Selectivity of Catalyst and its Activity

Selectivity of catalyst and its activity is an important concept.Catalytic surface is generally inactive in nature. When a reactant is adsorbed on the catalyst surface, it makes the catalyst active. This incites the further process of reaction. Catalysts are highly specific in nature, that is, what may be a catalyst for one can be an inhibitor for another. Some catalysts with same set of reactants give different products in comparison to other catalysts.

Selectivity of Catalyst

Some basic properties of catalyst are explained below:

Activity of Catalyst

Catalyst has an ability to increase the rate of reaction. This ability of catalyst is known as the activity of catalyst. It depends upon adsorption of reactants on the surface of catalyst. Chemisorption is the main factor governing the activity of catalysts. The bond formed during adsorption between the catalytic surface and the reactants must not be too strong or too weak.

It must be strong enough to make the catalyst active whereas, not so strong that the reactant molecules get immobilized on the catalytic surface leaving no further space for the new reactants to get adsorbed. Generally for the hydrogenation reaction, from Group 5 to Group 11 metals, the catalytic activity increases. The catalytic activity is found to be highest for group 7-9 elements of the periodic table.

\( 2H_2(g)~+~O_2(g)~~~\underrightarrow{Pt}~~~2H_2O(l)\)

Selectivity of Catalyst

Catalysts are highly specific compounds. They have an ability to direct the reaction to yield a particular product. The reaction with same reactants but different catalyst may yield different products. This is termed as the selectivity of catalyst. Catalysts are highly selective in nature. They can accelerate a particular reaction while inhibit another reaction. Hence, we can say a particular catalyst can catalyse one particular reaction only. It may fail to catalyse another reaction of the same type. For example: reaction of hydrogen and carbon monoxide yields methane when nickel is used as catalyst, methanol when a mixture of zinc oxide and chromium oxide is used as catalyst and methanal when only copper is used as catalyst.

(i) \(CO(g)~+~3H_2(g)~~~\underrightarrow{Ni}~~~CH_4(g)~+~H_2O(g)\)
(ii) \(CO(g)~+~2H_2(g)~~~\underrightarrow{Cu/ZnO-Cr_2O_3}~~~CH_3OH(g)\)
(iii)\(CO(g)~+~H_2(g)~~~\underrightarrow{Cu}~~~HCHO(g)\)

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