Abstract

ISO 9277:2010 specifies the determination of the overall specific external and internal surface area of disperse (e.g. nano-powders) or porous solids by measuring the amount of physically adsorbed gas according to the Brunauer, Emmett and Teller (BET) method. It takes account of the International Union for Pure and Applied Chemistry (IUPAC) recommendations of 1984 and 1994.

The BET method is applicable only to adsorption isotherms of type II (disperse, nonporous or macroporous solids) and type IV (mesoporous solids, pore diameter between 2 nm and 50 nm). Inaccessible pores are not detected. The BET method cannot reliably be applied to solids which absorb the measuring gas. A strategy for specific surface area determination of microporous materials (type I isotherms) is described in an annex.

Terms and definitions

For the purposes of this document, the following terms and definitions apply.

• adsorption: enrichment of the adsorptive gas at the external and accessible internal surfaces of a solid material

• physisorption: weak bonding of the adsorbate, reversible by small changes in pressure or temperature

• adsorbate: adsorbed gas

• adsorptive: gas or vapour to be adsorbed

• adsorbent: solid material on which adsorption occurs

• isotherm: relationship between the amount of gas adsorbed and the equilibrium pressure of the gas, at constant temperature

• volume adsorbed: volumetric equivalent of adsorbed amount expressed as gas at standard conditions of temperature and pressure (STP)

• adsorbed amount: quantity of gas adsorbed at a given pressure and temperature

• monolayer amount: number of moles of the adsorbate that form a monomolecular layer over the surface of the adsorbent

• surface area: extent of available surface area as determined by a given method under stated conditions

• specific surface area: absolute surface area of the sample divided by sample mass

• molecular cross-sectional area: molecular area of the adsorbate, i.e. the area occupied by an adsorbate molecule in the complete monolayer

• macropore: pore with width greater than approximately 50 nm

• mesopore: pore with width between approximately 2 nm and 50 nm

• micropore: pore with width of approximately 2 nm or less

• relative pressure: ratio of the equilibrium adsorption pressure, p, to the saturation vapour pressure, p0, at analysis temperature

• equilibrium adsorption pressure: pressure of the adsorptive gas in equilibrium with the adsorbate

• saturation vapour pressure: vapour pressure of the bulk liquefied adsorptive gas at the temperature of adsorption

Principle

The BET method is applicable only to adsorption isotherms of type II (disperse, nonporous or macroporous solids) and type IV (mesoporous solids, pore diameter between 2 nm and 50 nm) (see Figure 1). Inaccessible pores are not detected. The BET method cannot reliably be applied to solids which absorb the measuring gas. A strategy for specific surface area determination of microporous materials (type I isotherms) is described in Annex C.

The method specified involves the determination of the amount of adsorbate or adsorptive gas required to cover the external and the accessible internal pore surfaces of a solid (see Figure 2) with a complete monolayer of adsorbate. This monolayer amount can be calculated from the adsorption isotherm using the BET equation [see Equation (1)]. Any gas may be used, provided it is physically adsorbed by weak bonds at the surface of the solid (van der Waals forces), and can be desorbed by a decrease in pressure at the same temperature.

Nitrogen at its boiling point (about 77,3 K) is usually the most suitable adsorptive. Very often, argon at liquid argon temperature (i.e. 87,27 K) is a good alternative adsorptive for specific surface area determination (especially in the case of graphitized carbon and hydroxylated oxide surfaces, see Table A.1, footnote a) because it is a chemically inert monoatomic gas with a symmetrical electron shell configuration quite different from that of nitrogen, although the polarizabilities of argon and nitrogen are remarkably similar.

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