Enhancing the Selective Catalytic Reduction of NO_<i>x</i> at Low Temperature by Pretreatment of Hydrocarbons in a Gliding Arc Plasma

The removal of NOx over a Ag/γ-Al2O3 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 NOx removal efficiency increased substantially by approximately 10% at temperatures of [165–225 °C] owing to the presence of the OHCs. The dependence of the NOx 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 NOx removal at low temperatures. However, enhancement of the NOx removal efficiency after plasma pretreatment was not observed at high plasma discharge power. This is because NOx is formed from the air and a significant amount of n-heptane is completely oxidized to CO2 when the gliding arc plasma is operated at high power. Besides, the plasma treatment of n-heptane did not improve the NOx 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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