Soot Oxidation by O ₂ and/or NO ₂ 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 CRT^TM (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.