On the Reasons for High Activity of CeO2 Catalyst for Soot Oxidation

Abstract

The present work has demonstrated the reasons why CeO2 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, CeO2, ZrO2, Pr6O11, and a CeO2−ZrO2 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 CeO2 ≫ Pr6O11 ≈ CeO2−ZrO2 > ZrO2. The reactivity of oxygen species involved in the redox cycles would rather be important. The ESR measurement showed that admission of O2 to the pre-reduced CeO2 surface generated superoxide ions (O2−). Such reactive oxygen species were less abundant on CeO2−ZrO2 and were not detected on ZrO2 and Pr6O11. The labeled and unlabeled O2 pulse experiments demonstrated that reactive oxygen species on pre-reduced CeO2 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 O2 adsorbed at the three-phase boundary between soot, reduced CeO2, and the gas phase, but another active oxygen species, which is formed from the lattice oxygen at the CeO2/soot interface, contributes much more to the total soot oxidation. Silver loading onto CeO2 enhanced further the generation of superoxide and thus the catalytic activity for soot oxidation.