Theory of Heterogeneous Catalyst

Theory of Heterogeneous Catalyst

The catalysis in which reactant and catalyst are in different phases is called heterogeneous catalysis. In Haber's process of manufacture of ammonia, Nitrogen and Hydrogen are in the gaseous phase and Iron is present in solid-state, which acts as a catalyst.

             Fe
3H₂     +     N₂     ⇌     2NH₃

The theory of heterogeneous catalysis is based upon the phenomenon of adsorption. The action of a heterogeneous catalyst is due to the presence of free valencies present on the surface. These free valencies are responsible for the reactant molecules to undergo chemical reaction on the surface of the catalyst.
According to adsorption theory, the mechanism of heterogeneous catalysis involves the following steps-

Diffusion

When the reactant molecules come into contact with the solid catalyst in the reaction vessel, they diffuse on the surface of the catalyst.

Adsorption

The surface of the solid catalyst possesses some active centres which are having free unsatisfied valencies. Due to this unsatisfied valency, it is having a residual affinity for the molecules of the reactants. These molecules are adsorbed to form a layer of the reactànts with unimolecular thickness. Adsorption leads to higher concentration of the reactants on the surface; thereby rate of the reaction is decreased. Since adsorption is an exothermic process, the heat of adsorption decreases the energy of activation.

Formation of activated complex

Since the molecules are very close on the surface of the catalyst they interact among themselves to form an activated complex accompanied by an increase in its energy.

Decomposition of activated complex

Since the energy of the activated complex is high as soon as it is formed, it starts undergoing decomposition to give the products on the free surface of the catalyst accompanied by a decrease in its energy.

Diffusion of the products

The molecules of reaction products diffuse away from the catalyst surface. The heat of adsorption helps in this connection. Then the fresh molecules may again undergo reaction on the surface.
All the processes mentioned above are shown below-

Example: Addition of hydrogen to ethene in the presence of Ni catalyst.
Mechanism-


With the help of this theory, we can explain the following facts-

Action of finely divided catalyst

With the increase of divisions, the free surface increases, thereby more and more number of free valencies or active centres which are responsible for the adsorption of the reactants are created. Hence in the case of finely divided catalyst, catalytic activity is greatly enhanced because of increase in active centres.

Action of Promoters

Promoters get adsorbed on the surface of the catalyst, thereby, produce discontinuity of unevenness on the surface. This increases the number of active centres resulting in greater adsorption leading to enhanced catalytic activity.

Action of Poisons

Poisons are preferentially adsorbed by the active centres present on the surface of the catalyst, thereby, reducing the number of active centres available for reactant molecules.

Specific action of the catalyst

The specific action of the catalyst is due to the affinity between the catalyst and the reactants. This affinity mainly determines the adsorption of the reactant molecules on the surface of the catalyst and their subsequent conversion into the products.

Which of the following is not an example of heterogeneous catalysis?

A. Enzyme catalysis       ✓
B. Hardening of animal and vegetable oils
C. Haber’s process
D. Cracking of heavy oils for a synthesis of gasoline

The factor which determines the activity of a heterogeneous catalyst is

A. Total surface area only
B. The number of active sites per unit amount of catalyst only
C. Method of preparation, prior treatment only
D. Total surface area, number of active sites and method of preparation       ✓