Efficient exhaust gas cleaning from NO _x (NO and NO ₂ ) by absorption and adsorption based methods requires the oxidation of NO. The application of non-thermal plasma is considered as a promising oxidation method but the oxidation of NO by direct plasma remains limited due to the back-reaction of NO ₂ to NO mediated by O radicals in plasma. Indirect NO oxidation by plasma produced ozone allows to circumvent the back-reaction and further oxidize NO ₂ to N ₂ O ₅ but the slow reaction rate for the latter process limits the efficiency of this process. Present paper gives an overview of the role of metal-oxide catalysts in the improvement of oxidation efficiency for both direct and indirect plasma oxidation of NO _x . The plasma produced active oxygen species (O, O ₃ ) were shown to play an important role in the reactions taking place on the catalyst surfaces while the exact mechanism and extent of the effect were different for direct and indirect oxidation. In the case of direct plasma oxidation, both short and long lifetime oxygen species could reach the catalyst and participate in the oxidation of NO to NO ₂ . The back-reaction in the plasma phase remained still important factor and limited the effect of catalyst. In the case of indirect oxidation, only ozone could reach the catalyst surface and improve the oxidation of NO ₂ to N ₂ O ₅ . The effect of catalyst at different experimental conditions was quantitatively described with the aid of simple global chemical kinetic models derived for the NO _x oxidation either by plasma or ozone. The models allowed to compare the effect of different catalysts and to analyze the limitations for the efficiency improvement by catalyst.

C5 perfluorinated ketone (C5 PFK with UIPAC chemical name 1,1,1,3,4,4,4-heptafluoro-3-(trifluoromethyl)-2-butanone and sold by 3M as Novec ^™ 5110) has a high dielectric strength and a low global warming potential, which makes it interesting as an insulating gas in medium and high-voltage applications. The study was carried out to determine the effective Townsend ionization coefficient α _eff as a function of electric field strength and gas density for C5 PFK and for its mixtures with air. The non-self-sustained Townsend discharge between parallel plate electrodes was initiated by illuminating the cathode by UV radiation. The discharge current, I , was measured as a function of inter-electrode distance, d , at different gas densities, N , and electric field strengths, E . The effective ionization coefficient α _eff was determined from the semi-logarithmic plots of I / I ₀ against d . For each tested gas mixture, the density normalized effective ionization coefficient α _eff / N was found to be a unique function of reduced electric field strength E / N . The measurements were carried out in the absolute pressure range of 0.05–1.3 bar and E / N range of 150–1200 Td. The increasing fraction of C5 PFK in air resulted in the decrease of effective ionization coefficient. The limiting electric field strength ( E / N ) _lim where the effective ionization coefficient α _eff became zero was 770 Td (190 kV cm ⁻¹ at 1 bar) for pure C5 PFK and decreased to 225 Td (78 kV cm ⁻¹ at 1.4 bar) for 7.6% C5 PFK/air mixture. The latter value of ( E / N ) _lim is still more than two times higher than the ( E / N ) _lim value of synthetic air and about two-thirds of the value corresponding to pure SF ₆ . The investigated gas mixtures have the potential to become an alternative to SF ₆ in numerous high- and medium-voltage applications.

Over the years, ever more stringent requirements on the pollutant emissions, especially NO_X, from combustion systems burning natural gas are introduced by the European Union (EU). Among all NO_X reduction methods, the flue gas treatment by plasma is widely applied and could be used for both small scale and domestic combustion systems. However, the removal efficiency depends on concentrations of oxygen, water vapor, traces of hydrocarbons, and nitrogen oxides in flue gas. In order to analyze the application of the NO_X reduction for small-scale or domestic combustion systems, experiments of NO_X reduction by non-thermal plasma from real flue gases originating from premixed methane combustion at different equivalence ratio (ER) values were performed. It was determined that the residual oxygen in flue gas plays an important role for improvement of NO to NO₂ oxidation efficiency when O₂ concentrations are equal to or higher than 6%. The power consumption for the plasma oxidation constituted approximately 1% of the burner power. In the case of ozone treatment, the addition of O₃ to flue gas showed even more promising results as NO formed during combustion was fully oxidized to NO₂ at all ER values.

First Townsend ionization coefficient α in gas mixture of He with N ₂ is relevant for modeling of the development and chemical activity of atmospheric pressure plasma jets. This study determined α from measurements of current versus electrode separation in a steady-state non-self-sustaining Townsend discharge set-up. The measurements were carried out in absolute pressure range of 10–800 Torr and reduced electric field E/N range of 20–1000 Td. The effect of the N ₂ concentration in the gas mixture on the density normalized ionization coefficient α/N depended on the reduced electric field strength E/N . At E/N values below 200 Td, increased N ₂ content reduced the α/N while at E/N values above 200 Td, increased N ₂ content increased the plateau value of α/N . Reasonably good coincidence was observed between calculation performed with Bolsig+ and measured α/N . The discrepancy between experimental results and calculations at N ₂ percentage in the mixture below 5% and E/N values below 200 Td can be attributed to the Penning effect.

Mixtures of Ar with N ₂ are relevant for many areas of plasma applications. In particular, modeling of atmospheric pressure plasma jets for biomedical applications requires precise knowledge of fundamental gas parameters such as the Townsend ionization coefficient α . This study determines α in mixtures of Ar with N ₂ in a dark non-self-sustaining Townsend discharge. The discharge current versus electrode spacing was measured in the absolute pressure range of 10–800 Torr and reduced electric field strength E / N range of 40–1000 Td. The density normalized ionization coefficient α / N increases with E / N and decreased with N ₂ content in the gas mixture. A good coincidence between measurements and calculations with Bolsig+ was observed. In case when N ₂ concentration was between 10% and 50% in the mixture, experimental results were 20%–30% lower than predicted by Bolsig+ calculations. The results of this work were compared with α / N determined in mixtures of He with up to 10% of N ₂

Precise knowledge of the fundamental ionization properties of gases, such as the effective ionization coefficient, is crucial for discharges in mixtures of Ar:O₂, which are significant for a wide range of plasma applications. This study determined the effective ionization coefficient in electronegative gas mixtures of Ar:O₂ in the pressure range of 10–800 Torr and reduced electric field strength E/N range of 40–1200 Td utilizing a steady-state non-self-sustaining Townsend discharge. The reduced effective ionization coefficient α_e/N increased with E/N and decreased with increasing O₂ content in the gas mixture. The experimental results were compared with a model which was based on calculating the ionization and attachment coefficients with BOLSIG+. The ion conversion of O⁻ to O₂⁻, detachment from O₂⁻, and formation of O₃ were accounted for similarly as has been done with N₂:O₂ mixtures. Reasonably good agreement between the measurements and the model calculations was achieved for Ar:O₂ mixtures with the O₂ content between 20% and 70%. A discrepancy of more than 20% between measurement and calculations was observed at low E/N values when the O₂ content was below 20% and at high E/N values when the O₂ content was above 70%. Several possible explanations were proposed for the observed discrepancy; however, more elaborate models are required. The reduced critical electric field E/N_crit, where the apparent effective ionization coefficient is zero, was determined as a function of the O₂ content in the Ar:O₂ mixtures. E/N_crit increased with increasing O₂ content in the mixture.