Berzelius suggested the term catalyst, substance which alter (change) the rate of a chemical reaction and themselves remain chemically and quantitatively unchanged after the reaction are known as catalyst and the phenomenon is known as catalysis.
A Substance that enhances the activity of catalyst called as promoter.
It decreases the activity of catalyst.
Homogeneous Catalysis: –
When the reactants and the catalyst are in the same phase (i.e. liquid or gas) is termed as homogeneous catalyst..
Heterogeneous catalysis: –
The catalytic process in which the reactant and the catalyst are in different phase is known as heterogeneous catalyst.
Shape Selectivity catalysis by Zeolite: –
The catalytic reaction depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called as Shape Selectivity catalysis.
“Zeolite” is good shape selective catalysts because of their honey comb like structure. They are micro porous alumino silicates with three dimensional networks of silicates in which some silicone atoms is replaced by aluminum atoms giving AL-O-Si framework. An important Zeolite catalyst used in petroleum industry is ZSM-S and it converts directly Alcohols into Gasoline (Petrol) by dehydrating them to give a mixture of hydrocarbons.
Enzyme Catalyst: –
Enzymes are complex nitrogenous organic compound which are produced by living plants and animals work as a catalyst in many life process termed as Biochemical Catalysts (enzyme) and the phenomenon is known as Biochemical Catalysis.
Characteristics of Enzyme Catalysis:-
- Most highly efficient: – One molecule of enzyme may transform one million molecule of reactant per minute.
- Highly specific nature: – Each enzyme is specific for a given reaction.
- Highly active under optimum temperature: – The rate of an enzyme reaction is maximum at definite temperature called optimum temperature (298K-310K).
- Highly active under optimum PH :- Rate of enzyme reaction is maximum at optimum
- PH (5-7)
- Increasing activity in presence of activators and co-enzymes:- The enzymatic activity in presence of certain substance called co-enzymes (vitamins) and activators are generally Na+, CO2+, Mn2+, Cu2+, etc.
- Inhibitors and poisons decrease or stop the rate of enzyme reaction.
Mechanism of Enzyme Catalysis: –
There are number of cavities present on the surface of colloidal particles of enzymes. The molecules of the reactant (substrate) which have complementary shape and can fit into the theses cavities just like a key fits into a lock on the account of the presence of actives groups . Activated complex is formed which then decompose as to yield the products .