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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 CeO2 (5wt% 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 NO x storage, NO x release and NO x 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 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 CeO2 or CeO2-ZrO2 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 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. [Copyright &y& Elsevier]

Abstract: Using model powder catalysts and fully formulated monolithic lean NO x trap (LNT) catalysts, the effect of ceria on LNT desulfation behavior was investigated. TPR experiments performed on the model catalysts showed that each of the oxide phases present (BaO, CeO2, and Al2O3) is able to store sulfur and that they possess distinct behavior in terms of the temperatures at which desulfation occurs. For a Pt/BaO/Al2O3 catalyst, addition of ceria was found to reduce the degree of sulfur accumulation on the BaO phase, particularly at high sulfur loadings, by contributing to sulfur capture. The importance of maintaining the Pt and Ba phases in close proximity for efficient LNT desulfation was also demonstrated; physically separating the Pt and Ba sites resulted in a shift of the desulfation temperature of the surface BaSO4 by 20–40°C towards higher temperature, i.e., towards the position characteristic of bulk BaSO4. From the monolith studies, it was found that relative to a sample containing no ceria, samples containing La-stabilized CeO2 or CeO2–ZrO2 showed a greater resistance to deactivation during sulfation (as reflected by the NO x storage efficiency), and required lower temperatures to restore the NO x storage efficiency to its pre-sulfation value. In addition to the ability of ceria to store sulfur and release it at relatively low temperatures under reducing conditions, these findings can be attributed to the high water–gas shift activity displayed Pt/CeO2, which result in increased intra-catalyst concentrations of H2 under rich conditions. The results also showed that precious metal loadings can significantly impact desulfation efficiency and that both high Rh and Pt loadings are beneficial for catalyst desulfation. [Copyright &y& Elsevier]

Abstract: The effect of La2O3-stabilized ceria incorporation on the functioning of fully formulated lean NO x trap catalysts was investigated. Monolithic catalysts were prepared, corresponding to loadings of 0, 50 and 100g CeO2/L, together with a catalyst containing 100g/L of ceria–zirconia (Ce0.7Zr0.3O2). Loadings of the other main components (Pt, Rh and BaO) were held constant. Catalyst evaluation was performed on a bench flow reactor under simulated diesel exhaust conditions, employing NO x storage/reduction cycles. NO x storage efficiency in the temperature range 150–350°C was observed to increase with ceria loading, resulting in higher NO x conversion levels. At 150°C, high rich phase NO x slip was observed for all of the catalysts, resulting from an imbalance in the rates of nitrate decomposition and NO x reduction. Optimal NO x conversion was obtained in the range 250–350°C for all the catalysts, while at 450°C high rich phase NO x slip from the most highly loaded ceria-containing catalyst resulted in lower NO x conversion than for the ceria-free formulation. N2O was the major NO x reduction product at 150°C over all of the catalysts, although low NO x conversion levels limited the N2O yield. At higher temperatures N2 was the main product of NO x reduction, although NH3 formation was also observed. Selectivity to NH3 decreased with increasing ceria loading, indicating that NH3 is consumed by reaction with stored oxygen in the rear of the catalyst. [Copyright &y& Elsevier]