Activity and Selectivity

Activity of Catalyst:

The ability of the catalyst to increase the rate of reaction is known as the activity of the catalyst.

· The activity of a catalyst depends upon the strength of chemisorption to a large extent

· The reactants must get adsorbed reasonably strongly on to the catalyst to become active

· The hydrogenation reaction, the catalytic activity increases from Group 5 to Group 11 metals with maximum activity

Characteristics:

· It depends upon adsorption of reactants on the surface of catalyst. .

· Chemisorption is the main factor governing the activity of catalysts.

· 2H₂ (g) + O₂ (g) → 2H₂O (l)

Selectivity of Catalyst:

The selectivity of a catalyst is its ability to direct a reaction to yield a particular product selectively.

Characteristics:

· Selectivity of different catalysts for same reactants is different.

· With different catalyst different products are obtained.

CO (g) + 3H₂ (g) CH4 (G) + H₂o(g)

Shape Selective Catalysis by Zeolite:

The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape-selective catalysis

· Zeolites are good shape-selective catalysts because of their honeycomb-like structures.

· They are microporous alumina silicates.

· The reactions taking place in zeolites depend upon the size and shape of reactant and product molecules as well as upon the pores and cavities of the zeolites.

· Zeolites are being very widely used as catalysts in petrochemical industries for cracking of hydrocarbons and isomerisation.