Characteristics and Differences of Physisorption and Chemisorption

To determine the correct statement(s) regarding the adsorption of a gas on a solid surface, let us analyze each option individually:

1. Analysis of Statement A:
   Adsorption is a surface phenomenon in which gas molecules accumulate at the surface of a solid, resulting in a reduction of residual surface forces and surface energy. This excess energy is liberated in the form of heat.

   From a thermodynamic perspective:
   $\Delta G = \Delta H - T\Delta S$
   When a gas is adsorbed on a solid surface, the movement of the gas molecules becomes restricted, which leads to a decrease in entropy ($\Delta S < 0$).
   For a process to be spontaneous, the change in Gibbs free energy must be negative ($\Delta G < 0$).
   Since $\Delta S < 0$, the term $-T\Delta S$ is positive ($> 0$). Therefore, for $\Delta G$ to be negative, the enthalpy change $\Delta H$ must be negative ($\Delta H < 0$) and its magnitude must exceed $|T\Delta S|$.
   Thus, adsorption is inherently an exothermic process.
   Statement A is correct.

2. Analysis of Statement B:
   At low temperatures, gas molecules are held onto the adsorbent surface by weak van der Waals forces (physisorption). As the temperature increases, the molecules gain kinetic energy to overcome the activation energy barrier necessary for chemical bond formation. Consequently, physisorption can transform into chemisorption at higher temperatures. For example, dihydrogen ($H_2$) is physically adsorbed on Nickel ($Ni$) surface at low temperatures, but as temperature increases, it dissociates into hydrogen atoms ($H$) which form chemical bonds with the surface.
   Statement B is correct.

3. Analysis of Statement C:
   Since physisorption is exothermic and has a very low activation energy, increasing temperature supplies energy to break the weak van der Waals interactions, thereby causing physisorption to decrease with increasing temperature.
   Chemisorption requires activation energy, so it initially increases with temperature up to a maximum before decreasing at very high temperatures. Thus, stating that physisorption increases with temperature is incorrect.
   Statement C is incorrect.

4. Analysis of Statement D:
   Chemisorption involves the formation of chemical bonds (covalent or ionic) between the adsorbent and adsorbate, yielding a high heat of adsorption ($\Delta H_{\text{adsorption}} \approx 80\text{--}240 \text{ kJ mol}^{-1}$). Physisorption involves weak van der Waals forces and has a much lower heat of adsorption ($\Delta H_{\text{adsorption}} \approx 20\text{--}40 \text{ kJ mol}^{-1}$).
   Because chemisorption involves the formation of chemical species, it requires a significant activation energy ($E_a$), making it a slower process compared to physisorption.
   Statement D is correct.

Conclusion:
The correct statements are A, B, and D.