NO oxidation on catalyzed soot filters

• Filter regeneration using CSF-generated NO2 results in a two-stage ΔP decrease.
• Multiple NO turnovers take place during passive filter regeneration.
• NO2 back diffusion is a key to soot cake oxidation.
• Pt/Pd-CSF decreases NO2/NOx variability relative to Pt-CSF.
• Calcination of the Pt/Pd powder can further decrease this variability.

In addition to its filtration function, a catalyzed soot filter (CSF) also plays a variety of catalytic roles in modern diesel emission control systems. This paper focuses on NO oxidation on CSF for passive filter regeneration and for optimizing the performance of a downstream SCR catalyst. Using a Pt-catalyzed CSF loaded with diesel soot, we conducted a filter regeneration experiment with NO2 and found that soot oxidation started within the filter walls then extended to the soot cake. The NO2 used to oxidize the soot can be continuously regenerated on the filter after it is consumed, i.e. multiple NO turnovers. By inserting a capillary sampling probe into an inlet channel of a soot-free Pt-CSF, we demonstrated that NO2 generated within the filter walls can diffuse back to the inlet channels against the bulk gas flow, which makes the passive regeneration of the soot cake possible. For CSF-catalyzed NO oxidation in a SCR system, we found a large variability in NO conversion on a Pt-CSF resulting from different aging conditions. A Pt/Pd-CSF can reduce the NO conversion variability to a degree. High-temperature calcination of the supported Pt/Pd powder, before the slurry process, can further reduce this variability.

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