The effect of changing the gas composition on soot oxidation over DPF and SCR-coated filters

Abstract

In this study, we have investigated the effect of changing the gas feed composition on soot oxidation over diesel particulate filter (DPF) and SCR-coated filter during temperature ramp experiments from 200 to 750°C. For DPF, the increasing NO2/NOx ratio promoted soot oxidation across a fairly low temperature range (200–500°C) and, in addition, soot oxidation was enhanced by water. Interestingly, the presence of NH3 in the gas feed inhibited the soot oxidation under various feed compositions for the DPF. Moreover, ammonia oxidation was also observed over the filter and based on these experiments together with experiments without soot, we conclude that ammonia oxidation is actually occurring on soot particles and that the interaction of the ammonia with the soot is inhibiting soot oxidation. Further, fast and NO2 SCR reactions were found to occur on soot particles, while no reactions with NO under standard SCR conditions was observed. Thus, the NH3 and NO2 molecules are interacting with the soot. When comparing the DPF and SCR-filter we found that the SCR material enhanced the soot oxidation under standard and fast SCR conditions, because of the fact that soot oxidation was inhibited by NH3 and would be rapidly consumed under SCR reactions with the Cu-zeolite in the SCR-filter. Furthermore, during fast SCR conditions, because NO2 is consumed during fast SCR reaction for the SCR-filter, the opposite results were found where soot oxidation occurred at lower temperature for the DPF.