Controlled synthesis of NO and helium metastable measurement in atmospheric pressure RF plasma

Non-thermal plasma catalytic technology has potential to create versatile gas conversion concepts. Our research focuses on nitrogen oxide synthesis, especially NO production as a target molecule for nitrogen fixation. NO_x species are generated in an atmospheric pressure RF plasma from N₂/O₂ admixed to helium. The concentrations are measured by FTIR with a multi-pass cell. It shows NO is further oxidized with increasing O₂ admixture and O₃ generation, which can be controlled by changing surface temperature and catalytic material.

Helium metastable species play a crucial role during the discharge. Broadband absorption spectroscopy is used to measure absolute densities of He(2³S₁) and He₂(a³Σ⁺_u) metastables. A 1D global model is developed to fit the experimental data and analyze metastable generation and destruction mechanisms. The helium metastable induced desorption of adsorbed water causes a decay of the metastable density along the plasma channel. Surface materials with a lower work function exhibit stronger secondary electron emission, thus have a higher rate coefficient for He^* to He₂^* conversion. Phase-resolved optical emission spectroscopy (PROES) is used to measure the time- and space-resolved He emission in an RF cycle. Different discharge modes are observed under varied driving voltage and gas mixture. The impact of plasma–surface interaction mechanisms on the efficiency of plasma generation can be monitored thereby. A strong impact of the choice of the catalytic materials is observed.