Difference between physical adsorption and chemical adsorption - Database & Sql Blog Articles

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According to the difference between the surface of the adsorbent and the adsorbate, the adsorption can be divided into physical adsorption and chemical adsorption.

Physical adsorption is the interaction between the adsorbed fluid molecules and the solid surface molecules is the intermolecular attraction, the so-called Vanderwaals

. Therefore, physical adsorption, also known as van der Waals adsorption, is a reversible process. When the gravitational force between the solid surface molecules and the gas or liquid molecules is greater than the gravitational force between the gas or liquid internal molecules, the molecules of the gas or liquid are adsorbed on the solid surface. From the point of view of molecular motion, these molecules adsorbed on the solid surface will also detach from the solid surface and enter the gas due to molecular motion.

Or liquid

In the middle, there is no chemical change in itself. As the temperature increases, the gas

Or liquid

As the kinetic energy of the molecule increases, the molecules are less likely to stay on the surface of the body, and more and more gases escape.

Or in the liquid, the so-called

"

Desorption

”

. This adsorption

-

The reversible phenomenon of desorption is present in physical adsorption. Industrially, this phenomenon is utilized, and by changing the operating conditions, the adsorbed substances are desorbed, the adsorbent is regenerated, and the adsorbed substances are recovered to achieve separation. The physical adsorption is characterized in that no chemical reaction occurs in the adsorbed material, and the adsorption process proceeds extremely rapidly, and the equilibrium between the phases participating in the adsorption can be instantaneously achieved.

Chemical adsorption is the result of a chemical bond between the solid surface and the adsorbate. This type of adsorption requires a certain amount of activation energy, so it is also called

"

Activated adsorption

”

. The size of this chemical bond affinity can vary widely, but it greatly exceeds the van der Waals force of physical adsorption. The adsorption heat released by chemical adsorption is much larger than the heat of adsorption released by physical adsorption, reaching the order of magnitude of chemical reaction heat. The adsorption heat released by physical adsorption is usually close to the heat of liquefaction of the gas. Chemical adsorption is often irreversible, and after desorption, the desorbed material often undergoes chemical changes that are no longer the original traits, so the process is irreversible. The rate of chemisorption is mostly slow, and the adsorption equilibrium takes a long time to reach. Increasing the temperature can greatly increase the adsorption rate. Desorption of such adsorption is also difficult to perform, often requiring high temperatures to drive out the adsorbed molecules. It has also been found that the same substance, when it is at a low temperature, is physically adsorbed on the adsorbent. As the temperature rises to a certain extent, chemical changes begin to change to chemisorption, and sometimes both adsorptions occur simultaneously. Chemical adsorption plays an important role in the catalysis process.

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