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The Brunner-Emmett-Teller theory, sometimes known as the BET theory, is used to calculate the surface area of solid or porous materials. Given that the size of a material's surface has an impact on how that solid will interact with its environment, it provides crucial information about their physical structure. 

The surface area of a substance is frequently related to many aspects, including dissolving rates, catalytic activity, moisture retention, and shelf life. 

Surface area analysis is one of the most frequently used techniques in material characterization and is essential to the design and production of solids. The process of BET surface analysis and its use in industry are explained in this article.

About Brunauer-Emmett-Teller (BET) 

The Brunauer-Emmett-Teller (BET) theory is a key analytical tool for the measurement of the specific surface area of materials. It seeks to explain the physical adsorption of gas molecules on a solid surface. Physical adsorption or physisorption is a term that is frequently used to describe observations. 

In the Journal of the American Chemical Society, Stephen Brunauer, Paul Hugh Emmett, and Edward Teller published their idea in 1938. The BET theory is applicable to multilayer adsorption systems that typically employ a probing gas (referred to as the adsorbate) that does not chemically react with the adsorptive (the substance that the gas adheres to; the gas phase is referred to as the adsorptive).

The most popular gaseous adsorbate for exploring surfaces is nitrogen (s). Due to this, routine BET analysis is often carried out at N2's boiling point (77 K). In order to quantify surface area at various temperatures and measurement scales, other probing adsorbates are also used, albeit less frequently. 

These consist of water, carbon dioxide, and argon. The amount of specific surface area that may be computed using the BET theory may vary on the adsorbate molecule used and its adsorption cross section because specific surface area is a scale-dependent feature and cannot be defined at a single true value.

With the following hypotheses, the theory's concept is an extension of the Langmuir theory, a theory for monolayer molecule adsorption, to multilayer adsorption.

Gas molecules physically adsorb on a solid in an endless number of layers; 
they only interact with layers next to them; and each layer is subject to the Langmuir hypothesis.
First layer's constant and higher than second layer's adsorption enthalpy (and higher).
For the second (and higher) layers, the enthalpy of adsorption is equal to the enthalpy of liquefaction.

Analysis of Brunauer-Emmett-Teller (BET) 

The specific surface area is calculated using the theory of gas molecule monolayer synthesis on a solid surface. The experimental adsorption isotherm is then converted into a BET plot using the BET model, which employs a linearized version of the BET equation, allowing the monolayer volume to be calculated. 

The total specific surface area expressed in m2/g is then calculated in this BET analysis using the gas molecule's cross section area and the monolayer volume.

In addition to BET analysis, gas adsorption can be used to determine the presence of pores in terms of both pore volume and pore size distribution. Since capillary condensation in pores is pore size dependent, the pore size distribution can be calculated using the capillary condensation in pores principle.

In order to cause condensation in bigger holes, a higher pressure is required. Similar to this, smaller holes are filled at lower adsorbing gas partial pressures.

The sample is pre-treated at a high temperature in a vacuum or a flowing gas before the measurement in order to remove any impurities. Pre-treatment settings must be tailored to the qualities of the materials since too low or too high a temperature might significantly alter the BET surface area obtained from the subsequent BET analysis.

The following adsorption analysis can be distinguished between two general techniques:

The flow technique employs a TCD detector to gather data on the volume of total pores and/or the amount of adsorbed gas resulting in a given BET surface area.
A comprehensive isotherm containing information on BET surface area, pore volume, and pore size distribution can be obtained using the volumetric approach, which is based on pressure measurements in an enclosed volume and gives additional information.
A Qsurf M3 analyzer can be used to make the flow measurements, and the report only includes a single number for the total pore volume or BET area. Quantachrome Autosorb-6B (N2 or CO2), Micromeritics TriStar 3000 (N2), Micromeritics Gemini, or Micromeritics ASAP 2020 (Ar or Kr) are just a few examples of the instruments that can be used to measure volumetric adsorption. 

The report can range from a single value for the BET surface area to a full report on isotherm, specific BET surface area, total pore volume, and pore size distribution, going beyond

Application of Brunauer-Emmett-Teller (BET) 

An inert gas, often nitrogen, is adsorbed on the surface of a solid substance in physical gas adsorption. This happens on the external surface as well as the inside surface caused by pores in porous materials. The process of measuring the BET surface area by gas adsorption, often known as BET analysis, is the most well-known. 

A so-called adsorption isotherm, or BET isotherm, is produced when nitrogen is absorbed at liquid nitrogen temperature, or 77 K, and it can be measured over porous and non-porous materials. Non-porous materials have a type II isotherm, which can be used to calculate the BET surface area.

Depending on the kind of pores present, porous materials will often provide a type I or type IV isotherm. To effectively determine the BET surface area, BET analysis over type I isotherm materials needs a particular technique. The use of argon or carbon dioxide adsorption rather than nitrogen adsorption to precisely probe the tiny micropores is frequently favoured in certain circumstances, such as for zeolites or activated carbons. 

A BET analysis using krypton gas adsorption can effectively characterise samples with low BET surface area. Even though just a small portion of the isotherm can be measured in these circumstances, the BET surface area is nevertheless accurately determined.

Carbon activated

Numerous gases have a great affinity for activated carbon, and it possesses an adsorption cross section.

for nitrogen adsorption at liquid-nitrogen temperature is 0.162 nm2 (77 K). Activated carbon has a significant specific surface area, even reaching 3000 m2/g, according to the BET theory's estimation of the specific surface area from experimental data. 

But because of the higher adsorption in micropores, this surface area is significantly exaggerated, hence more accurate methods, like the subtracting pore effect (SPE) approach, should be employed to calculate it.

Concrete and cement

In addition to the calcined limestone that makes concrete harden, other minerals used in its production, such as fly ash, silica fume, and other substances, also affect how quickly concrete cures. The nitrogen BET method is also employed, while Blaine air permeability is frequently chosen due to its ease of use and inexpensive cost.

The calcium silicate hydrate, also known as C-S-H, is what causes the hardening reaction in hydrated cement and has a large specific surface area due to its high porosity.

The strength and permeability of the material, as well as its porosity, are related to a number of significant characteristics of the substance, which in turn influence the properties of the final concrete. 

Comparing various cements can be done by measuring the specific surface area using the BET method. This can be done by measuring several adsorption isotherms, such as the adsorption of nitrogen at 77 K and the adsorption of water vapor at temperatures close to ambient (the boiling point of liquid nitrogen). 

Although the findings from a single approach can still be used to compare various cements, different ways of measuring cement paste surface areas frequently yield substantially varied numbers.

Catalysis

The surface area of catalysts is a significant determinant of catalytic activity in the field of solid catalysis. According to the BET technique, inorganic substances like mesoporous silica and layered clay minerals have high surface areas of several hundred m2/g, suggesting the potential for use as effective catalytic materials.