Adsorption Theory of Heterogeneous Catalysis

Because heterogeneous catalytic action occurs specifically on the surface of the solid catalyst, the adsorption theory explains that mechanism as a five-step cycle. (i) Diffusion: the reactant molecules diffuse from the bulk gas or liquid phase toward the catalyst's surface. (ii) Adsorption: the reactant molecules are adsorbed onto the catalyst's surface (typically by chemisorption, since a real interaction with the surface is needed for the next step). (iii) Reaction: the adsorbed reactant molecules undergo the actual chemical reaction on the catalyst surface, forming an intermediate species that is itself still attached to the surface. (iv) Formation of products: the intermediate converts into the final product molecules, still on the surface. (v) Desorption: the product molecules leave (desorb from) the catalyst surface, freeing that surface site. Schematically: Diffusion -> Adsorption -> Intermediate formation -> Product formation -> Desorption, and fresh reactant molecules immediately move in to replace the just-departed products, restarting the cycle from step (i). This continuous cycling is exactly why a catalyst is recovered chemically unchanged in mass and composition at the end of the overall reacti …