Effect of the loading sequence of CeO2 and Pd over Al2O3 on the catalytic performance of Pd-only close-coupled catalysts

Abstract

The Pd-only close-coupled catalysts of the different loading sequence of CeO2 and Pd over Al2O3 were prepared using consecutive wetness impregnation method. Differences in structural, surface properties and catalytic performance, together with the thermal aging behaviors of catalyst at 1000 °C were investigated systematically by XRD, TPR, XPS, ICP, CO adsorption and TEM. Consequently, the Ce/Pd/Al catalysts loaded Ce after Pd over the alumina show superior structural properties before and after aging treatment, suggesting the Ce/Pd/Al catalysts possess better thermal resistance than Pd/Ce/Al catalysts loaded Pd after Ce. Moreover, the aged Ce/Pd/Al catalysts possess stronger reducible probed by TPR, and HRTEM and CO adsorption show smaller particles size of Pd and higher palladium dispersion. As a result, for Ce/Pd/Al catalysts, more palladium sites play a role as active site, which promotes low temperature conversion of pollutants effectively. The catalytic performance of the catalysts is forcefully subject to palladium dispersion and reduction property, especially for NO conversion, and the light-off temperature of NO over aged Ce/Pd/Al is 27 °C lower than aged Pd/Ce/Al. The results have demonstrated that novel loading sequence is beneficial to catalytic activity which is maintained even after aging treatment.