Comparison of the catalytic activity of MO2 (M = Ti, Zr, Ce) for soot oxidation under NOx/O2

The catalytic activity of TiO2, ZrO2, and CeO2 for soot oxidation under NOx/O-2 was compared. Characterisation of the oxides by N-2 adsorption at -196 degrees C, Raman spectroscopy, and XRD revealed that the predominant crystalline phase of TiO2 (rutile or anatase) and ZrO2 (monoclinic or tetragonal) had no significant affect on the catalytic activity of these oxides for soot oxidation with NOx/O-2. CeO2, with the only allowed fluorite structure, is the most active oxide because accelerates the NO conversion to NO2, whereas TiO2 and ZrO2 do not catalyse this reaction. Once NO2 is produced by CeO2, part of it reacts with soot, and a certain amount is stored on CeO2. NOx stored on CeO2 under reaction conditions (around 450 degrees C) does not accelerate soot oxidation and can be evolved under N-2 flow but not under a NOx/O-2 stream. NOx storage on TiO2 and ZrO2 under reaction conditions was not detected. An additional benefit of CeO2 with regard to TiO2 and ZrO2 is that the CeO2-catalysed soot oxidation yields mainly CO2, whereas TiO2 and ZrO2 yield higher percentages of CO. This can be attributed to the lowest CeO2-catalysed soot oxidation temperature and to the ability of CeO2 to catalyse CO oxidation to CO2. The effect of thermal sintering at 800 degrees C (decreased BET surface area and increased crystal size) on CeO2 is more important than on TiO2 and ZrO2, but 800 degrees C-calcined CeO2 is Still more active for soot combustion than the other oxides studied. (c) 2007 Elsevier Inc. All rights reserved.