Heterogeneous catalysis and the Langmuir isotherm

Diffusion of the reactants to the surface, adsorption on the active sites, surface reaction, desorption of the products, diffusion back to the bulk. The rate-determining step is often adsorption or the surface reaction. Physisorption: van der Waals forces, of the order of $20\ \text{kJ/mol}$, reversible. Chemisorption: a real bond to the surface, of the order of $200\ \text{kJ/mol}$, monolayer, often dissociative.