Lean NOx reduction on LNT-SCR dual-layer catalysts by H2 and CO

Authors:   Yi Liu, Yang Zheng, Michael P. Harold, Dan Luss

Abstract:
Lean reduction of NO_x (NO & NO₂) by H₂ and CO was conducted over monolithic catalysts consisting of a selective catalytic reduction (SCR) catalyst layer deposited on top of a lean NOx trap (LNT) catalyst. An increase in the CO/H₂ ratio decreased the cycle-averaged NOx conversion for the ceria-free LNT catalyst. CO poisoning was especially significant below 250 ^◦C. The low-temperature NO_x reduction was increased either by use of an LNT-SCR dual-layer catalyst or deposition of CeO₂ on the LNT catalyst. However, the ceria decreased the high-temperature reductive conversion of NO_x due to promotion of the undesired NH3 oxidation. Ceria zoning enhanced the monolith NO_x conversion. Downstream loading of ceria led to the highest NO_x reduction at both low- and high- temperatures due to the beneficial interaction of the ceria and H₂. The low-temperature NO_x conversion of an aged dual-layer catalyst could be increased by a higher SCR catalyst loading. However, at high temperatures NO_x reduction was independent of the SCR loading. The ratio of the lean to rich feed duration and the total cycle time were optimized to improve the NO_x conversion in a temperature range from 150 to 400^◦C. The highest cycle-averaged NO_x conversion was obtained with a 30s:5s lean-rich cycle containing 1.25% total reductant for all CO/H₂ ratios for a lean feed containing 500 ppm NO and 5% O₂.

Keywords:
Dual-layer catalyst
Monolith Lean
NO_x trap
NO_x
Selective catalytic reduction
NO_x storage and reduction
Zoning ceria
Cu
ZSM-5
NH₃
Pt
CO
H₂

Published in: Applied Catalysis B: Environmental (Volumes 132–133, Pages 1-552, 27 March 2013)

Publisher: Elsevier  

ISSN Information: 0926-3373

Lean NOx reduction on LNT-SCR dual-layer catalysts by H2 and CO

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