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Catalyst:- Catalyst is a substance which can change (enhance) the speed of a chemical reaction without itself undergoing any change in its mass and chemical composition. And the phenomena is called Catalysis.
Poisons:- Cerain substances like arsenic Co, if present lowers the activity of the catalysts. These substance are to be called as catalytic poisons.
Catalytic promoter:- A substance if present along with catalyst enhances the activity of the catalyst, it is called as a Catalytic promoter.
Types of catalysis:-
Homogenous Catalysis:-if the catalyst is present in same phase as the reactants, it is called homogenous catalyst and the type of catalysis is called Homogenous Catalysis.
Example:- oxidation of sulphar dioxide to trioxide in presence of nitric oxide.
Heterogeneous Catalysis:-if the catalyst is present in a different phase than that of reactants, it is called a Heterogeneous catalyst and this type of catalysis is known as Heterogeneous Catalysis.
Ex. Manufacture of ammonia from N2 and H2 by Haber’s process using iron as catalyst
(ii) Manfacture of sulphuric acid by oxidation of SO2 to SO3 using pltinised asbestos or V2O5 as catalyst
Theory of Heterogeneous Catalysis:-
(Modern adsorption Theory):- According to this theory, there are free valencies, on the surface of solid catalysts (generally metals) and mechanism involves these steps:-
Some important Features of
Solid Catalysts (or heterogeneous Catalysis):-
Zeolites as shape selective Catalysts:- zeolites are aluminosilicates with the general formula Mx/n [(AlO2)x(SiO2)y]. ZH2O when n is charge on metal cation Mn+. Which is usually Na+, K+, Ca+. they are microporous three dimensional network silicates in which some silicon atoms are replaced by aluminium atoms. Giving Al-O-Si frame work.
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