Potential rare earth modified CeO2 catalysts for soot oxidation I.: Characterisation and catalytic activity with O2

Ceria (CeO2) and rare-earth modified ceria (CeReOx with Re = La, Pr, Sm, Y) catalysts are prepared by nitrate precursor calcination and are characterised by BET surface area, XRD, H-2-TPR, and Raman spectroscopy. Potential of the catalysts in the soot oxidation is evaluated in TGA with a feed gas containing O-2. Seven hundred degree Celsius calcination leads to a decrease in the surface area of the rare-earth modified CeO2 compared with CeO2. However, an increase in the meso/macro, pore volume, an important parameter for the soot oxidation with O-2, is observed. Rare-earth ion doping led to the stabilisation of the CeO2 surface area when calcined at 1000 degrees C. XRD, H2-TPR, and Raman characterisation show a solid solution formation in most of the mixed oxide catalysts. Surface segregation of dopant and even separate phases, in CeSmOx and CeYOx catalysts, are, however, observed. CePrOx and CeLaOx catalysts show superior soot oxidation activity (100% soot oxidation below 550 degrees C) compared with CeSmOx, CeYOx, and CeO2. The improved soot oxidation activity of rare-earth doped CeO2 catalysts with O-2 can be correlated with the increased meso/micro pore volume and stabilisation of external surface area. The segregation of the phases and the enrichment of the catalyst surface with unreducible dopant decrease the intrinsic soot oxidation activity of the potential CeO2 catalytic sites. Doping CeO2 with a reducible ion such as Pr4+/3+ shows an increase in the soot oxidation. However, the ease of catalyst reduction and the bulk oxygen-storage capacity is not a critical parameter in the determination of the soot oxidation activity. During the soot oxidation with 02, the function of the catalyst is to increase the 'active oxygen' transfer to the soot surface, but it does not change the rate-determining step, as evident from the unchanged apparent activation energy (around 150 kJ mol(-1)), for the catalysed and un-catalysed soot oxidation. Spill over of oxygen on the soot surface and its subsequent adsorption at the active carbon sites is an important intermediate step in the soot oxidation mechanism. (c) 2007 Elsevier B.V. All rights reserved.