This document discusses diffusion and reactions within porous catalyst particles. It begins by describing the processes that can occur within a catalyst, including external diffusion, internal diffusion, adsorption, desorption, and surface reactions. It then analyzes diffusion and reaction at the level of an individual catalyst particle. The document derives equations for species flux, diffusion coefficients, and external mass transfer coefficients. It also provides examples of internal diffusion within catalyst pellets, foils, and channels. Finally, it presents solutions for internal diffusion and reaction within a porous catalyst, defining terms like effectiveness factor and Thiele modulus that characterize these processes.