BET specific surface area determination method

The BET theoretical calculation is based on the multi-molecular layer adsorption formula derived from the classic statistical theory by Brunauer, Emmett and Teller, namely the famous BET equation:

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P: Partial pressure of adsorbate P0: Saturated vapor pressure of adsorbent

V: actual sample adsorption capacity Vm: single layer saturation adsorption capacity

C: constant related to the adsorption capacity of the sample

It can be seen from the above formula that the BET equation establishes the quantitative relationship between the single-layer saturated adsorption amount Vm and the multi-layer adsorption amount V, which provides a good theoretical basis for the measurement of specific surface area.

The BET equation is based on the theory of multi-layer adsorption, which is closer to the actual adsorption process of many substances, so the test results are more reliable. In the actual testing process, usually measure 3-5 groups of tested samples under different gas partial pressures, the multi-layer adsorption capacity V, with P / P0 as the X axis, as the Y axis, linear fit from the BET equation to draw a line The slope and intercept of the measured value of Vm to calculate the specific surface area of ​​the sample. Theory and practice show that when the P / P0 point is in the range of 0.05-0.35, the BET equation is consistent with the actual adsorption process, and the graph linearity is also very good, so the point selection in the actual test process needs to be within this range. Since 3-5 groups of P / P0 are selected for measurement, we usually call it multi-point BET. When the adsorption capacity of the tested sample is very strong, that is, the C value is very large, the intercept of the straight line is close to zero, and it can be approximated that the straight line passes through the origin. At this time, only a set of P / P0 data can be measured and connected to the origin to find the specific surface area. , We call it single point BET. Compared with multi-point BET, the error of single-point BET results will be larger.

If the flow method is used for BET determination, the measurement system needs to be equipped with a device that can accurately adjust the gas partial pressure P / P0 to achieve the determination of the adsorption amount under different P / P0. For each point P / P0, the process of BET adsorption and desorption is similar to the direct comparison method. The difference is that the BET method needs to calibrate the volume of the actual gas adsorbed by the sample, but the direct comparison method does not.

Features: BET theory is closer to the actual adsorption process of substances, and the range of samples that can be measured is wide

The test results have high accuracy and credibility, and are especially suitable for scientific research and production units.