In a typical monolith reactor for the conversion of automobile exhaust gases one has to consider processes on several length-scales: monolith length and diameter on the order of 10 cm, the channel diameter approx. 1 mm, the porous catalytic washcoat layer with thickness on the order of 10-100 µm, the particles forming the supporting material (typically gamma-Al2O3) – diameter on the order of 1 µm, meso- and micro-pores of the supporting material with diameter on the order of 1-10 nm, the active catalytic sites with size in nanometres, and the reacting molecules with size in angstroms. The modeling of such reactor is a multi-scale problem [1].

In this paper we focus on the length-scale of the washcoat layer and present a methodology for modelling of reaction-transport processes in a digitally reconstructed, porous heterogeneous catalyst [2]. Microkinetics of the CO oxidation on Pt/gamma-Al2O3 with an explicit consideration of the surface-deposited species has been employed in the model [3,4]. The reaction takes place on the Pt sites located on the Al2O3 surface and in the meso-pores, simultaneously with the transport of gaseous reaction components. The transport inside the meso-porous Al2O3 particles is characterised by the effective diffusivity based on Knudsen diffusion.

Several 3D porous structures have been digitally reconstructed from the typical SEM images of Pt/gamma-Al2O3 catalytic washcoats of monoliths. The dependences of overall CO reaction rate and the effectiveness factor on the temperature and properties of the Pt/Al2O3 catalytic porous structure (washcoat macro-porosity, characteristic size of the macro-pores, the size of Al2O3 particles and the noble metal loading) have been studied.

References

[1] Kosek, J., Stepanek, F. & Marek, M. (2005). Modelling of transport and transformation processes in porous and multiphase bodies. Adv. Chem. Eng. 30, in press.

[2] Stepanek, F., Marek, M., Hanika, J. & Adler, P.M. (2001). Mesoscale modeling in multiphase catalysis. Catal. Today 66, 249.

[3] Koci, P., Kubicek, M. & Marek, M. (2004). Multifunctional aspects of three-way catalyst: Effects of complex washcoat composition. Chem. Eng. Res. Des. 82, 284-292.

[4] Koci, P., Kubicek, M. & Marek, M. (2004). Modelling of TWC monolith converters with microkinetics and diffusion in the washcoat. Ind. Eng. Chem. Res. 43, 4503-4510.