# Adsorption Theory of Heterogeneous Catalysis

Well you will be amazed to know that long before chemists recognized catalysts and the process of catalysis, common people were already using it for a number of purposes such as fermenting wine to produce vinegar or raising bread. Uses of catalysts are countless. Catalysts are playing a vital role in many processes or manufacturing techniques on which industries are based on. It was late 18th century when chemists started to recognize the catalysis process. The concept of catalysis was invented by chemist Elizabeth Fulhame and was reported in her book in 1794. Although the term catalysis was first used by Jons Jakob Berzelius in 1835. In the 1880s, Wilhelm Ostwald investigated reactions catalyzed by the presence of acids and bases, for this he was awarded the Nobel Prize in Chemistry in 1909.

### What is Catalysis?

The process of increasing the rate of chemical reaction by adding a substance which does not take part in the reaction is called catalysis and the substance which is added and increases the rate of reaction is called a catalyst. A very small amount of catalyst is required to alter the rate of reaction. For example, in the reaction of converting hydrogen peroxide into water and oxygen gas, potassium permanganate is used as a catalyst which increases the rate of reaction.

2H₂O₂ $\overset{\text{Potassium permanganate}}{\rightarrow}$ 2H₂O + O₂

### Types of Catalysis

On the basis of phases of catalysts and reactants, catalysis can be divided into following two types –

• Homogeneous Catalysis

• Heterogeneous Catalysis

### What is Homogeneous Catalysis and Catalyst?

The catalyst who is present in the same phase as of the reactants in the reaction is called homogeneous catalyst and this type of catalysis process is called homogeneous catalysis.

Examples of Homogeneous Catalysis and Catalysts –

1. Hydrolysis of Sugar – In hydrolysis of sugar reactants sugar (sucrose solution) and water are used in liquid states and the catalyst sulfuric acid is also used in the liquid state. Reaction is given below –

C₁₂H₂₂O₁₁₍ₗ₎ + H₂₍ₗ₎ $\overset{\text{H₂SO₄₍ₗ₎}}{\rightarrow}$ C₆H₁₂O₆₍ₗ₎ + C₆H₁₂O₆₍ₗ₎

Sucrose                           Glucose       Fructose

2. Hydrolysis of the Ester – In hydrolysis of the ester, ester is taken in liquid state with water (liquid) for the reaction in presence of catalyst hydrochloric acid which is also taken in liquid state. Reaction is given below –

CH₃COOCH₃₍ₗ₎ + H₂O₍ₗ₎ $\overset{\text{HCl₍ₗ₎}}{\rightarrow}$ CH₃COOH₍ₗ₎ CH₃OH₍ₗ₎.

### What is Heterogeneous Catalysis and Catalysts?

The catalyst whose phase differs from that of the reactants in the reaction is called heterogeneous catalyst and this type of catalysis process is called heterogeneous catalysis.

Examples of Heterogeneous Catalysis and Catalysts –

1. In Haber’s process of formation of ammonia, nitrogen and hydrogen are used in gaseous forms while catalyst iron is used in solid form.

$N_{2(g)}$ + 3$H_{2(g)}$ $\overset{\text{Fe₍ₛ₎}}{\rightarrow}$ 2NH₃

2. Formation of Sulfuric Acid – In this process sulfur dioxide (gas) is oxidized to sulfur trioxide (gas) by heterogeneous catalysis in presence of solid V2O5 catalyst. Then sulfur trioxide is hydrolyzed to sulfuric acid.

$SO_{2(g)}$ + $O_{2(g)}$ $\overset{\text{V₂O₅₍ₛ₎}}{\rightarrow}$ 2 $SO_{3(g)}$

### What is Adsorption Theory of Heterogeneous Catalysis?

Modern Adsorption theory of heterogeneous catalysis is the mixture of moderate compound hypothesis and the old adsorption hypothesis or old adsorption theory. Old adsorption theory lacked specificity so there was a need for modern adsorption theory.

According to adsorption theory of heterogeneous catalyst, there are free valencies in the catalyst on which reactant molecules get attached. The mechanism of adsorption theory of heterogeneous catalysis involves following steps –

• Step 1. Diffusion of reactant molecules

• Step 3. Intermediate complex formation

• Step 4. Desorption

• Step 5. Diffusion of product molecules

Step 1. Diffusion of Reactant Molecules – In this step reactant molecules get diffused towards the surface of the catalyst.

Step 2. Adsorption – In this step reactant molecules get adsorbed on the surface of the solid catalyst or form loose bonds with the free valencies of the catalyst.

Step 3. Intermediate Complex Formation – In this step adsorbed reactant molecules on the surface of the catalyst react with each other and form new stronger bonds with each other which leads to the formation of an intermediate.

Step 4. Desorption – In this step intermediate converts into product as it loses its affinity towards the catalyst. The product molecule gets desorbed from the surface of the catalyst.

Step 5. Diffusion of Product Molecules – In this step desorbed product molecules from the surface of the catalyst get diffused away from the catalyst.