Classification of adsorption isotherms and analysis of adsorption mechanisms

Adsorption isotherms are a source of information about the pore structure, adsorption heat, and other physical and chemical characteristics of adsorbents. The adsorption isotherm of the adsorbate can be obtained under constant temperature and a wide range of relative pressures. In order to better understand the information contained in adsorption isotherms, the following is a brief introduction to the classification and adsorption mechanism of adsorption isotherms [1,8,10,19,20,33,53-55]: Numerous adsorption isotherms can be classified into six types (IUPAC classification), as shown in Figure 1-11. For microporous adsorbents with a very small surface area, their adsorption behavior is characterized by an I-type adsorption isotherm. The I-type adsorption isotherm is concave with the partial pressure P/Po line and is characterized by the formation of a platform, which is horizontal or nearly horizontal. As the saturation pressure reaches the adsorption isotherm or directly with P/Po = Intersection or manifested as a 'tail'. The initial part of the adsorption isotherm represents the filling process of narrow micropores in the adsorbent, and its ultimate adsorption capacity depends on the accessible micropore volume rather than surface area. At higher relative pressures, the slope of the platform is caused by multi-layer adsorption on non microporous surfaces (such as mesopores or macropores and outer surfaces). Type II adsorption isotherms are normally caused by non porous or macroporous adsorbents, resulting in loosely single-layer to multi-layer adsorption. The existence of inflection points indicates the transition from single-layer adsorption to multi-layer adsorption, that is, the completion of single-layer adsorption and the beginning of multi-layer adsorption. Type III adsorption isotherms are typically associated with weaker adsorbent adsorbate interactions and stronger adsorbate adsorbate interactions. In this case, the synergistic effect leads to the formation of multiple layers of adsorption before a uniform single adsorption layer is completed, resulting in a rapid increase in adsorption capacity as adsorption progresses. The interaction between adsorbates plays a crucial role in the adsorption process. The adsorption of water vapor on non porous surfaces is an example of type III adsorption isotherms. The obvious feature of type IV adsorption isotherm is the existence of hysteresis loop, which is closely related to the occurrence of capillary condensation, and maintains a constant adsorption capacity in a high and wide range of partial pressures. Its initial part is similar to type II adsorption isotherm, corresponding to single-layer to multi-layer adsorption on the mesoporous wall. Some mesoporous or microporous carbons exhibit V-shaped water in few adsorbents

Adsorption isotherms, like type III adsorption isotherms, have very weak interactions between adsorbents and adsorbates compared to interactions between adsorbates and adsorbates, including the formation of hydrogen bonds between water molecules. The VI type adsorption isotherm is equivalent, but has special theoretical significance. It represents the multi-layer adsorption gradually formed on a uniform non porous surface such as graphitized carbon, and each step height provides different adsorption capacities of the adsorption layers.

Fig．1-11 IUPAC classification of adsorption isotherms

Figure 1-11 Adsorption isotherm types classified by the Union of Pure Theory and Applied Chemistry (IUPAC)

The adsorption isotherm is the superposition of adsorption by different pore sizes in proportion to their total surface area or pore volume. For narrow slit pores, the adsorption isotherm N (P) can be expressed by the following equation [56-60]:

among which

N (P) is the number of adsorbed moles at a relative pressure of P;

Hmin and Hmax are the smallest and largest pore sizes in the adsorbent;

ρ (P, H) is the molar density of N when the pore size is H and the relative pressure is P;

F (H) is the pore volume function corresponding to the pore width H, which characterizes the pore size distribution of the adsorbent.

Evans [58] and colleagues used the statistical thermodynamic approximation method of mean field density function theory to study the properties of liquids adsorbed in pores Density Functional Theory conducted research. The theory of mean field density function is an approximate theory that divides the interactions between liquid molecules in a homogeneous liquid phase into short-range repulsion and long-range attraction. The influence of remote interactions on liquid properties can be approximated by the theory of mean field density function; The short-range repulsive effect is modeled through the equivalent arrangement of hard balls. The theory of mean field density function is equivalent to the adsorption model proposed based on thermodynamic analysis methods for predicting the adsorption equilibrium phase in larger pores. However, the description of adsorbed molecules in smaller pores is closer to the real situation, especially in predicting the thickness of the adsorption layer on the pore wall and showing the change from capillary condensation to pore filling at the critical pore size. The intermolecular interactions are calculated using the Lennard Jones potential energy equation as previously described. In 1989, Seaton et al. used density function theory to measure the pore size distribution of carbonaceous adsorbents from micropores to mesopores. follow

Later, Oliver and Conklin [571] (Micromeritics) Ins. Corp. proposed a more general calculation method, which uses non local or uniform density approximation (Non local) or smoothed density The regularization method of approximation is further expanded to the macroporous range (0.4nm to 400nm). The author [55] and some foreign scholars [59-62] used DFT method to characterize the pore size distribution of carbonaceous adsorbents. The results showed that DFT is a simple algorithm based on high-resolution adsorption isotherms, and only one method is needed to characterize the pore range (micropores to macropores) of the entire porous solid. In addition, the commonly used molecular simulation method currently includes Monte Carlo method [63-65], which is similar to the DFT method mentioned above and will not be further elaborated here. In summary, based on current research on carbonaceous adsorbents and papers published in relevant journals, molecular simulations and pore size distribution of carbonaceous adsorbent adsorption are currently hot topics in research.