N2 oxidation kinetics in a ns-pulsed discharge above a liquid electrode

In this work, the kinetics of nitrogen fixation via plasma-induced N2 oxidation in a 10-ns pulsed atmospheric pressure water-contacting discharge sustained in air is investigated. Two pulse regimes, a single pulse and a 3-pulse burst of 100 kHz, are considered. The densities of relevant radicals (NO, O) are studied by time-and space-resolved laser-induced fluorescence spectroscopy. It is concluded that in a single pulse mode, O atoms are mainly generated by O2 reacting with electronically excited states of N2 (𝐴3𝛴𝑢+,𝐵3𝛱𝑔,𝐶3𝛱𝑢) and are primarily reduced as a result of O3 formation. The O density shows a maximum at~100 ns after the plasma pulse with number density of ~10^23 m^(-3). NO radicals, on the other hand, are primarily formed by reacting with the 𝑁2(𝐴3𝛴𝑢+)state (up to~ 1 μs after the pulse) and with OH radicals (up to ~ 10’s of μs), peaking at approximately 60 μs with a peak density of ~10^21 m^(-3). The NO loss pathway is represented by the reversed Zeldovich mechanism at short time delays (~ 10’s μs), whereas at longer delays (>100’s of μs) HNO2 and NO2 formation causing NO loss start to be dominant. In the burst mode, the energy efficiency of NO formation decreases despite higher N2 conversion, for which three reasons are suggested: (1) NO removal by the generated 𝑂(𝐷1) after the discharge pulse through the reverse Zeldovich mechanism, (2) NO oxidation via the accumulated O3, (3) pre-ionization induced by high repetition rate (100 kHz) leading to shrinkage of the plasma bulk.