NO x storage–reduction characteristics of Ba-based lean NO x trap catalysts subjected to simulated road aging

Abstract

In order to study the effect of washcoat composition on lean NO x trap (LNT) aging characteristics, fully formulated monolithic LNT catalysts containing varying amounts of La-stabilized CeO 2 (5 wt% La 2O 3) or CeO 2-ZrO 2 (Ce:Zr = 70:30) were subjected to accelerated aging on a bench reactor. Subsequent catalyst evaluation revealed that aging resulted in deterioration of the NO x storage, NO x release and NO x reduction functions, whereas the observation of lean phase NO 2 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 NH 3 in the temperature range 250–450 °C. TEM, H 2 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 BaSO 4, decreased catalyst NO x storage capacity. Second, sintering of the precious metals in the washcoat was observed, which can be expected to decrease the rate of NO x reduction. Additionally, sintering is hypothesized to result in segregation of the precious metal and Ba phases, resulting in less efficient NO x spillover from Pt to Ba during NO x adsorption, as well as decreased rates of reductant spillover from Pt to Ba and reverse NO x spillover during catalyst regeneration. Spectacular improvement in LNT durability was observed for catalysts containing CeO 2 or CeO 2-ZrO 2 relative to their non-ceria containing analog. This was attributed to (i) the ability of ceria to participate in NO x storage/reduction as a supplement to the main Ba NO x storage component; (ii) the fact that Pt and CeO 2(-ZrO 2) are not subject to phase segregation; and (iii) the ability of ceria to trap sulfur, resulting in decreased sulfur accumulation on the Ba component.