Activity and Selectivity of a Catalyst

Catalyst is those chemicals that change the rate of a chemical reaction but do not undergo any change itself in the process. The function of a catalyst has several implications. It has specific roles in several biological processes, like in growth, immunity, and general metabolism. Biological catalysts are called enzymes. The efficiency of a catalyst depends on the selectivity and activity of the catalyst.

In nature, the catalyst remains inactive until a reactant binds to it. The catalyst is highly specific for its reactants, and that is why some catalysts can change the reaction rate for some chemical reactions, while are rendered ineffective for other chemical reaction. Some catalyst can enhance the reaction rate for some reaction and can also inhibit other reactions. 

Activity and selectivity of catalyst determine the efficiency of the catalyst.

Activity of catalyst

As per the catalytic activity definition, the ability of a catalyst to increase the rate of a chemical reaction is called its activity. The catalytic activity meaning is described by the physical interaction of the reactants with the catalyst by chemisorption. The bond formed between the catalyst and reactant determines the reaction rate. The strength of such bonds has to be optimal for the catalyst to perform. Therefore, the reaction rate or the activation energy measures for the catalytic activity. 

However, the other two parameters can be used to measure catalytic activity.

Turnover Number

The turnover number is defined by the number of catalytic cycles that can be performed by the catalyst before it deteriorates. Commonly used industrial catalysts have a turnover number ranging from 10 to 105.

Turnover Frequency

The turnover frequency is the number of times the reaction takes place per catalyst per unit time. 

The activity of a biological catalyst is determined by the physical fit of the reactant on the catalyst molecule. The lock and key hypothesis best describes the model, where the reactant binds to the groove of the catalyst in the form of a key fitting into the lock. However, the current scenario is not the same. Enzymes, the biological catalyst, do not have a groove where the reactant molecule perfectly fit. Rather the reactant intermediately fits into the groove of the catalyst and induces conformational changes in the structure of the catalyst. These conformational changes restructure the groove for a perfect fit with the reactant molecule. The activity of the catalyst increases with the perfect fit with the reactant.

Selectivity of catalyst

Another important property that determines the efficiency of a catalyst is its selectivity. A catalyst is highly selective for the reactant on which it acts. Moreover, the type of catalyst acting on the reactant determines the product thus formed. Like the activity of a catalyst, selectivity is also determined by the fit of the catalyst with the reactant. The reaction cannot proceed if the reactant does not fit properly into the groove of the catalyst. Moreover, the groove of different may be similar and, therefore, can bind with the same reactant. Still, the conformational changes induced may be different, which is the reason why different products can be generated from the same reactant source by different catalysts.

For example, Carbon monoxide reacts with Hydrogen in the presence of Nickel to form methane and water. However, they form methanol in the presence of chromium oxide and zinc oxide. However, it forms formaldehyde in the presence of Copper.

The catalytic properties of transition elements, especially the activity, are high. Mainly the oxides of these transition elements have high catalytic activity. Vanadium pentaoxide, Platinum, Nickel are all good catalysts. The high catalytic activity of transition elements are mainly due to 

• Presence of vacant d-orbitals.

• Exhibits variable oxidation states.

• Can form reaction intermediates.