Catalytic Activities of Transition Metal Oxides for Plasma PM Removal

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

Although diesel engines have a higher combustion efficiency and a lower CO2 emission than gasoline engines, the harmful particulate matter (PM) emitted by the diesel engines is required to be removed. Plasma technologies have a potentiality as an effective method for PM removal. We have developed a dielectric barrier discharge (DBD) reactor driven by a pulse power supply to remove PM from diesel engines. It was found that PM was trapped within the DBD reactor due to electrostatic precipitation effects. In order to improve the PM removal ability of the DBD reactor, catalysts that can effectively promote the oxidation of PM trapped in the DBD reactor are desirable.

The efficiencies of PM oxidation catalyzed by ten different catalysts (seven transition metal oxides, two noble metals and aluminum oxide) have been measured using a thermo-gravimetrical analyzer under an atmosphere containing O3 and NO2 that can be generated in plasma discharges. The PM oxidation efficiency by MnO2 has been found to exceed that of the noble metal catalysts even at a temperature less than 300 C. The PM oxidative activities by transition metal oxides changed due to the presence of active oxygen species of O3 and NO2.

The efficiencies of PM oxidation catalyzed by seven transition metal oxides under a plasma discharge condition have been evaluated using a batch type of DBD reactor. The DBD reactor mainly consists of dielectric alumina plates and aluminum plates. The powders of each catalyst were deposited and fixed on the surfaces of alumina plates. PM collected from the exhaust gases of a diesel engine was then fixed on the surface of alumina plates with deposited catalyst powders. Experiments were carried out at 200 C.

The efficiency of PM oxidation using the DBD reactor with deposited Fe2O3 is approximately 35% higher than that without any catalysts. Fe2O3 deposited in the DBD reactor has the highest catalytic activity for the PM removal under the plasma discharge condition. The PM oxidative activities of the transition metal oxides under the plasma discharge condition are different from that observed by the thermo-gravimetrical analysis without plasma discharges. This difference is mainly due to photocatalytic effects by ultraviolet-light radiation generated within plasma discharges. The experimental results and discussion will be presented in detail.

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