Catalysis

Most reactions that occur in nature or synthetically require a lot of time for completion. In order to speed up the reaction, a substance called the catalyst has to be added. Thus catalysis is the process of speeding up a reaction. Let us explore more about the phenomenon of catalysis, catalysts and the role of surface chemistry in catalysis.

What is Catalysis?

Catalysis is the increase in the rate of a chemical reaction due to the participation of an additional substance called a catalyst. which is not consumed in the catalyzed reaction and can continue to act repeatedly. Usually, even a small amount of catalyst is sufficient to bring about this effect. In catalyzed mechanisms, the catalyst usually reacts to form a temporary intermediate which then regenerates the original catalyst in a cyclic process.

Mechanism of Catalysis

In the presence of a catalyst, less free energy is required to reach the transition state, but the total free energy from reactants to products does not change. A catalyst may participate in multiple chemical transformations. The effect of a catalyst may vary due to the presence of other substances known as inhibitors  (which reduce the catalytic activity) or promoters (which increase the activity and also affect the temperature of the reaction).

Catalyzed reactions have a lower activation energy (rate-limiting free energy of activation) than the corresponding uncatalyzed reaction, resulting in a higher reaction rate at the same temperature and for the same reactant concentration. As in the case of any other chemical reaction, the reaction rate depends upon the frequency of contact of the reactants in the rate-determining step.  Usually, the catalyst participates in this slowest step. The rate of the reaction depends upon the amount of catalyst.Although catalysts are not consumed by the reaction itself, they may be inhibited, deactivated, or destroyed by secondary processes.

Types of Catalysts

Depending upon whether a catalyst exists in the same phase as the substrate catalysts can be classified as Heterogenous and Homogenous catalysts. Biocatalysts  (enzymes) are often seen as a separate group.

Heterogeneous Catalysts

Heterogeneous catalysts act in a different phase than the reactants. Most heterogeneous catalysts are solids that act on substrates in a liquid or gaseous reaction mixture. Diverse mechanisms for reactions on the surface are known, depending on how the adsorption takes place.

The total surface area of solid has an important effect on the reaction rate. The smaller the catalyst particle size, the larger the surface area for a given mass of particles. A heterogeneous catalyst has active sites, which are the atoms or crystal faces where the reaction actually occurs.  Most of the surface of the catalyst is catalytically inactive.

Homogeneous Catalysts

Homogeneous catalysts function in the same phase as the reactants, but the mechanistic principles involved in heterogeneous catalysis are generally applicable. Typically homogeneous catalysts are dissolved in a solvent with the substrates. One example of homogeneous catalysis involves the influence of H⁺ on the esterification of carboxylic acids, such as the formation of methyl acetate from acetic acid and methanol.

Enzymes and Biocatalysts

In biology, enzymes are protein-based compounds that catalyze metabolism as well as other biochemical reactions. Though enzymes are the most commonly known biocatalysts, certain non-protein-based classes of biomolecules also exhibit catalytic properties. These include ribozymes as well as synthetic deoxyribosymes.

Biocatalysts are considered to be an intermediate between homogeneous and heterogeneous catalysts, strictly speaking, soluble enzymes can be considered to be homogeneous catalysts whereas membrane-bound enzymes can be put under the category of heterogeneous enzymes.  Several factors affect the activity of enzymes (as well as other catalysts). Some of these include temperature, pH, the concentration of enzyme, substrate, and products. An important reagent that acts as a catalyst is water. It acts in various bond forming as well as bond breaking reactions.

Solved Questions For You

Que: Which one of these is a non-protein based biocatalyst?

a. Ribobozyme

b. Hydrolases

c. Glucose-6- Phosphate

d. Phosphatidylcholine

Ans: The correct option is option a. Ribozyme