Heterogeneous catalysis is a catalytic process in which the catalyst and reactants are present in different phases. This form of heterogeneous catalytic action is explained by the adsorption theory.
The following steps are involved in the mechanism of catalysis :
(I) The reactant molecules are adsorbed on the catalyst surface.
(II) Chemical reaction is accompanied by the formation of an intermediate.
(III) The products are desorbed from the surface of the catalyst.
(IV) The products diffuse away from the surface of the catalyst.
The catalyst is present in the solid phase in this process, while the reactants are present in the gaseous state. The gaseous molecules are then adsorbed on the catalytic surface. When the concentration of reactants on the catalytic surface increases, the reaction rate increases. In such reactions, the products have a low affinity towards the catalyst and are quickly desorbed, allowing new reactants to access the surface.