Coating Distribution in a Commercial SCR Filter

Abstract

A commercial SCR filter, deployed in the USA in 2015, was sectioned and examined using techniques including mercury porosimetry, electron microscopy, and micro-X-ray computed tomography. The catalyst washcoat was found to be consistent with Cu/SSZ-13, possibly including some zirconia and alumina. Three distinct regions were observed with respect to catalyst loading and location. A region at the inlet end of the filter, comprising 15 to 21% of the total effective filter length, was relatively lightly coated. Most of the catalyst present in this region was observed inside the porous filter walls, and the catalyst concentration was generally greater near the upstream filter wall surfaces. Moving axially down the monolith toward the outlet, a second region comprising 14 to 20% of the total effective filter length was more heavily coated, with catalyst present throughout the thickness of the porous filter walls, as well as coatings on both the upstream and downstream filter wall surfaces. The final region at the outlet end of the monolith, which accounted for 65 to 70% of the filter length, had an intermediate catalyst loading. Most of the catalyst here was again observed inside the porous filter wall. Concentrations in this region were higher near the downstream filter wall surfaces. Detailed models of multi-functional aftertreatment devices, such as the one examined here, have included representations of catalyst distribution within the filter bricks and indicate that catalyst distribution may have an impact on flow distribution, soot loading patterns, local concentrations, and ultimately conversion efficiency. Previous work has also shown that catalyst distribution across the thickness of an exhaust filter wall can have significant impacts on backpressure during soot loading.