Gas Adsorption for Surface Area (BET) and Pore Size Analysis (BJH)

Introduction

Gas adsorption Analysis based on the BET and BJH methods is now widely used to characterize the adsorbent-specific surface area, pore size distribution, and pore volume. These methods, named after their respective developers, Brunauer, Emmett, and Teller for BET, and Barrett, Joyner, and Halenda for BJH, rely on the analysis of gas adsorption and desorption isotherms to provide valuable information about the porous structure of materials and activated carbon gas adsorption.

This technique involves exposing solid materials to gases or vapors under various conditions and measuring the weight uptake or volume of the sample. Consequently, analyzing these measurements provides insights into the solid’s physical attributes. Specifically, this analysis reveals information about skeletal density (ρS), porosity, total pore volume (TOPV), and pore size distribution.

Importance of BET and BJH Theory

The BET (Brunauer–Emmett–Teller) theory explains the physical adsorption of gas molecules on a solid surface. Unlike Langmuir’s theory (which deals with monolayer molecular adsorption), BET extends to multilayer adsorption. 

Key hypotheses include:

• Gas molecules are physically adsorbed in infinitely stacked layers.
• Gas molecules interact only with adjacent layers.
• The Langmuir theory applies to each layer.
• The BJH (Barrett, Joyner, and Halenda) method complements BET by calculating pore size distributions from experimental isotherms. It uses the Kelvin model of pore filling and inversely links relative pressures to pore sizes.

Read more: ASTM D6761 Determination of the Total Pore Volume of Catalysts and Catalyst Carriers 

Scope of BET and BJH Analysis

Physisorption

Physisorption pertains to the physical interest between gas molecules and a solid surface, commonly through weak van der Waals or London distribution forces. This interchange is reversible and does not affect the construction of chemical bonds between the gas molecules and the surface. Physisorption arises at low temperatures and tensions and is scrutinized with non-polar gasses and materials.

Chemisorption

Conversely, chemisorption entails the construction of strong chemical bonds between the gas molecules and the exterior of the solid material. This procedure is distinguished by the exchange or sharing of electrons, which directs the formation of new chemical bonds. Chemisorption is often irreversible and occurs at increased temperatures and pressures. It is generally scrutinized with reactive gases and materials.

Sample Size

The on-hand sample volume for gas adsorption methods usually varies from 0.1g to 1g. Specifically, BET (Brunauer-Emmett-Teller), which is meant for surface area testing, and BJH (Barrett-Joyner-Halenda), which is applied in pore size distribution analysis, both fall within this range. Consequently, this volume range ensures that the methods can effectively assess surface area and pore size distribution.

Pros and Cons of the BET – BJH Method

• BET Analysis

• BJH Analysis

Conclusion

Gas-based absorption methods, such as the BET method for scanning the surface area and the BJH tool for pore size distribution, are essential for material identification. These surface area and pore size analysis methods furnish important information about the surface characteristics and porosity of materials that are crucial for developments in fields that include the process of catalysis and adsorption studies, as well as the production of new materials. Knowing a material’s porosity and specific surface area also allows us to tailor it suitably for diverse applications. Hence, these tools are now the sine qua non in material science.

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