Effect of ceria on the desulfation characteristics of model lean NO _x trap catalysts

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 Al ₂O₃) 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 BaSO ₄ 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.