LNT–SCR dual-layer catalysts optimized for lean NOx reduction by H2 and CO

Abstract

Monolithic catalysts consisting of a layer of selective catalytic reduction (SCR) catalyst deposited on top of a lean NOx trap (LNT) catalyst were optimized to provide high NOx conversion at both low and high temperatures with minimal precious group metal (PGM) loading using H2/CO reductant mixtures. The optimized dual-layer catalyst circumvents the need for urea feed and has the potential to reduce the expensive PGM loading by up to 38% from that of LNT only catalyst under laboratory test. We investigated the impact of catalyst design variables, such as SCR and LNT zoning, the ceria level in LNT, as well as SCR zeolite type (ZSM-5, SSZ-13) layer thickness. Zoning of either or both the SCR and LNT in the dual-layer catalysts enables an increase of the low-temperature NOx conversion, and minimizes the high temperature (300–400°C) conversion loss caused by the SCR diffusion resistance and undesired NH3 oxidation by the LNT. High ceria loading of the LNT enhanced NH3 generation, NOx adsorption and mitigated CO poisoning at low temperatures (150–250°C). Commercial Cu-SSZ-13 exhibited a higher NH3 storage capacity and better low-temperature SCR activity than the in-house synthesized Cu-ZSM-5, and improved the low-temperature NOx conversion of the dual-layer catalysts. The diffusion resistance in the top active Cu-zeolite layer inhibited the overall NOx reduction as shown by replacing it with an inert Na-ZSM-5 layer with a high Si/Al ratio. Washcoat diffusion limitations adversely affect the high temperature performance more than the NH3 oxidation to NOx. The experiments revealed that diffusion limitations in the top SCR layer loading of 1.0g/in.3 started at 150°C using a pure H2 feed and at 250°C using a CO/H2 feed.