Heterogeneous Catalysis

The concept of heterogeneous catalysis involves some basic knowledge about the catalysts, phases, types of catalysis, etc. So, let’s understand these terms first to get a better idea about Heterogeneous catalysis.

• Phases- A phase of a substance is its physical form where it has uniform physical properties throughout. Phases include solid, liquid and gas.
• Catalysts- These are the substances that are intentionally added to a chemical reaction to increase the rate of that reaction. It does not combine with the reactants and only aids the process.
• Types of Catalysis- The process of adding a catalyst is catalysis and can be divided into two categories. Heterogeneous and homogeneous catalysis.
• Heterogeneous catalysis- It is the process where the phase of the catalyst added is different from that of the reactants or the products. In most cases, the catalyst is in the solid phase, while the reactants are in the gas or liquid phase in heterogeneous catalysis.

Process Cycle of a Heterogeneous Catalysis-

Under this process, the phase of the catalyst is typically solid while the reactants and the products are in the gaseous state. The entire process of Heterogeneous Catalysis can be further divided into a 3-step cycle. This involves Adsorption, Reaction and then Desorption. All three steps occur at the catalyst’s surface.

The first step is the Adsorption of the reactants (also called the adsorbates). The reactants that are in the gas phase would find the catalyst (called the adsorbent) surface and make bonds with the atoms on its surface. The adsorbates attach to the solid catalyst which is why a larger surface area is preferable.

In the next step, the reaction occurs. On the surface of the catalyst, the reactants combine and the desired products are formed. The catalyst does not react and is not consumed under this process.

Then finally, Desorption occurs, which is exactly the opposite of Adsorption. Now the final products detach from the catalyst’s surface and the process stops here.

Types of Adsorption- Physisorption and Chemisorption

There are two possible ways in which the adsorption could occur in heterogeneous catalysis. This is based on the strength of their bond formed with the catalyst. Let’s consider them.

1. Heterogeneous catalysis with Physisorption-

This one is weak bond adsorption of the reactants on the surface of the catalyst. This adsorption occurs as a result of the weak Van Der Waals forces of attraction. The forces that may contribute to physisorption involve dipole-dipole forces, interactive forces between a dipole and an induced dipole, or even the London dispersion forces.

Under this type of adsorption, there are no chemical bonds formed. These reactants on the surface of the catalyst are free to move across the surface as well. The atoms are in an intermediate energy state when physisorption occurs. From here, there are two possibilities. It could either undergo chemisorption and form a stronger bond or desorption and split from the catalyst surface.

2. Heterogeneous catalysis with Chemisorption-

Now, if the molecule of the reactant reaches even closer to the catalyst surface, a much stronger bond is formed. This is called Chemisorption and here, even their electron clouds overlap. Both the adsorbate and the adsorbent share electrons at this stage and successfully form a strong chemical bond, unlike in physisorption.

Again, this process of chemisorption can happen in two ways. Either by molecular adsorption in which the chemical structure of the reactant molecule is unaltered or dissociation adsorption where the chemical bonds within the reactant are affected. Both of these ways are used depending upon the reactants and the products.

Examples of Heterogeneous Catalysis-

Heterogeneous catalysis is used widely across the chemical and energy industry to manufacture various chemicals. Catalysts with phases different from the reactants are used. Some common examples for the same are listed below:

1. The Haber-Bosch Process- It is the most famous process used for the production of ammonia. Hydrogen and Nitrogen react together under the presence of the catalyst oxides of iron on alumina.
2. The process for the synthesis of Sulphuric acid- Oxygen and Sulphur dioxide react together catalysed by the oxides of vanadium. This is the Contact-Process.
3. The Ostwald Process- Next process in line is for the synthesis of Nitric Acid. The reactants for the same are ammonia and oxygen and the unsupported platinum-rhodium gauze acts as the catalyst in the solid phase.
4. Steam Reforming- This is the process for the production of Hydrogen. Steam reforming of methane involves methane and water as reactants and is catalysed by nickel or potassium oxide.
5. The synthesis of Ethylene Oxide- This process requires ethylene and oxygen catalysed by silver on alumina for its production.

Exceptions under Heterogeneous catalysis-

Even though almost all heterogeneous catalysis has catalysts in the solid phase, there are a few practical cases where this is not true. It is possible to have a solid-liquid and a liquid-liquid chemical reaction where the catalyst is in a liquid state. In the case of two immiscible liquids, one liquid may be the catalyst while the other carries the reactants.

Conclusion-

Heterogeneous catalysis is a very important concept and holds a lot of value due to its wide practical applications. This makes it very important to understand it well. The article covers almost everything you need to know about heterogeneous catalysis. But still, if you need help, you can always consider Unacademy. The detailed notes will help you clear the concepts that will stay with you for a long time. Any candidate preparing for JEE- Mains or Advance should explore the in-depth knowledge and questions of almost every type provided by Unacademy.