Why does physisorption decrease with the increase of temperature?

Physisorption, also known as physical adsorption or Van der Waals adsorption, is a type of adsorption that occurs when gas or liquid molecules adhere to the surface of a solid through weak Van der Waals forces. The strength of physisorption is directly related to the energy of interaction between the adsorbate molecules and the solid surface. When you increase the temperature, several factors come into play that can decrease the extent of physisorption:

Increased thermal energy: As you raise the temperature, the thermal energy of both the adsorbate molecules and the solid surface increases. This increased thermal energy can disrupt the weak Van der Waals forces that hold the adsorbate molecules to the surface, making it easier for them to escape and reducing the overall extent of physisorption.

Increased desorption rate: At higher temperatures, the rate of desorption (the process by which adsorbate molecules leave the surface) typically increases. This is because adsorbate molecules have higher kinetic energy and are more likely to overcome the energy barrier required to break the Van der Waals bonds with the surface. As a result, more adsorbate molecules will leave the surface, reducing the amount of physisorbed molecules.

Reduced adsorption energy: The strength of Van der Waals forces between molecules generally weakens with increasing temperature. This means that the energy of interaction between the adsorbate molecules and the solid surface is reduced at higher temperatures, making it less favorable for molecules to stay physisorbed.

Competitive effects: At higher temperatures, other processes like chemisorption (strong chemical bonding between the adsorbate and the surface) may become more prominent and compete with physisorption. This can further reduce the overall extent of physisorption.

In summary, increasing temperature decreases physisorption because it increases the thermal energy, desorption rate, and weakens the Van der Waals forces between the adsorbate molecules and the solid surface. As a result, fewer molecules will be physisorbed, and the extent of physisorption decreases.