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The high removal efficiency of NOxfrom diesel engine exhaust gas at low temperatures remains challenging due to the poor activity of catalysts under this condition. This work investigated the effectiveness of gliding arc plasma in the NOxremoval over the Ag/γ-Al2O3catalyst with n-heptane as a reducing agent source for NOxreduction. The plasma conjugated with the catalyst was performed like an injection method, i.e., a small part flow (1/26) that consisted of n-heptane was reformed to H2and oxygenated hydrocarbons (OHCs) by gliding arc plasma before mixing it with the gas containing NOx. Then, the overall gas was passed through the catalyst stage at various temperatures from 127 to 253 °C. The experimental data revealed that NOxremoval efficiency significantly increased with plasma treatment. The enhancement by plasma treatment increased with temperature from 127 to 226 °C, but the enhancement reduced at a temperature of 253 °C. The result came from formation of H2and OHCs in the treated plasma gas, which reactivated NOxremoval over the catalyst at low temperatures. Moreover, the enhanced NOxremoval efficiency in a wide low-temperature range is obtained due to considerable hydrogen formation by the gliding arc plasma. To sum up, the highest NOxremoval efficiency was 68.5% under a temperature of 226 °C and specific energy input of 34.6 J/L, while it was 20.3% for the catalyst alone; in other words, the removal efficiency increased by 48.1% with using plasma under these conditions. Owing to its low energy consumption, high-throughput gas treatment, and effectiveness at low temperatures, the technology is promising in practical applications for refining diesel exhaust gases.

A two-stage plasma catalyst system for high-throughput NOxremoval 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 NOxdid not occur at room temperature; instead, NO was only oxidized to NO2and 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 NO2occurs 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 O3decreases as the amount of water vapor in the feed gas increases. Compared to the catalyst alone, the two-stage plasma catalyst system increased NOxremoval by 29% at a temperature of 200 °C and an energy density of 25 J/L.

The removal of NOxover a Ag/γ-Al2O3catalyst 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 NOxremoval efficiency increased substantially by approximately 10% at temperatures of [165–225 °C] owing to the presence of the OHCs. The dependence of the NOxremoval 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 NOxremoval at low temperatures. However, enhancement of the NOxremoval efficiency after plasma pretreatment was not observed at high plasma discharge power. This is because NOxis formed from the air and a significant amount of n-heptane is completely oxidized to CO2when the gliding arc plasma is operated at high power. Besides, the plasma treatment of n-heptane did not improve the NOxremoval 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.