Transient Spark Discharge and Ozone-Driven Nitrogen Fixation to Water

Abstract

Fixation of nitrogen and the generation of plasma-activated water are currently a significant focus within the low-temperature plasma research community. This study examines the enhancement of nitrogen fixation in water, by converting the weakly soluble nitrogen oxides (NO and NO₂) generated by transient spark (TS) to highly soluble dinitrogen pentoxide (N2​O5​) and nitric acid (HNO3​) in the gas phase. This is achieved by mixing ozone (O3) with air that has been treated by a TS discharge. Without O3, only nitrite ions (NO2−)​ are detected in the water, formed primarily due to reaction between solvated NO and NO2. With addition of O3 (400 ppm), the composition of species in water significantly changes depending on the initial NO/O3 ratio. An excess of O3 enables formation of N2​O5​ and HNO3​ in the gas and a high concentration of nitrate ions (NO3−​) in the water. With an excess of NO, the dominant gas phase product is NO2 and a mixture of NO2− and NO3− is formed in the water by reaction between solvated NO2 molecules. Despite the additional energy required for O3​ generation, the overall energy efficiency for the formation of NOx​− (NO2−​ + NO3−​) in the water increases fourfold, when enough N2O5 is formed. Further improvements are possible by optimizing the use of O3​ and ensuring all N2​O5​ is captured from the gas phase.