The different NOx trap performance on ceria and barium/ceria containing LNT catalysts below 200 °C

Abstract

The low-temperature NO_x storage efficiency of Ce-containing lean NO_x trap catalysts was investigated using temperature programmed adsorption. The incorporation of ceria either as a NO_x storage phase or as the support for barium oxides improves the NO_x storage capacity from 200 °C to 350 °C. However, as temperature increases from 100 °C to 150 °C, the rate of NO_x storage slows down over the catalysts containing ceria as a storage phase, while this phenomenon is less obvious if ceria is used as the support for barium oxides. By investigating the individual components of catalysts using NO-TPD, we found that the NO_x release was mainly from ceria. Based on the in situ DRIFT data, we conclude that nitrite species are formed on ceria during the NO_x storage at low temperatures, and the nitrite species can be activated and transformed to nitrate species upon further oxidation, accompanied by NO release; on the other hand, NO release occurs weakly on barium/ceria as the nitrites are more stable on the more basic Ba phase than on the ceria. In addition, the dispersion of barium species on ceria is higher than that on γ-Al₂O₃, resulting in the formation of a larger amount of basic sites. The addition of barium onto ceria not only improves the stability of ad-NO_x species but also positively impacts the amounts of NO_x trapped.