In terms of chemistry, catalyzed reactions are those reactions which occur in the presence of a catalyst. Now, the question arises. What is this catalyst? Well, catalyst is a chemical substance that either increases or decreases the rate of reaction as per the requirements. The major difference between other reagents and catalyst is that they are never consumed by the catalyzed reactions, and they can participate in further chemical transformations. For instance, palladium is a catalyst which plays a vital role in the reduction of ethyne to ethene, but it is never consumed by this reaction. Moreover, after achieving the ethene it is necessary to deactivate this catalyst otherwise the ethene will further be reduced to ethane.
On the basis of their ability to increase or decrease the rate of reactions; catalyst is known as positive and the negative catalysts. These catalysts provide the lower rate-limiting free energy change to catalyzed reactions, which results into a larger reaction than the un-catalyzed reaction at the same temperature. However, kinetically these catalyzed reactions are similar to typical chemical reactions in which the reaction rate completely depends upon the frequency of the contact of the substances participating in it.
Well, that was about the catalyst and its physical and chemical properties. Now, let’s consider the various types of the catalyst and considerably their effects on the catalyzed reactions. The first and the most common type of catalyst is heterogeneous catalyst. As the name depicts these catalysts act in an opposite phase as compared with the reactants participating in the catalyzed reactions. It means that if the reactants are in the solid form then the catalyst will be in the form of gas or solid. For example, in case of Haber process the finally divided iron acts as the catalyst in the synthesis process of ammonia from hydrogen and nitrogen.
The next type of catalysts is a homogenous catalyst which functions in the same phase as that of substances participating in catalyzed reactions. However, their mechanism and the functioning procedure are same as heterogeneous catalysts in the catalyzed reactions. The example of such a type of catalysts is the influence of H+ in the esterification of esters, which include achieving methyl acetate from methanol and acetic acid.
Moreover, there are few more types of catalysts, which include electro-catalysts. These electro-catalysts are also used to enhance the rate of catalyzed reactions, especially half reactions, which comprise the fuel cell in the fuel cell engineering. Apart from these all catalysts there are some organic molecules available that work as the catalysts. Usually, these catalysts require superior loading (i.e. higher amount of catalyst in per unit reactant) as compared with the metallic catalysts used in the catalyzed reactions.