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 are subject to aging effects, resulting in decreased performance of the NO oxidation reaction. The life span of DPFs 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 catalyzed DPF or accumulation of soot above critical levels. In this study, a new exhaust aftertreatment system has been developed to accommodate laboratory-scale catalysts and DPFs for testing with full-size heavy-duty engines. The modified exhaust aftertreatment set-up was used together with a rig for accelerated soot and ash loading to assess the impact of catalyst aging on regeneration performance under real conditions. Experiments were conducted with and without diesel oxidation catalyst to limit or increase the concentration of 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 catalyzed DPF, a temperature increase from about 390 °C to 450 °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 leading to more comparable soot oxidation rates with a temperature difference of 30 °C for soot ignition. These results highlight the importance of the catalytic activity for the functionality of the system, which should be considered for future catalyzed DPF design and regeneration strategies.