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

Abstract

The low-temperature NOx storage efficiency of Ce-containing lean NOx trap catalysts was investigated using temperature programmed adsorption. The incorporation of ceria either as a NOx storage phase or as the support for barium oxides improves the NOx storage capacity from 200 °C to 350 °C. However, as temperature increases from 100 °C to 150 °C, the rate of NOx 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 NOx release was mainly from ceria. Based on the in situ DRIFT data, we conclude that nitrite species are formed on ceria during the NOx 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-NOx species but also positively impacts the amounts of NOx trapped.