A 4-way Heavy-Duty Diesel (HDD) emissions control aftertreatment
system typically consists of diesel oxidation catalyst (DOC),
catalyzed soot filter (CSF), urea-based selective catalytic NOx
reduction (SCR) and NH₃ slip control catalyst (AMOX). Incorporating
the SCR functionality into the soot filter (SCRoF) has great
potential to reduce system costs and package volume/weight.
In this paper, we discuss some of the recent Cu-Zeolite-based
SCR on filter (SCRoF) developments targeting Passive filter
regeneration applications. The on-engine investigation of complete
DOC+SCRoF+AMOX system focused on three major areas: 1) SCR
performance of NOx conversion efficiency and NH₃ slip under both
steady state and transient testing conditions; 2) SCRoF system
response to sulfur exposure and subsequent sulfur removal for
activity recovery; and 3) Characteristics of filter soot load,
pressure drop, and passive soot oxidation in SCRoF.
Overall, the SCRoF system showed comparable NOx conversion
efficiency and sulfur response as the conventional DOC+CSF+SCR+AMOX
system. A 92+% cycle cumulative NOx conversion efficiency was
demonstrated in a transient emission testing cycle while the peak
tailpipe NH₃ slip was well controlled below 5 ppm. With the tested
system, soot load does not adversely affect NOx reduction. After
sulfur exposure, system performance was fully recovered by
desulfation at 500°C - often the threshold temperature for passive
systems. It is also important to point out that passive soot
oxidation (i.e., C+NO₂) in a SCRoF will be considerably lower than
in a catalyzed soot filter (CSF). Passive regeneration capability
is inhibited due to SCR reactions competing for NO₂. In this work,
intrinsic mechanisms and possible approaches to further enhance
passive soot oxidation are reviewed and discussed.