Modelling of micro/nano-scale concentration and temperature gradients in porous supported catalysts

Meso-scale mathematical model of local reaction and transport processes in a porous catalyst with bimodal pore-size distribution is used for the evaluation of internal concentration and temperature gradients. The model takes into account reaction steps on active sites (microkinetics), diffusion of reactants in both macro- and meso-pores (molecular and Knudsen diffusion), and heat generation and transport. The processes are modelled within a 3D domain (≈10×10×10μm3) of computer-reconstructed porous catalyst. Methods of digital reconstruction, based on the analysis of electron microscope images (SEM and TEM) are employed. The methodology is demonstrated on CO oxidation on Pt/γPt/γ-Al2O3Al2O3. Overall reaction rate and effectiveness factor are studied in dependence on the morphological properties of the catalyst—macro- and meso-porosity, the sizes of support γγ-Al2O3Al2O3 particles, and characteristic diameter of the pores. Relative importance of diffusion in macro- and meso-pores and reaction heat effects are discussed. The volume-averaged parameters (e.g. effectiveness factor) calculated at this detailed spatial scale can be used as input values for simulations at a larger scale. Parametric study is performed for different γγ-Al2O3Al2O3 particles sizes, mixing ratios and levels of sintering, and local optima of effectiveness factor are found.