Potential rare-earth modified CeO2 catalysts for soot oxidation part II: Characterisation and catalytic activity with NO + O2

Ceria (CeO2) and rare-earth modified ceria (CeReOx with Re = La3+, Pr3+/4+, Sm3+, Y3+) supports and Pt impregnated supports are studied for the soot oxidation under a loose contact with the catalyst with the feed gas, containing NO + O2. The catalysts are characterised by XRD, H2-TPR, DRIFT and Raman spectroscopy. Among the single component oxides, CeO2 is significantly more active compared with the other lanthanide oxides used in this study. Doping CeO2 with Pr3+/4+ and La3+ improved, however, the soot oxidation activity of the resulting solid solutions. This improvement is correlated with the surface area in the case of CeLaOx and to the surface area and redox properties of CePrOx catalyst. The NO conversion to NO2 over these catalysts is responsible for the soot oxidation activity. If the activity per unit surface area is compared CePrOx is the most active one. This indicates that though La3+ can stabilise the surface area of the catalyst in fact it decreases the soot oxidation activity of Ce4+. The lattice oxygen participates in NO conversion to NO2 and the rate of this lattice oxygen transfer is much faster on CePrOx. In general, the improvement of the soot oxidation is observed over the Pt impregnated CeO2 and CeReOx catalysts, and can be correlated to the presence of Pt°. The surface reduction of the supports in the presence of Pt occurred below 100 °C. The surface redox properties of the support in the Pt catalysts do not have a significant role in the NO to NO2 conversion. In spite of the lower surface area, the Pt/CeYOx and Pt/CeO2 catalysts are found to be more active due to larger Pt crystal sizes. The presence of Pt also improved the CO conversion to CO2 over these catalysts. The activation energy for the soot oxidation with NO + O2 is found to be around 50 kJ/mol.