The modern adsorption theory is a combination of the old theory of adsorption and the intermediate theory of compound formation, the process of heterogeneous catalysis takes place in five steps-
The reactants diffuse to the surface of the catalysts.
These molecules then get adhered to the suitable sites available for adsorption.
The reactants, when bound to the surface have a higher probability of reacting with each other, and after the reaction, they form an intermediate compound.
The intermediate compound gets desorbed from the surface, which again becomes available for adsorption for other molecules to come.
The intermediate compound then disintegrates to form the final products, which then diffuse out of the internal pores and the external surface of the catalyst.