PNNL

Abstract

A novel step-response technique was developed for the kinetic study of diesel soot oxidation. 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. Due to its high throughput, the technique allows broad parametric studies to be performed rapidly and in a kinetically rigorous manner. The technique was applied to soot oxidation by O2, one of the major regeneration mechanisms for the catalytic soot filter systems. It was found 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 Arhenius 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.