Adsorption properties are the primary property of activated carbon. Activated carbon has crystallites like graphite grains but irregularly arranged. During the activation process, pores with different shapes and sizes are produced between the crystallites. It is assumed that the pores of the activated carbon are cylindrical pore shapes. The radius of the pores calculated according to a certain method can be divided into two categories:
(1) According to IUPAC:
Micropore <1.0nm
Middle hole 1-25nm
Large pores > 25 nm.
(2) According to custom:
Micropores <150nm
Middle hole 150-20 000nm
Large pores > 20 000 nm.
Due to these pores, especially micropores provide a large surface area.
The pore volume of the micropores is generally only 0.25-0.9 mL/g, the number of pores is about 1020/g, and the total pore surface area is about 500-1500 m2/g, which is usually measured by the BET method, and is also called up to 3500-5000 m2/g. of. Almost 95% of the surface area of ​​activated carbon is in the micropores, so in addition to some macromolecules, micropores are an important factor in determining the adsorption performance of activated carbon. The pore volume of the mesopores is generally about 0.02-1.0 mL/g, and the surface area is up to several hundred square meters, generally only about 5% of the total silkworm species of activated carbon. Its function can adsorb steam and provide a channel for the adsorbate to enter the micropores, and can directly adsorb larger molecules.
The pore volume of macropores is generally about 0.2-0.5 mL/g, and the surface area is only about 0.5-2 m2/g. The first effect is to make the adsorbate molecules rapidly penetrate into the smaller pores inside the activated carbon; the second is to act as a catalytic carrier. The catalyst is usually precipitated in the micropores in a small amount, and most of them are precipitated in the macropores and mesopores.
The surface area of ​​the activated carbon should include the internal surface area and the external surface area. In fact, the adsorption property mainly comes from the huge internal surface area, so it cannot be mistaken that the grinding of the activated carbon will significantly increase the surface area and increase the adsorption force.
Many adsorptions are reversible physical adsorption, that is, the adsorbed material is a fluid, which is adsorbed by activated carbon under a certain temperature and pressure, and is desorbed by the adsorbate at high temperature and low pressure, and the inner surface of the activated carbon is restored to its original state. This is a widely used physical adsorption, also known as van der Waals adsorption.
How to judge the adsorption performance of activated carbon can be based on the following criteria:
1. The richer the capillary pores on the surface and inside of the activated carbon, the stronger the adsorption capacity;
2. The same weight of activated carbon, the smaller the particle size, the larger the surface area, the larger the contact area of ​​uey6kp with the outside world, and the stronger the adsorption capacity;
3. The surface of the activated carbon and the internal pore size distribution are reasonable, and the pore size is slightly larger than the diameter of the harmful gas molecules in the decoration, so that it can have a strong adsorption capacity for the decoration harmful gases.
Chemical properties of activated carbon
In addition to physical adsorption, adsorption of activated carbon also involves chemical adsorption. The adsorption of activated carbon depends on both the pore structure and the chemical composition.
Activated carbon not only contains carbon, but also contains a small amount of chemically bonded, functional group-starting oxygen and hydrogen, such as carbonyl, carboxyl, phenols, lactones, terpenes, ethers. Some of the oxides and complexes contained on these surfaces are derived from derivatives of raw materials, some of which are formed by the action of air or water vapor upon activation and activation. Surface sulfides and chlorides are sometimes formed. In the activation, the minerals contained in the raw materials are concentrated into activated carbon to become ash, and the main components of the ash are salts of alkali metals and alkaline earth metals, such as carbonates and phosphates.
These ash contents can be lowered by washing with water or pickling.
The inorganic components in activated carbon are evident from the analysis of the four types of powdered charcoal products in Table 3-1. (scheduled list) catalytic
Catalytic activity of activated carbon
Activated carbon is accompanied by catalytic catalysis in many adsorption processes and exhibits catalyst activity. For example, activated carbon adsorption of sulfur dioxide is catalytically oxidized to sulfur trioxide.
Since activated carbon has a specific surface oxygenate or complex, it has catalyst activity for various reactions, such as chlorine gas and carbon monoxide to generate phosgene.
Since a complex is formed between the activated carbon and the support, the complex catalyst greatly increases the catalytic activity, for example, the activated carbon carrying the palladium salt, even if the catalyst without the copper salt is present, the oxidation reaction of the olefin can be catalyzed. And the speed is fast and the selectivity is high.
Since activated carbon has a developed pore structure, a large internal surface area, and good heat resistance, acid resistance, and alkali resistance, it can be used as a carrier for the catalyst. For example, in the reaction of hydrogenation, dehydrocyclization, isomerization or the like in organic chemistry, activated carbon is an excellent carrier for platinum and palladium catalysts.
Mechanical properties of activated carbon
Several projects have been downloaded to indicate the mechanical properties of activated carbon, which is valued by the application of activated carbon, especially for a large number of industrial users.
(1) Particle size: A set of standard sieve sieving method is used to determine the weight of activated carbon remaining in and through each sieve to indicate the particle size distribution.
(2) Static density or bulk density: the weight of activated carbon per unit volume of dietary pore volume and interparticle void volume.
(3) Bulk density and particle density: the weight of the activated carbon per unit volume of the dietary pore volume without the intergranular void volume.
(4) Strength: The fracture resistance of activated carbon.
(5) Abrasion resistance: that is, wear resistance or anti-friction performance.
These mechanical properties directly affect the application, for example: density affects the size of the container; the thickness of the powder affects the filtration; the particle size distribution of the carbon affects the fluid resistance and pressure drop; the fracture affects the service life and waste carbon regeneration.
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