Question

Why does physisorption decrease with the increase of temperature?

Answer 1

Le Chatelier’s principle is used to predict the behaviour of a system due to changes in the pressure, temperature, or the concentration. It states that changes in the temperature, pressure, volume, or the concentration of a system will result in predictable and opposing changes in the system in order to achieve a new equilibrium state. Since the physical adsorption process is exothermic, it occurs readily at low temperature and decreases with increasing temperature.

Complete answer:
For an endothermic reaction, the heat is absorbed in the reaction and thus $\Delta H$ is positive. For an exothermic reaction, the situation is just the opposite. Heat is released in the reaction and thus $\Delta H$ is negative.
If we see the heat as a reactant or a product, we can apply Le Chatelier’s principle. For example, if we raise the temperature on an endothermic reaction, it is essentially like adding more reactant to the system, and therefore, by Le Chatelier’s principle, the equilibrium will shift to the right. Conversely, lowering the temperature on an endothermic reaction will shift the equilibrium to the left, since lowering the temperature in this case is equal to removing a reactant. For an exothermic reaction, heat is a product and therefore, increasing the temperature will shift the equilibrium to the left, whereas decreasing the temperature will shift the equilibrium to the right.
Hence, physisorption is exothermic in nature and therefore according to the Le-Chatelier's principle, it decreases with an increase in temperature which means that physisorption occurs more readily at a lower temperature.

Note:
Physisorption involves weak forces of attraction between adsorbent and adsorbate molecules which is generally of Vander Wall’s type and these forces decrease rapidly with the increase in the temperature.