A comparison between Pt and Pd in NOx storage catalysts

The importance of Pt and Pd in noble metal-barium oxide type NOx storage catalysts was investigated. Model Pt/BaO/Al2O3 and Pd/BaO/Al2O3 catalysts were prepared and evaluated with respect to NOx storage capacity, activity towards NO reduction under lean conditions and NO oxidation capacity using synthetic lean burn exhausts containing NO, O2, C3H6 and N2. The study was carried out by performing static and transient flow reactor experiments and temperature-programmed desorption studies. At 300 °C, the Pd/BaO/Al2O3 sample shows a higher NOx storage capacity than Pt/BaO/Al2O3, i.e. more NOx is stored during the lean periods and almost all NOx is released and reduced during the subsequent rich periods. At this temperature (300 °C), the NO reduction is not complete during the rich phase for the Pt-based catalyst suggesting poisoning of Pt-sites by adsorbed species. At 400 °C, Pt/BaO/Al2O3 stores slightly more NOx than its Pd-based counterpart. XPS measurements on pre-treated catalysts, show some changes in oxidation state for Pd between the rich and lean phases. The oxidation of NO is much more limited on Pd based samples compared to Pt containing catalysts. The importance of NO2 as an intermediate in the storage of NOx as nitrate under lean conditions is confirmed in this study.