Question

Define adsorption theory of heterogeneous catalysis.

Answer 1

Catalysts are of two types depending upon the phases of catalyst and reactants. Homogeneous catalyst is a catalyst in which the reactant and catalyst are present in the same phase. Heterogeneous catalysts are catalysts in which both reactant and catalyst are in different phases.

Complete answer:
Catalyst influences the rate of reaction by providing the reaction a different pathway. Heterogeneous catalysis typically involves solid phase catalysts and gas phase reactants.

In Adsorption theory of catalysis, the reactants in their gaseous state or in dissolved state in solution get adsorbed on the surface of the catalyst in its solid state because of the physisorption or chemisorption.
There is a cycle of molecular adsorption, reaction and desorption occurs at the surface of the catalyst.
So there are a total of five stages in accordance with adsorption theory of heterogeneous catalysis. Diffusion of reactants to the surface- in the first step the diffusion of reactant molecules occurs at the surface of the catalyst.

Adsorption of reactants on the catalyst- there are some free valencies present at the surface of catalyst The adsorption of reactants occurs by forming loose bonds with the catalysts because of those free valencies. Reaction between reactants- after adsorption, the chemical reaction takes place between the reactants.

Desorption of products - After the reaction has completed, bonds between the catalyst and product are broken down and desorption occurs. Diffusion of products away from the surface of the catalyst- products get released from the surface of the catalyst.

Note: Heterogeneous catalysis is very important because it enables faster, large scale production and selective product formation. There are two types of adsorption- physisorption which is weakly bonded adsorption, the catalyst and molecules of reactants are bonded through Van der Waals forces. Chemisorptions that are strongly bonded adsorption, the adsorbate reactant molecules and adsorbent catalyst shares electrons and form chemical bonds.