In heterogeneous catlaysis the catalyst is in the differernt phase than the reactants.

According to Langmuir a gaseous reaction taking place on the surface of a solid catalyst can be divided into five different stages. They are diffusion of reactants to surface, adsorption of reactants at the surfaces, chemical reaction at the surface, desorption of products from the surface, diffusion of the products from the surface.

All these steps are consecutive. If any of them has a slower rate constant than the others, it will become the rate-determining step. Adsorption and desorption steps are generally faster than the other stages. It is generally beleived that the kinetics of the surface reactions can be treated successfully on the basis of the following assumptions:

1. The rate determining step is the adsorption step 2. In chemisorption, chemical bonds are formed between the adsorbate and the surface resulting in the formation of a monolayer.