Comparison of the catalytic activity of MO 2 (M = Ti, Zr, Ce) for soot oxidation under NO x/O 2

Abstract

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