Experimental Microkinetic Approach of the Catalytic Oxidation of Diesel Soot by Ceria Using Temperature-Programmed Experiments. Part 2: Kinetic Modeling of the Impact of the Ceria/Soot Contacts on the Rate of Oxidation

Abstract

An experimental microkinetic approach of the catalytic oxidation of diesel soots is developed considering mechanical mixtures of ceria and soot (ceria/soot weight ratio R ) to mimic the situation of a catalyst-coated filter. Three ceria/soot mixtures denoted as TC- R = 10, TC- R = 1, and LC- R = 10 have been prepared according to the tight and loose contacts, respectively. Temperature-programmed experiments (without O 2 ) performed on stabilized ceria/soot mixtures provide the rate of the CO 2 production, R CO 2 ( T ), via the reaction between carbon defect sites C f of the soot and oxygen species transferred by the ceria particles. The R CO 2 ( T ) curves present two peaks at low and high temperatures. The study is dedicated to the kinetic modeling of the R CO 2 ( T ) curves at low temperatures. According to the experimental microkinetic approach, two plausible kinetic models of the reaction have been selected on the basis of literature data; they differ from the implication or not of oxygen diffusion on the ceria surface. The mathematical kinetic formalisms, associated with the models, consider that the ceria/soot contacts constitute key kinetic parameters of the soot oxidation and their properties, such as the average number of contacts N c between a ceria particle and the soot particles and the average surface s c of a contact, included in the kinetic equations. Different kinetic parameters have been obtained using experimental procedures, such as the activation energy of oxidation of the carbon defect sites (187 kJ/mol) and the N c values for the different ceria/soot mixtures (i.e., N c = 4.3 for TC- R = 10). It is shown that the microkinetic approach permits obtaining theoretical R CO 2 ( T ) curves consistent with the experimental data for the three mixtures, considering similar values of the kinetic parameters, except the N c and R values. Moreover, the kinetic models have been extended to catalytic soot oxidation in the presence of O 2 by including its adsorption on ceria. This study confirms the interest of the experimental microkinetic approach for the understanding of the key kinetic parameters controlling the catalytic soot oxidation. Moreover, it provides a mathematical formalism for the comparison of the performances of different solid catalysts, in particular for the development of new formulations.