH functions as the oxidant of NO to HNO2, exclusively yielding NO2− in solution with no effective second-step oxidation to NO3 or HNO3. On the other hand, in the N2-O2 gas system, O3 functions as the major oxidant in the successive oxidation from NO to NO2 to NO3, producing NO3− predominant PAW; however, the second-step oxidation in the gas phase is competing with the oxidation reactions in the liquid phase.