Experimental Microkinetic Approach of the Catalytic Oxidation of Diesel Soot by Ceria Using Temperature-Programmed Experiments. Part 1: Impact and Evolution of the Ceria/Soot Contacts during Soot Oxidation
Résumé
The present study is dedicated to an experimental microkinetic approach of the catalyst oxidation of the diesel soot using a filter coated with ceria. To mimic the situation encountered in this process, mechanical ceria/soot mixtures have been prepared according to the tight and loose contact concepts described in the literature with ceria/soot ratio R > 1. Diesel soots prepared on an engine test bench and commercial soot have been used. The evolution of the ceria/soot contacts (via the amount of oxygen transferred from ceria to soot) with the progressive oxidation of the soot is followed using temperature-programmed experiments (denoted as TPEs) that provide the rate of CO2 and CO productions [denoted as R-CO2(T) and R-CO(T)] during the increase in the temperature in helium in the range of 300-1100 K. During the first TPE, different surface processes implying pure soot and ceria contribute to R-CO2(T) and R-CO(T), making the evaluation of the oxygen transfer difficult. It is shown that these difficulties are suppressed by performing, on the same ceria/soot sample, successive cycles constituted by a TPE followed by adsorption of O-2 at 300 K that leads to the progressive oxidation of the soot. After three cycles, it is shown that, whatever the ceria/soot mixtures, the amount of oxygen that can be transferred from ceria to soot remains constant. This indicates that the ceria/soot surface contacts do not change during the soot oxidation, which is a conclusion consistent with recent literature data on environmental transmission electron microscopy (TEM). However, the amount of oxygen provided by ceria and available for the soot oxidation is dependent upon the type of ceria/soot mixtures, and this controls the performances of the catalyst evaluated by the decrease of the light-off temperature of the soot in a flow rate of 30% O-2/He. These conclusions are used in part 2 (10.1021/ef100582w) to provide a detailed kinetic modeling of the TPE experiments for the different ceria/soot mixtures, focusing on the key role of the ceria/soot contacts on the rate of soot oxidation. This provides a consistent formalism to understand the impact of the types of catalyst/soot mixtures on the performances.