Catalyst-free oxidation of nitrogen fixation by underwater bubble discharge: performance optimization and mechanism exploration

The environmental and energy challenges associated with the Haber-Bosch nitrogen fixation process present significant ecological concerns. In contrast, low-temperature plasma technology has emerged as a highly promising alternative for nitrogen fixation, capable of converting air to NOx and producing NOx- in the liquid phase using only electrical energy. In this study, nanosecond pulsed power is employed to drive an underwater microporous coaxial reactor, generating bubble spark discharges for the efficient synthesis of NOx- in water. The variation in the concentration of gas-liquid two-phase products is systematically investigated by adjusting key parameters, including pulse voltage, gas flow rate, and gas composition. Optimal nitrogen fixation is achieved at a rate of 153 µmol/min, with energy consumption as low as 4.93 MJ/mol for gas-liquid nitrogen fixation products. Results indicated that increasing the pulse voltage enhanced the NOx- yield, promoting the formation of HNO3 and NO2. However, excessive air flow rates reduced nitrogen fixation efficiency due to inadequate activation and decreased mass transfer efficiency. The addition of an optimal O2 ratio significantly improved the NOx- yield. Plasma emission spectroscopy and energy loss pathway analysis are employed to investigate the formation mechanisms of gas-phase reactive species, and potential reactions in the liquid phase are explored through modifications in reactor design. This work provides valuable insights into the regulation of gas-liquid two-phase product formation, highlighting the impact of the controlled parameters on nitrogen fixation performance.