Mechanistic role of hydroxyl radicals in alkali-enhanced nitrogen fixation via underwater air bubble discharge.

Plasma-assisted nitrogen fixation provides a sustainable alternative to the energy-intensive Haber-Bosch process, but the role of alkaline media in gas-liquid systems remains unclear. Here, a nanosecond pulse-driven underwater air bubble discharge reactor was developed for nitrogen fixation in the absence of solid catalysts. The highest nitrogen fixation synthesis rate reached 54.9 µmol min-1, and the lowest energy consumption was approximately 11.1 MJ mol-1. Optical emission spectra indicate that an alkaline condition increases the plasma vibrational temperature, promoting nitrogen activation and the formation of reactive nitrogen species. The mechanistic study indicates that strongly alkaline conditions enhance plasma-phase N2 activation and the reactive absorption of gaseous NOx, while ˙OH primarily mediates the secondary oxidation of NO2- to NO3-, thereby regulating product selectivity. This work reveals that an alkaline condition reshapes the formation, retention, and deep oxidation pathways of reactive nitrogen species by regulating the OH- concentration at the gas-liquid interface, converting reactive oxygen species, and stabilizing the state of NOx in the liquid phase. It also provides a feasible pathway for sustainable nitrogen fertilizer production.