Compare the similarities and differences between physical adsorption and chemical adsorption

Activated carbon has a well-developed pore structure, and the adsorbed molecules enter the adsorption pores through the macropores (R 50nm) and mesopores (2nm R 50nm) through the two transportation channels Micropores (R 2nm), and the molecules and surface groups on the pore surface are combined together by van der Waals force to complete adsorption. Founded in 1996, Nanjing Mulinsen Carbon Industry Co., Ltd. is a professional activated carbon production enterprise integrating scientific research, development, production and sales. Over the past 20 years, the company has been focusing on the production of activated carbon, and has now developed into a subsidiary of Nanjing Mulinsen (deep processing base, mainly producing high-end products such as high-grade pure activated carbon and activated carbon catalysts), Hainan Mulinsen (Southeast Asia coconut shell raw material base, mainly producing coconut shell activated carbon), Gansu Mulinsen (Northwest coconut...).

There is an adsorption heat, the adsorption surface occurs, the larger the surface area, the more adsorption capacity.

Attached to the characteristics of both.

Physical adsorption has the following characteristics: The physical adsorption of gas is similar to the liquefaction of gas and the condensation of vapor, so the heat of physical adsorption is small, which is similar to the heat of liquefaction of the corresponding gas; The higher the boiling point or the lower the saturated vapor pressure of gases or vapors, the easier it is for them to liquefy or condense, and the greater the physical adsorption capacity; Physical adsorption generally does not require activation energy, so the adsorption and desorption rates are fast. Any gas can be physically adsorbed on any solid as long as the temperature is suitable, and there is no selectivity; Physical adsorption can be monolayer adsorption or multilayer adsorption; The structure of the adsorbed molecule does not change much, and no new chemical bonds are formed, so no new absorption peaks appear on the infrared and ultraviolet spectrograms, but there can be displacements. Physical adsorption is reversible; Most of the adsorption in solid self-solution is physical adsorption.

Compared with physical adsorption, chemisorption mainly has the following characteristics: The forces involved in adsorption are comparable to the chemical bonding forces and much stronger than the van der Waals forces. The heat of adsorption is approximately equal to the heat of reaction.

Adsorption is monolayer. Therefore, it can be described by Langmuir isotherm and sometimes by Freudlich's formula. Zemkin adsorption isothermal formula is only suitable for chemical adsorption:

v/vm=1/a·㏑cop。where v is the adsorption volume at the equilibrium pressure p; VM is the saturated adsorption volume of a monolayer; A and C0 are constants. Selective.

Irreversible to temperature and pressure. In addition, chemisorption often requires activation energy. To determine whether an adsorption is chemisorption, it is mainly based on the heat of adsorption and irreversibility.

Physical adsorption (intermolecular).

It is caused by the intermolecular force between the adsorbate and the adsorbent, also known as the van der Waals force. So the binding force is weaker.

Characteristics: The binding force is weak, the adsorption heat is small, and the adsorption and desorption speed are also fast. The adsorbed substance is also easier to desorb, so the physical adsorption is reversible.

For example, activated carbon adsorbs many gases, and the adsorbed gases are easily released without changing their properties.

Chemical adsorption (intramoleculeal).

The adsorption of adsorbate molecules and solid surface atoms (or molecules) undergoes electron transfer, exchange or commonality to form adsorption chemical bonds.

Characteristics: Only monolayer adsorption occurs; The heat of adsorption is comparable to the heat of chemical reaction; Selective. Most of them are irreversible adsorption; The adsorption layer can remain stable at higher temperatures, etc. Chemical adsorption can be divided into activated adsorption (activated) that requires activation energy

adsorption) and non-activated adsorption without activation energy

adsorption), the former has a slower adsorption rate, and the latter has a faster adsorption rate.

Physical adsorption is the force of attraction between the adsorbed fluid molecules and the molecules on the surface of the solid, the so-called Vanderwaals force. Therefore, physical adsorption, also known as van der Waals adsorption, is a reversible process. When the gravitational force between the molecules on the surface of the solid and the molecules of the gas or liquid is greater than the gravitational force between the molecules inside the gas or liquid, the molecules of the gas or liquid are adsorbed on the surface of the solid.

From the point of view of molecular motion, these molecules adsorbed on the surface of the solid will also detach from the surface of the solid and enter the gas (or liquid) due to the molecular movement, without any chemical changes themselves. As the temperature increases, the kinetic energy of the gas (or liquid) molecules increases, and the molecules are less likely to remain on the surface of the body, and more and more escape into the gas (or liquid), the so-called "desorption". This reversible phenomenon of adsorption-desorption exists in physical adsorption.

This phenomenon is used in industry to change the operating conditions to desorb the adsorbed substance, so as to achieve the purpose of regenerating the adsorbent and separating the adsorbed substance. The characteristic of physical adsorption is that the adsorbed substance does not undergo any chemical reaction, the adsorption process is carried out very quickly, and the equilibrium between the phases involved in the adsorption can be reached instantaneously.

Chemical adsorption is the result of the chemical bonding force between the surface of the solid and the adsorbate. This type of adsorption requires a certain amount of activation energy, so it is also called "activation adsorption". The magnitude of this chemical bond affinity can vary greatly, but it greatly exceeds the van der Waals force of physical adsorption.

The heat of adsorption emitted by chemical adsorption is much larger than that emitted by physical adsorption, reaching the order of magnitude of the heat of chemical reaction. The heat of adsorption emitted by physical adsorption is usually similar to the heat of liquefaction of gases. Chemical adsorption is often irreversible, and after desorption, the desorption substance often undergoes chemical changes and is no longer the original trait, so its process is irreversible.

The rate of chemisorption is mostly slower, and the adsorption equilibrium also takes a considerable time to reach, and increasing the temperature can greatly increase the adsorption rate. The desorption of this type of adsorption is also not easy to carry out, and it often requires a high temperature to expel the adsorbed molecules. It has also been found that the same substance, at low temperature, it undergoes physical adsorption on the adsorbent, and as the temperature rises to a certain extent, it begins to undergo chemical changes to chemical adsorption, and sometimes the two adsorptions occur at the same time.

Chemical adsorption plays an important role in the catalytic process.

You can distinguish what type of adsorption is by following the differences in the table above.

The difference between chemisorption and physical adsorption is as follows:

1. Physical adsorption is produced due to the van der Waals attraction between molecules, which can be single-layer adsorption or multi-layer adsorption. Chemical adsorption is a kind of adsorption caused by the force of chemical bonds, which is monolayer adsorption.

2. The process of physical adsorption is relatively fast, while the speed of chemical adsorption is relatively slow. Physical adsorption can reach equilibrium at the moment of adsorption, while chemical adsorption takes a relatively long time to reach equilibrium.

When a fluid comes into contact with a porous solid, one or more components of the fluid accumulate on the surface of the solid, a phenomenon called adsorption. Adsorption also refers to the phenomenon in which the surface of a substance (mainly a solid substance) absorbs molecules or ions from the surrounding medium (liquid or gas).

A phenomenon in which a layer of atoms or molecules is formed on the surface of a liquid or gas. The adsorbed atoms or molecules are often firmly attracted by chemical bonds, i.e., chemical adsorption. In chemisorption, the adsorbed layer is often a thin layer as thick as a molecule.

Adsorption can also occur through weaker physical forces, known as physical adsorption, usually forming several molecular layers.

Adsorption belongs to a mass transfer process, in which the molecules inside the substance and the surrounding molecules have a gravitational attraction to each other, but the molecules on the surface of the substance do not exert the force relative to the outside of the substance.

Therefore, the surface of liquid or solid substances can adsorb other liquids or gases, especially in the case of a large surface area, this adsorption force can have a great effect, so large areas of substances are often used in industry for adsorption, such as activated carbon, water film, etc.

Physical adsorption is generally multi-layer adsorption, and chemical adsorption is generally single-layer liquid absorption, which is correct.

Physical adsorption is adsorption by physical action, mainly through polarity and dispersion forces.

With the increase of the number of layers, the adsorption force is greatly reduced, but multi-layer adsorption can be carried out. Chemical adsorption is done through chemical bonds.

For example, nickel metal adsorbs hydrogen, and hydrogen dissociates on the surface of nickel metal to form nickel-hydrogen bonds. ，

Chemical adsorption can also be made into a single-layer desorption of birds. As long as the carrier is juxtaposed and made into a relatively porous structure, it is fine. So this statement is not true. It is also used in reality. For example, some catalysts made of porous materials and so on.