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A two-stage plasma catalyst system for highthroughput NO x removal was investigated. Herein, the plasma stage involved the large-volume plasma discharge of humidified gas and was carried out in a sandwich-type honeycomb monolith reactor consisting of a commercial honeycomb catalyst (50 mm high; 93 mm in diameter) located between two parallel perforated disks that formed the electrodes. The results demonstrated that, in the plasma stage, the reduction of NO x did not occur at room temperature; instead, NO was only oxidized to NO 2 and n-heptane to oxygenated hydrocarbons. The oxidation of NO and n-heptane in the honeycomb plasma discharge state was largely affected by the humidity of the feed gas. Furthermore, the oxidation of NO to NO 2 occurs preferably to that of n-heptane with a tendency of the NO oxidation to decrease with increasing feed gas humidity. The reason is that the generation of O 3 decreases as the amount of water vapor in the feed gas increases. Compared to the catalyst alone, the two-stage plasma catalyst system increased NO x removal by 29% at a temperature of 200 °C and an energy density of 25 J/L.

The removal of NO x over a Ag/γ-Al 2 O 3 catalyst coupled with gliding arc plasma at low temperatures is demonstrated. Specifically, n-heptane (the reducing agent) was pretreated by exposure to gliding arc plasma (the outlet gas temperature of 73.4 °C) before injecting into the simulated diesel exhaust gas and passing it through the catalyst zone. As a result of the plasma treatment, the feed gas consisted of oxygenated hydrocarbons (OHCs), which serve as reducing agents, instead of only n-heptane without plasma treatment. Consequently, the NO x removal efficiency increased substantially by approximately 10% at temperatures of [165−225 °C] owing to the presence of the OHCs. The dependence of the NO x removal efficiency on typical reducing agents was examined; these results agreed with our hypothesis that aldehyde derivatives were more effective than the parent compound (n-heptane) for NO x removal at low temperatures. However, enhancement of the NO x removal efficiency after plasma pretreatment was not observed at high plasma discharge power. This is because NO x is formed from the air and a significant amount of n-heptane is completely oxidized to CO 2 when the gliding arc plasma is operated at high power. Besides, the plasma treatment of n-heptane did not improve the NO x removal under high operating temperature conditions at which the catalyst itself exhibits high catalytic activity. This led us to surmise that boosting the effectiveness of the OHCs generated during plasma pretreatment would require the ratio of the exhaust gas flow rate to the reducing agent flow rate to be high, which is challenging to realize in laboratory-scale experiments. This method would lower the energy consumption of the plasma stage.

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The dependence of the plasma discharge performance on the size of the honeycomb monolith in a sandwich-type honeycomb monolith plasma reactor operated under humidified air conditions was investigated. In addition, the effect of the feed gas temperature on the plasma discharge was also examined in the low-temperature range (25 °C–42 °C), which is similar to the typical temperature of the actual surrounding ambient air. The results showed that variation of the temperature significantly affects the discharge power, i.e., the discharge power decreases with increasing temperature. The results also indicated that, in the absence of the honeycomb monolith in the reactor, the plasma discharge did not occur inside the discharge zone created by two parallel perforated disks. However, when the honeycomb monolith was sandwiched between the two electrodes, the discharge developed between them because of the generated surface discharge spread through the honeycomb channels. Interestingly, a parallel relationship exists between monoliths with two different diameters in terms of their energy density and energy efficiency for O3 generation. These results suggest that the use of a monolith with a small diameter, instead of the original large commercial monolith, is sufficient when conducting research on the honeycomb discharge, as it facilitates experimental design.