Physical adsorption test analysis - Database & Sql Blog Articles

â—†hysteresis loop
1. The reason for the hysteresis loop is that the capillary condensation causes the N2 molecules to condense and fill the mesoporous pores below the normal pressure, and the capillary condensation occurs at the liquid surface of the annular adsorption membrane on the pore walls. The desorption is started from the spherical meniscus of the orifice, so that the adsorption and desorption isotherms do not coincide, often forming a hysteresis loop. There is another saying that the contact angle between liquid nitrogen entering the channel and the material during adsorption is the advancing angle, and the retreat is the receding angle. These two angles lead to differences in the Kelvin equation. Of course, there may be a combination of the two, individuals tend to identify the former, at least intuitively (Xuan Ji?) the former can make sense.
2. Types of Hysteresis Loops The characteristics of the hysteresis loop correspond to specific pore structure information, and this comparison is tested to understand the Kelvin equation.
H1 is a uniform hole model that can be considered as a straight hole for easy understanding. However, some students will say that the S1-15 has an ordered hexagonal mesoporous structure from the H1 hysteresis loop, which is a mistake. The H1 type hysteresis loop can be seen as an ordered mesopores, but it is not known whether it is hexagonal, tetragonal, or triangular. The hexagonal is what the small angle XRD sees. This is the obvious Zhang Guan Li Dai;
H2 is more difficult to explain. It is generally considered to be caused by porous adsorbate or uniform particle packing pores. It is considered to be “ink bottle”. After liquid nitrogen desorption in a small pore size bottleneck, the liquid nitrogen gas bound in the bottle will suddenly escape. ;
H3 has a larger adsorption amount at the high pressure end than H4, and is considered to be a slit hole formed by the accumulation of flaky particles;
H4 is also a slit hole, which is different from particle stacking and is a hole similar to that produced by a layered structure.
3. When the medium pressure part has a large adsorption amount but does not produce a hysteresis loop, when the relative pressure is about 0.2-0.3, according to the Kelvin equation, the pore radius is small, and the effective pore radius is only a few adsorbent molecules. Capillary agglomeration occurs, and the adsorption and desorption isotherms coincide. When the pore size of MCM-41 is 2 or 3 nm, there is no hysteresis loop in the ordered mesoporous adsorption and desorption.
â—†The mesoporous analysis usually adopts the BJH model (Barrett-Joiner-Halenda), which is the application of the Kelvin equation in the cylinder model. It is suitable for the mesoporous range, and the obtained result is smaller than the actual one.
KJS (Kruk-Jaroniec-Sayari) with higher precision for MCM-41 and SBA-15 pore structure analysis and its modification method. When KJS came out, the highly ordered MCM41 was used as the material for pore analysis, combined with XRD results. There is a higher precision KJS equation than BJH, and the applicable pore size analysis range is between 2 and 6.5 nm. Later, it was promoted to make it have a larger scope of application, which can be used for the characterization of SBA-15 pore structure (4.6-30nm).
◆About the t-Plot and αs methods is the treatment method for the whole adsorption or desorption curve. t-Plot can be understood as the thickness pattern method, and the adsorption amount of the monolayer is plotted by the nitrogen adsorption amount, and the adsorption film formed during the aggregation is formed. The average thickness is the average number of adsorbed layers multiplied by the thickness of the monolayer (0.354 nm), specific surface area = 0.162 * monolayer adsorption amount * Avogadro constant. When the sample is a non-porous material, t-Plot is a straight line passing through the origin. When the sample contains micropores, mesopores, and large pores, the straight line becomes several fold lines, which need to be analyzed separately. The subscript in the αs method is standard. Sing proposes to use the amount of adsorption at a relative pressure of 0.4 instead of the amount of adsorption of the monolayer, and then to map, using this method to specify a standard or to do it on the instrument. A standard, processing method and graphical interpretation are similar. The two can be transformed into each other, t=0.538αs
â—†Microporous analysis Microporous analysis of microporous materials has different requirements for vacuum degree, control system and temperature sensor. The test time is also longer, and the time may be ten times or even twenty times that of ordinary samples. Due to the limited difference between micropore size and probe molecular size, some microporous probe molecules can not enter, the analytical method should be determined according to different samples. If necessary, refer to relevant literature methods for reference, and then do a batch of samples. It is an analytical method, and the trend of the results is mostly correct. It is still difficult to analyze the pore size distribution of all ranges with a model. The theory of nonlinear density generalized theory (NLDFT) is ok, but it is used less in the paper.