The Effect of CeO2 on the Performance of Lean NOx Trap Catalysts

Yaying Ji1, Todd J. Toops2, Jae-Soon Choi2 and Mark Crocker1*

1 Center for Applied Energy Research, University of Kentucky,

2540 Research Park Drive, Lexington, KY 40511-8479

2 Fuels, Engines, and Emissions Research Center, Oak Ridge National Laboratory, 2360 Cherahala Blvd., Knoxville, TN 37932-1563

*crocker@caer.uky.edu

Ceria is an important component of Lean NOx Trap catalysts (LNTs) formulated for lean burn gasoline applications, its role being principally to provide the necessary oxygen storage capacity when the engine is operating at the stoichiometric point (i.e., the catalyst is functioning as a conventional three-way catalyst). However, there have been few reports specifically concerning the effect of ceria on LNT performance. In this study, we investigated the effect of CeO2 on the performance of model LNTs in powder and monolithic form.

NOx storage and regeneration behavior was first investigated in a microreactor equipped with a mass spectrometer. Two model powder catalysts were used in this study: 1 wt% Pt/20 wt% BaO/Al2O3 (hereafter denoted as PBA) and a physical mixture of 1 wt% Pt/20 wt% BaO/Al2O3 and 1 wt% Pt/CeO2 in a 74:26 weight ratio (denoted as PBAC). NOx storage measurements under continuous lean flow indicated the addition of ceria was advantageous to NOx storage at T „T 300 oC. Furthermore, the beneficial effect of ceria addition on NOx conversion was evident over the entire range of 200 to 400 oC under lean-rich cycling conditions (6 min lean, 0.5 min rich). In these experiments the ceria-containing catalyst (PBAC) displayed a superior selectivity to N2 over the ceria-free LNT (PBA) in the entire temperature range. The regeneration behavior of stored NOx (at 300 oC) was investigated using the temperature programmed reduction (TPR) technique. N2 release commenced at higher temperatures during reduction with CO compared to reduction with H2 and extended up to 500 oC for PBA and 431 oC for PBAC. The addition of H2O to the CO feed resulted in an initiation of N2 release at a lower temperature for both catalysts. However, PBAC exhibited much earlier N2 release and higher H2 formation (via the water-gas shift reaction) than PBA.

To further explore the impact of ceria on LNT performance, two fully formulated monolithic catalysts containing either no CeO2 or 100 g/l La-stabilized CeO2 were evaluated on a bench flow reactor under realistic cycling conditions (60 s lean and 5 s rich). Measurements confirmed that the ceria-containing monolithic catalyst exhibits higher NOx conversion than the ceria-free one in the temperature range 150-350 oC. Significant improvements were also observed with respect to the lean phase NOx storage capacity of the catalysts, as well as their selectivity to N2 during rich phase reduction.

Overall, these findings demonstrate that the incorporation of ceria in LNTs not only improves NOx storage efficiency but also positively impacts LNT regeneration behavior both in terms of conversion and selectivity, albeit at the expense of additional reductant consumed by the stored oxygen.