Major Differences Between Physical & Chemical Adsorption

In the related applications of adsorption, the adsorption capacity of the adsorbent is an important index to evaluate the applicability, however, it is not absolute. In different applications, due to the different environmental conditions and various functional requirements, we may need some trade-offs in different characteristic indicators. For example, in the environmental protection-related applications, if the adsorbent can only be used once, no matter how good the adsorption capacity it is, we can expect a lot of waste from adsorbent will be generated. In such kind of application, this kind of adsorbent is not welcome, because it will cause more environmental troubles than adsorbate itself.
Previously we’ve introduced two types of adsorptions in "Introduction to Materials Adsorption Science" . In this article, we will learn the major differences of these two adsorption methods. This article focuses on the differences between the physical adsorption and chemical adsorption.

Properties of Physical Adsorption

Low enthalpy: The attractive force (Van der Waals force) between the adsorbent and the adsorbate molecules is weak, so the mole enthalpy of adsorption is low.

Reversibility: The physical adsorption process is usually reversible, so equilibrium can be reached quickly:
Adsorbate + gas <==> gas/adsorbate + heat
As we increase the pressure (or decrease the volume), the equilibrium shifts positively, thus adsorption increases, and more gas is adsorbed. Conversely, reduce the pressure in order to remove the gas.

Effect of temperature: Since the adsorption process is exothermic, physical adsorption occurs readily at low temperatures. According to Le-Chatelier's Principle (https://en.wikipedia.org/wiki/Le_Chatelier%27s_principle), the adsorption capacity of the adsorbent will decrease while temperature increasing. It is because in a high temperature environment, the kinetic energy of gas molecules increases, and more adsorbate leaves the surface of the adsorbent.

Lack of specificity: Physical adsorption is not selective in nature, that is, all gases can be adsorbed with the same level on all solid adsorbents. This is because of the van der Waals forces on the adsorbate surface do not have any preference for a specific gas.

Property of the adsorbate: Gases that are easily liquefied (have higher critical temperature) are easily adsorbed, because the van der Waals forces are stronger near the critical temperature.

State of Adsorbate: In physical adsorption, the state of the adsorbate on the adsorbent remains the same state as before adsorption.

Properties of Chemical Adsorption

High enthalpy: The attraction between the molecules of the adsorbent and the adsorbate is a strong chemical bond, so the molar adsorption enthalpy is very high.

Irreversible: Chemisorption involves the formation of compounds and the process is irreversible.

Effect of temperature: The chemical adsorption capacity is initially increased with the raising temperature, and after reaching a certain temperature, it will decrease regularly. Gases adsorbed at low temperatures by physical adsorption may transfer to chemical adsorption at higher temperature.

Highly specific: Chemical adsorption involves the formation of chemical bonds between adsorbate molecules and the adsorbent surface. Therefore, it is highly specific. For example, hydrogen is adsorbed on transition metals via the formation of hydrides.

State of Adsorbate: Since the adsorbate undergoes a chemical reaction in chemical adsorption, the state of the adsorbed substance may change after adsorption.

Comparison of Physical & Chemical Adsprotion

The following table quickly compares the difference between physical adsorption and chemical adsorption:

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