Formation of Nitrogen Oxides by Nanosecond Pulsed Plasma Discharges in Gas–Liquid Reactors

Abstract

A gas–liquid-film flow reactor with a nanosecond pulsed power supply was utilized to produce nitrogen oxides from Ar/N2 mixtures (gas phase) and deionized water (liquid phase). Chemical analysis of the stable products found in both the gas and liquid phases was performed and chemical quenching was incorporated for the liquid phase samples in order to eliminate post plasma reactions. Significant amounts of NO and NO2 in the gas phase and $${\text{NO}}_{2}^{ - }$$ and $${\text{NO}}_{3}^{ - }$$ in the liquid phase were determined using FTIR spectroscopy and ion chromatography, respectively. The production rate of all nitrogen oxides produced increased significantly with N2 concentration while H2O2 formation decreased slightly. The gas temperature of the plasma was approximately 525 K and was unaffected by N2 concentration while the electron density ranged from 1 × 1017 cm−3 in pure Ar to 5.5 × 1017 cm−3 in 28% N2. The role of the $${\cdot {\text{OH}}}$$ in the reaction pathway was assessed by adding CO as a gas phase radical scavenger showing that $${\cdot {\text{OH}}}$$ is essential for conversion of the gas phase NO and NO2 into water soluble $${\text{NO}}_{2}^{ - }$$ and $${\text{NO}}_{3}^{ - }$$ . Conversely, atomic oxygen originating from water is likely responsible for NO and NO2 generation. Experiments with N2/O2/Ar mixtures and air showed a significant increase in NO2 production caused by the additional generation of reactive oxygen species. An overall energy yield for all nitrogen oxides produced in the most efficient case was 50 eV/molecule.