A brief introduction to the specific surface area

A brief introduction to the specific surface area of gas adsorption method

in the test of specific surface area and pore size distribution (porosity) of gas adsorption method, several factors will have a very important impact on the test process and results. These factors need to be considered in the effective analysis of test results. These factors include: sample processing conditions, adsorbate gas characteristics, different test methods, etc., which are introduced in detail below

sample processing conditions

because the specific surface area and porosity of dandelion are divided into many varieties, the determination is closely related to the outer surface of the particles, and the key of the adsorption method is that the adsorbate gas molecules are effectively adsorbed on the surface of the tested particles or filled in the pores, so whether the surface of the sample particles is clean is very important. The purpose of sample treatment is to release the surface occupied by non adsorbate molecules as much as possible, so as to facilitate the surface adsorption of adsorbate molecules in the test process. Generally, pretreatment is required before sample determination, and the treatment method is selected according to the characteristics of the measured sample. Generally, what most samples need to remove is the water molecules adsorbed on their surfaces. Therefore, drying at atmospheric pressure above 100 ℃ (generally 105 ℃ -120 ℃) can achieve this purpose, which is conducive to simplifying the operation process. For samples containing micropores or with strong adsorption characteristics, it is easy to adsorb impurity molecules at room temperature and pressure, or many other molecules are adsorbed on the surface during the manufacturing process. Generally, it is necessary to conduct degassing treatment under vacuum conditions, and sometimes inert protective gas must be introduced in the pretreatment process to facilitate the desorption of impurities on the sample surface. In short, the purpose of sample pretreatment is to make the sample surface clean, so as to ensure the accuracy and effectiveness of the measurement results of specific surface area and pore size (porosity)

adsorbate gas characteristics

in the analysis and test of specific surface area and pore size distribution of gas adsorption method, the most basic requirement for adsorbate gas is that its chemical properties are stable, and it will not have any impact on the performance and surface adsorption characteristics of the sample itself during the adsorption process, and it must be reversible physical adsorption. Nitrogen is the most commonly used adsorbent. Practice shows that nitrogen is selected as the adsorbent for the determination of most substances, and the accuracy and repeatability of the test results are ideal. For samples containing micropores, if the micropore size is very small and basically close to the diameter of nitrogen molecules, on the one hand, nitrogen molecules are difficult or impossible to enter the micropores, resulting in incomplete adsorption; On the other hand, the adsorption characteristics of gas molecules in pores with the same diameter are very complex, which are affected by many additional factors. Therefore, the adsorption capacity cannot fully reflect the size of the sample surface area. For such samples, argon or krypton gas with smaller molecular diameter or combined with different substrates (such as film or spunbonded fabric) in other situations is generally used as adsorbent to facilitate the adsorption of samples and ensure the effectiveness of test results

test method factors

different test methods will also have a great impact on the test results, and different test methods have their own advantages and disadvantages. In the continuous flow method, due to the principle of comparison, compared with the volumetric method, it can effectively reduce the impact of sample treatment on the test results. Through the comparison method, to some extent, the error caused by the imperfect treatment of the standard sample and the tested sample can be offset, provided that the surface structure and adsorption characteristics of the two samples are similar, and the treatment conditions are the same. This is very valuable for the detection for the purpose of on-site control of product quality. Reducing the sample processing time can greatly improve the detection efficiency. If the same material is used as the standard sample and the tested sample, because the surface structure and adsorption characteristics are similar, the specific surface area test results will be insensitive to the sample processing conditions, in other words, the error will be offset. Therefore, the continuous flow method is very suitable for the on-site inspection of product quality. On the contrary, volumetric method can be said to be very sensitive to sample treatment, because it adopts the principle of absolute adsorption capacity measurement. Any unclean surface or other factors that affect the adsorption process of adsorbate will directly affect the measurement results

in summary, whether the specific surface area and aperture data are tested correctly or not has an important connection with the preprocessing operation, and the performance of the preprocessor must be considered in the selection of analytical instruments

specific surface area is the surface area per gram of solid material, in m2/g; The national standard for specific surface area test is the bet multipoint method of low temperature nitrogen adsorption based on bet theory (the last point of gb/t 195). The key points of measuring the specific surface area by nitrogen adsorption bet multipoint method are: within the range of% nitrogen partial pressure, measure the adsorption capacity of adsorbent (powder to be measured) to nitrogen at different nitrogen partial pressure points, make the adsorption isotherm, and calculate the saturated adsorption capacity of monolayer corresponding to the monolayer when the surface of adsorbent is covered with nitrogen molecules through bet formula, so as to calculate the specific surface area of adsorbent. 3h-2000bet-m specific surface instrument is a high-precision analytical instrument completely based on the national "specific surface measurement" test standard, with 7 leading technologies in China; For example, the only integrated in-situ purging treatment function in China, aiming at the problem of purging treatment between different nitrogen partial pressure points measured by chromatography, makes the purging treatment between different nitrogen partial pressure points more convenient and efficient, and reduces the impact of continuous testing on accuracy; The only program-controlled pneumatic and thermal desorption aid device in China ensures sharp and fast desorption peaks and reduces background error after full automation; The only chromatographic concentration detection system in China makes the detection accuracy of nitrogen partial pressure 10 times higher than that of flow method; Six way valve sampler program control, quantitative tube program control switching; The instrument parameters are displayed on large LCD screen, so that the working state parameters of the instrument are clear at a glance; As well as the functions of liquid nitrogen temperature monitoring, detector gas cut-off protection, detector constant temperature device, important link sound prompt, particle size report, etc., the 3h-2000bet-m specific surface instrument achieves and partially surpasses the performance of similar foreign instruments in terms of test accuracy, stability, safety, operation convenience and so on. It is a new product of the 3h-2000 series specific surface instrument with a large number of customers in China