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NIRE Annual Report
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 1999

Catalytic Oxidation of Soot from Diesel Engines

Environmental Technology Division
Atmospheric Environmental Protection Department
Objectives
The emission of soot from the diesel engines is a serious environmental problem. Placing a filter device in the exhaust stream can reduce it. The soot collected in the filter must be periodically removed by combustion to prevent plugging of the filter. Oxidation catalysts can lower the combustion temperature of the soot. This research aims at the development of a practical particulate oxidation catalyst system for diesel engine exhausts.
Results
In our exploratory work, we have found platinum to be a promising soot oxidation catalyst. The catalytic activity was strongly affected by the composition of reactant gas. As shown in Fig. 1, the addition of NO to dry air brought about an acceleration of carbon oxidation. Furthermore, low concentration of SO2 in addition to NO substantially promoted the oxidation. Finally, it showed the highest activity in NO + SO2 + H2O + O2 + N2, which are typical components of diesel exhaust.
Fig.1
Fig. 1 Temperature programmed reactions of carbon 1wt%Pt/SiO2. Conditions; catalyst = 0.5g, carbon black = 0.005g, flow rate = 0.5dm3min-1, heating rate = 10°C/min.
Fig. 2 shows the speculated reaction scheme. NO2, which is formed by the oxidation of NO over Pt, is regarded as an oxidizer that directly attacks the carbon and turns itself to NO. The effect of SO2 suggests that SO3, produced from SO2 over Pt, plays a role as a catalyst that accelerates the oxidation of carbon by NO2.
Fig. 2 Speculated reaction scheme of carbon oxidation with Pt/SiO2
Fig. 2 Speculated reaction scheme of carbon oxidation with Pt/SiO2
Selected Publications
1) Outstanding effect of SO2 addition on the rate of carbon oxidation with a Pt catalyst, Uchisawa et al., J. Chem. Soc., Chem. Commun. (1998) 2255.
2) Carbon oxidation with platinum supported catalysts, Uchisawa et al., Appl. Cat. B 18 (1998) L183.

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