Enhanced soot oxidation by lattice oxygen via La 3+-doped CeO 2

Abstract

The catalytic behaviours of CeO 2 and a series of La 3+-doped CeO 2 catalysts (La 3+ loading between 5 and 50 wt%) have been studied for soot oxidation by O 2. XRD and Raman spectroscopy characterisation indicated that solid solutions are formed in the studied Ce/La ratio, in which La 3+ cations replace Ce 4+ cations in the CeO 2 lattice. Thermogravimetric analysis showed that La 3+ significantly improves CeO 2 catalytic activity for soot oxidation with O 2. The best catalytic activity was found with 5 wt% La 3+-doped CeO 2 catalyst (CeO 2-5La), in both loose and tight contact conditions. This improvement seems to be related to the increase in BET surface area and the change in the catalyst redox properties of CeO 2 brought about by doping with La 3+. La 3+ decreases the onset temperature of Ce 4+ to Ce 3+ reduction by H 2 from 580 °C (CeO 2) to 325 °C (CeO 2-5La) and increases the amount of Ce 4+ that can be reduced by H 2 (maximum amount for CeO 2-5La catalyst). An advanced TAP reactor is used for the first time to study catalysed soot oxidation with labelled oxygen. In the absence of catalyst, oxidation starts above 500 °C, and mainly labelled oxidation species (CO and CO 2) were found. In the presence of catalyst, it is shown that the gas-phase labelled oxygen replaces nonlabelled lattice oxygen, creating the highly active nonlabelled oxygen. This highly active nonlabelled oxygen reacts with soot, giving CO and CO 2. The creation of such active oxygen species starts from 400 °C and thereby decreases the soot oxidation temperature. CeO 2-5La produces more such active species, for example, leading to 98% oxygen conversion at 400 °C compared with 37% over CeO 2 alone under identical circumstances.