MECHANISM OF HETEROGENEOUS CATALYTIC REACTION

It's believed that a heterogeneous catalytic reaction takes place through the following sequence of steps.

1) Diffusion of the reactant molecules and towards the surfacing of the catalyst.

2) Adsorption of the reactant molecules on the surfaces into the solid.

3) Chemical reaction of the adsorbed molecules with the surface of the catalyst resulting into the stretching and weakening of bonds formation an activated complex with the catalyst which then decomposes or breaking of the bonds of reactant molecules.

Steps (2) and (3) are jointly called chemisorption.

(4) Desorption of the products all are from the surface.

(5) Diffusion mechanism of the products away from the surface.

The varies steps are illustrated with the following examples :

A) HYDROGENATION OF ETHYLENE IN PRESENCE OF FINELY DIVIDED Ni, Pt, Pd. 

                                                                                        It's illustrated as under

Mechanism for the reaction between ethylene and Hydrogen on a catalytic surface. 

1) Diffusion of C₂H₄ and H₂ towards surface. 

2) Adsorption of C₂H₄ and H₂ at the active sites.

3)

(a) Dissociation of H₂ into H atoms.

(b) Linking of the H- atom to C₂H₄ to form

C₂H₆

4) Desorption of C₂H₆ from the surface.

5) Diffusion of C₂H₆ away from the surface.

6) DECOMPOSITION OF HI IN THE PRESSURE OF PLATINUM :-

2 HI (g) pt → H₂(ɡ) + I₂ (ɡ)

The mechanism of this reaction below the figure as follows :