Impact of the Catalyst/Soot Ratio on Diesel Soot Oxidation Pathways

The transition between catalytic and thermal soot oxidation mechanisms is of importance in diesel particulate filters (DPFs). The objective of this study is to develop and validate a new detailed kinetic model predicting reactivity and selectivity of soot oxidation by NOx + O-2 over Pt/Ce0.73Zr0.27O2 over a wide range of catalyst/soot (c/s) ratios. On the basis of a hierarchical development approach, we merged previously developed kinetic subsets with reactions describing oxygen spillover between ceria and soot. The kinetic parameters for these new steps were estimated by fitting concentration profiles of the major product species. This mechanism allows us to obtain insight into the interplay between catalytic and purely thermal oxidation pathways as a function of the c/s ratio. Overall, our results stress the importance of non-catalytic reactions even at high c/s ratios. It is shown that, in the case of a NO2-rich gaseous feed, a purely thermal reaction between NO2 and soot contributes significantly to soot oxidation, even at high c/s ratios. Among the CO2 production reactions, the reaction of spilt-over oxygen with molecular oxygen shows a complex behavior with respect to the c/s ratio, which can be explained by a competition of O-2 with NO2 for spilt-over oxygen. NOx consumption pathways over the catalyst were also investigated. For low NO2/NOx ratios, it was found that oxygen spillover plays a more important role than NO2 in soot oxidation.