Interplay between nitrogen oxides and ozone in a filamentary dielectric barrier discharge at various frequencies

Pursuing more efficient nitrogen fixation in the context of the current climate crisis is of prime importance. The Haber-Bosch process (NH3 production) accounts for at least 2% of the yearly released greenhouse gases. Alternatively, different atmospheric nonthermal discharges have proven their ability to convert nitrogen and oxygen from the air into nitrogen oxides (NO, NO2, and HNO3 hereafter referred as NOx). Among them, dielectric barrier discharges (DBDs) in N-2/O-2 are very interesting as they are characterized by microdischarges (MDs) developing into filaments under specific conditions (gap length, voltage, frequency, nature of the dielectrics). In this work, we used a sinusoidal voltage at 4, 12, 17.5, and 30 kHz as a key parameter to manipulate the MDs behaviors in an N-2/O-2 DBD and tailor the chemistry of the discharge. We characterized the MDs distribution by high-speed camera imaging. Two different nitrogen emission systems, that is, the second positive system (SPS) and the first negative system (FNS), were used to assess the filament temperature and what we define as the delayed-glow temperature, respectively. The influence of the filament's distribution on the overall discharge temperature has also been evaluated, by using a novel approach allowing the generation of heat distribution maps directly on a thermosensitive paper exposed to the discharge. Finally, the effect of the filaments on ozone (O-3) and NOx production has been monitored by in situ Fourier-transform infrared (FTIR). The DBD can be tuned between homogenously distributed MDs at 4 and 12 kHz to highly localized filaments at 17.5 and 30 kHz. A high filament temperature combined with a cold delayed-glow is observed at 17.5 kHz while both the filament and the delayed-glow temperature are high at 30 kHz. Low frequencies lead to heat spreading into the whole discharge volume, while localization of heat along the filament's paths is observed at high frequencies. High-temperature filaments in the 30 kHz discharge and in the globally heated discharge at low frequencies quickly remove O-3 from the discharge and consequently limit the overall NOx production by removing important oxidation pathways. On the contrary, filaments surrounded by cold gas observed at 17.5 kHz allowed to maintain a high concentration of O-3 necessary for a faster oxidation of NO and thus increase the HNO3 production, even after 250 s, by avoiding backward reactions.