Adsorption hysteresis loop during

Figure 3.3.44 Hysteresis loop during adsorption and desorption of N2 in a q lindrical pore with a radius of 1 nm [Cbet= 100, solid line calculated by Eq. (3.3.49) for pjp < 0.7, dashed curves represent, schematically, capilla condensation and pore draining].

The shape of adsorption-desorption isotherms gives the first pieces of information on a porous solid texture. Existence of an inflexion point in the low-pressure region of the high-resolution isotherm is a sign of the microporous character of a solid [6]. NaY zeolite appears microporous (type I isotherm) regardless of probe molecule and temperature. Similarly, USY zeolite is mainly microporous,but a small hysteresis loop at high P/Po values reveals the presence of some mesopores created during dealumination of Y zeolite [8,9]. 

During desorption, as the relative vapor pressure is reduced, pore solids in which capillary condensation occurs often show a hysteresis loop. The simplest interpretation of this phenomenon is given by the ink-bottle model (6). In the framework of this model (Fig. 12) the adsorption and desorption processes are controlled, respectively, by the void and neck sizes. Thus, desorption from a given pore occurs at a lower pressure than adsorption. 

The type II isotherm is associated with solids with no apparent porosity or macropores (pore size > 50 nm). The adsorption phenomenon involved is interpreted in terms of single-layer adsorption up to an inversion point B, followed by a multi-layer type adsorption. The type IV isotherm is characteristic of solids with mesopores (2 nm < pore size < 50 nm). It has a hysteresis loop reflecting a capillary condensation type phenomenon. A phase transition occurs during which, under the eflcct of interactions with the surface of the solid, the gas phase abruptly condenses in the pore, accompanied by the formation of a meniscus at the liquid-gas interface. Modelling of this phenomenon, in the form of semi-empirical equations (BJH, Kelvin), can be used to ascertain the pore size distribution (cf. Paragr. 1.1.3.2). 

From what is said above, it follows that condensation of a gas in the cylindrical pores of a porous material will occur at a different relative pressure p/po than desorption, leading to a different dependence of vs. p/po during adsorption and desorption (see Fig. 13.10). This leads to a hysteresis loop. From the appropriate Kelvin equation, Eqn. (13.21) and Eqn. (13.23), it follows that, for a t material with identical pores of the diameter r, the point of pore filling and emptying are related by equation (13.24). 

When capillary condensation of nitrogen in pores present in the catalyst proceeds, there is a positive deviation from the f-plot because at a given relative pressure more nitrogen is taken up than is adsorbed on a flat surface. Because the curvature of the meniscus of the capillary condensed nitrogen is different during adsorption and desorption, the sorption isotherm usually exhibits hysteresis, unless only open slit-shaped pores are present. The shape of the hysteresis loop indicates... 

Fig. 1(a) shows N2 adsorption and desorption isotherm of Pt/C. At a relative N2 pressure of 0.4-0.7, an increase in the amount of adsorbed N2 with a hysteresis loop corresponds to the filling of mesopores. This result suggests that not only micropores less than 1 nm but also mesopores were generated under pyrolysis. The BET surface area was calculated to be 623 mVg. The pore size distribution of mesopores was calculated using the BJH model for the desorption branch and is shown in Fig. 1(b). The average pore size was 3.5 nm. The neutral surfactants molecule must play an important role to generate micropores and mesopores during the carbonization. We expect that the existence of mesopores would improve the diffusion of reactants and products in selective CO oxidation. 

Qualitative explanations for the hysteresis in sorption for cellulosic materials have been advanced. Urquhart [23] hypothesized that hysteresis could be caused by a differential availability of hydroxyl groups in cellulose during the adsorption and desorption branches of the RH cycle. A second explanation for hysteresis stems from the observed plasticity of the cellulose gels, which swells upon adsorption. Desorption results in some irreversible (plastic) deformation, which results in a higher moisture content of the structure as compared to the adsorption process. His theory postulated that the lost work during a stress strain cycle is equal to the lost work represented by the hysteresis loop between two relative humidity levels. 

For example, a mesopore with a radius of 10 nm is filled during adsorption for pn2/ p, 2 = 0.97 and drained for a value of 0.94. For a micropore with a radius of 1 nm filling would already occur for Pn2/Pn2 = 0.72 and draining for a value of 0.52. Thus during an adsorption and subsequent desorption experiment we see a hysteresis loop such as shown in Figure 3.3.44 for a pore with a radius of 1 nm and a Cbet of 100. 

Figure 4.61. Types of hysteresis loop (irreversible adsorption-desorption processes) observed during adsorption into mesoporosity. Mesoporous carbons generally belong to the Type-Hl hysteresis loop (Sing et ai, 1985).

With many adsorbents a hysteresis loop occurs between the adsorption and desorption branches of the isotfierm (Hgure 3.1). This is due to capillary condensation augmenting multilayer adsorption at the pressures at which hysteresis is present, the radii of curvature being different during adsorption from the radii of curvature during desorption. Since the desorption branch is thermodynamically more stable than the adsorption branch it is usual to use the desorption branch for pore size determinatioa... 

For a type III, the slope of the adsorption branch, within a great extension of the hysteresis loop, is higher than that corresponding to the desorption branch. Here cooperative phenomena during adsorption are more intense than during desorption. This has not been mentioned by previous authors. 

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