Plasma-assisted nitrogen fixation: the effect of water presence

Electrification of the nitrogen fixation industry via plasma technology shows promising prospects to overcome growing environmental and economic shortcomings. The basic idea relies on using renewable energy sources and water to replace natural gas as the source of energy and hydrogen, thus, minimizing the tremendous CO2 footprint. However, the effect of water on plasma-assisted nitrogen fixation remains unclear in terms of the energy efficiency of the process. In this work, we investigate the efficiency of plasma-assisted nitrogen fixation toward NOx species production using two plasma systems. A dry pin-to-pin plasma is compared with a pin-to-liquid configuration, quantifying nitrogen-containing species in both gas and liquid phases, using infrared spectroscopy and ion chromatography, respectively. The main gaseous products detected in the systems are NO, NO2, HNO2, and N2O, while the liquid phase shows the presence of NO2- and NO3- ions. The main mechanisms of these species' generation are illustrated, emphasizing the effect of the plasma/liquid interface. Particularly, the experiments with isopropanol used as a scavenger of OH radicals revealed that these radicals are responsible for approximate to 30% of the generated NOx-. Despite this, the presence of water in the reactive zone yet decreases the nitrogen fixation energy efficiency by approximate to 20% in comparison with nitrogen fixation in dry air. Among the possible reasons, the energy loss on water evaporation, the quenching of N-2 excited states, and the less efficient extended Zeldovich mechanism are proposed and discussed.

Automated summary

Electrification of the nitrogen fixation industry via plasma technology using renewable energy sources and water as the source of energy and hydrogen shows promising prospects. However, the effect of water on plasma-assisted nitrogen fixation remains unclear. In this study, the efficiency of plasma-assisted nitrogen fixation was investigated using two plasma systems. The presence of water in the reactive zone decreased the energy efficiency of nitrogen fixation by approximately 20%.