Principle of selecting multiple BET specific surface area points

When calculating the specific surface area, the first step is to determine whether the sample is suitable for BET theory. The preset pressure point calculation range of the instrument (0.05-0.35) is only suitable for most mesoporous samples and not for microporous samples. At the same time as looking at the BET results, it is necessary to determine whether the selection range and C constant are reasonable.

Do not use pressure data points that are too low, as they are not sufficient to form a monolayer and cannot be used for calculating specific surface area.

Do not use excessively high relative pressure points, as incorrect point selection can cause differences in correlation coefficients and negative intercepts (with a negative C constant) in linear regression. The upper limit of sampling for certain samples increases with increasing pressure and cannot be calculated based on the maximum BET value at a single point.

Do not use excessively high relative pressure points, as incorrect point selection can cause differences in correlation coefficients and negative intercepts (with a negative C constant) in linear regression. The upper limit of sampling for certain samples increases with increasing pressure and cannot be calculated based on the maximum BET value at a single point.

Confirm the BET behavior characteristics of the sample at higher pressures. If the BET curve curves upwards within the "normal" linear range, it is necessary to discard the high adsorption point and calculate with a lower adsorption point. If the BET curve bends downwards and deviates from the extrapolated linear extension, it indicates that the sample adsorption requires more adsorption than the calculated BET value, and additional testing sample loading is needed.

Based on long-term practical experience, it is recommended to calculate the specific surface area according to the following range:

① Mesoporous material: 5 points with a conventional specific surface area P/P0 between 0.05-0.3.

② The specific surface P/P0 of microporous materials is between 0.005-0.1 at 8 points.

③ Microporous and mesoporous materials: 8 points with a specific surface area P/P0 between 0.01-0.2.