Low Temperature Ceria-Based Lean NO x Traps

Abstract

Using temperature-programmed desorption after NO adsorption at 200 °C, a variety of catalytic materials as potential low-temperature lean NO x traps were tested after hydrothermal aging. For the non Pt/ceria related materials tested, there was a strong relationship between NO oxidation activity and NO x storage capacity, regardless of the type of storage phase. While for Pt/ceria containing samples, they exhibited relatively high NO x trapping ability although they showed relatively poor NO oxidation activity. DRIFTS results indicated that NO x can be stored via a “nitrite” route for Pt/ceria materials and via a “NO2” route for the other materials. Among all the materials tested, a Pt/CeO2/Al2O3 sample was found to have the best activity in terms of NO x trapping and NO oxidation, as compared with samples like Pt/Al2O3 and Pt/CeO2, as well as a commercial LNT. Characterization data showed that the addition of CeO2 to Pt/Al2O3 helped maintain metallic Pt dispersion on Al2O3 after aging. Pt sintering on Pt/Al2O3 and Pt oxidation on Pt/CeO2, induced by strong interaction with the support, limited NO oxidation and trapping in the corresponding samples. Besides the stabilization effect on Pt, the addition of CeO2 introduced more NO x adsorption sites, which contribute to the enhanced NO x trapping observed. In addition, Pt dispersion was found to affect stored nitrate stability. Due to enhanced Pt dispersion by ceria modification, nitrates on Al2O3 decomposed at much lower temperature on Pt/CeO2/Al2O3 compared to Pt/Al2O3.