Meso-scale modelling of CO oxidation in digitally reconstructed porous Pt/γ-Al2O3 catalyst

Abstract In this paper we present a methodology for modelling of local reaction-transport processes in a digitally reconstructed, porous heterogeneous catalyst. Microkinetics of the CO oxidation on Pt / γ - Al 2 O 3 with an explicit consideration of the surface-deposited species has been employed in the model. The reaction takes place on the Pt sites located on the γ - Al 2 O 3 surface and in the meso-pores, simultaneously with the transport of gaseous reaction components. Several 3D porous structures have been digitally reconstructed from the typical Scanning electron microscope images of Pt / γ - Al 2 O 3 catalytic washcoats of monoliths. Typical dependences of overall CO reaction rate and the effectiveness factor on the temperature and properties of the porous Pt / γ - Al 2 O 3 structure (washcoat macro-porosity, characteristic size of the macro-pores, the size of γ - Al 2 O 3 particles, noble metal loading and distribution) are evaluated. Non-monotonous dependence of the averaged reaction rate on the macro-porosity of the washcoat is observed. It is shown that the dependence of the average reaction rate on the amount of Pt in the porous layer is strongly non-linear under certain conditions and the spatial distribution of the catalytically active sites within the porous structure of the support controls the effectiveness factor to a significant extent. In the combination with semi-deterministic methods of the reconstruction (simulation of the catalyst preparation process) the results can be used for the optimisation of the washcoat structure. The methodology presented in this work is also a contribution to the development of multi-scale simulation techniques whereby volume-averaged parameters calculated at one spatial scale are used as input values for simulations at a larger scale.

[1]  Jc Jaap Schouten,et al.  Acetylene and carbon monoxide oxidation over a Pt/Rh/CeO2/γ-Al2O3 automotive exhaust gas catalyst: kinetic modelling of transient experiments , 2001 .

[2]  R. Farrauto,et al.  Automobile exhaust catalysts , 2001 .

[3]  Morphology and thermal conductivity of model organic aerogels , 1996, cond-mat/9612044.

[4]  F. Štěpánek,et al.  Effective thermal conductivity of wet particle assemblies , 2004 .

[5]  M. Kubicek,et al.  Periodic forcing of three-way catalyst with diffusion in the washcoat , 2004 .

[6]  R. Hayes,et al.  Evaluating the effective diffusivity of methane in the washcoat of a honeycomb monolith , 2000 .

[7]  Ian T. Cameron,et al.  Classification and analysis of integrating frameworks in multiscale modelling , 2004 .

[8]  Paolo Fornasiero,et al.  Automotive catalytic converters: current status and some perspectives , 2003 .

[9]  F. Keil.  Modelling of phenomena within catalyst particles , 1996 .

[10]  Juraj Kosek,et al.  Modeling of Transport and Transformation Processes in Porous and Multiphase Bodies , 2005 .

[11]  M. Kubicek,et al.  Multifunctional Aspects of Three-Way Catalyst: Effects of Complex Washcoat Composition , 2004 .

[12]  F. Štěpánek,et al.  Modeling capillary condensation hysteresis cycles in reconstructed porous media , 1999 .

[13]  G. Ertl.  Heterogeneous catalysis on the atomic scale. , 2002, Chemical record.

[14]  R. Hayes,et al.  A New Technique to Measure the Effective Diffusivity in a Catalytic Monolith Washcoat , 2004 .

[15]  Gbmm Guy Marin,et al.  Development of a transient kinetic model for the CO oxidation by O2 over a Pt/Rh/CeO2/γ-Al2O3 three-way catalyst , 1998 .

[16]  Karthik Ramanathan,et al.  Light-off criterion and transient analysis of catalytic monoliths , 2003 .

[17]  František Štěpánek,et al.  Computer simulation of granule microstructure formation , 2005 .

[18]  R. Hayes,et al.  Calculating effectiveness factors in non-uniform washcoat shapes , 2005 .

[19]  Petr Kočí,et al.  Modeling of Three-Way-Catalyst Monolith Converters with Microkinetics and Diffusion in the Washcoat , 2004 .