Impact of reduced catalytic activity on passive regeneration of catalyzed diesel particulate filters

Oxidation catalysts can greatly improve the regeneration efficiency of diesel particulate filters (DPF) by providing sufficient levels of NO2 for low-temperature soot oxidation. As for other automotive catalysts, catalyzed DPFs (CDPFs) are subject to aging effects, resulting in decreased performance of the NO oxidation reaction. The life span of CDPFs generally only considers the elevated back pressure as a consequence of the accumulation of ash. However, with reduced catalytic activity and impaired functionality of the regeneration process there is a risk of premature replacement of the CDPF or accumulation of soot above critical levels. In this study, a new exhaust aftertreatment system has been developed to accommodate sub-scale catalysts and DPFs for testing with full-size heavy-duty engines. The modified exhaust aftertreatment system was used together with a rig for accelerated soot and ash loading to assess the impact of CDPF aging on regeneration performance under real conditions. Experiments were conducted with and without diesel oxidation catalyst to limit or increase the concentration NO2. It could be demonstrated that the impaired catalytic activity can have a significant impact on the regeneration process. With a limited upstream concentration of NO2 fed to the CDPF, a temperature increase from 360 °C to 440 °C was required to initiate the oxidation of soot. Furthermore, an overall lower oxidation rate was observed. With the addition of a diesel oxidation catalyst, resulting in elevated upstream concentrations of NO2, the effect of aging could be partially mitigated where close to equivalent soot oxidation rates could be achieved with a temperature difference of 15 °C. These results highlight the importance of the CDPF activity for the functionality of the system, which should be considered for future CDPF design and regeneration strategies.