The pore size of a catalyst influences the diffusion mechanism. For large pores, molecular diffusion is predominant, where molecules collide more frequently with each other than with the pore walls. In small pores, Knudsen diffusion becomes significant, where molecules collide more frequently with the pore walls than with each other. The transition between these two regimes can be described by the Knudsen number, which is the ratio of the mean free path of the molecules to the pore diameter.
