Catalysis:-

A substance which changes the speed of a chemical reaction without being consumed is called catalyst and the phenomenon is called catalysis. Usually the substances which increased the rate of reaction are covered under the category of catalyst. The catalysts may be classified into two categories:

1. Homogeneous Catalysts: When the catalyst is in the same phase as the reactants and products, it is called homogenous catalysts .e.g.

2.  Heterogeneous Catalysis: when the catalyst is in the different phase then the reactants and products, it is called the heterogeneous Catalyst. e.g.

Some characteristics of heterogeneous (Solid) Catalysts:-

1. Activity:- Activity of catalyst means its capacity to increase the speed of a chemical reaction. e.g. Pt in the reaction between H₂(g) and O₂(g) increases the rate of reaction by 10¹⁰ times.
2. Selectivity:- Selectivity of a catalyst means its ability to direct the reaction to form particular products excluding the other. e.g. acetylene on reaction with H2 in presence of Pt as catalyst gives ethane, while in presence of Lindlar’s catalysis (palladium, BaSO4 & quinoline or Sulphur) gives ethylene.              

Shape or structure Selective Catalysis (By Zeolites):-

The catalytic action that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is known as shape- selective catalysis. Zeolites Exhibit shape selective catalisis because of their porous structure. Zeolites are micro porous aluminosilicates of General formula M_x/n[(AlO₂)_x(SiO₂)_y].mHO₂ where n is the valency of the action Mⁿ⁺. They are three dimensional network silicates in which some silicon atoms are replaced by aluminium atoms. Zeolites to e used as catalysts are heated in vacuum so as to push out molecules of hydration. The loss of molecules of hydration cause cavities of molecular dimensions in the network structure and makes if porous. The molecules of reactants which can be absorbed in these pores only will react. E.g. ZSM-5(Zeolite Sieve of Molecularity-5), has formula H_x[(AlO₂)_x(SiO₂)_90-x]. 16HO₂, it is used for the conversion of alcohol into Gasoline.

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