Sustainable nitrogen fixation with nanosecond pulsed spark discharges: insights into free-radical-chain reactions

Abstract

Considering the increasing demand for fertilizers to support the global food supply, as well as the high-energy consumption and environmental concerns caused by industrial nitrogen fixation (i.e., Haber–Bosch process), there is a critical need to develop and integrate more sustainable, green-chemistry-based processes of nitrogen fixation. As a prominently electrified Power-to-X (where X stands for chemicals, fuels, and other high-value products) approach, non-thermal plasmas generated by the spark and gliding arc discharges are highly promising to meet the required criteria, especially when the power and the electrified chemical reactions are precisely dosed and timed using nanosecond repetitively pulsed plasmas. However, the underlying mechanism for NO_x formation in plasmas is a stumbling block for further green chemistry process developments. Here, a nanosecond pulsed spark discharge based on a plate-to-plate configuration was developed for sustainable nitrogen fixation at ambient conditions. Outstanding energy efficiency of 4–11 g kW h⁻¹ and a concentration of 960–10 900 ppm for NO_x formation were obtained with the varied airflow rates ranging from 40 to 340 mL min⁻¹. Using the optical emission spectroscopy and the chemical kinetics model, we found that the key intermediate species involved in NO_x reaction pathways strongly depend on the plasma parameters and species residence time in spark discharges. It is revealed that more than 50% of the generated NO originate from the chain reactions of O and N radicals with vibrationally excited N₂ and O₂ molecules (O + N₂(υ) → NO + N and N + O₂(υ) → NO + O). Most NO₂ molecules are formed by further oxidation of NO species (NO + O → NO₂). The presence of O and N spectral lines at the post-discharge stage further confirmed the important role of free-radical-chain reactions. These results provide new insights into sustainable and decentralized NO_x production, and deeper understanding of the plasma-based green chemistry will guide the optimization and practical applications of the plasma-based power-to-chemical nitrogen fixation.