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Mechanism of the Diesel PM Removal by Dielectric Barrier Discharges

Satoshi Kodama, Shuiliang Yao, Shin Yamamoto, Chieko Mine, and Yuichi Fujioka. Chemical Research Group, Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizugawa-shi, Kyoto 619-0292, Japan

It is desirable to change vehicles powered by gasoline to those by diesel to reduce the emission of the greenhouse gas CO2, as the energy efficiency of vehicles powered by diesel is better than those by gasoline. Unfortunately, the emission of harmful particulate matter (PM) and nitrogen oxides (NOx) from diesel vehicles is a problem; innovative after-treatment systems for removals of diesel PM and NOx are required. Diesel particulate filters (DPF) have been developed for PM removal from diesel vehicles and it can remove PM efficiently. However, the pressure loss due to PM deposition in the DPF and additional fuel required for DPF regeneration create a fuel penalty. To resolve these problems, we have developed a plasma PM removal system aiming to reduce PM emission with a low fuel penalty. The plasma discharges are generated using a dielectric barrier discharge (DBD) reactor to activate O2, H2O and NO in diesel exhaust gases to O, OH, O3, and NO2. PM (the main component is carbon) reacts with these activated oxygen species to yield CO and CO2. In this study, the mechanism of the diesel PM removal by the plasma discharges was investigated.

The PM samples were collected from the exhaust gases of a diesel engine. A lab-scale DBD reactor was used to generate plasma discharge. This DBD reactor consists of two pieces of flat alumina plates, two pieces of aluminum flat plates and spacers. Several parallel grooves are engraved on one side of each alumina plate. Each aluminum plate is placed on the flat side of the alumina plates. Each two alumina plates of grooves on their surfaces are assembled face to face, and between which the spacers are inserted to form discharge spaces. The aluminum plates are connected to the output side and earth terminal of a high-voltage pulse power supply. A mixture gas of PM, H2O, N2 and O2 was introduced to the discharge spaces of the DBD reactor. PM removal ratios were calculated from concentrations of PM oxidation products (CO and CO2) in the gas from the DBD reactor using gas chromatographs. After plasma discharges, PM samples were collected and analyzed using XPS to find the changes of carbon bonding ratios. The oxidation of PM (before and after plasma discharges) with O3 was also measured by using a thermo gravimetric analyzer.

It was found that the ratio of sp2 bonding carbon in the PM sample decreased while the ratio of sp3 bonding carbon increased after plasma discharges. This finding suggested that the active oxygen species generated by plasma discharges reacted with the sp2 bonding carbon to form sp3 bonding carbon. On the other hand, the ratio of sp3 bonding carbon decreased while the ratio of sp2 bonding carbon increased after the reaction with O3. This fact implied that PM is further oxidized by O3 from the sp3 bonding carbon to CO and CO2. The detail mechanism of plasma PM removal and the further development of this technology will be given.

This work was supported by the New Energy and Industrial Technology Development Organization, Japan.