Differential kinetic analysis of diesel particulate matter (soot) oxidation by oxygen using a step-response technique

The effects of a catalytic coating on the oxidation of captured soot over diesel particulate filters (DPF) is debated in the literature, since a catalyzed filter wall appears to lack sufficiently tight contact with soot deposits to exercise direct catalytic action. The topology of soot-catalyst contact may change with progressive oxidation of the soot layer; hence, a technique capable of probing catalytic action via detailed kinetic analysis at different stages of oxidation is required to conclusively resolve this problem. A novel step-response technique was developed in this work as a methodological foundation for such study. Using this technique, various aspects of the oxidation process can be probed while consuming only differential amounts of carbon, and the impact of the reaction heat on the measured rates can be minimized. This technique was applied to soot oxidation by O2 and showed that, after decoupling effects due to the sample history, carbon oxidation by O2 in the absence of H2O can be well-described by an unmodified Arrhenius equation, with similar activation energy values for diesel and model soot samples (137 ± 8.7 and 132 ± 5.1 kJ/mol, respectively). The reaction order in O2 for these samples was found to be 0.61 ± 0.03 and 0.71 ± 0.03, respectively, and was remarkably independent of the temperature, suggesting that the fractional order is not due to mixed kinetic control. The reaction mechanism was also found to be independent of carbon conversion. The density of the reaction sites, however, appeared to increase with oxidation. This increase could not be accounted for by the changes in the specific surface area, either directly measured or derived from such simplified models as the shrinking-core formalism. The entire set of obtained experimental results can be described using a kinetically uncomplicated model in a broad range of temperatures, partial pressures of oxygen and degrees of soot oxidation.