Specific surface area and pore size test method classification - Database & Sql Blog Articles

There are many kinds of specific surface area analysis test methods (), in which the gas adsorption method is widely used in various industries at home and abroad because of the scientific nature of its test principle, the reliability of the test process, and the consistency of test results. And gradually replaced other specific surface area testing methods, becoming the most authoritative test method. Many international standards organizations have listed gas adsorption as a specific surface area test standard, such as D3037 of ASTM in the United States and ISO-9277 of the International ISO Standards Organization. There are many industry standards for specific surface area testing in China, the most representative of which is the national standard GB/T 19587-2004 "Gas adsorption BET method for the determination of solid surface specific surface area".
The principle of determining the specific surface area by the gas adsorption method is based on the adsorption characteristics of the gas on the solid surface. Under a certain pressure, the surface of the sample particles (adsorbent) has reversible physical adsorption on the gas molecules (adsorbed matter) at ultra-low temperature, and There is a certain equilibrium adsorption amount corresponding to a certain pressure. By measuring the equilibrium adsorption amount, the specific surface area of ​​the sample to be tested is equivalently obtained by a theoretical model. Due to the irregularity of the actual outer surface of the particle, strictly speaking, the method determines the sum of the outer surface of the particle and the total surface area of ​​the internal through hole that the adsorbate molecule can reach, as shown in the figure.
Nitrogen is the most commonly used adsorbate due to its easy availability and good reversible adsorption characteristics. The specific surface area measured by this method is called the "equivalent" specific surface area. The concept of "equivalent" means that the specific surface area of ​​the sample is the number and molecular number of nitrogen molecules coated (adsorbed) through the surface of the sample. Characterized by the maximum cross-sectional area. Actually, the saturated adsorption amount (V) of nitrogen molecules on the surface of the sample was determined. The saturated adsorption capacity (Vm) of the single layer was calculated by different theoretical models, and the number of molecules was obtained. The hexagonal model of surface close-packed was used to calculate the molecular equivalent of nitrogen. The maximum cross-sectional area (Am) is used to determine the specific surface area of ​​the sample to be tested. Calculated as follows:


Sg: specific surface area of ​​the sample to be tested (m2/g)
Vm: single layer saturated adsorption capacity of nitrogen molecules under standard conditions (ml)
Am: equivalent maximum cross-sectional area of ​​nitrogen molecules (closed hexagonal theoretical value Am = 0.162 nm2)
W: mass of the sample to be tested (g)
N: Avogadro constant (6.02x1023)
Substituting the above data, the basic formula for calculating the specific surface area by the nitrogen adsorption method is obtained:



It can be seen from the above formula that accurately determining the single layer saturated adsorption amount Vm on the surface of the sample is the key to the determination of specific surface area.

Test method classification

There are two classification criteria for the specific surface area test method (). First, according to the method of measuring the amount of adsorbed gas in the sample, it can be divided into: continuous flow method, volume method and gravimetric method. The weight method is basically rarely used; in addition, according to the calculation of specific surface area theory, it can be divided into: Direct comparison method specific surface area analysis, Langmuir method specific surface area analysis and BET specific surface area analysis and the like. At the same time, the two classification standards have a certain relationship. The direct comparison method can only use the continuous flow method to determine the amount of adsorbed gas. The BET method can use either the continuous flow method or the volumetric method to determine the amount of adsorbed gas. The relationship is as shown.




Continuous flow method

The continuous flow method () is relative to the static method, the entire test process is carried out under normal pressure, and the adsorbent is adsorbed while being in a continuous flow state. The continuous flow method is developed on the basis of the principle of gas chromatography, and the amount of adsorbed gas in the sample is determined by a thermal conductivity detector. The continuous dynamic nitrogen adsorption uses nitrogen as the adsorption gas and helium or hydrogen as the carrier gas. The two gases are mixed in a certain ratio to make the nitrogen reach the specified relative pressure and flow through the surface of the sample particles. When the sample tube is placed in a liquid nitrogen environment, the powder material physically adsorbs nitrogen in the mixed gas, and the carrier gas is not adsorbed, causing a change in the proportion of the mixed gas component, thereby causing a change in the thermal conductivity coefficient. When the signal voltage is detected from the thermal conductivity detector, an adsorption peak appears. After the adsorption is saturated, the sample is returned to room temperature, and the adsorbed nitrogen is desorbed to form a desorption peak opposite to the adsorption peak. The area of ​​the adsorption peak or the desorption peak is proportional to the amount of nitrogen adsorbed on the surface of the sample, and the amount of nitrogen represented by the peak area can be quantified by the quantitative gas. By measuring the amount of nitrogen adsorbed by a series of nitrogen partial pressure P/P0, an isothermal adsorption or desorption curve of nitrogen can be drawn to determine the specific surface area. The desorption peak is usually used to calculate the specific surface area.



Features: The continuous flow method test process is simple to operate, and the system error is eliminated. At the same time, the direct surface contrast method and the BET method can be used to calculate the specific surface area.
In the volumetric method, the amount of adsorbed gas in a sample is determined by the gas equation. A certain amount of adsorbed gas is introduced into the pre-vacuum sealed system, and the change in the gas pressure in the closed system caused by the sample suction and desorption is measured, and the change in the number of moles of the adsorbed gas is converted by the gas equation P*V/T=nR.

Direct comparison

The direct comparison method specific surface area analysis test uses the continuous flow method to determine the amount of adsorbed gas. The standard sample (stabilized substance with strict specific surface area) is selected during the measurement. Parallel to the test gas path exactly the same as the sample to be tested, and simultaneously desorbed by adsorption with the sample to be tested, and the respective desorption peaks are determined. Under the same adsorption and desorption conditions, the specific surface area of ​​the sample to be tested and the standard sample is proportional to the peak area. Calculated as follows:



Sx: specific surface area of ​​the sample to be tested S0: standard sample specific surface area,
Ax: desorption peak area A0 of the sample to be tested: standard sample desorption peak area
Wx: the mass of the sample to be tested W0: the quality of the standard sample. Advantages: the specific surface area can be obtained without actually calibrating the volume of adsorbed nitrogen and performing complicated theoretical calculations; the test operation is simple, the test speed is fast, and the efficiency is high. Disadvantages: when the standard sample is When the surface adsorption characteristics of the sample are very different, if the number of adsorption layers is different, the error of the test result will be large.
The direct comparison method is only applicable to sample measurement close to the adsorption characteristics of standard samples. Since the BET method has a more reliable theoretical basis, the BET specific surface area measurement is more generally accepted at home and abroad.

BET specific surface area test



The BET theoretical calculation is based on the multi-molecular layer adsorption formula deduced by classical statistical theory by Brunauer, Emmett and Teller, namely the famous BET equation:

P: adsorbent partial pressure P0: adsorbent saturated vapor pressure
V: actual adsorption amount of sample Vm: single layer saturated adsorption amount
C: The constant related to the adsorption capacity of the sample can be seen from the above formula. The BET equation establishes the quantitative relationship between the single-layer saturated adsorption amount Vm and the multi-layer adsorption amount V, which provides a good theoretical basis for the determination of specific surface area.
The BET equation is based on the theory of multilayer adsorption and is closer to the actual adsorption process of many materials, so the test results are more reliable. In the actual test process, the multi-layer adsorption amount V of 3-5 sets of tested samples under different gas partial pressures is usually measured, and P/P0 is taken as the X-axis. For the Y-axis, a linear fit is performed by plotting the BET equation to obtain the slope and intercept of the line, and the Vm value is obtained to calculate the specific surface area of ​​the sample to be tested. The theory and practice show that when the P/P0 is in the range of 0.05-0.35, the BET equation is consistent with the actual adsorption process, and the linearity of the graph is also very good. Therefore, the selection point in the actual test process should be within this range. Since 3-5 sets of P/P0 are selected for measurement, we usually call it multi-point BET. When the adsorption capacity of the sample to be tested is very strong, that is, when the C value is large, the intercept of the straight line is close to zero, and the straight line can be approximated as passing through the origin. At this time, only one set of P/P0 data can be measured and connected to the origin to determine the specific surface area. We call it a single point BET. Compared with multi-point BET, the single-point BET result error will be larger.
If the flow method is used for the BET measurement, the measuring system needs to have a device capable of precisely adjusting the gas partial pressure P/P0 to achieve the adsorption amount measurement under different P/P0. For each point P/P0, the BET adsorption and desorption process is similar to the direct comparison method. The difference is that the BET method needs to calibrate the volume of the actual adsorbed gas volume of the sample, and the direct comparison rule does not.
Features: The BET theory is closer to the actual adsorption process of the substance. It can measure a wide range of samples, and the test results are highly accurate and reliable. It is especially suitable for scientific research and production units.

Pore ​​size distribution test

Gas adsorption method pore size (porosity) distribution measurement is utilized

Capillary condensation

with

Volume equivalent substitution

The principle is that the amount of liquid nitrogen filled in the hole to be measured is equivalent to the volume of the hole. The adsorption theory assumes that the shape of the pores is a cylindrical shape, thereby establishing a capillary condensation model. It is known from the capillary aggregation theory that the pore size range in which capillary aggregation occurs under different P/P0 is different, and as the P/P0 value increases, the pore radius at which aggregation can occur also increases. Corresponding to a certain P/P0 value, there is a critical pore radius Rk, all pores with a radius smaller than Rk are capillaryly coagulated, liquid nitrogen is filled therein, and pores larger than Rk do not undergo capillary condensation, and liquid nitrogen does not exist therein. filling. The critical radius can be given by the Kelvin equation:

Rk is called the Kelvin radius and it depends entirely on the relative pressure P/P0. The Kelvin formula can also be understood as a hole in which condensation has occurred. When the pressure is lower than a certain P/P0, the condensed liquid in the hole having a radius larger than Rk will be vaporized and desorbed. Theory and practice show that when P/P0 is greater than 0.4, the capillary condensation phenomenon will occur. By measuring the amount of nitrogen condensed in the sample under different P/P0, the isothermal adsorption-desorption curve can be drawn, which can be determined by different theoretical methods. The pore volume and pore size distribution curves are obtained. The most common calculation method is to use the BJH theory, commonly referred to as the BJH pore volume and pore size distribution.