In relation to the diesel particulate filter (DPF),which is a promising technique for substantially reducing the particualte matter (PM),the screening and development of catalysts to improve the oxidation of PM, especially the solid carbon (soot),have been reviewed. improvement in the engine combustion technique have been conventionally the main countermeasure for the reduction of PM, but stringent regulations, which should be met in the near future, are demanding developments for practical exhaust treatment technique using the DPF. The use of a catalyst, which substantially lowers the soot ignition temperature, is effective to put the DPF technique in practice. In order to influence the catalytic activity upon solid carbon (soot),two types of catalysts, mobile-molten-salt-type and indirect-oxidation-type using coexisting NO in the exhaust gas, are thought to be promising. Concerning the latter type catalyst, the authors have found that SO_3, made on Pt catalysts from SO_2 also coexisting in the exhaust gas,has a strong catalysis for the promotion of soot oxidation with NO2.
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