Diffusion in catalysis refers to the movement of reactant molecules towards the active sites of the catalyst and the subsequent removal of products. This process is often described by two primary mechanisms: molecular diffusion and Knudsen diffusion. Molecular diffusion occurs when the mean free path of the molecules is much smaller than the pore diameter of the catalyst, while Knudsen diffusion happens when the mean free path is comparable to or larger than the pore diameter.
