There are two classification criteria for the specific surface area test method. First, according to the different methods of determining the amount of gas adsorbed by the sample, it can be divided into: continuous flow method, volume method and gravimetric method (gravimetric method is rarely used now);The other is according to the different theoretical methods of calculating the specific surface area, which can be divided into: direct comparison method specific surface area analysis determination, Langmuir method specific surface area analysis and determination of specific surface area analysis and determination. At the same time, these 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, while the bet method can use both the continuous flow method and the volume method to determine the amount of adsorbed gas.
Continuous flow method.
Compared with the static method, the whole test process is carried out under normal pressure, and the adsorbent is adsorbed in a continuous flow state. The continuous flow method is developed on the basis of the principle of gas chromatography, and the thermal conductivity detector determines the amount of gas adsorbed by the sample. Continuous dynamic nitrogen adsorption is to use nitrogen as the adsorption gas and helium or hydrogen as the carrier gas, and the two gases are mixed in a certain proportion 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 the nitrogen in the mixture, while the carrier gas will not be adsorbed, resulting in a change in the proportion of the mixture gas composition, resulting in a change in the thermal conductivity coefficient, and then the signal voltage can be detected from the thermal conductivity detector, that is, the adsorption peak appears. After adsorption saturation, the sample is brought back to room temperature, and the adsorbed nitrogen is desorbed, forming a desorption peak opposite to the adsorption peak. The area size of the adsorption peak or desorption peak is proportional to the amount of nitrogen adsorbed on the sample surface, and the amount of nitrogen represented by the peak area can be calibrated by quantifying the gas. By measuring the amount of nitrogen adsorbed by the sample at a series of nitrogen partial pressures p p0, the nitrogen isothermal adsorption or desorption curves can be plotted, and then the specific surface area can be calculated. The desorption peak is often used to calculate the specific surface area.
Features: The continuous flow method is simple to operate, has a strong ability to eliminate systematic errors, and has the ability to use the direct comparison method and the bet method to calculate the specific surface area theory.
Capacity method. In the volumetric method, the amount of gas adsorbed in a sample is determined using a gaseous equation. A certain amount of adsorbed gas is introduced into the closed system with pre-vacuuming, and the change in the number of moles of the adsorbed gas is calculated by measuring the change of gas pressure in the closed system caused by sample adsorption and desorption, and the gaseous equation p*v t=nr is used to convert the mole change of the adsorbed gas.
Direct contrast method.
The direct contrast method is used to determine the amount of adsorbed gas by continuous flow method, and standard samples (stable substances with strict specific surface area calibration) are selected during the determination process. It is connected in parallel to the test gas path exactly the same as the tested sample, and adsorption is carried out separately by adsorption with the measured sample, and the desorption peaks are determined. Under the same adsorption and desorption conditions, the specific surface area of the measured sample and the standard sample is directly proportional to their peak area size. The calculation formula is as follows:
sx: specific surface area of the measured sample s0: specific surface area of the standard sample,
ax: desorption peak area of the measured sample a0: desorption peak area of the standard sample
wx: quality of the sample to be measured w0: standard sample quality
Advantages: The specific surface area can be obtained without the need to actually calibrate the adsorbed nitrogen volume and perform complex theoretical calculationsThe test operation is simple, the test speed is fast, and the efficiency is high.
Disadvantages: When the surface adsorption characteristics of the standard sample and the measured sample are very different, such as the number of adsorption layers is different, the error of the test result will be large.
The direct comparison method is only suitable for sample measurement with adsorption characteristics similar to that of standard samples, and the specific surface area determination of the BET method is more widely recognized at home and abroad because the BET method has a more reliable theoretical basis.
bet specific surface area determination.
The Bet theory calculation is based on the multilayer adsorption formula derived from classical statistical theory by Brunauer, Emmett and Teller, which is the famous Bet equation:
p: Partial pressure of adsorbate P0: Saturated vapor pressure of adsorbent
v: Actual adsorption capacity of the sample vm: saturated adsorption capacity of a single layer
c: Constant related to the adsorption capacity of the sample
As can be seen from the above equation, the bet equation establishes the quantitative relationship between the saturated adsorption capacity vm of the single layer and the adsorption capacity v of the multilayer, which provides a good theoretical basis for the determination of the specific surface area.
The bet equation is based on the theory of multilayer adsorption, which is closer to the actual adsorption process of many substances, so the test results are more reliable. In the actual test process, usually 3-5 groups of measured samples under different gas partial pressures multi-layer adsorption V are measured, with p p0 as the x-axis and y-axis, and the BET equation is used for linear fitting to obtain the slope and intercept of the straight line, so as to obtain the vm value and calculate the specific surface area of the measured sample. Theory and practice show that when p p0 takes the point at 005-0.In the range of 35, the bet equation is consistent with the actual adsorption process, and the graphic linearity is also very good, so the point selection needs to be within this range during the actual test process. Since 3-5 groups of p p0 are selected for the assay, we usually call them multi-point bets. When the adsorption capacity of the measured sample is very strong, that is, when the c value is very large, the intercept of the straight line is close to zero, and it can be approximated that the straight line passes through the origin, at this time, only a set of p p0 data can be measured and connected to the origin to find the specific surface area, which we call a single point bet. Compared with multi-point bets, single-point bets will have a larger error in the result.
If the flow method is used for BET determination, the measurement system needs to have a device that can accurately adjust the partial pressure p p0 of the gas to achieve the determination of the adsorption capacity at different p p0. For each point P P0, the desorption and desorption process of bet is similar to that of the direct comparison method, except that the bet method needs to calibrate the volume of the actual adsorbed gas amount of the sample, while the direct comparison method does not.
Features: The theory is closer to the actual adsorption process of substances, a wide range of samples can be measured, and the accuracy and credibility of the test results are high, which is especially suitable for scientific research and production units.