Soot Oxidation by O
in the Presence of Catalysts Under Lean-Burn and Rich Atmospheres
Exhaust gas from diesel engines will be subjected to new and stringent emissions standards. To lower the emissions of carbon particulates (soot), a filtration system has to be fitted in the exhaust line. Diesel Particulate Filters (DPF) trap the soot, which then has to be removed by combustion. Soot doped with cerium-based additives leads to an enhancement of reactivity with O2. It is also well known that the presence of NO2 significantly accelerates the carbon oxidation rate. The CRTTM (Continuous Regeneration Trap) system is based on this property. More recently, the DPNR system (Diesel Particulate NOx Reduction system), which is a DPF impregnated with a NOx trap catalyst, was proposed.
The aim of the present study was to investigate the effect of different catalytic materials on soot combustion. Two catalysts were tested: a commercial Oxygen Storage Catalyst (OSC) and a Lean NOx trap catalyst (laboratory made). Different types of soot, both with and without an additive were tested. The experiments were performed in a fixed bed reactor under different atmospheres which mimicked lean-burn conditions, with or without the presence of NO2. Different NO2 concentrations were tested, ranging from 0 to 1000 ppm. Another atmosphere simulated lean-burn/rich cycling conditions in the presence of propene. The catalyst and soot were mixed and then ground together. The effect of the gas temperature was also considered (from 573 K to 723 K). The presence of OSC enhances soot combustion by the C-O2 reaction. The presence of NO and/or NO2 in the gas phase also produces an increase the rate of combustion. An hierarchic classification of the effect on soot combustion of the different catalysts and parameters tested is proposed.