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Two features to facilitate chemical reactions at low temperature, non-thermal plasma and the weak dependency of photocatalyst on temperature, have been exploited by many researchers to effectively decompose hydrocarbon emissions emitted until the light-off of a three-way catalyst in spark ignition engines. To develop a realizable emissions reduction reactor, as part of such effort, this study investigates for the three model gases, propylene, n-butane and acetylene: 1) the conversion efficiency of the emissions reduction reactor, which utilizes the effect of dissociation, ionization-by-collision of the non-thermal plasma and the photocatalytic effect of TiO2, and 2) the concentrations of the products such as acetaldehyde, acetic acid, polymerized hydrocarbons and NO2. The operating parameters to obtain the plasma energy density ranging from 7.8 to 908 J/L were varied.

NO2 is an effective soot oxidizer operating at lower temperatures than O2. The effect of pure NO2 on soot oxidation was evaluated and compared with the gas treated by plasma, which initially consisted of NO, O2, and hydrocarbons. The cutout of a commercial DPF was used and the pressure difference across the DPF was monitored for an hour. The concentration of NO/NO2, CO, CO2 at the outlet of the DPF was measured as a function of time. CO and CO2 concentration was measured periodically by gas chromatography. The experiment was performed at 230, 250, 300, 350°C. When NO2 only was used as an oxidizing agent, there was a close relationship between the decrease of the pressure difference across the DPF, the CO and CO2 concentration at the outlet of the DPF, and the back conversion of NO2 to NO.

NOx reduction by a plasma/catalyst system was tested with modeled gas and real exhaust gas. Ag/Al2O3 was used as the catalyst. The oxidation of NO to NO2 by the plasma was increased as HC concentration and input energy density increase. The presence of H2O in the reactant gas led to the production of acid by plasma. The catalytic activity for NOx reduction was enhanced by the assistance of plasma especially in the lower temperature region. This activity was a little suppressed in the presence of H2O, but the acid was not detected in the effluent gas treated by the plasma/catalyst system. The NOx conversion to N2 was evaluated by a gas chromatography in the model gas with helium as the balance gas. The result in helium balance gas showed the selectivity to N2 depended on the catalyst temperature and was also enhanced by the assistance of plasma. The 50% of NOx removal from the diesel exhaust gas was achieved by the plasma /catalyst system.