Effect of the electrode material on the atmospheric plasma conversion of NO in air mixtures

Non-thermal atmospheric pressure plasma is widely used for conversion of hazardous gases. Results from different laboratories confirm importance of energy non-equilibrium in the plasma where dominant energy carriers are electrons and a dominant chemistry is based on formation and interactions of radicals. Because of rather high electric fields required for generation and sustaining of air discharges at atmospheric pressure many plasma systems were found rather to create a lot of NO instead of removing it. A widely supported way to clean NO and NO2 from air mixtures is a plasma assisted catalytic reduction where the cold plasma is combined with the solid-state catalyst. In an ideal case the plasma acts as an oxidation catalyst where an atomic oxygen from air oxidizes NO to NO2 and the solid-state catalysts are then capable to convert all NO2 to N2 and O2. In most cases it is also necessary to involve auxiliary gases, e.g., propylene, to make the process efficient enough. This work introduces an original cold plasma system based on atmospheric hollow cathodes generated by a nanopulse DC power with controllable voltage and pulse frequency. The system was optimized in both the geometry and the applied power. However, the material of electrodes was found to be the most important factor affecting the plasma performance and consequently the chemical kinetics. A 100% conversion of NO to NO2 was achieved with a graphite electrode, without using any auxiliary gas and without catalyst. Plasma performance and conversion efficiency are compared for several electrode materials.