Factors Affecting Adsorption

The extent to which a gas is adsorbed onto a solid surface — how much adsorbate accumulates per unit mass of adsorbent — depends, qualitatively, on four factors: the nature of the adsorbent, the nature of the adsorbate, the pressure of the gas, and the concentration at a given temperature.

Surface area of the adsorbent. Since adsorption is fundamentally a surface phenomenon, the amount adsorbed depends directly on how much surface area the adsorbent presents: the higher the surface area (for instance, using a finely powdered or highly porous adsorbent rather than a solid lump of the same mass), the higher the amount of adsorbate that can be held.

Nature of the adsorbate. Gases that are easily liquefiable — such as $\text{SO}_2$, $\text{NH}_3$, HCl and $\text{CO}_2$ — have comparatively strong van der Waals forces of attraction, and this same strong intermolecular attraction that makes them easy to liquefy also makes them adsorb readily and strongly. By contrast, so-called permanent gases such as $\text{H}_2$, $\text{N}_2$ and $\text{O}_2$ have low critical temperatures, cannot be liquefied easily, and correspondingly adsorb only slowly and weakly; in general, gases with a higher critical temperature are adsorbed more readily than gases with a lower critical temperature.

Effect of temperature. As covered in detail in the adsorption-isobar section that follows, raising the temperature makes chemisorption first increase (supplying the activation energy the chemical bond needs to form) and then decrease (as desorption sets in once the adsorbate's kinetic energy grows large enough to break the bond), whereas physisorption decreases steadily with rising temperature throughout, since heat simply works against the weak physical forces holding the adsorbate. …