The exothermic nature of adsorption reactions is due to the following reasons. 

(i) Adsorption leads to a decrease in the residual forces on the surface of the adsorbent. This causes a decrease in the surface energy of the adsorbent. Therefore, adsorption is always exothermic.

(ii) ΔH of adsorption is always negative. When a gas is adsorbed on a solid surface, its movement is restricted leading to a decrease in the entropy of the gas i.e., ΔS is negative. Now for a process to be spontaneous, ΔG should be negative.

ΔG = ΔH - TΔS

Since ΔS is negative, ΔH has to be negative to make ΔG negative. Hence, adsorption is always exothermic.