As the surface particles of the adsorbent are unstable, adsorption is an exothermic mechanism and as the adsorbate is adsorbed on the surface, the adsorbent energy decreases, resulting in heat evolution. The adsorption, thus, is still exothermic.
For the reaction to take place, the particles are affixed to the surface of another medium. The residual forces on the surface of the adsorbent decrease during this process which in turn reduces surface energy. In the form of heat, this energy occurs, so adsorption is exothermic in nature.