Diesel Oxidation Catalyst Pt–Pd/MnOx–Al2O3 for Soot Emission Control: Effect of NO and Water Vapor on Soot Oxidation

The operation of diesel engines is accompanied by the emission of soot particles and nitrogen oxides, which have a harmful effect on the environment. In this paper, the effect of the composition of a gas feeds simulating the diesel engine exhaust gases on the oxidation of diesel soot particles on the surface of a diesel oxidizing catalyst Pt–Pd/MnO x –Al 2 O 3 with a total Pt–Pd content of 15 g/f 3 is considered. The rate and effective activation energy of catalytic oxidation of diesel soot with oxygen and NO x were studied in the isothermal mode with varying concentrations of O 2 in the presence of NO and/or water vapor. It was revealed that the rate of catalytic oxidation of soot depends on the concentration of O 2 (2.5–10 vol.%) and NO (300–450 ppm), but this effect is significant at soot conversion below 45–50%. It was found that the decrease in the rate of soot oxidation slows down after reaching 25–30% of soot burnout, probably due to the accumulation of O-containing groups on the surface of graphite-like soot structures and an increase in soot defectiveness. Water vapor (10 vol.%) alone did not catalyze the oxidation of soot up to temperatures of 475 °C. In the presence of oxygen, the soot oxidation with a wet feed proceeded more easily than with a dry mixture at temperatures of 400 and 425 °C; the promoting effect of water weakened at higher temperatures, and at a temperature of 475 °C had a negative character. In a wet mixture containing O 2 and NO, the soot oxidation was inhibited by water vapor at temperatures of 375–450 °C. The reason was low generation of NO 2 over catalyst Pt–Pd/MnO x –Al 2 O 3 under wet conditions.