Introduction: The Bet test theory is based on the multilayer adsorption model proposed by three scientists, Brinauer, Emmett and Teller, and derives the Bet equation for the relationship between the single-layer adsorption amount VM and the multilayer adsorption capacity V. It can be used to test the specific surface area, pore volume, pore size distribution, and nitrogen adsorption desorption profile of particles.
The bet test method is widely used in the study of particle surface adsorption performance and the data processing of related testing instruments, and is now the most widely used method in the industry with the strongest reliability of test results. The measurement of specific surface area not only plays an important role in the study of particle properties, but also has great significance in scientific research and industrial production.
The principle of BET method determination: nitrogen is used as the adsorbate, helium or hydrogen is used as the carrier gas, and the two gases are mixed in a certain proportion to reach the specified relative pressure, and then flow through the solid substance. When the sample tube is insulated with liquid nitrogen, the sample is physically adsorbed to the nitrogen in the gas mixture, while the carrier gas is not. At this point, an adsorption peak appears on the screen. When the liquid nitrogen is removed, the sample tube is back at room temperature, and the adsorbed nitrogen is desorbed, and a desorption peak appears on the screen. Finally, a known volume of pure nitrogen is injected into the gas mixture to obtain a corrected peak. Based on the peak areas of the corrected and desorption peaks, the adsorption capacity of the sample at this relative pressure can be calculated. By changing the mixing ratio of nitrogen and carrier gas, the adsorption capacity at the relative pressure of several nitrogen can be measured, so that the specific surface can be calculated according to the bet formula. Generally, porous substances with large specific surface area and large activity have strong adsorption capacity.
Explanation of terms
Specific surface area: The total surface area of particles per unit volume or unit mass. Generally, porous substances with large specific surface area and large activity have strong adsorption capacity. Solids have a certain geometric shape, and their surface area can be calculated by instrumental calculations. However, the determination of the surface area of powders or porous substances is difficult because they have not only irregular outer surfaces, but also complex inner surfaces.
Bet Test:It refers to the nitrogen isothermal adsorption-desorption curve, but it is p p0=0 in the nitrogen isothermal adsorption-desorption curve05~0.35 between a short segment, with the bet formula to obtain a single layer adsorption capacity data vm, and then calculate the specific surface area accordingly.
The instrument has three conventional test modes: mesoporous, microporous, and full bore (including microporous and mesoporous). Therefore, the physical adsorption meter is to test the pore size of the pore less than 50nm, although the test will provide data between 50-100nm, but the results are only used as a reference. (Macroporous samples can be measured by mercury pore size distribution, pore volume, specific surface area, etc.) ）
1. Since the diameter of the injection tube of the instrument is relatively small (the microphone is generally a 12mm diameter tube, and the general diameter of the conta is a 9mm tube), please provide a sample of appropriate size. For example, fiber samples need to be cut as much as possible;Large, lumpy samples need to be physically broken.
2. The test sample is generally powder, if it is granular, please try to physically crush it relatively small, generally below 3mm. For easy sampling, do not use ziplock bags for sampling.
3. The weight should be more than 100mg as much as possible to reduce the weighing error. The specific surface area is greater than 1000 m2 g, which can be called 005-0.08g。Because the adsorption capacity of the sample in the sample tube should not be too large, otherwise the adsorption equilibrium time will be too long, resulting in too long the experimental time. If the specific surface area is small and the sample volume is small, the sample adsorption in the sample tube is too small, which is likely to result in a negative adsorption-desorption curve. Therefore, if you do not know the specific surface area, please provide as many samples as possible.
a.Under the premise that the specific surface area is roughly expected, the formula can be used: specific surface area (m2 g) * sample amount (g) = 15-20m2 to roughly determine the required sample size, and the test engineer will determine the test sample amount according to the actual situation. For samples with a specific surface area greater than 400 m2 g, it is recommended that the sample volume should not be less than 50 mg in order to reduce weighing errors. Please write down the specific surface area range when making an appointment, if the results are inaccurate due to the small sample size, we generally do not arrange a retest.
b.In the case of not knowing the specific surface area, the mass of the whole pore and micropore mode is generally more than 100 mg (provided that there are micropores in the sample), and the test mesoporous mode needs to be more than 250 mg.
4. The degassing temperature is required to be within the stable temperature range of the sample, and cannot exceed one-half of the melting point temperatureThe actual temperature of vacuum degassing is higher than that of the oven, if the sample is easy to decompose or **, please inform the staff in advance, otherwise you will have to bear the follow-up problems. However, if the degassing temperature is below 100, it is likely that the degassing is not complete, resulting in a low result. The maximum degassing temperature of the device is 300 !!If the degassing temperature is higher than the synthesis temperature of the sample, make sure that the sample does not decompose (it is best to provide the sample TG data), otherwise you will be responsible for any subsequent problems.
5. The maximum pressure of the conventional test is 1 atmosphere, and if you need high-pressure adsorption, you need to communicate separatelyIf you would like to make an appointment for H2 and later, please contact the staff first. Generally, the degassing time is 6-8h, if the degassing time exceeds 12h, please contact the staff.
6. Sample delivery reminder: when drying and sending samples, it is best to explain the type of sample, and it is best to give the approximate specific surface area, and it is best to provide a sample with a surface area of 20 m2 (weight approximate specific surface area) (if the specific surface of activated carbon is generally about 700m2 g, it is greater than 0.)03 g sample). If the catalyst used is usually molded and then reacted, the sample will be sent after molding, so that the result is more objective. Molecular sieve samples generally do not need to be formed, especially when mesoporous molecular sieves are formed, large pressure cannot be used, otherwise the hard work may be ruined.
Adsorption isotherm: take the relative pressure as the x-axis, the nitrogen adsorption capacity as the y-axis, and then roughly divide the relative pressure of the x-axis into low pressure (0.).0-0.1), medium pressure (0.3-0.8), high voltage (090-1.0) Three paragraphs. Then the adsorption curve is at:
The deviation of the y-axis at the low pressure end indicates that the material has a strong force with nitrogen (type, type, type), and when there are more micropores, due to the strong adsorption potential in the micropores, the adsorption curve is shaped at the beginningThe low voltage end deviates from the x-axis and the material is weak (type, type).
The medium pressure end is mostly the condensation accumulation of nitrogen in the pores of the material, and mesoporous analysis is the best in this data, including the pores generated by the accumulation of sample particles, and the pores in the mesoporous range of order or gradient. The BJH method is based on the pore size data derived from this segment;
The high pressure section gives a rough indication of the degree of particle accumulation, such as the last upward movement in type I, the particles may not be uniform. The total pore volume usually obtained is usually taken with a relative pressure of 0The condensation value of the nitrogen adsorption capacity at about 99.
6 types of isotherm analysisThe adsorption capacity of type I isotherm increases rapidly at low relative pressure, and the adsorption saturation value appears after reaching a certain relative pressure, which is similar to the Langmuir type adsorption isotherm. In general, the I-type isotherm often reflects the microporous filling phenomenon on the microporous adsorbent (molecular sieve, microporous activated carbon), and the saturated adsorption value is equal to the filling volume of the micropores.
Type II isotherms reflect the typical physisorption process on non-porous or macroporous adsorbents, which is most commonly described by the Bet formulation. Due to the strong interaction between the adsorbate and the surface, the adsorption capacity rises rapidly at a low relative pressure, and the curve is convex. The isotherm inflection point usually appears near the single-layer adsorption, and with the continuous increase of the relative pressure, the multi-layer adsorption is gradually formed, and when the saturated vapor pressure is reached, the adsorption layer is infinite, which makes it difficult to determine the accurate limit equilibrium adsorption value in the test.
Type III isotherms are rare. The isotherm is concave and there is no inflection point. The amount of adsorbed gas increases with the increase of component partial pressure. The concave curve is because the interaction between adsorbate molecules is stronger than that between adsorbate and adsorbent, and the heat of adsorption in the first layer is smaller than that of adsorbate, so that the adsorbate is difficult to adsorb in the initial stage of adsorption, and with the progress of the adsorption process, the adsorption appears self-accelerating, and the number of adsorption layers is not limited. The bet formula C value is less than 2, and type III isotherms can be described.
Type IV isotherms are similar to type II isotherms, but the latter part of the curve is convex again, and there may be adsorption hysteresis loops in the middle section, which corresponds to a system with capillary agglomeration of porous adsorbents. At moderate relative pressures, type IV isotherms rise faster than type II isotherms due to the occurrence of capillary condensation. After the mesoporous capillary is filled with condensation, if the adsorbent still has large pores or the adsorbate molecules interact strongly, it may continue to adsorb to form a multimolecular layer, and the adsorption isotherm continues to rise. However, in most cases, after the capillary condensation is completed, an adsorption termination platform appears, and no further multilayer adsorption occurs.
The V-type isotherm is similar to the III isotherm, but the adsorption layer is limited when the saturated vapor pressure is reached, and the adsorption capacity tends to a limit. At the same time, due to the occurrence of capillary condensation, the isotherm rises faster at medium relative pressure, accompanied by hysteresis loops.
Type VI isotherms are a special type of isotherm that reflect the results of multilayer adsorption on a non-porous uniform solid surface (e.g., a clean metal or graphite surface). The actual solid surface is mostly inhomogeneous, so it is difficult to encounter this situation.
The hysteresis loop is commonly found in type IV adsorption isotherms, which refers to the adsorption branches measured when the adsorption amount increases with the equilibrium pressure and the desorption branches measured when the pressure decreases, which do not coincide in a certain relative pressure range and separate to form a ring. The adsorption capacity of the desorption branch is greater than that of the adsorbed branch at the same relative pressure. The theory explained is mainly the capillary condensation theory, and the hysteresis loop is mostly found in type IV adsorption isotherms, and according to the latest IUPAC classification, there are the following six (the 1985 standard is mainly H1, H2A, H3, and H4).
There are saturated adsorption platforms on the adsorption isotherms of H1 and H2 hysteresis loops, which reflect a uniform pore size distribution.
Type H1 reflects cylindrical pores with uniform diameter distribution at both ends, while type H1 hysteresis loops can be observed in mesoporous materials with relatively narrow pore size distribution, and spherical particle aggregates with uniform size (e.g., MCM-41, MCM-48, SBA-15, etc.).
The H2 type reflects a complex pore structure, which may include typical "inkwell" holes, tubular pores with uneven pore size distribution, and densely packed spherical particle gap pores. The pore size distribution and hole shape may be difficult to determine, and the pore size distribution is wider than that of H1 loops. The desorption branch is very steep in H2A, mainly due to pore-blocking percolationin a narrow range of pore necks or c**itation-induced evaporation, and H2A hysteresis rings are commonly found in silicone gels and some ordered three-dimensional mesoporous materials, such as SBA-16 and Kit-5. Compared with H2A, H2B type has a much wider size distribution of neck width, which is commonly found in mesoporous silicon foam (MCFS) and some ordered mesoporous silicon materials (such as FDU-12) after hydrothermal treatment.
The H3 and H4 hysteresis loop isotherms do not have obvious saturated adsorption plateaus, indicating that the pore structure is very irregular.
The adsorption branch of the H3 hysteresis loop is similar to the type II adsorption isotherm, and the lower limit of the desorption loop is generally located at C**itation-InducedP P0. The holes reflected in the H3 type include flat slit structures, cracks, and wedge structures. The H3-type hysteresis loop is given by a flake granular material, such as clay, or by a fractured pore material, and does not exhibit adsorption saturation in the region of higher relative pressure.
The H4 hysteresis loop is a composite of type I and type II adsorption isotherms. The H4 form is found in mixed microporous and mesoporous adsorbents, and in solids containing narrow fissure pores, such as activated carbon, and in molecular sieves.
H5 hysteresis rings are rare and usually contain holes that are open at both ends and blocked at one end, such as PHTS (Plugged Hexagonal Templated Silicas).
The adsorption isotherm of SBA-15 zeolite is taken as an example.
These isotherms belong to the IUPAC classification type IV, the H1 hysteresis loop. It can be seen from the figure that the adsorption capacity increases gently in the low-pressure section, and the N2 molecules are adsorbed on the inner surface of the mesopores in a single layer to multiple layers, and the relative pressure p p0 = 010~0.29 is more suitable. at p p0 = 05~0.8. There is a sudden increase in the adsorption capacity (only capillary condensation has this phenomenon, which is mesoporous, ordered or stacked). The position of this segment reflects the size of the sample pore size (r' 2 vm [rt*ln(P0 P)], P0 P: relative pressure at which capillary condensation occurs), and the variation in width can be used as a measure of mesopore homogeneity. A third segment sometimes rises at higher p p0, which can reflect macropores or particles packing up in the sample. The specific surface area, pore volume and pore size distribution can be determined by the N2 adsorption-desorption isotherm. The analysis of its specific surface area is generally performed using the Bet (Brunauer-Emmett-Teller) method. The pore size distribution is usually BJH (Barrett-Joiner-Halenda) model.
Kelvin equation
BJH is suitable for mesoporous, >5nm, cylindrical models. In this method, the contribution of the adsorption film to the chemical potential of the liquid is omitted, and the Kelvin equation is the basis of the BJH model, and the diameter derived from the Kelvin equation plus the thickness of the liquid film is the pore diameter. The radius of curvature of the bending liquid surfacer＝2γvm/[rt*ln(p0/p)], to calculate the pore size r produced by the bending liquid surface, then there is r'cos r, due to the different contact angles of different materials, the following figure gives the corresponding diagram of the radius of curvature of the bending liquid surface r' and the relative pressure p po without considering the contact angle:
This is due to capillary condensation that the N2 molecule condenses at a lower than atmospheric pressure to fill the mesoporous pores, because capillary condensation begins to occur on the liquid surface of the annular adsorption membrane on the pore wall, and desorption starts from the spherical meniscus surface of the orifice, so that the adsorption-desorption isotherms do not coincide, often forming a hysteresis ring. There is also another theory that the contact angle between liquid nitrogen entering the pores and the material during adsorption is the forward angle and the desorption angle is the backward angle, and the difference between these two angles leads to a difference when using the Kelvin equation. Of course, it is possible that the combination of the two, and individuals tend to identify with the former, at least intuitivelyThe former makes sense.
The medium pressure part has a large adsorption capacity but does not produce a hysteresis loop.
At a relative pressure of 02-0.Around 3, according to the Kelvin equation, it can be seen that the pore radius is very small, the effective pore radius is only a few adsorbate molecules, there will be no capillary condensation, the adsorption-desorption isotherm coincides, and the ordered mesoporous adsorption and desorption does not appear a lag loop when the mcm-41 pore size is one nm.
Mesoporous analysis. The BJH model (Barrett-Joiner-Halenda) is usually used, which is an application of the Kelvin equation to the cylinder model, which is applicable to the mesoporous range, and the results obtained are smaller than the actual ones. For the KJS (Kruk-JaronieC-Sayari) with higher precision for the structure analysis of MCM-41 and SBA-15 holes and its correction method, KJS was analyzed with highly ordered MCM41 as the material, and combined with the XRD results, the KJS equation with higher accuracy than BJH was obtained, and the applicable pore size analysis range was 2-6between 5nm. Later, it was promoted to make it have a large range of application and can be used for SBA-15 hole structure (46-30 nm).
Microwell analysis. The micropore analysis of microporous materials has different requirements for vacuum, control system, and temperature sensor, and the test time is relatively long, which may be ten or even twenty times that of ordinary samples. Due to the limited difference between the size of the micropore and the size of the probe molecule, some micropore probe molecules can not enter, the analytical method should be determined according to different samples, and the relevant literature methods can be used for reference when necessary. Nowadays, it is still a bit difficult to analyze the pore size distribution of all ranges with one model, and the nonlinear density generalization theory (NLDFT) is said to be possible, but it is rarely used in **.
1. Why should the sample be degassed?
The purpose is to remove the impurities adsorbed on the surface of the sample, such as water, oil, etc., and the sample is generally heated under vacuum.
2. How to choose the degassing temperature?
The first principle is not to damage the structure of the sample. Generally, the degassing temperature should not be higher than the melting point of the solid or the phase transition point of the glass, and it is recommended not to exceed half of the melting point temperature. The selectable temperature range of our test instruments is generally -300 °C.
Considering that a high degassing temperature can lead to irreversible changes in the sample structure, and that a low degassing temperature may lead to incomplete degassing treatment and a small result, the degassing temperature can be increased appropriately without damaging the sample structure, and you can also refer to chemical manuals (e.g., The Handbook of Chemistry and Physics) and standard methods published by various standards organizations (e.g., ASTM).
3. How to choose the degassing time?
The determination of degassing time is related to the degassing temperature and the complexity of the sample pore. The more complex the pore, the more difficult it is for the impurities in the pore to come out, and the longer it takes to degasseThe degassing temperature is low, the molecular diffusion rate is slow, and the degassing time needs to be extendedThe degassing time of general samples can be set to no less than 6h, and the degassing time of some samples containing complex micropores needs longer. However, there are exceptions, the United States Pharmacopeia stipulates that the degassing time of magnesium stearate is only 2 hours
Note: Since the degassing temperature, degassing time and degassing vacuum are all related to the specific surface area value, it is inevitable that there will be errors in the bet results, so the sample needs to be compared with fixed treatment conditions.
4. What experimental data can be obtained from gas adsorption and desorption experiments?
Specific surface area, pore volume, and pore size distribution of the sample.
5. Why is N2 commonly used for adsorption and desorption experiments?
Requirements for adsorbate molecules in gas adsorption and desorption experiments: the adsorbate molecules should be as small as possible, nearly spherical and inert to the surface. The molecules that meet the requirements are: nitrogen, krypton and argon.
Advantages of N2: Cheap, high-purity N2 is readily available, and N2 is most commonly used because it can generate meaningful type II and IV adsorption isotherms on most surfaces and has a well-recognized molecular cross-sectional area.
6. Under what circumstances can N2 not be used as an adsorbate?
1) N2 can have strong interaction with the sample surface or chemisorption;
2) The adsorption of N2 and the sample is too weak (C < 1) to form type II and IV adsorption isotherms
3) The specific surface area of the sample is very small, and the pressure change caused by adsorption is too small (p p0 < 0.).05), when N2 is used as the adsorbate, the systematic error is large, and the krypton and argon with lower saturated vapor pressure should be used as the adsorbate.
7. What is adsorption, what are the types of adsorption, what are the differences, and what are the uses of each?
Broadly speaking, the enrichment of molecules, atoms, or ions near the interface of matter can be called adsorption (if strictly distinguished, it includes adsorption and absorption, omitted). Adsorption can be divided into physical adsorption and chemical adsorption, the main difference between the two is whether there is a chemical bond (strict identification is more troublesome, interested students please see the relevant information), the difference in the characteristics shown is shown in the following table:
Physical adsorption provides a method for determining the surface area, average pore size, and pore size distribution of catalysts (generally referred to as N2 adsorption and desorption experiments).Chemical adsorption is an important part of heterogeneous catalytic processes, and is often used to study the catalytic mechanism and determine the surface area of specific catalyst components (e.g., the surface area of PT by CO adsorption, etc.).
8. What is microporefilling, what is capillary condensation?
Micropore filling: Due to the enhancement of the adsorption potential, there is a significant adsorption enhancement in the micropores, which has a strong ability to capture adsorbate molecules at low relative pressure. This facilitated adsorption mechanism at very low relative pressures caused by the overlapping adsorption potential of the relative pore walls in the micropores is called micropore filling.
Capillary condensation: In the porous adsorbent, if the concave liquid surface can be formed in the initial stage of adsorption, according to the kelvin formula, the vapor pressure on the concave liquid surface is always less than the saturated vapor pressure on the flat liquid surface, so when it is less than the saturated vapor pressure, the concave liquid surface has reached saturation and vapor condensation occurs, and the effect of this vapor condensation is always from small pores to large pores, and with the increase of gas pressure, the capillary pores of gas condensation are getting larger and largerIn the case of desorption, the desorption pressure is always less than the adsorption pressure at the same adsorption capacity, because the radius of curvature of the liquid surface after capillary condensation is always smaller than that before capillary condensation.
Micropore filling is similar to capillary condensation in the phenomenon of filling pores, but is different in nature. Micropore filling is a microscopic phenomenon that depends on the enhanced potential energy interaction between the adsorbed molecule and the surface, which occurs within the micropores and the relative pressure is very low;Capillary coagulation is a macroscopic phenomenon that depends on the characteristics of the meniscus of the adsorbed liquid, and the necessary condition for capillary coagulation is that at least two layers of particles can be accommodated in the pores, which occur in the middle pores and under the relative pressure in the middle. With nitrogen as the adsorbate, the general radius is about 16nm。
9. What is the effect of adsorbate gas on the experiment?
The specific surface area determination by the Bet method, IUPAC makes a clear recommendation compared to the adsorbed gas measured on the surface:
1) General mesoporous materials: N2 adsorption at 77K (liquid nitrogen) or AR adsorption at 87K (liquid argon);
2) Microporous materials: AR adsorption at 87K (liquid argon);
3) For 0Ultra-low specific surface area determination below 5m2 g: KR adsorption at 77K (liquid nitrogen).
N2 at 77K is the most commonly used sorbate for micropore and mesoporous analysis, but N2 adsorption is not suitable for quantification of micropores, especially ultrapores (pore size <7). Therefore, both IUPAC and ISO15901 recommend and CO2 as molecular probes to replace N2. Although N2, AR and CO2 kinetic diameters are similar (0., respectively36，0.34 and 033), but the adsorption behavior of these three adsorbed substances is completely different. Due to the absence of quadrupole moment action, AR does not interact specifically with most surface functional groups and exposed ions, so it is important to interact specifically with most surface functional groups and exposed ions, so at the boiling point temperature (873K) can give more accurate pore size information for many microporous systems, especially molecular sieves, SOCMOF, etc. In the case of FAU zeolites, AR can be used at high relative pressures (10 -510, pore volume, and pore size distribution).
The determination of pore structure generally includes gas adsorption method and mercury intrusion method, etc., the physical adsorption of gas is applied to pores with a diameter of less than 50 nm, and the mercury intrusion method can be applied to a diameter greater than about 35 nm pore system. Closed pores that cannot be accessed by molecules can be determined by small-angle X-ray scattering or small-angle neutron scattering.
Note: Do not test samples with pore sizes greater than 50 nm with gas adsorption-desorption experiments.
11. In general, the aperture should be calculated using desorption data
a.For ideal cylindrical pores with openings at both ends, the adsorption and desorption supports coincide;
b.For cylindrical pores with openings at both ends, the meniscus curvature corresponding to the adsorption branch is cylindrical, while the desorption branch corresponds to the spherical meniscus formed at the orifice
c.Flat pores and slit-shaped pores formed by flake particles do not undergo capillary agglomeration during adsorption, and the desorption data reflect the true pores
d.For holes with throat orifice, such as the "inkwell" hole with a large mouth and abdomen (the larynx size is smaller than the cavity size), adsorption is a process of gradually filling the cavity of the hole, and the pore size distribution in the cavity can be obtained according to the adsorption branch data, but the desorption branch can reflect the pore size of the larynx (here it provides a theoretical basis for the calculation of the window and pore size of three-dimensional mesoporous materials such as FDU-12). The internal diffusion rate of a porous catalyst is limited precisely by the size of the throat at the narrowest pore, rather than by the size of the extended cavity. Therefore, desorption rami is a better measure of pore size. In particular, when the intertwined pore structure has several parallel throats, the desorption branch reflects the thickest throat size, which happens to correctly reflect the limiting effect of the pore structure on the diffusion within the catalyst.
For adsorption, some degree of supersaturation may be required prior to capillary agglomeration, and the thermodynamic equilibrium assumed by the Kelvin formula may not be achieved. In addition, during desorption, the condensed liquid in the capillary pores is close to the liquid body, but during adsorption, the physical force (especially the first layer) is not the same as the intermolecular force of the liquid body, so the kelvin formula uses the surface tension of the liquid body and the molar volume of the liquid body is relatively reluctant.
12. What are the application limitations of calculating specific surface area from the bet equation?
The multilayer adsorption theory (BET equation for short) is currently the most popular method for calculating the specific surface. The initial bet work was established on nitrogen adsorption isotherms, and various non-porous adsorbents could be found at p p0 005-0.A linear bet plot is given in the range of 3, which in turn calculates the specific surface value.
Porosity, such as the presence of micropores and mesopores, has an important impact on the applicability of the Bet equation. The bet equation can be applied to the specific surface analysis of non-porous and mesoporous materials with wide pore diameters, but strictly cannot be used for microporous adsorption materials, since the monolayer-multilayer adsorption process is usually less than 0It is done in 1 hour, and it is very difficult to distinguish between them during the microvia filling process. On the other hand, the calculation results of bet are related to the volume and shape of the adsorbed substance molecules, that is, the effective benchmark scale for evaluating the surface area will be problematic, because the molecular cross-sectional area of nitrogen, which is commonly used as an adsorbed gas, will change due to the quadrupole moment during the microporous adsorption process, which destroys the basis of the calculation of the bet equation.
13. What is the "equivalent bet surface area"?
The surface area value calculated from the adsorption curve of the microporous material by the bet method does not reflect the true internal surface area of the material, but can be considered as "apparent" or "equivalent bet surface area".