The principle of BET method

Abstract

The meaning of specific surface area: a solid has a certain geometric shape, and its surface area can be obtained by ordinary instruments and calculations. However, it is difficult to measure the surface area of ​​powders or porous substances, which have not only irregular outer surfaces but also complex inner surfaces. The measurement of specific surface area is of great significance in both scientific research and industrial production. Generally, porous materials with large specific surface area and high activity have strong adsorption capacity.

The significance of specific surface area

Specific Surface Area: The total surface area of ​​a particle per unit volume or unit mass.

The meaning of specific surface area: A solid has a certain geometric shape, and its surface area can be obtained by ordinary instruments and calculations. However, it is difficult to measure the surface area of ​​powders or porous substances, which have not only irregular outer surfaces but also complex inner surfaces. The measurement of specific surface area is of great significance in both scientific research and industrial production. Generally, porous materials with large specific surface area and high activity have strong adsorption capacity.

What is BET?

BET: Abbreviation for the names of Brunauer, Emmett and Teller

The BET test theory is based on the multi-molecular layer adsorption model proposed by these three scientists, and deduces the relationship equation between the single-layer adsorption Vm and the multi-layer adsorption V, which is the famous BET equation; The BET specific surface area test can be used to measure the specific surface area, pore volume, pore size distribution and nitrogen adsorption and desorption curves of particles. It plays an important role in studying the properties of particles.

The principle of BET method

With nitrogen as the adsorbate and helium or hydrogen as the carrier gas, the two gases are mixed in a certain proportion to reach the specified relative pressure, and then flow through the solid material. When the sample tube is placed in liquid nitrogen for heat preservation, the sample will physically adsorb the nitrogen in the mixed gas, but the carrier gas will not be adsorbed. The adsorption peak appears on the screen.

When the liquid nitrogen is removed, the sample tube is at room temperature again, and the adsorbed nitrogen is desorbed, and a desorption peak appears on the screen. Finally, a known volume of pure nitrogen was injected into the mixture to obtain a corrected peak. According to the peak area of ​​the corrected peak and the desorption peak, the adsorption amount of the sample under the relative pressure can be calculated. By changing the mixing ratio of nitrogen and carrier gas, the adsorption amount of several nitrogen relative pressures can be measured, so that the specific surface can be calculated according to the BET formula.

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Reference

• Wiki: BET Theory