This is because the capillary condensation causes the N2 molecules to condense and fill the mesoporous pores below atmospheric pressure. Since capillary condensation begins to occur on the liquid surface of the annular adsorption membrane on the pore wall, the desorption is from the orifice The spherical meniscus starts, so that the adsorption and desorption isotherms do not coincide, often forming a hysteresis loop. There is another argument that the contact angle between the liquid nitrogen entering the pores and the material during adsorption is the advancing angle, and the receding angle is the desorption angle. The difference between these two angles leads to a difference when using the Kelvin equation. Of course there may be a joint effect of the two. Individuals tend to agree with the former, at least intuitively (Xuanhu?) The former makes sense. ※ Types of hysteresis loop The characteristics of the hysteresis loop correspond to specific hole structure information, and analysis of this comparison tests the understanding of the Kelvin equation. H1 is a uniform hole model, which can be regarded as a straight hole for easy understanding. However, some students will say that the H1 type hysteresis loop shows that SBA-15 has an ordered hexagonal mesoporous structure when de-spectruming. This is a wrong statement. H1 type hysteresis ring can see ordered mesopores, but it is not known whether it is hexagonal, tetragonal, or triangular. The hexagonal is what is seen by the small angle XRD. This is obviously Zhang Guanli Dai; H2 is more difficult to explain, generally considered to be porous adsorption Caused by the accumulation of qualitative or uniform particles, it is mostly considered to be an "ink bottle." After the liquid nitrogen in the small-bore bottleneck is desorbed, the liquid nitrogen gas bound in the bottle will suddenly escape; H3 compared with H4. Large, it is considered to be a slit hole formed by the accumulation of lamellar particles; H4 is also a slit hole, which is different from the accumulation of particles, and is similar to the holes produced by the layered structure. ※ The medium pressure part has a larger adsorption capacity but does not produce a hysteresis loop When the relative pressure is about 0.2-0.3, according to the Kelvin equation, it can be seen that the pore radius is very small, the effective pore radius is only a few molecules of adsorbate molecules, and capillary condensation does not occur. The adsorption and desorption isotherms coincide. The pore diameter of MCM-41 is 2. There is no hysteresis loop in ordered mesoporous desorption at 3 nm
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