Gas adsorption techniques based on the BET and BJH methods are 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. Nitrogen is commonly employed as the adsorbate due to its inert nature, although Argon or Krypton may be used for more sensitive measurements. The experimental protocol outlined in ASTM D5160 is a guideline for conducting gas adsorption studies.
Principle and Methodology of Gas Adsorption Test, BET
The gas adsorption technique begins with removing any impurities from the active sites of the adsorbent through heating and vacuum treatment. Subsequently, the adsorbent is placed in an analytical tube under vacuum and chilled to liquid nitrogen (77 K). Nitrogen gas is then progressively charged onto the solid, then incrementally charged to the solid, starting at low pressures, and the volume of adsorbed gas is measured after each increment. This process continues until saturation pressure is reached and adsorption ceases. An isotherm is plotted, depicting the volume uptake versus relative pressure.
The low-pressure region of the isotherm corresponds to monolayer adsorption, while higher-pressure regions indicate multilayer adsorption. The BET method uses the monolayer segment of the isotherm to determine surface area using the BET equation, which connects adsorbed gas volume to monolayer volume and system pressure. The BJH approach, on the other hand, uses the Kelvin equation to calculate pore volumes and pore size distributions using experimental isotherms, making it especially useful for mesopore and small macropore size ranges.
To calculate the BJH volume of pores and the pore size distribution, Nitrogen pressure is incrementally increased, followed by measuring the adsorbed volume in the multilayer adsorption region. Capillary condensation occurs at higher relative pressures, filling the pores with liquid Nitrogen. The process is reversed by lowering the relative pressure and determining the desorption equilibrium. Hysteresis may occur between the adsorption and desorption curves. The pore size distribution is then computed using the desorption isotherm, providing valuable insights into the material’s porous structure.
