BET theory

Concept

The concept of the theory is an extension of the Langmuir theory, which is a theory for monolayer molecular adsorption, to multilayer adsorption with the following hypotheses:

1. gas molecules physically adsorb on a solid in layers infinitely;
2. there is no interaction between each adsorption layer; and
3. the Langmuir theory can be applied to each layer.

The resulting BET equation is

where p and p 0 are the equilibrium and the saturation pressure of adsorbates at the temperature of adsorption, v is the adsorbed gas quantity (for example, in volume units), and v m is the monolayer adsorbed gas quantity. c is the BET constant,

where E 1 is the heat of adsorption for the first layer, and E L is that for the second and higher layers and is equal to the heat of liquefaction.

Equation (1) is an adsorption isotherm and can be plotted as a straight line with 1 / v [ ( p 0 / p ) − 1 ] on the y-axis and φ = p / p 0 on the x-axis according to experimental results. This plot is called a BET plot. The linear relationship of this equation is maintained only in the range of 0.05 < p / p 0 < 0.35 . The value of the slope A and the y-intercept I of the line are used to calculate the monolayer adsorbed gas quantity v m and the BET constant c . The following equations can be used:

The BET method is widely used in surface science for the calculation of surface areas of solids by physical adsorption of gas molecules. The total surface area S t o t a l and the specific surface area S B E T are given by

where v m is in units of volume which are also the units of the monolayer volume of the adsorbate gas, N is Avogadro's number, s the adsorption cross section of the adsorbing species, V the molar volume of the adsorbate gas, and a the mass of the solid sample or adsorbent.

Derivation

The BET theory can be derived similarly to the Langmuir theory, but by considering multilayered gas molecule adsorption, where it is not required for a layer to be completed before an upper layer formation starts. Furthermore, the authors made five assumptions:

1. Adsorptions occur only on well-defined sites of the sample surface (one per molecule)
2. The only molecular interaction considered is the following one: a molecule can act as a single adsorption site for a molecule of the upper layer.
3. The uppermost molecule layer is in equilibrium with the gas phase, i.e. similar molecule adsorption and desorption rates.
4. The desorption is a kinetically-limited process, i.e. a heat of adsorption must be provided:
5. these phenomenon are homogeneous, i.e. same heat of adsorption for a given molecule layer.
6. it is E₁ for the first layer, i.e. the heat of adsorption at the solid sample surface
7. the other layers are assumed similar and can be represented as condensed species, i.e. liquid state. Hence, the heat of adsorption is E_L is equal to the heat of liquefaction.
8. At the saturation pressure, the molecule layer number tends to infinity (i.e. equivalent to the sample being surrounded by a liquid phase)

Let us consider a given amount of solid sample in a controlled atmosphere. Let θ_i be the fractional coverage of the sample surface covered by a number i of successive molecule layers. Let us assume that the adsorption rate R_ads,i-1 for molecules on a layer (i-1) (i.e. formation of a layer i) is proportional to both its fractional surface θ_i-1 and to the pressure P, and that the desorption rate R_des,i on a layer i is also proportional to its fractional surface θ_i:

where k_i and k_−i are the kinetic constants (depending on the temperature) for the adsorption on the layer (i−1) and desorption on layer i, respectively. For the adsorptions, these constant are assumed similar whatever the surface. Assuming an Arrhenius law for desorption, the related constants can be expressed as

where E_i is the heat of adsorption, equal to E₁ at the sample surface and to E_L otherwise.

Cement paste

By application of the BET theory it is possible to determine the inner surface of hardened cement paste. If the quantity of adsorbed water vapor is measured at different levels of relative humidity a BET plot is obtained. From the slope A and y-intersection I on the plot it is possible to calculate v m and the BET constant c . In case of cement paste hardened in water (T = 97 °C), the slope of the line is A = 24.20 and the y-intersection I = 0.33 ; from this follows

From this the specific BET surface area S B E T can be calculated by use of the above-mentioned equation (one water molecule covers s = 0.114 n m 2 ). It follows thus S B E T = 156 m 2 / g which means that hardened cement paste has an inner surface of 156 square meters per g of cement. However, the article on Portland cement states that "Typical values are 320–380 m²·kg⁻¹ for general purpose cements, and 450–650 m²·kg⁻¹ for "rapid hardening" cements."

Activated Carbon

For example, activated carbon strongly adsorbs many gases and has an adsorption cross section s of 0.162 nm² for nitrogen adsorption at liquid nitrogen temperature (77 K). BET theory can be applied to estimate the specific surface area of activated carbon from experimental data, demonstrating a large specific surface area, even around 3000 m² g⁻¹. However, this surface area is largely overestimated due to enhanced adsorption in micropores, and more realistic methods should be used for its estimation, such as SPE method.

Catalysis

In the field of solid catalysis, the surface area of catalysts is an important factor in catalytic activity. Porous inorganic materials such as mesoporous silica and layered clay minerals have high surface areas of several hundred m² g⁻¹ calculated by the BET method, indicating the possibility of application for efficient catalytic materials.

Specific surface area calculation

The ISO 9277 standard for calculating the specific surface area of solids is based on the BET method.