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How does the BET surface area analyzer measure pore size distribution?

05 11 月, 2025From: BSD Instrument

BET surface area analyzer is a cornerstone technique in materials science for characterizing the surface properties of porous materials. By combining adsorption isotherm measurements with theoretical models, it provides critical insights into both specific surface area and pore size distribution. This article elucidates the principles, procedures, and analytical methods underlying pore size distribution determination using BET analyzers.

1. Theoretical Foundations of BET Analysis

The BET method is rooted in multilayer adsorption theory, which describes how gas molecules adsorb onto a material’s surface in successive layers. At cryogenic temperatures (typically 77 K using liquid nitrogen), nitrogen gas is introduced into the analyzer, and its adsorption behavior is measured under controlled pressures. The BET equation relates the amount of adsorbed gas to the relative pressure (

P / P_{0}

), enabling calculation of the monolayer capacity (

V_{m}

) and specific surface area:

 $V ( P ^{0} - P ) P = V ^{m} C 1 + V ^{m} C C - 1 \cdot P ^{0} P$

Here,

C

is a constant related to adsorption heat, and the linear region of the BET plot (

1/ V \cdot P / (P_{0} - P)

vs.

P / P_{0}

) is used to determine

V_{m}

and

C

. The specific surface area is then calculated using the cross-sectional area of nitrogen molecules (

0.162 nm^{2}

) and Avogadro’s number .

2. Experimental Workflow

Sample Preparation

Degassing: Samples are pretreated under vacuum or inert gas at elevated temperatures (e.g., 200–350°C) to remove physisorbed contaminants like water or hydrocarbons. This step ensures accurate adsorption measurements .
Sample Loading: The degassed sample is loaded into a sample cell, which is then sealed and cooled to cryogenic temperatures.

Adsorption Isotherm Measurement

Nitrogen Adsorption: The analyzer gradually increases the relative pressure ( $P / P_{0}$ ) from 0.05 to 0.995, measuring the equilibrium adsorbed gas volume at each step. This generates an adsorption isotherm, which reflects the material’s pore structure .
Desorption Isotherm: After saturation, pressure is decreased to record desorption data, which helps analyze pore filling and emptying mechanisms.

3. Pore Size Distribution Analysis

The BET analyzer employs theoretical modelsto convert adsorption/desorption data into pore size distributions:

Barrett-Joyner-Halenda (BJH) Method

Principle: The BJH model uses Kelvin’s equation to relate adsorption isotherms to pore diameters. It assumes cylindrical pores and calculates pore volumes based on capillary condensation at specific $P / P_{0}$ values.
Application: Primarily used for mesopores (2–50 nm). BJH plots adsorbed volume vs. pore diameter, creating a distribution curve .

Non-Local Density Functional Theory (NLDFT)

Advancements: For micropores (<2 nm), BJH underestimates pore volumes due to interactions between adsorbed layers. NLDFT addresses this by incorporating quantum mechanical effects, providing accurate micropore characterization. It is now standardized in ISO 15901-3 .

Data Interpretation

Adsorption vs. Desorption: Adsorption isotherms are preferred for pore size analysis to avoid artifacts like the "4 nm spike" caused by desorption hysteresis in composite pore systems .
Software Tools: Modern analyzers use software (e.g., Gold APP’s V-Sorb) to automate data fitting, generate pore size histograms, and export reports .

4. Key Applications

Catalysts: Evaluating pore structures to optimize active site accessibility .
Energy Materials: Analyzing battery electrodes and fuel cells for pore-related performance metrics .
Environmental Science: Characterizing adsorbents (e.g., activated carbon) for pollutant removal .

5. Limitations and Considerations

Closed Pores: BET cannot assess pores inaccessible from the surface .
Sample Requirements: Requires sufficient sample quantity (0.5–6 cm³) and homogeneous particle size .
Pressure Range: Accuracy diminishes outside $P / P_{0} = 0.05-0.35$ for BET and 2–50 nm for BJH .

Conclusion

The BET surface area analyzer bridges theoretical models and experimental data to decode pore structures with nanometer precision. While BJH remains the workhorse for mesopores, NLDFT and DFT (Density Functional Theory) are revolutionizing micropore analysis. As materials science advances, integrating BET with complementary techniques (e.g., electron microscopy) will further enhance its utility in designing next-generation porous materials.

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