The Micromechanics of Catalytic Soot Oxidation in Diesel Particulate Filters

Despite the great effort devoted to the modeling of the operation of catalytic DPFs, even today very simple expressions are used for the soot oxidation rate. In the relevant to DPF operation case of a gas phase rich in oxygen, the structure of the soot-catalyst geometry and its evolution during oxidation determines the reaction rate. An extensive set of controlled experiments (isothermal or with linear temperature increase) using fuel borne catalysts and catalytic coatings has been performed in order to obtain corresponding soot oxidation rate-conversion curves. The shape of the resulting curves cannot be described by the typical theories for solid phase reactions posing the need for microstructural models for the micromechanics of soot catalyst interactions. The model in the case of soot-catalytic coating interaction is based on a population balance approach for different classes of soot particles which have different levels of contact to the catalyst sites and the dynamic evolution of these populations during oxidation, taking into account migration and fragmentation phenomena. For the case of fuel borne catalysts the initial aggregation state between the soot and catalyst particles is shown to largely determine the evolution of the system during oxidation. The micromechanical models of soot-catalyst interaction are able to describe the experimentally observed oxidation rate-conversion dependencies and they are readily integrated into DPF simulation codes in order to improve their performance.