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氮气吸附脱附曲线的种类都有哪些？
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请教大家： 做完了样品的比表面积测定，但是不太会分析氮气的脱附吸附曲线，现在想找到关于氮气脱附吸附曲线的类型，请大家帮忙，谢谢！！ [search]吸附脱附曲线[/search] |
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liudanokokok
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buffaloli(金币+3,VIP+0):多谢，辛苦了！
buffaloli(金币+3,VIP+0):多谢，辛苦了！
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主要有五种（可参见附件PDF文件） Experimental gas adsorption isotherms usually fall into six categories,out of which five (Types I-V according to the IUPAC classification,65 see Figure 1) are relevant to the present discussion. Type I isotherms exhibit prominent adsorption at low relative pressures (the relative pressure is defined as the equilibrium vapor pressure divided by the saturation vapor pressure) and then level off. Type I isotherm is usually considered to be indicative of adsorption in micropores or monolayer adsorption due to strong adsorbentadsorbate interactions (which may be the case for chemisorption, which involves chemical bonding between the adsorbate and the adsorbent surface; we will not discuss chemisorption here). It should be noted that pores are classified herein on the basis of their diameter (or width) as micropores (below 2 nm), mesopores (between 2 and 50 nm), and macropores (above 50 nm).65 In the case of nonpolar gases commonly used for characterization of porous solids (nitrogen, argon),64-68 chemisorption is unlikely and therefore a classical interpretation would associate Type I with microporosityHowever, Type I isotherms may also be observed for mesoporous materials with pore sizes close to the micropore range. In particular, in the case of adsorption of N2 at 77 K or Ar at both 77 and 87 K in cylindrical pores, a Type I isotherm would have to level off below the relative pressure of about 0.1 for the material to be exclusively microporous, as inferred from the results of recent studies of siliceous OMMs. Consequently, when a Type I isotherm does not level off below the relative pressure of 0.1, the sample is likely to exhibit an appreciable amount of mesopores or even be exclusively mesoporous. However, such Type I behavior may be indicative of some degree of broadening of the mesopore size distribution. This is because materials with highly uniform cylindrical pores may exhibit discernible steps on adsorption isotherms (and therefore these isotherms are classified as Type IV, as discussed later) at relative pressures down to 0.1 or perhaps even lower (for N2 at 77 K and Ar at 77 and 87 K).Type I isotherms are quite common for OMMs with organic groups bonded to a silica framework, both prepared via chemical bonding47 and co-condensation. Adsorption on many macroporous solids proceeds via multilayer formation in such a manner that the amount adsorbed increases gradually as the relative pressure increases, although the multilayer buildup close to the saturation vapor pressure may be quite abrupt. This unrestricted multilayer formation process gives rise to Type II and III isotherms. In this case, the adsorption and desorption branches of the isotherm coincide; that is, there is no adsorption-desorption hysteresis. Depending on the surface properties of a given solid, there may be a pronounced stage of the monolayer formation (Type II) or the adsorption isotherm may be convex in the whole pressure range (Type III). The latter behavior can be observed when lateral interactions between adsorbed molecules are strong in comparison to interactions between the adsorbent surface and adsorbate. N2 adsorption isotherms similar to Type II were reported for several as-synthesized (surfactant-containing) OMMs. Type III adsorption isotherms were reported for water adsorption on certain OOINs with hydrophobic surfaces. Adsorption on mesoporous solids proceeds via multilayer adsorption followed by capillary condensation (Type IV and V isotherms). Therefore, the adsorption process is initially similar to that on macroporous solids, but at higher pressures the amount adsorbed rises very steeply due to the capillary condensation in mesopores. After these pores are filled, the adsorption isotherm levels off. Capillary condensation and capillary evaporation often do not take place at the same pressure, which leads to the appearance of hysteresis loops. However, it was suggested long ago and unequivocally confirmed after the discovery of OMMs that the capillary condensation-evaporation in mesopores may also be reversible (this behavior will be denoted herein as Type IVc). Type IV in general, and IVc in particular, is typical for many OOINs with accessible mesopores, although when the size of the pores is close to the micropore range or PSD is broad, Type I isotherms can be observed. The distinction between Types IV and V is analogous to that between Types II and III. Finally,it should be noted that some OOINs may exhibit adsorption isotherms that can be regarded as a combination of the aforementioned five types of isotherms as a result of the presence of several different types of pores in the structure. [ Last edited by liudanokokok on 2008-5-19 at 17:02 ] |
2楼2008-05-19 16:58:57
simonzrh
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3楼2008-05-19 17:16:08
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