Classification of the hottest nbsp adsorption isot

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nbsp; Classification of adsorption isotherms and brief analysis of adsorption mechanism nbsp; nbsp;

The adsorption isotherm of BR is the source of information about the pore structure, adsorption heat and other physicochemical characteristics of adsorbents. Under the condition of constant temperature and wide range of relative pressure, the adsorption isotherm of the adsorbate can be obtained. In order to better understand the information contained in the adsorption isotherms, the following is a brief introduction to the classification of adsorption isotherms and adsorption mechanism [1, 8, 10, 19, 20, 33, 53 ~ 55]: many adsorption isotherms can be divided into six types (IUPAC classification), as shown in the figure is the type of adsorption isotherms. For microporous adsorbents with a very small surface area, the adsorption performance is type I adsorption isotherm. The type I adsorption isotherm and the partial pressure P/PO line are concave and high-frequency experiments are characterized by the formation of a platform, which is horizontal or nearly horizontal. As the saturation pressure reaches, the adsorption isotherm either directly intersects with P/po=1 or shows a "tail". The initial part of the adsorption isotherm represents the filling process of narrow micropores in the adsorbent, and its limit adsorption capacity depends on the accessible micropore volume rather than the surface area. The slope of the platform under high relative pressure is caused by multilayer adsorption on non microporous surfaces (such as mesopores or macropores and outer surfaces). The normal type II adsorption isotherm is a loose single-layer to multi-layer adsorption caused by nonporous or macroporous adsorbents. The existence of inflection point indicates the transition from monolayer adsorption to multilayer adsorption, that is, the completion of monolayer adsorption and the beginning of multilayer adsorption. The type III adsorption isotherm is usually related to the weak adsorbent adsorbate interaction and the strong adsorbent adsorbate interaction. In this case, the synergistic effect leads to the formation of multi-layer adsorption before the completion of a uniform single adsorption layer, so the adsorption capacity increases rapidly with the progress of adsorption, The interaction between adsorbate and adsorbate plays an important role in the adsorption process. Water vapor adsorption on non porous surfaces is the best example of type III adsorption isotherms. The obvious feature of type IV adsorption isotherm is that it has a hysteresis loop, which is closely related to the occurrence of capillary condensation, and maintains a constant adsorption capacity in a high and wide partial pressure range. Its initial part is similar to type II adsorption isotherm, which corresponds to single-layer to multi-layer adsorption on the mesopore wall. Some mesoporous or microporous carbons in few adsorbents show V-type water "br"

adsorption isotherms. Like type III adsorption isotherms, the interaction between adsorbent and adsorbate is very weak compared with that between adsorbent and adsorbate, which of course includes the formation of hydrogen bonds by water molecules. Type VI adsorption isotherms are quite rare, but they have special theoretical significance. They represent multilayer adsorption gradually formed on homogeneous non porous surfaces such as graphitized carbon, and each step height provides the adsorption capacity of different adsorption layers. Br





Fig, The adsorption isotherm n (P) can be expressed by the following formula [56 ~ 60]: "br"

where "br"

n (P) is the number of adsorption moles when the relative pressure is p; Br

Hmin and Hmax are the smallest and largest pore sizes of adsorbents; 《br》

ρ (P, H) is the molar density of n when the aperture is h and the relative pressure is p; Br

f (H) is the pore volume function corresponding to the pore width h, which characterizes the pore size distribution of the adsorbent. "Br"

Evans [58] and his colleagues studied the performance of the liquid adsorbed in the pores by using the approximate method of statistical thermodynamics, namely the mean field density function theory. The mean field density function theory is an approximate theory that divides the interaction between liquid molecules in the mean liquid phase into short-range repulsion and long-range gravitation. The effect of long-range interaction on liquid properties can be treated approximately according to the theory of mean field density function; The short-range repulsion effect is modeled by the equivalent arrangement of hard spheres. The prediction of the adsorption equilibrium phase in the larger pores by the mean field density function theory is equivalent to the adsorption model proposed based on the thermodynamic analysis method, while the description of the adsorbed molecules in the smaller pores is closer to the real situation. In particular, it can predict the thickness of the adsorption layer on the pore wall, and can show the change from capillary condensation to pore filling under the critical pore size. The interaction between molecules is calculated by Lennard Jones potential energy equation as described above. In 1989, seaton[56] and others first used the density function theory to measure the pore size distribution of carbon adsorbent from micropore to mesopore. With br

, Oliver and conklin[571] (micromeriticsins.corp.) proposed a more general calculation method. They used the regularization method of nonlocal or uniform density approximation (nonlocalorsmootheddensityapprox is 28 pin dual in-line package image) to further expand to the large hole range (0.4nm to 400nm). Using DFT method, the author [55] and some foreign scholars [59 ~ 62] characterized the pore size distribution of carbonaceous adsorbents. The results show that DFT is a simple algorithm based on high-resolution adsorption isotherm, and it can characterize the pore range (micropore to macropore) of the whole porous solid with only one method. In addition, the current general molecular simulation method is Monte Carlo method [63 ~ 65], which is similar to the aforementioned DFT method and will not be repeated here. In a word, from the current research of carbon adsorbents and the papers published in relevant publications, the molecular simulation and pore size distribution of carbon adsorbents are the current research hotspots. Br

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