BET (Brunauer–Emmett–Teller)
Brunauer–Emmett–Teller (BET) theory is used to measure the surface area of solid or porous materials. It gives important information on their physical structure as the area of a material’s surface affects how that solid will interact with its environment. Many properties such as dissolution rates, catalytic activity, moisture retention, and shelf life are often correlated to a material’s surface area. Critical to the design and manufacture of solids, surface area analysis is one of the most widely used methods in material characterization.
To determine the surface area, the solid sample is cooled, under vacuum, to cryogenic temperature (using liquid nitrogen). Nitrogen gas (as a typical adsorbate) is dosed to the solid sample (adsorbent) in controlled increments. After each dose of nitrogen gas, the relative pressure (P/P0) is allowed to equilibrate, and the weight (W) of nitrogen adsorbed is determined. The BET equation strictly describes a linear plot of 1/((P0/P)-1) vs. P/P0 which for most solids, using nitrogen as the adsorbate, is restricted to a limited region of the adsorption isotherm, usually in the P/P0 range of 0.05 to 0.35. From this plot the weight of nitrogen (Wm) constituting a monolayer of surface coverage is determined. The total surface area of the sample can be calculated from the slope and intercept of the BET plot using the BET equation and the known molecular cross-sectional area of the nitrogen molecule.
1/(W((P0/P-1)) = 1/(WmC) + ((C-1)/(WmC))(P/P0)
The term C, the BET C constant, is related to the energy of adsorption in the first adsorbed layer and consequently its value is an indication of the magnitude of the adsorbent/adsorbate interactions.
To determine the pore volume and pore size distribution, the gas pressure is increased further incrementally until all pores are filled with nitrogen molecules. Next, the gas pressure is reduced incrementally, evaporating the condensed nitrogen gas from the system. Evaluation of the adsorption and desorption isotherms reveals information about the pore volume and pores size distribution.
APPLICATIONS
- Surface area analysis of solid materials (such as carbon black, catalysts, batteries and ceramics)
- Simultaneous acquisition of surface area and pore size data
- Non-destructive method
LIMITATIONS
- Closed pores are not accessible via the material surface. Therefore, gas adsorption cannot be used for their assessment.
Quantachrome AUTOSORB-1
The AUTOSORB-1 operates by measuring the quantity of gas adsorbed onto or desorbed from a solid surface at some equilibrium vapor pressure by the static volumetric method. The data are obtained by admitting or removing a known quantity of adsorbate gas into or out of a sample cell containing the solid adsorbent maintained at a constant temperature below the critical temperature of the adsorbate. As adsorption or desorption occurs the pressure in the sample cell changes until equilibrium is established. The quantity of gas adsorbed or desorbed at the equilibrium pressure is the difference between the amount of gas admitted or removed and the amount required to fill the space around the adsorbent (void space).
The AUTOSORB-1 has the capability of measuring adsorbed or desorbed volumes of nitrogen at relative pressures in the range 0.001 to slightly under 1.0. This volume-pressure data can be reduced by the AUTOSORB-1 software into BET surface area (single and/or multipoint), Langmuir surface area, adsorption and/or desorption isotherms, pore size and surface area distributions, pore volume and surface area using an extensive set of built-in data reduction procedures.
The Quantachrome AUTOSORB Software interfaces the AUTOSORB to a computer for data acquisition, reduction and archiving.
TECHNICAL SPECIFICATIONS
- Total Surface Area: at least 0.5 m2
- Pore Volume: 4 x 10-6 cm3/g and higher
- Pore size: 2 – 50 nm
- Relative Pressure Range: 0.001 – 0.95
- Sample cells: 6 mm, 9 mm and 12 mm stem cells
Rod, K. A., Smith, A. P., Leng, W., Colby, S., Kukkadapu, R. K., Bowden, M., Qafoku, O., Um, W., Hochella, M. F., Jr., Bailey, V. L. and Renslow, R. S. (2020). Water-dispersible nanocolloids and higher temperatures promote the release of carbon from riparian soil. Vadose Zone Journal, 19(1), e20077. https://doi.org/10.1002/vzj2.20077
Video: Gas sorption show made by Quantachrome