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

Abstract

Lean reduction of NOx (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 NOx 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 NOx due to promotion of the undesired NH3 oxidation. Ceria zoning enhanced the monolith NOx conversion. Downstream loading of ceria led to the highest NOx reduction at both low- and high- temperatures due to the beneficial interaction of the ceria and H2. The low-temperature NOx conversion of an aged dual-layer catalyst could be increased by a higher SCR catalyst loading. However, at high temperatures NOx 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 NOx conversion in a temperature range from 150 to 400°C. The highest cycle-averaged NOx 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 500ppm NO and 5% O2.