An atmospheric pressure series arc discharge (SAD) plasma jet was designed, and an air/O2/Ar gas mixture was used for discharge production aimed at enhancing N2 rotational and vibrational state populations for nitrogen fixation. The optical emission spectroscopic (OES) diagnostic was performed to estimate the vibrational (Tv) and rotational (Tr) temperatures of nitrogen molecules and electronic excitation temperature (Tex) and electron density (ne) in the discharge, for understanding underlying physicochemical phenomena. The dissipated power (Pdiss) in the discharge was estimated using the discharge voltage and current. The relative population density of the N2 vibrational state was increased ∼11.5% by four pairs of electrodes with respect to a single electrode for the gas flow rate of 5.6 l min−1. The plasma parameters determined were in the range of Tr ≈ 1621–1850K, Tv ≈ 2222–2613 K, Tex ≈ 9862–11878 K, and ne ≈ (5.53–10.93) × 1014 cm−3 for Pdiss ≈ 8.61–37.81 W for the experimental conditions considered. Further, the SAD plasma jet was applied to distilled water for N2 fixation with different treatment durations. The NOx (NO2− + NO3−) concentration was increased ∼166% by four pairs of electrodes compared to a single electrode for the gas flow rate of 2.8 l min−1. The estimated energy cost was in the range of ENOx ≈ 60–140 MJ mol−1 with three pairs of electrodes for different gas flow rates, and the ENOx ≈ 60 MJ mol−1 was the same for different pairs of electrodes with a constant gas flow rate of 2.8 l min−1. The enhanced N2 vibrational state and nearly constant energy cost for different pairs of electrodes might reflect the potentiality of this innovative design for the mass production of NOx in water.
Rahman et al. (2025) studied this question.