Absorption Chemistry of Plasma-Generated NOx Gas for Green Nitric Acid Production

Nitric acid (HNO3) is predominantly produced in large production plants using the Ostwald process. In view of its widespread application as synthetic fertilizer, small- scale and local production has become of interest. The chemical precursor of nitric acid is NOx gas, which can be produced from air at percentage-level concentrations using small-scale, electrically powered warm plasma reactors. Using gas-phase plasma, the downstream conversion of NOx into fertilizer is a crucial, but as yet understudied step. This work aims to help close this gap and support the further development of plasma-driven nitrogen fixation and subsequent NOx scrubbing. The chemistry of NOx absorption through gas scrubbing is investigated at a 1% NOx concentration using a synthetic mimic of a plasma-produced NOx stream in order to maintain maximal controllability. pH values of the scrubber solution were kept in the range of 1 to 5 and it was recirculated for up to 30 h. The kinetics of NOx absorption were found to be strongly pH-dependent, requiring several hours of recirculation to reach steady state conditions. Once at steady state, the NOx removal efficiency turned out to be rather pH independent and reached around 84% for experiments at pH 1–4. The formation of nitrous acid (HNO2) byproduct reached a constant value of around 3.43 mM based on a dynamic equilibrium of its formation and decomposition. Approaches to minimize undesired nitrite and nitrous acid byproduct formation are discussed.