Paper PS-TuP20
NO_x Fixation by Atmospheric Pressure N₂/O₂ Filamentary DBD Plasma over Water: Physicochemical Mechanisms of Plasma-Liquid interactions

Tuesday, October 22, 2019, 6:30 pm, Room Union Station B

NO_x formation from N₂/O₂ gas or air through nonthermal plasmas has become an important research topic for researchers in the last decades, considering both environmental and industrial importance. In this study, an atmospheric pressure filamentary dielectric barrier discharge (DBD) plasma has been produced over the water surface by a sinusoidal20-30 kV, 24.5 kHz power supply. In order to study the physicochemical mechanism of plasma-liquid surface interaction for NO_x production, the characterization of the plasma has been performed using optical emission spectroscopy (OES) and by recording voltage-current curves (with a high voltage probe, a Rogowski coil and a digital oscilloscope). The concentration of nitrates and nitrites ions in water was determined by ion chromatography. The power absorbed by the plasma discharge has been determined by Volt-Charge (V-Q) Lissajous curves in different experimental conditions. It is shown that the absorbed power increases sharply with increasing applied voltage and with the content of N₂ in the gas mixture. OES diagnostics allows identifying the exited species, to study the relative emission intensity of NO (A-X) while varying the plasma gas composition, and to determine the rotational (T_rot≈320-380 K) and vibrational (T_vib≈2600-3800 K) temperatures under different experimental conditions. The Boltzmann plot method has been employed for the estimation of T_rot and T_vib by using the OH (A-X) and the N₂ (C-B) bands respectively. Both Trot and T_vib increase with increasing applied voltage and with increasing the O₂ content in the gas mixture. In the liquid phase, the concentration of NO₂^- is maximum at low applied voltages, treatment times and with pure N₂ discharges, whereas in these conditions the concentration of NO₃^- is minimum. The concentration of NO₃^- increases with increasing treatment times, applied voltages and O₂ content in the gas mixture. It is shown that the nitrites formed during the plasma treatment transform rapidly, and completely into nitrates. From the total amount of nitrites and nitrates synthesized in solution (named NO_x below), the energy yield for the conversion was estimated. Depending on the starting gas composition and applied powers, yields of ~180 W/mg NO_x to ~ 9.5 W/mg NO_x were obtained.