Modeling reaction-diffusion processes within catalyst washcoats: II. Macroscale processes informed by microscale simulations

This paper first develops a two-dimensional Thiele-type cylindrical-pore model that predicts catalytic washcoat performance, albeit for idealized cylindrical pores. The primary purpose for the cylindrical-pore model is to serve as a basis of comparison with three-dimensional models of catalytic performance in actual geometrically complex washcoat pores that are tomographically reconstructed from focused-ion-beam-scanning-electron-microscopy (FIB-SEM) measurements. In both models, the reaction-diffusion processes are characterized by a Damköhler number that is based on a pore diffusion coefficient and a single first-order reaction rate. Performance metrics include effective product flux from the pores, pore effectiveness, and reaction depth within the pore. In all cases, the models are generalized by casting the conservation equations and performance metrics as dimensionless variables. The paper then derives expressions to upscale individual pore performance to full-scale washcoats. The new understanding that emerges from these studies provides qualitative and quantitative insight that can assist the design and fabrication of improved washcoat microstructures.