Catalytic coating of a particulate filter for exhaust gas aftertreatment affects the filter performance in multiple ways. Not only it enables conversion of gaseous pollutants such as CO, hydrocarbons or NOx, but its presence on or inside the filter wall significantly influences soot filtration efficiency and pressure drop of the device. Understanding these effects from pore to device scale is essential for optimization of the coating structure aiming on a balanced mix between high catalytic conversion and filtration efficiency on one hand, and low pressure drop on the other. This contribution presents results that combine systematic washcoating experiments, characterization, testing and mathematical modeling of the coated filters. Cordierite filters are coated with slurry of a model Pt/Al2O3 catalyst, using the particle size distribution and viscosity as the main parameters controlling the catalyst structure and location in and/or on the porous filter wall. Series of samples with similar catalyst loading but different distributions are prepared. The resulting microstructure is characterized in 3D by X-ray tomography (XRT) and by cross-section scanning electron microscopy (SEM). Time-resolved XRT allows observing the formation of final coating structure with possible cracks during the washcoat drying as well as filtration cake build-up during subsequent tests. Samples performance is tested in lab reactor. Pressure drop and filtration efficiency are the measures most sensitive to the catalyst microstructure and distribution in the filter. The catalytic reaction light-off is determined mainly by total catalyst loading, however, the differences in conversion may arise above the light-off temperature due to mass transfer limitations leading to undesired slip of unreacted pollutants from some of the samples at higher gas flow rates. Finally, the results are complemented with pore-scale CFD computations of gas flow through filter wall, component diffusion and reaction. Transient simulations are used for soot particle transport and trapping, providing predictions of filtration efficiency and pressure drop evolution as the soot cake builds up. The simulation results are in line with trends observed in the measured data, which enables rationalization of the coating configuration and provides feedback for further optimization of the filter washcoating procedure.

3D reconstruction, modeling and optimization of catalytic coating structure in particulate filters

ACS Spring 2022

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