Multi-scale modelling of reaction and transport in porous catalysts

Abstract

Mathematical models of reaction and transport in porous catalyst on three different scales (nano, micro, and macro) are presented. Methodology is demonstrated on CO oxidation in porous Pt / γ - Al 2 O 3 catalyst, washcoated on monolith. Morphological characteristics are obtained from TEM and SEM images of the washcoat cross-section. Methods for passing the model results from the bottom to the top level are discussed. On the nano level (I), spatially 3D model of a single meso-porous γ - Al 2 O 3 particle ( ≈ 0.1 – 1 μ m ) is used. Digital reconstruction of porous medium is here based on virtual agglomeration of Al 2 O 3 nano-particles and consequent deposition of individual Pt crystallites. The model considers the effects of catalytic sites sizes and diffusion in small meso-pores ( d ≈ 10 nm ) . On the micro level (II), spatially 3D model of porous catalytic washcoat ( thickness ≈ 30 μ m ) is used. Here the reconstruction is based on virtual packing of Pt / γ - Al 2 O 3 micro-particles described in detail in the model (I). From this model, spatially averaged reaction rate and effectiveness factor are evaluated in dependence on temperature, concentration of reactants, and macro-porous structure of the washcoat (influenced by γ - Al 2 O 3 micro-particles size and sintering). On the macro level (III), spatially 1D model of catalytic monolith ( ≈ 10 cm ) with plug flow and solution of mass and heat transport in monolith channel is used. Here the results of the detailed models (I, II) are employed in the form of local reaction rates. The results of the multi-scale modelling approach are compared with standard catalytic monolith models.