BET specific surface area measurements and analysis

The gas adsorption analysis method described by Brunauer, Emmett and Teller (BET) is a technique used to determine the specific surface area of materials. It is based on the adsorption of gas onto the surface of a solid material and the measurement of the quantity of gas adsorbed at different pressures.

The BET method is based on the assumption that the adsorbed gas molecules are found in monomolecular layers on the surface of the material. The amount of gas adsorbed at a given pressure is measured and used to calculate the specific surface area using the BET equation.

The BET method is generally used to characterise the specific surface area of porous materials:

• Characterization of catalysts: Catalysts are widely used in the chemical industry to accelerate chemical reactions. The BET method can be used to characterise the specific surface area of catalysts and to determine their capacity to adsorb reagents.
• Characterization of adsorbents : Adsorbents are materials used to remove pollutants and impurities from water, air and flue gases. The BET method can be used to characterise the specific surface area of adsorbents and to determine their capacity to adsorb pollutants.
• Characterization of zeolites: Zeolites are microporous materials used as catalysts, adsorbents and ion exchangers. The BET method can be used to characterise the specific surface area of zeolites and to determine their ion exchange capacity.
• Characterization of nanoporous materials: Nanoporous materials such as membranes and separation materials are used in a variety of applications, including water and gas purification. The BET method can be used to characterise the specific surface area of these materials and to determine their separation capacity.

In summary, the BET method is useful in any application where the specific surface area of a porous material needs to be accurately characterised and measured

The BET specific surface area corresponds to the total accessible surface area (in m²) for 1 gram of solid material. The greater the surface area, the more active zones the material has to interact (with a liquid, a gas, a molecule, etc.).

For manufacturers, this data is useful for :

• Check the quality of powders or catalysts,
• Optimise the effectiveness of an active ingredient or support,
• Understand a failure linked to poor surface interaction.

Specific surface area plays an essential role in a material's functional properties: adsorption, reactivity, dissolution or dispersion. Uncontrolled variations can have a direct impact on the performance of the finished product. BET analysis can be used to validate batch conformity, detect manufacturing drift or control critical raw materials.

Of these analytical methods, the gas adsorption determination described by Brunauer, Emmett and Teller, known as the BET method or technique, is by far the most important and widely used.

Accurate and highly reproducible, BET analysis involves determining the amount of gas needed to form a single-molecule layer around the solid sample. As the cross-sectional area of the molecules is a specific characteristic of each gas, it is then possible to determine the total surface area of the solid.

A failure may be linked to an abnormally low or high specific surface area, altering the interaction of the material with its environment (humidity, active agents, temperature). FILAB identifies these deviations using a precise BET analysis, in order to trace the root cause of the defect and propose corrective measures.

The BET method is applicable to a wide variety of divided or porous solids. The most commonly analysed products include:

• Metal powders and metal oxides (TiO₂, ZnO, Al₂O₃...)
• Technical ceramics
• Catalysts and catalyst supports
• Activated carbons and adsorbents
• Pigments and mineral fillers
• Active pharmaceutical ingredients and excipients