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.

Abstract The elementary processes during the fixation of nitrogen by plasma catalysis are studied in a low-pressure plasma experiment with N $$_2$$ 2 and O $$_2$$ 2 as source gases. The formation of surface groups on an iron oxide foil is monitored with infrared reflection absorption spectroscopy. Surface nitrates (NO $$_3^-$$ 3 - ) are formed when the substrate is exposed to a 1:1 N $$_2$$ 2 :O $$_2$$ 2 plasma, as well as N $$_2$$ 2 O(g), NO(g), NO $$_2$$ 2 (g), and O $$_3$$ 3 (g) in the gas phase. It is postulated that NO $$_{1,2}$$ 1 , 2 (g) species created by the plasma, adsorb at the surface and create these nitrates. This constitutes an intermediate step for nitrogen oxidation by plasma catalysis.

We utilized a combination of experimental alongside data-driven and theoretical modelling techniques to study non-thermal plasma properties and observables including optical emission spectral intensities, electron temperature, species concentrations, degree of ionization, and reaction rates. As a case study we measured the plasma properties of Argon gas in the low-pressure regime using optical emission spectroscopy (OES) while varying plasma input power and gas flow rate. We used data-driven and drift-diffusion modeling techniques to obtain complementary information, including electron temperature, reduced electric field, and species densities. The calculated density number of excited argon has a linear correlation to measured emission intensity, and we found that the dominant effect on Ar I intensity is the applied power with the gas flow (or pressure) the secondary factor (77% and 20%, respectively). The electron temperature increases with power but decreases with flow (or pressure). Combining the measured and modelling results help to understand the cold plasma dynamics and chemistry towards more complex plasma chemistry applications.