Question

Define adsorption with an example. What is the role of adsorption in heterogeneous catalysis?

Answer 1

Hint : Start with the definition of adsorption, its main components and highlight the basic difference between adsorption and absorption. Select a suitable example which can showcase the phenomenon clearly. The role of adsorption can be explained with the five-step mechanism of heterogeneous catalysis.

Complete Step By Step Answer:
Adsorption is described as a surface phenomenon where particles get concentrated only on the surfaces of materials. Adsorption restricts the adsorbates only to the surface of the materials.
Adsorption has two components, that is, adsorbent, which provides the surface and adsorbate, which accumulates over the surface.
Adsorption is different from absorption as there is a thorough penetration of particles into the materials.
For example, when a chalk stick is dipped into an ink pot then, the chalk stick becomes colored only from outside. If we break the chalk stick in half, we will notice that it is only wet from the inside but there is no trace of any color. Hence, we can conclude that the colored component of ink pot undergoes adsorption while the solvent molecules of the ink pot undergo absorption.
In the above example, chalk stick acts as an adsorbent while the color components act as an adsorbate.
The reverse of adsorption is desorption in which removal of adsorbate from adsorbent takes place.
Adsorption is an endothermic process while desorption is an endothermic process.
Role of adsorption in heterogeneous catalysis:
Heterogeneous catalysis is the type of catalysis where the phase of the catalyst is different from that of the reactants or products. It typically involves solid phase catalysts and gas phase reactants.
Adsorption forms an essential part of heterogeneous catalysis. This can be easily seen in the five-step mechanism of heterogeneous catalysis:
Diffusion of the reactant gas molecules to the surface of the catalyst.
Adsorption of reactants on the catalysis surface due to unbalanced or residual attractive forces. Here the catalyst acts as the adsorbent and reactants as adsorbate.
A chemical reaction occurs on the catalyst’s surface to form an intermediate. This intermediate is called an ‘activated complex’. The rate of this reaction increases with the increase in concentration of the reactants.
Desorption of reaction products from the catalyst surface, thereby, making the surface available for more reaction to occur.
Diffusion of reaction products away from the surface of the catalyst.
The above mechanism is also called ‘adsorption theory of heterogeneous catalysis. The above mechanism also explains why catalysts remain unchanged in a reaction and are effective even in small quantities.

Note :
When giving an example, highlight what is adsorbent and what is adsorbate. Also pay close attention to the spelling as there is always a chance to mix these two terms. When explaining the mechanism of heterogeneous catalysis, keep in mind the five steps that are, diffusion, adsorption, chemical reaction, desorption, and again diffusion.