NOx storage-reduction characteristics of Ba-based lean NOx trap catalysts subjected to simulated road aging

Yaying Ji, Courtney Fisk, Vencon Easterling, Uschi Graham, Adam Poole, Mark Crocker, Jae-Soon Choi, William Partridge, Karen Wilson

Research output: Contribution to journalArticlepeer-review

Abstract

In order to study the effect of washcoat composition on lean NOx trap (LNT) aging characteristics, fully formulated monolithic LNT catalysts containing varying amounts of La-stabilized CeO2 (5 wt% La2O3) or CeO2-ZrO2 (Ce:Zr = 70:30) were subjected to accelerated aging on a bench reactor. Subsequent catalyst evaluation revealed that aging resulted in deterioration of the NOx storage, NOx release and NOx reduction functions, whereas the observation of lean phase NO2 slip for all of the aged catalysts indicated that LNT performance was not limited by the kinetics of NO oxidation. After aging, all of the catalysts showed increased selectivity to NH3 in the temperature range 250–450 °C. TEM, H2 chemisorption, XPS and elemental analysis data revealed two main changes which can explain the degradation in LNT performance. First, residual sulfur in the catalysts, present as BaSO4, decreased catalyst NOx storage capacity. Second, sintering of the precious metals in the washcoat was observed, which can be expected to decrease the rate of NOx reduction. Additionally, sintering is hypothesized to result in segregation of the precious metal and Ba phases, resulting in less efficient NOx spillover from Pt to Ba during NOx adsorption, as well as decreased rates of reductant spillover from Pt to Ba and reverse NOx spillover during catalyst regeneration. Spectacular improvement in LNT durability was observed for catalysts containing CeO2 or CeO2-ZrO2 relative to their non-ceria containing analog. This was attributed to (i) the ability of ceria to participate in NOx storage/reduction as a supplement to the main Ba NOx storage component; (ii) the fact that Pt and CeO2(-ZrO2) are not subject to phase segregation; and (iii) the ability of ceria to trap sulfur, resulting in decreased sulfur accumulation on the Ba component.

Original languageEnglish
Pages (from-to)362-375
Number of pages14
JournalCatalysis Today
Volume151
Issue number3-4
Early online date18 Jan 2010
DOIs
Publication statusPublished - 19 Jun 2010

Bibliographical note

The authors thank Shelley Hopps for sulfur measurements, and Rob Spicer and Tonya Morgan for assistance with the rapid aging experiments. This project was funded by the U.S. Department of Energy (DOE) under award No. DE-FC26-05NT42631. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily
reflect the views of the DOE.

Keywords

  • lean NOx trap
  • NOx storage
  • catalyst aging
  • precious metal sintering
  • ceria

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