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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.

The present study investigated the electrical characteristics and radical production efficiency of a coplanar barrier discharge (CBD) device manufactured by Kyocera by multilayer ceramic technology. The device consisted of a number of linear electrodes with electrode and gap widths of 0.75 mm, immersed into a ceramic dielectric barrier. A closed flow-through system necessary for the measurements was prepared by placing a quartz plate at a height of 3 mm from the ceramic barrier. The production of nitrogen radicals was determined from the removal of a trace amount of NO in pure N2 gas, while the production of oxygen radicals was determined by ozone production in pure O2 or synthetic air. The production efficiency of N and O radicals and NO oxidation in synthetic air was comparable with the efficiency of a volume barrier discharge device. The power density per unit of surface area of the CBD device was more than two times larger than that of a similar volume barrier discharge setup, which makes the CBD device a compact alternative for gas treatment. The production of ozone and different nitrogen oxides was also evaluated for the open system of the CBD which is usable for surface treatment. The ozone concentration of this system was nearly independent from the input power, while the concentration of nitrogen oxides increased with input power. The open system of the CBD was additionally tested for the treatment of a silicon surface. An increase of applied power decreased the time required to reduce the water contact angle below 10 degrees but also started to have an impact on the surface roughness.

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