On the Reasons for High Activity of CeO 2 Catalyst for Soot Oxidation

The present work has demonstrated the reasons why CeO 2 becomes an active catalyst for diesel particulate (soot) abatement, which attracts recent worldwide attention in the development of clean diesel automobiles. Four typical fluorite-type oxides, CeO 2, ZrO 2, Pr 6 O 11, and a CeO 2 −ZrO 2 solid solution have been studied as model catalysts for soot oxidation in conjunction with the redox property and the reactivity of solid oxygen species. It was found that the redox property measured in terms of oxygen storage/release capacity was not the sole determining factor for the observed catalytic activity decreasing in the order of CeO 2 ≫ Pr 6 O 11 ≈ CeO 2 −ZrO 2 > ZrO 2 . The reactivity of oxygen species involved in the redox cycles would rather be important. The ESR measurement showed that admission of O 2 to the pre-reduced CeO 2 surface generated superoxide ions (O 2 − ). Such reactive oxygen species were less abundant on CeO 2 −ZrO 2 and were not detected on ZrO 2 and Pr 6 O 11 . The labeled and unlabeled O 2 pulse experiments demonstrated that reactive oxygen species on pre-reduced CeO 2 caused a temporal oxidation of soot even at quite a low temperature of 150 °C, compared to more than 350 °C required for successive catalytic soot oxidation. The reactive oxygen is formed from gaseous O 2 adsorbed at the three-phase boundary between soot, reduced CeO 2, and the gas phase, but another active oxygen species, which is formed from the lattice oxygen at the CeO 2 /soot interface, contributes much more to the total soot oxidation. Silver loading onto CeO 2 enhanced further the generation of superoxide and thus the catalytic activity for soot oxidation.