Adsorption theory: This theory can be applied to heterogeneous catalysis only. As the name suggests the reactant is adsorbed on the surface of the catalyst and the effective concentration of the reactant is generally increased on the surface, the reactants are brought closer and the reaction speed is increased. However, it is now fairly well known that simple physical adsorption on the surface of the solid has practically nothing to do with catalysis. It is chemisorptions that is mainly responsible for catalytic activity. As mentioned the unused bonding capabilities of the surface atoms or ions may be utilized to bond molecules from the gas or solution phase to the surface of the solid. In practice not all the atoms or ions on the surface are reactive and can adsorb gases or ions. The places where reacting molecules can get adsorbed are called active sites.
The extent of chemisorptions will, therefore, depend on the number of available active sites and on the nature of the reacting molecules. The number of active sites per unit of catalyst depends on the nature of the catalyst, on its method of preparation, and on its treatment before use.
It is believed that the mechanism of heterogeneous catalysis on the surface of solid catalysts consists of five steps as follows:
(i) Diffusion: Reactant molecules diffuse to the solid catalyst surface.
(ii) Adsorption: Reactant molecules are first physically adsorbed on the surface. They then become chemisorbed.
(iii) Chemical reaction: The chemisorbed molecules adjacent to each other react to produce the products.
(iv) Desorption: This is the reverse of adsorption. After the reaction the product molecules are at first chemisorbed on the surface. They then become physically adsorbed and finally break free from the surface.
(v) Diffusion: The product molecules diffuse away from the surface.