Reaction and Diffusion Phenomena in Catalyzed Diesel Particulate Filters

The objective of this study is to explain the physical and chemical mechanisms involved in the operation of a catalyzed diesel particulate filter. The study emphasizes on the coupling between reaction and diffusion phenomena (with emphasis on NO₂ "back-diffusion"), based on modeling and experimental data obtained on the engine dynamometer. The study is facilitated by a novel multi-dimensional mathematical model able to predict both reaction and diffusion phenomena in the filter channels and through the soot layer and wall. The model is thus able to predict the species concentration gradients in the inlet/outlet channels, in the soot layer and wall, taking into account the effect of NO₂ back diffusion. The model is validated versus engine dyno measurements. Two sets of measurements are employed corresponding to low-temperature "controlled" regenerations as well as high-temperature "uncontrolled" conditions. Good agreement is observed between measured and computed results in both regeneration modes. It is concluded that at low temperatures, the main reaction mechanism involves NO₂ produced on the catalytic sites and partially diffusing back to the soot layer (re-cycling). At high temperature regeneration modes, NO₂ is not favored due to chemical equilibrium limitations and therefore the main reaction path involves oxygen reaction with soot.