Integrated Selective Catalytic Reduction–Diesel Particulate Filter Aftertreatment: Insights into Pressure Drop, NOx Conversion, and Passive Soot Oxidation Behavior

Integrating urea-selective catalytic reduction (SCR) and diesel particulate filter (DPF) technologies into a single device has the potential to reduce the complexity of current diesel aftertreatment strategies. Fundamental studies were performed to shed light on the pressure drop and reaction behavior of integrated SCR and DPF systems. Details of SCR washcoat amount and location were investigated for effect on pressure drop during soot filtration. The SCR catalyst primarily impacted depth filtration of soot, promoted by increased catalyst located within the upstream portion of the porous filter wall. This effect is believed to be related to the nature of the porous filter substrate and pore network and changing of the rate at which pores plug in the presence of catalyst. SCR catalyst on the wall of the inlet filter channel also had an effect on the pressure rise during cake filtration of soot. NO x reduction efficiency measurements were performed to determine the nature and magnitude of the effect of soot on SCR performance. The effect of soot on the SCR performance is primarily attributed to the contribution of passive soot oxidation, and the propensity for soot oxidation to shift the NO 2 /NO x fraction relative to 0.5. SCR performance at NO 2 /NO x < 0.5 is adversely affected by the presence of soot oxidation by increasing the SCR dependency on standard (NO only) SCR reactions; conversely, at NO 2 /NO x > 0.5, the SCR performance is positively impacted by a decreased dependency on NO 2 -only SCR reactions. Temperature-programmed oxidation studies were performed to evaluate the impact of SCR on passive soot oxidation. SCR adversely impacts soot oxidation performance via NO 2 diffusive effects, decreasing NO 2 concentration in the inlet channel. This impact can be minimized or recovered at higher NO 2 concentration and NO 2 /NO x fractions >0.5.