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Chemistry

Adsorption Theory in Heterogeneous Catalysis Explained

Adsorption Theory in Heterogeneous Catalysis Explained

Q: 1. What is adsorption theory in heterogeneous catalysis?

The adsorption theory of heterogeneous catalysis states that a reaction occurs on the surface of a solid catalyst after the reactant molecules are first adsorbed onto it. In this theory:Reactant molecules attach to the catalyst surface by adsorption.The adsorbed molecules form an activated complex with lower activation energy.The product is formed and then desorbed from the surface.This theory explains how surface phenomena increase reaction rate in heterogeneous catalytic reactions such as hydrogenation and oxidation.

Q: 2. What is meant by heterogeneous catalyst?

A heterogeneous catalyst is a catalyst that exists in a different phase from the reactants, usually a solid with gaseous or liquid reactants. Key features include:The reaction occurs on the surface of the solid catalyst.The catalyst provides active sites for adsorption.It can be easily separated and reused.For example, in the Haber process, solid iron catalyzes the reaction N2(g) + 3H2(g) → 2NH3(g).

Q: 5. What is the difference between physisorption and chemisorption?

The main difference between physisorption and chemisorption is the type and strength of forces involved. Key differences include:Physisorption: Weak van der Waals forces, low enthalpy change, reversible, forms multilayers.Chemisorption: Strong chemical bonds, high enthalpy change, usually irreversible, forms monolayer.Chemisorption is generally responsible for catalytic activity in heterogeneous catalysis.

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Mechanism Steps Assumptions and Examples of Adsorption Theory of Heterogeneous Catalyst

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

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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 2. Adsorption
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.

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This ends our coverage on Adsorption Theory of Heterogeneous Catalysis. We hope you enjoyed learning and were able to grasp the concept. We hope after reading this article you will be able to answer questions based on this topic. If you are looking for solutions to NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.

FAQs on Adsorption Theory in Heterogeneous Catalysis Explained

1. What is adsorption theory in heterogeneous catalysis?

The adsorption theory of heterogeneous catalysis states that a reaction occurs on the surface of a solid catalyst after the reactant molecules are first adsorbed onto it. In this theory:

Reactant molecules attach to the catalyst surface by adsorption.
The adsorbed molecules form an activated complex with lower activation energy.
The product is formed and then desorbed from the surface.

This theory explains how surface phenomena increase reaction rate in heterogeneous catalytic reactions such as hydrogenation and oxidation.

2. What is meant by heterogeneous catalyst?

A heterogeneous catalyst is a catalyst that exists in a different phase from the reactants, usually a solid with gaseous or liquid reactants. Key features include:

The reaction occurs on the surface of the solid catalyst.
The catalyst provides active sites for adsorption.
It can be easily separated and reused.

For example, in the Haber process, solid iron catalyzes the reaction N₂(g) + 3H₂(g) → 2NH₃(g).

3. How does adsorption occur on a catalyst surface?

Adsorption on a catalyst surface occurs when reactant molecules bind to active sites through physical or chemical interactions. The process involves:

Diffusion of reactants to the catalyst surface.
Attachment to active sites by physisorption or chemisorption.
Formation of an activated complex.

In chemisorption, strong chemical bonds are formed, which is essential in most heterogeneous catalytic reactions.

4. What are the steps involved in adsorption theory of catalysis?

The adsorption theory of catalysis involves three main steps: adsorption, reaction, and desorption. These steps are:

Step 1: Adsorption – Reactants are adsorbed onto the catalyst surface.
Step 2: Surface reaction – Adsorbed molecules react to form products.
Step 3: Desorption – Products leave the surface, freeing active sites.

This cyclic process lowers activation energy and increases the rate of heterogeneous catalytic reactions.

5. What is the difference between physisorption and chemisorption?

The main difference between physisorption and chemisorption is the type and strength of forces involved. Key differences include:

Physisorption: Weak van der Waals forces, low enthalpy change, reversible, forms multilayers.
Chemisorption: Strong chemical bonds, high enthalpy change, usually irreversible, forms monolayer.

Chemisorption is generally responsible for catalytic activity in heterogeneous catalysis.

6. Why is adsorption important in heterogeneous catalysis?

Adsorption is important in heterogeneous catalysis because it concentrates reactant molecules on the catalyst surface and lowers the activation energy of the reaction. Specifically:

It increases effective collisions between reactants.
It weakens existing bonds in reactants.
It forms a lower-energy activated complex.

Without adsorption, the surface reaction in a heterogeneous catalyst would not occur efficiently.

7. What is an example of adsorption theory in industrial chemistry?

An example of adsorption theory in industrial chemistry is the hydrogenation of ethene using a nickel catalyst. The reaction is C₂H₄(g) + H₂(g) → C₂H₆(g). In this process:

Ethene and hydrogen are adsorbed onto the nickel surface.
The H–H bond and π-bond of ethene weaken.
Ethane forms and desorbs from the surface.

This demonstrates how adsorption facilitates bond breaking and formation.

8. What are active sites in heterogeneous catalysts?

Active sites are specific locations on a catalyst surface where adsorption and reaction occur. These sites:

Are often surface defects, edges, or corners.
Provide proper orientation and bonding for reactants.
Determine the catalytic activity and selectivity.

The number and nature of active sites directly influence the efficiency of heterogeneous catalysis.

9. How does surface area affect heterogeneous catalysis?

An increase in surface area increases the rate of heterogeneous catalysis by providing more active sites for adsorption. This means:

Finely divided or powdered catalysts are more effective.
More adsorption leads to higher reaction rates.
Porous materials enhance catalytic performance.

Greater surface area improves contact between reactants and the solid catalyst.

10. What are the limitations of adsorption theory in catalysis?

The adsorption theory has limitations because it does not fully explain all catalytic behaviors, especially complex reaction mechanisms. Key limitations include:

It may not describe multi-step surface mechanisms in detail.
It assumes uniform active sites, which is not always true.
It does not fully explain catalyst poisoning or deactivation.

Despite these limitations, the adsorption theory remains the fundamental model for understanding heterogeneous catalysis.