Diagnostics of a pulsed microwave discharge used for nitrogen fixation by means of infrared absorption and emission spectroscopy*

Nowadays, plasma-assisted nitrogen fixation processes have been demonstrated as a highly promising alternative to the environmentally impacting Haber-Bosch process [1]. The community develops abundant attempts to optimize these processes to attain a higher order of fixation at lower energy cost. One of the best potential candidates, low pressure pulsed microwave (MW) discharges those based on surface wave launchers [2,3], remain particularly unexplored for nitrogen fixation.

In this work, aiming at better insights to the chemical mechanisms occurring in these discharges, we have studied a pulsed MW surfaguide discharge operating at 2.45 GHz to convert nitrogen molecules into N-containing compounds (e.g. NO) in a N₂:O₂ gas mixture. The ro-vibrational absorption bands of the generated molecular species are monitored by a Fourier transform infrared apparatus to evaluate the relative number density, especifically targeting NO. The role of vibrational excitation on overcoming the high energy barrier of N₂ dissociation are also examined in this work. The rotational temperatures are determined based on NO (A,0 – X,2) and N₂ (C,0 – B,0) rotational emission bands, respectively, at 248 nm and 337 nm. Particular attention is devoted to estimate the evolutions of the NO yield and energy cost at different discharge conditions.

This research is supported by the FNRS project “NITROPLASM”, EOS O005118F.