Mechanism of Homogeneous Catalytic Reactions
The catalyst combines with one of the reactants to form an intermediate. Intermediate compound being unstable either decomposes or combines with the other reactant form the product and the catalyst is regenerated.
For Example: the combination of SO2 and O2 to form SO3 is a slow process. However, in the presence of NO (catalyst) the reaction becomes fast.
2 SO2 (g) + O2 (g) ——> 2 SO3 (g)
In this reaction nitric oxide combines with one of the reactants to form intermediate compound (N02). This intermediate (N02) combines readily with S02 to form S03 and the catalyst NO is regenerated in the last step:
1) 2NO+ O2 ——> 2 NO2 (fast)
2) NO2 + SO2 ——> SO3 + No (fast)
2SO2 + O2 ——> 2 SO3 (fast)
When the catalyst is in different phase than the reactants, it is called heterogeneous catalyst. Such reactions are called heterogeneous catalytic reactions.
In heterogeneous catalysis, catalyst is generally a solid and the reactants are generally gases. This is also known as surface catalysis because the reaction starts at the surface of the solid catalyst. These catalysts have very large surface area of the order of 1 to 500 m2 per gram for contact.
Thus, despite an enormous surface area, once the reactant gas molecules cover the surface, the rate does not increase on increasing the reactant concentrations.
Some other examples are:
(1) Manufacture of NH3 from H2 and N2 by Haber’s process using finely divided iron catalyst.
3H2(g) + N2(g) ——> 2 NH3 (g)
Here reactants are in the gaseous state while the catalyst is in the solid state.
(2) Methanol is prepared from CO and H2 by using a mixture of copper, zine oxide as catalyst and Cr2O3 as promoter:
CO(g) + 2 H2 (g) ——> CH3OH (l)
(3) Manufacture of S03 from S02 in the Contact process using platinised asbestos or V2O5 as catalyst.
2 SO2 + O2 ——-> 2 SO3
(4) Dehydrogenation of ethanol by using nickel catalyst:
CH3CH2OH ——> CH3CHO + H2
(5) Oxidation of ammonia into nitric oxide in the presence of platinum gauze in Ostwald’s process:
4NH3 (g) + 5 O2 ——> 4 NO (g) + 6 H2 O (g)
(6) Hydrogenation of vegetable oils in the presence of finely divided nickel as catalyst is also an example of heterogeneous catalytic reaction because one of the reactants is in liquid state and the other in gaseous state, while the catalyst is in the solid state.
Vegetable oils (l) + H2 (g) ——-> Vegetable ghee (s)