This paper develops a detailed analysis of reaction-diffusion processes within a porous rhodium-alumina catalyst washcoat. Focused-ion-beam-scanning-electron-microscope (FIB-SEM) techniques are developed and applied to reconstruct the actual catalyst-support microstructure. Three-dimensional transport processes within washcoat micro-pore structures are modeled using a dimensionless representation of reaction and diffusion rates based on the Damkohler number. Three-dimensional computational solutions for particular porous microstructures are modeled and interpreted. In a companion paper, these microstructural results are used to assist development of larger-scale models that can be incorporated into reactor-scale simulations. (C) 2016 Elsevier Ltd. All rights reserved.