Structure characterisation of catalytic particulate filters for automotive exhaust gas aftertreatment

Modern automobiles with internal combustion engine are equipped with several different converters of exhaust gas and a particulate filter. The size and cost of exhaust aftertreatment system can be reduced by coating the catalytically active material on or into the porous filter walls. The characterisation of filter morphology including the distribution of catalytic material inside the filter wall is a key prerequisite for the development of porous structures with optimum performance with respect to pressure loss, filtration efficiency and catalytic activity. Three SiC filter samples with different amount of SCR catalyst were characterised by X-ray tomography (XRT), scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). Combination of the techniques provides detailed and consistent information on filter and coating hierarchical pore structure. The segmented XRT image revealed 3D distribution of catalytic material and larger macropores inside filter wall. The 3D XRT images were further used as the input for mathematical models evaluating porosity, pore size distribution (PSD), effective diffusivity and tortuosity. Maximum sphere inscription method applied to XRT data gave PSD shifted to larger pore sizes in comparison with the MIP results, in line with the bottleneck limitation of MIP. On the other hand, MIP was able to determine also smaller internal pores in the catalyst layer. Pressure loss of the filters measured in lab reactor increased non-linearly with the amount of catalytic coating. The PSD and XRT suggest that a large number of substrate pores is filled up completely by catalytic material.