Types of catalysts
The catalysts have been divided into different types according to their behaviour and pattern of action.
1. Positive catalyst
A catalyst which enhances the speed of the reaction is called positive catalyst and the phenomenon is known as positive catalysis. Various examples are given below :
i. Decomposition of H2O2 in presence of colloidal platinum
2H2O2 --- Pt --- > 2H2O + O2
ii. Decomposition of KClO3 in presence of manganese dioxide.
2KClO3 ---- MnO2 -- > 2KCl + 3O2
2. Negative Catalyst
There are certain substances which, when added to the reaction mixture, retard the reaction rate instead of increasing it. These are called negative catalysts or inhibitors and the phenomenon is known as negative catalysis. The examples are given below.
i. The oxidation of sodium sulphite by air is retarded by alcohol.
2 Na2SO3 + O2---- Alcohol -- > 2 Na2SO4
ii. The decomposition of hydrogen peroxide decreases in presence of glycerine.
2 H2O2 --- Glycerine --- > 2 H2O + O2
3. Auto catalyst
In certain reactions, it is observed that one of the products formed during the reaction acts as a catalyst for that reaction. Such type of catalyst is called auto catalyst and the phenomenon is known as auto catalysis.
In the oxidation of oxalic acid by potassium permanganate, one of the products MnSO4 acts as a auto-catalyst because it increases the speed of the reaction.
4. Induced Catalyst
When one reactant influences the rate of other reaction, which does not occur under ordinary conditions, the phenomenon is known as induced catalysis.
Sodium arsenite solution is not oxidised by air. If, however, air is passed through a mixture of the solution of sodium arsenite and sodium sulphite, both of them undergo simultaneous oxidation. Thus sulphite has induced the arsenite and hence is called induced catalyst.
The activity of a catalyst can be increased by addition of a small quantity of a second material. A substance which, though itself not a catalyst, promotes the activity of a catalyst is called a promoter. Some examples of the promoters are given below.
i. In the Haber's process for the synthesis of ammonia, traces of molybdenum increase the activity of finely divided iron which acts as a catalyst.
N2 + 3 H2 < - Fe--- -- +Mo- - > 2NM3
A substance which destroys the activity of the catalyst is called a poison and the process is called catalytic poisoning. Some of the examples are
(i) The platinum catalyst used in the oxidation of SO2 in contact process is poisoned by arsenious oxide.
SO2 + O2 < - Pt --- -- poisoned by As2O3- - > 2 SO3
(ii) The iron catalyst used in the synthesis of ammonia in Haber process is poisoned by H2S
N2 + 3 H2 < - Fe --- -- poisoned by H2S- - > 2 NH3
The catalytic surface has unbalanced chemical bonds on it. The reactant gaseous molecules are adsorbed on the surface by these free bonds. This accelerates the rate of the reaction. The distribution of free bonds on the catalytic surface is not uniform. These are crowded at the peaks, cracks and corners of the catalyst. The catalytic activity due to adsorption of reacting molecules is maximum at these spots. These are, therefore, referred to as the active centres. If a catalyst has more active centres, then its catalytic activity is increased.