Adsorption Theory of Catalysis




Removal of the adsorbed substance from the surface is known as desorption. Imagine a tank of water in equilibrium with its surroundings. The amount of oxygen entering and leaving the water from the air will be the same and the oxygen concentration in the water will be constant. If the water temperature increases, the equilibrium and solubility are changed, and the oxygen will desorb from the water lowering the oxygen content.


Modern adsorption theory

The modern adsorption theory is the combination of intermediate compound formation theory and the old adsorption theory. The catalytic activity is localised on the surface of the catalyst. The mechanism involves five steps:
(i) Diffusion of reactants to the surface of the catalyst.
(ii) Adsorption of reactant molecules on the surface of the catalyst.
(iii) Occurrence of chemical reaction on the catalysts surface through formation of an intermediate.
(iv) Desorption of reaction products from the catalyst surface, and thereby,making the surface available again for more reaction to occur.


Activity, selectivity and specificity of a catalyst

Activity: It means the capacity to increase the speed of the chemical reaction.
Selectivity: By selectivity of a catalyst we mean its ability to direct the reaction to form particular products excluding others.
Specificity: Action of a catalyst is highly specific in nature, i.e a given substance can act as a catalyst only in a particular reaction and not for all the reactions.


Adsorption theory of catalysts

Adsorption theory: According to this theory, reactants are adsorbed on the surface of the catalyst and form a film. Due to high concentration of the reactants on the film, reaction proceeds at a faster rate.
Modern adsorption theory: According to this theory, reactants are adsorbed at the active centers i.e. free valencies etc. on the solid surface and form activated complex which under strain forms new molecules and leaves the surface. This explains, why the finely divided catalyst has greater activity.


Describe different methods for increasing the catalytic activity of the catalyst

There are following methods to increase activity of the catalyst :
1.Treatment by hydrogen enhance catalytic activity.
2.Treatment by high temperature.