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 NO_<i>x</i> and producing NO_<i>x</i>^- 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 NO_<i>x</i>^- 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^-1, with energy consumption as low as 4.93 MJ mol^-1 for gas-liquid nitrogen fixation products. Results indicated that increasing the pulse voltage enhanced the NO_<i>x</i>^- 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 NO_<i>x</i>^- 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.