A method that physically adsorbs a gas such as nitrogen onto a powder at low temperature, fits the BET equation to the adsorption isotherm, and obtains the specific surface area from the monolayer capacity.
Japanese name: BET法
BET method appears in 2 practice questions across 1 topic, including Nanomaterials.
Field: Nanomaterials
The BET method is based on a theory by Brunauer, Emmett and Teller that extends Langmuir's monolayer adsorption to handle multilayer adsorption. After the specimen is heated and degassed to remove surface moisture and the like, it is cooled to liquid-nitrogen temperature (about 77 K), and the amount adsorbed is measured while the relative pressure p/p₀ of nitrogen gas is varied. Fitting the resulting adsorption isotherm with the BET equation gives the monolayer capacity vm, the amount needed to cover the surface with exactly one layer. Converting vm to a number of molecules and multiplying by the area occupied by one nitrogen molecule, about 0.162 nm², gives the surface area.
The BET equation usually holds well over a relative pressure range of about 0.05–0.3. At lower pressures, surface heterogeneity, and at higher pressures, capillary condensation in pores, become significant, causing deviations from linearity. vm and the constant C are found from the slope and intercept of the straight line in the BET plot, and you also check, for example, that C is positive. For highly porous materials the value can change with the choice of fitting range, so it is important to record the measurement conditions when comparing. To measure small surface areas precisely, krypton is sometimes used instead of nitrogen.
Assuming the particles are dense spheres of equal size, the particle size (BET diameter) can be estimated from the specific surface area S as d = 6/(ρS). The BET method counts all surface that gas molecules can reach, so even when primary particles are gathered with point contacts, nearly all their surface is measured. The BET diameter is therefore close to the primary particle size seen by TEM and usually comes out smaller than sizes from dynamic light scattering or laser diffraction, which measure whole agglomerates in liquid. Conversely, a BET diameter much smaller than the TEM size indicates open pores or roughness on the particle surfaces.
Answer: A. Nitrogen adsorption is measured at about 77 K at various pressures, and the monolayer capacity from the BET equation is converted to a surface area
The BET method physically adsorbs a gas such as nitrogen onto the powder at liquid-nitrogen temperature (about 77 K) and measures the amount adsorbed while varying the relative pressure. The BET equation, which treats multilayer adsorption, is usually fitted to data in the relative pressure range of about 0.05–0.3 to obtain the monolayer capacity vm, and multiplying by the area occupied by one nitrogen molecule (about 0.162 nm²) gives the surface area. Its distinctive feature is that it gives the actual surface area, including the inner surfaces of open pores that gas molecules can enter. What XRD peak widths give is the crystallite size by the Scherrer equation, not the surface area of the particles themselves. Laser diffraction and scattering give the size distribution of particles (including agglomerates), but do not reflect surface roughness or pores.
No. The inner surfaces of closed pores that gas molecules cannot enter from outside do not take part in adsorption, so they are not included in the BET specific surface area.
The BET diameter is a size derived from the surface area of the particles, while the Scherrer equation gives the size of coherent crystal domains (crystallites). When a single particle consists of several crystallites, the BET diameter is larger than the crystallite size.
If moisture or gases adsorbed on the surface remain, the area available for nitrogen adsorption is reduced and the specific surface area is underestimated. The temperature is chosen within a range that does not alter the specimen.
The BET method physically adsorbs a gas such as nitrogen onto the powder at liquid-nitrogen temperature (about 77 K) and measures the amount adsorbed while varying the relative pressure.
Which correctly describes the principle of the BET method f…The BET method counts surface that gas molecules can reach, so even when primary particles are gathered with point contacts, nearly all their surface is measured.
For an alumina nanopowder, the primary particle diameter ob…Specific surface area XRD (X-ray diffraction) Scherrer equation Bottom-up synthesis Carbon nanotube Graphene Melting point depression of nanoparticles Quantum dot Zeta potential