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Diffusion is the transport/movement of molecules/particles through space and time. While bulk, gas-phase diffusion occurs during heterogeneous catalysis, the more important phenomenon is surface diffusion. Surface diffusion refers to the movement of adsorbed species on the surface from one active site to another. Just as in the case of homogeneous chemistry (within a fluid phase), overall reaction chemistry can either be diffusion- or reaction-controlled. In the case of diffusion control, surface-bound species do not move into close enough proximity to undergo chemical reaction (not occupying neighboring active sites). In this limit, the overall rate of reaction is not determined by the intrinsic chemical reaction kinetics. On the other hand, if surface diffusion is fast, the chemistry is reaction-controlled, and depends entirely on the actual chemical kinetics of the steps involved in the reaction mechanism.