The process of adsorption is always exothermic. There are two ways to understand this which are as follows :-
(I) The residual forces on the adsorbent’s surface are reduced as a result of adsorption. As a result, the adsorbent’s surface energy is lowered as well. Adsorption is usually exothermic as a result.
(II) ΔH of adsorption is always negative. When a gas is adsorbed on a solid surface, its movement is constrained. As a result, the entropy of the gas decreases, and ΔS becomes negative. G should now be negative in order for a process to be spontaneous.
Therefore,
$$
\Delta G=\Delta H-T \Delta S
$$
Because ΔS is negative, ΔH must also be negative in order for G to be negative. As a result, adsorption is always an exothermic process