4.1. Catalysts

The catalysts used in this study were metallic Pd/Rh monoliths, as shown in Fig. 12, designed for Euro IV emission limits and, therefore, catalysts have close to commercial structure. The model catalysts consisted of a thin Fe-Cr-Al foil coated with a layered washcoat (see Fig. 13), mainly containing γ -Al₂O₃. The washcoat material content of the catalyst was 62.8 g/m² and the amounts of the active metals in the catalyst were Pd 0.29 wt-% and Rh 0.03 wt-%. The active metals, being palladium and rhodium, were added on the different washcoats of the catalyst. In addition to palladium and γ -Al₂O₃, the other washcoat (washcoat 2) also contained La₂O₃ and minor amounts of OSC material as stabilizers and promoters (additives). Another washcoat (washcoat 1) included rhodium, γ -Al₂O₃ and OSC compounds. CeO₂ and Ce_XZr_1-XO₂ mixed oxides were used as OSC material in the catalyst.

The catalysts were prepared by mixing the washcoat materials with precursor salts in a water slurry and by coating to a layered structure as mentioned above. Finally, the catalysts were dried at 100°–150°C and then calcined in air at 300°–550°C. The dried and calcined catalysts are referred to as ‘fresh’ in the following text. Due to the complex structure of the catalyst, separate samples were prepared from the washcoats of the catalyst to help with the interpretation of the results. Furthermore, some of the experiments, in particular NO-TPD experiments, were also carried out with simplified model catalysts (γ -Al₂O₃, modified γ -Al₂O₃, Pd/modified γ -Al₂O₃ and Rh/γ -Al₂O₃) with the active metal loadings of 1 wt-% and 3 wt-% in the washcoat.