Physical gas adsorption, also referred to as physisorption, is a standard technique for characterising the surface and pore structure of solid materials. An inert gas, in most cases nitrogen, is adsorbed on the surface of a powder or other solid. This occurs on the outer surface of the particles and, for porous materials, also on the internal surface provided by the pores. The amount of gas adsorbed at different pressures is a direct reflection of the surface area and pore volume of the material. When the adsorption analysis is performed at many different pressure points, also the pore size distribution can be obtained.
Because the technique can be applied to porous and non-porous materials alike, physical gas adsorption is used in many fields, such as catalysis, sorbents, pharmaceuticals, pigments, ceramics, battery materials and nanomaterials. The most widely known application is the determination of the specific surface area by BET analysis, but the measurement provides much more information than a single surface area value.
How physical gas adsorption works
In physical gas adsorption, gas molecules are held on the surface of the solid by relatively weak intermolecular forces. This is what distinguishes physisorption from chemisorption, where the gas reacts chemically with specific active sites. Because physisorption is a reversible process that does not alter the material, it is well suited to probe the total accessible surface and the porous structure of a sample. Chemisorption is discussed separately under Catalyst characterisation by chemical gas adsorption.
During a measurement, the sample is cooled to the boiling temperature of the adsorbing gas, which is 77 K for nitrogen, and the gas is dosed at progressively increasing relative pressures. The quantity of gas that is adsorbed at each pressure is recorded and plotted as an adsorption isotherm. Often the pressure is subsequently lowered again to record the desorption branch. The shape of this isotherm, and any difference between the adsorption and desorption branch, provides a more detailled fingerpint of the textural properties of the material.
Adsorption isotherms
The isotherm is the basis of every gas adsorption analysis. According to the IUPAC classification, different types of isotherms can be distinguished that are related to the pore structure of the material. Non-porous and macroporous materials typically show a type II isotherm. Materials with micropores generally display a type I isotherm, while mesoporous materials show a type IV isotherm, often with a hysteresis loop between adsorption and desorption caused by capillary condensation in the pores.
Recognising the isotherm type is the first step in a correct interpretation. It determines which evaluation methods are applicable and whether additional care is required, for example for materials with very small micropores or multiple pore sizes and complex interconnectivity of the pore network.
Choosing the adsorptive gas
Nitrogen at 77 K is the most commonly used adsorptive and is suitable for a wide range of materials. However, the best choice of gas depends on the material and on the information that is needed.
For materials with small micropores, such as zeolites and activated carbons, argon or carbon dioxide adsorption is often preferred. These gases can probe the smallest pores more accurately than nitrogen. For samples with a very low surface area, krypton adsorption offers the sensitivity that is required. In this case only a part of the isotherm can be measured, but it is the approach that gives the most accurate result for such materials.
Pore volume and pore size distribution
Besides the surface area, physical gas adsorption is used to determine the pore volume and the pore size distribution. The principle is that the filling of pores during the analysis depends on the pore size. Micropores are filled through micropore filling at very low relative pressures, whereas mesopores are filled by capillary condensation at higher relative pressures. Smaller pores are thus filled at a lower partial pressure of the adsorbing gas, and larger pores require a higher pressure.
Physical gas adsorption is therefore particularly suited for the micropore and mesopore range. The upper limit of pore size that can be assessed is approximately 100 nanometers pore diameter. Larger pores can be measured adequately by mercury intrusion porosimetry, a technique that is complementary to physical gas adsorption.
Sample pre-treatment
Before the adsorption measurement, the sample is pre-treated at elevated temperature, under vacuum or in a flowing gas. This step removes water, solvents and other contaminants from the surface and pores. The pre-treatment conditions must be tuned to the properties of the material, because a temperature that is too low leaves contaminants behind, while a temperature that is too high can change the material itself. In both cases the outcome of the subsequent adsorption analysis can be negatively affected.
Flow and volumetric techniques
In general, two different techniques can be distinguished for the adsorption analysis. The flow technique uses a TCD detector to determine the amount of adsorbed gas. It results in a single value for the specific surface area and/or the total pore volume, and is a fast approach when limited information is sufficient.
The volumetric technique is based on pressure measurements in an enclosed volume. Since many adsorption and desorption points are measured, a full isotherm is obtained, from which the surface area, pore volume and pore size distribution can be derived. The report can range from a single surface area value to a comprehensive overview of all these parameters and description of the overall porous properties of the material.
Our laboratory performs volumetric adsorption measurements on a variety of instruments, among which the Quantachrome Autosorb-6B (N₂ or CO₂), Micromeritics TriStar II 3020 (N₂), Micromeritics Gemini, Micromeritics ASAP 2020 and Quantachrome Autosorb iQ (Ar or Kr).
Combining gas adsorption with other techniques
Physical gas adsorption is often combined with other techniques to obtain a more complete picture of a material. A combination with helium pycnometry that gives the true density, makes it possible to calculate the volume specific surface area. This parameter is used for the identification of nanomaterials according to European Commission guidelines, as described under surface area of nanomaterials.
Please note that the results of gas adsorption are obtained via idealised models and do not necessarily represent geometrical values. They are therefore best interpreted in combination with information on particle size, morphology, and the intended application of the material.
No. Physical gas adsorption is the measurement technique, in which the amount of gas adsorbed on a solid is measured at different pressures. BET analysis is an evaluation method that is applied to part of the resulting isotherm to calculate the specific surface area. The same measurement can also be used to determine the pore volume and pore size distribution, so physical gas adsorption provides more information than the BET surface area alone.
The adsorption isotherm shows how much gas is adsorbed at each relative pressure. Its shape is related to the pore structure of the material and is classified according to IUPAC. Non-porous and macroporous materials typically show a type II isotherm, microporous materials a type I isotherm and mesoporous materials a type IV isotherm. A hysteresis loop between adsorption and desorption points to capillary condensation in mesopores. The isotherm type determines which evaluation methods can be used. From a full isotherm, the surface area, pore volume and pore size distribution can be derived.
Nitrogen at 77 K is the most commonly used adsorptive and is suitable for a wide range of materials. For materials with small micropores, such as zeolites and activated carbons, argon or carbon dioxide is often preferred because these gases can probe the smallest pores more accurately. For samples with a very low surface area, krypton offers the sensitivity that is required. The best choice depends on the material and on the information that is needed.
Physical gas adsorption is particularly suited for the micropore and mesopore range. Smaller pores are filled at a lower relative pressure, whereas larger pores require a higher pressure, which makes it possible to determine a pore size distribution. The upper limit is approximately 100 nm pore diameter. Larger pores can be characterised by mercury intrusion porosimetry, which is complementary to gas adsorption.
