THEORIES OF CATALYSIS
There are two main theories to explain catalysis.
1. Intermediate compound formation theory
2. Adsorption theory
In general, the intermediate compound formation theory applies to homogeneous catalytic reactions and the adsorption theory applies to heterogeneous catalytic reactions.
1. The Intermediate Compound Formation Theory
According to this theory, the catalyst first forms an intermediate compound with one of the reactants. The compound is formed with less energy consumption than needed for the actual reaction. The intermediate compound being unstable combines with other reactant to form the desired product and the catalyst is regenerated.
For example, a reaction of the type
A + B ---- c---- > AB
which occurs in presence of a catalyst C, may take place as
A+C (Catalyst) ---- --- > AC (Intermediate)
AC + B -- --- --- > AB(Product) + C(Catalyst)
Many catalytic reactions can be explained on the basis of this theory.
The catalytic oxidation of SO2 to SO3 in the lead chamber process probably takes place as;
2 NO (Catalyst) + O2 --- ---- > 2NO2 Intermediate Compound
NO2 + SO2 --- --- > SO3 (Product) + NO (Catalyst)
This theory explains the mechanism of heterogeneous catalysis. Here, the catalyst functions by adsorption of the reacting molecules on its surface.
In general, there are four steps involved in the heterogeneous catalysis.
A(g) + B(g) ----( Catalyst) -- > C(g) + D(g)
Step - 1. Adsorption of reactant molecules
The reactant molecules A and B strike the surface of the catalyst. They are held up at the surface by weak vanderwaal's forces or by partial chemical bonds.
Step - 2. Formation of Activated complex
The particles of the reactants adjacent to one another join to form an intermediate complex (A-B). The activated complex is unstable.
Step - 3. Decomposition of Activated complex
The activated complex breaks to form the products C and D. The separated particles of the products hold to the catalyst surface by partial chemical bonds.
Step - 4. Desorption of Products
The particles of the products are desorbed or released from the surface.
Applications of catalysis
The applications of catalysis are summarised as follows.
i. Haber's process for the manufacture of ammonia.
ii. Ostwald's process for the manufacture of nitric acid.
iii. Lead chamber process for the manufacture of sulphuric acid.
iv. Contact process for the manufacture of sulphuric acid.
v. Deacon's process for the manufacture of chlorine.
vi. Bosch's process for the manufacture of hydrogen.
vii. Hydrogenation of vegetable oils Oil + H2 -- > Vanaspati ghee
viii. Bergius process for the synthesis of petrol from coal.
i. Finely divided iron. Molybdenum as promoter.
ii. Platinished asbestos
iii. Nitric oxide.
iv. Platinised asbestos or vanadium pentoxide (V2O5).
v. Cupric chloride (CuCl2)
vi. Ferric oxide (Fe 2O3). Chromic oxide as a promoter.
vii. Nickel (finely divided).
viii. Ferric oxide (Fe2O3)