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 (N 2 ​O 5 ​) and nitric acid (HNO 3 ​) in the gas phase. This is achieved by mixing ozone (O 3 ) with air that has been treated by a TS discharge. Without O 3 , only nitrite ions (NO 2 − )​ are detected in the water, formed primarily due to reaction between solvated NO and NO 2 . With addition of O 3 (400 ppm), the composition of species in water significantly changes depending on the initial NO/O 3 ratio. An excess of O 3 enables formation of N 2 ​O 5 ​ and HNO 3 ​ in the gas and a high concentration of nitrate ions (NO 3 − ​) in the water. With an excess of NO, the dominant gas phase product is NO 2 and a mixture of NO 2 − and NO 3 − is formed in the water by reaction between solvated NO 2 molecules. Despite the additional energy required for O 3 ​ generation, the overall energy efficiency for the formation of NOx​ − (NO 2 − ​ + NO 3 − ​) in the water increases fourfold, when enough N 2 O 5 is formed. Further improvements are possible by optimizing the use of O 3 ​ and ensuring all N 2 ​O 5 ​ is captured from the gas phase.