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