Type VI isotherms

The majority of physisorption isotherms (Fig. 1.14 Type I-VI) and hysteresis loops (Fig. 1.14 H1-H4) are classified by lUPAC [21]. Reversible Type 1 isotherms are given by microporous (see below) solids having relatively small external surface areas (e.g. activated carbon or zeolites). The sharp and steep initial rise is associated with capillary condensation in micropores which follow a different mechanism compared with mesopores. Reversible Type II isotherms are typical for non-porous or macroporous (see below) materials and represent unrestricted monolayer-multilayer adsorption. Point B indicates the stage at which multilayer adsorption starts and lies at the beginning of the almost linear middle section. Reversible Type III isotherms are not very common. They have an indistinct point B, since the adsorbent-adsorbate interactions are weak. An example for such a system is nitrogen on polyethylene. Type IV isotherms are very common and show characteristic hysteresis loops which arise from different adsorption and desorption mechanisms in mesopores (see below). Type V and Type VI isotherms are uncommon, and their interpretation is difficult. A Type VI isotherm can arise with stepwise multilayer adsorption on a uniform nonporous surface. 

Type VI isotherms are typical of adsorbents having a very uniform nonporous surface. Each step represents an adsorbed monolayer (i.e., noble gas adsorption on graphitized carbon blacks). 

The Type VI isotherm, in which the sharpness of the steps depends on the system and the temperature, represents stepwise multilayer adsorption on a uniform nonporous surface. The step height now represents the monolayer capacity for each adsorbed layer and, in the simplest case, remains nearly constant for two or three adsorbed layers. Amongst the best examples of Type VI isotherms are those obtained with argon or krypton on graphitised carbon blacks at liquid nitrogen temperature. 

When the surface of a nonporous adsorbent is energetically uniform the isotherm may have a step-like shape (type VI). A good example of a type VI isotherm is found in the adsorption of krypton at 90 K on carbon black, graphitized at 2700°C [3], Type VI isotherms are of theoretical interest only. 

The Type VI isotherm, or stepped isotherm, is also relatively rare and is associated with layer-by-layer adsorption on a highly uniform surface. The sharpness of the steps is dependent on the system and the temperature. 

In the sub-monolayer range, three distinctive regions were identified and attributed by Thorny and Duval to 2-D gas , liquid and solid phases. These measurements provided the first unambiguous evidence for the existence of sub-steps in the mono-layer region of a stepwise, Type VI, isotherm. 

Furthermore, the theory is not able to account for the Type VI isotherm, which is the simplest type of layer-by-layer multilayer isotherm. The steps of a Type VI isotherm tend to lose their sharpness as the temperature is raised. However, die results of Prenzlow and Halsey (1957) showed that the mid-point (inflection) of the tread is rather insensitive to change of temperature. This is an indication that in this case it is the step height, and not Point B, which corresponds to the monolayer completion. 

Inspection of the common normalized isotherm in Figure 9.3 reveals a number of distinctive features. At very low plp°, the isotherm is slightly convex with respect to the p/p° axis and it is evident that the linear, Henry s law region does not extend above p/p° 5x 10-4. Although the isotherm is not truly stepwise (i.e. not a true Type VI isotherm), it does exhibit a characteristic monolayer step. This is followed by a wavy second layer region and then a smooth multilayer curve. Thus, as the multilayer coverage is increased, the isotherm appears to conform to the normal Type II shape. 

We conclude that, provided the temperature is not too high, the simplest organic adsorptives can undergo stepwise adsorption on the graphitic basal plane to give well-defined Type VI isotherms. However, most organic adsorptives give Type II... 

The highly distinctive form of a Type VI isotherm is due to a stepwise layer-by-layer adsorption process. Such isotherms are given by the adsorption of simple non-polar molecules (e.g. argon, krypton and xenon) on uniform surfaces (e.g. the basal plane of graphite). The steps become less sharp as the temperature is increased. The vertical risers can be regarded as the adsorbed layer boundaries and the centres of the treads (inflection points) as the layer capacities. When present, sub-steps are associated with two-dimensional phase changes in the monolayer. Useful information concerning the surface uniformity and adsorbate structure can be obtained from the relative layer capacities and the presence of sub-steps. 

Finally, there is the type VI isotherm, in one or more stages conesponding to groups of adsorption sites which are homogenous in terms of energy. 

As already indicated, a weU-defmed step-wise (Type VI) isotherm is obtained when a noble gas or lower hydrocarbon is adsorbed on a basal graphitic surface at an appropriate temperature [7, 11]. The regular steps can extend up to four or five molecular layers, but become less sharp with increased distance from the adsorbent surface. An increase in temperature also produces a progressive blurring of the layer-by-layer adsorption [7]. The appearance of such regular multilayer steps in isotherms on uniform surfaces supports the view that (a) the influence of the surface structure can extend well beyond the first adsorbed layer and (b) the multilayer steps are associated with a form of localized physisorption. 

Such isotherms are shown in Figure B 1,26.4 for the physical adsorption of krypton and argon on graphitized carbon black at 77 K [H] and are examples of type VI isotherms (Figure B 1,26.31. Equation (B 1.26.7)) further... 

The saturation of all adsorption sites on the solid surface (6 = 1) is characterized by a plateau in the isotherm. The Langmuir adsorption isotherm is based on the following assumptions the surface is uniform and every adsorption site is equivalent to the others, the substrate surface is saturated when aU adsorption sites are occupied and monolayer formation has occurred, there are no interactions between the adsorbed particles. In general, physisorption isotherms show various shapes. These are, according to the lUPAC classification, types II and III which describe adsorption on nonporous or macroporous adsorbent with strong and weak gas-solid interaction, respectively. Type IV and V are adsorption isotherms which show typically capillary condensation with hysteresis loops and type VI isotherm shows stepwise multilayer adsorption. 

Type VI isotherms are representative for a stepwise mrrltilayer adsorption on homogeneous solid surfaces. Examples are the adsorption of argon and krypton on graphite at the temperature of liquid nitrogen. 

Physical adsorption is the basis for the various techniques to measure surface area of ceramic powders. The surface area is determined in terms of the amount of the gas adsorbed by a given mass of solid powder at a given temperature, under different gas pressures p. In practice, gases with a fixed volume are used for the powder, so that the amount of gas adsorbed can be identified according to the decrease in pressure of the gas. The amount of gas adsorbed versus p, or p/po, when the gas is at pressures below its saturation vapor pressure po, can be plotted as a graph, which is known as the adsorption isotherm. Figure 4.3 shows the types of these isotherms, according to Brunauer, Emmett and Teller (BET) classification [35-38]. The Type VI isotherm is called stepped isotherm, which is relatively rarely observed, but has special theoretical interest. This isotherm offers the possibility to determine the monolayer capacity of a solid, which is defined as the amount of gas that is required to cover the surface of the unit mass of the solid with a monolayer, so as to calculate the specific surface area of the solid. 

FIGURE 4.6 Schematic plot of a type VI isotherm from lUPAC classification (Sing 1985) as a function of pressure (a) and the logarithm of pressure (b). 

Type VI isotherms present stepwise multilayer adsorbates, the layers becoming more pronounced at low temperatures. 

Krypton is a subcritical vapour at the nitrogen boiling temperature. As such, its adsorption on crystalline surfaces leads to eondensation steps, typical of type VI isotherms according to lUPAC, while its adsorption on rough surfaces is BET-like. Based on this property of krypton adsorption at 77 K, a methodology is proposed to determine the purity of carbon nanotubes samples. The method is tested on model samples obtained by mixing mechanically purified multi-walled carbon nanotubes with various amounts of the same catalyst as used for their synthesis. 

Krypton at 77 K is a subcritical vapour, the adsorption of which on crystalline surfaces is known to give rise to stepped isotherms, classified as type VI by lUPAC [5]. For such vapours, when a given pressure is reached the intermolecular forces between adsorbed molecules overwhelms their thermal energy, by which a phase transition occurs between and adsorbed gas-like and adsorbed dense phases [6, 7]. This phenomenon leads to a riser in the isotherms that corresponds to the complete coverage, at a given pressure, of the surface by a 2D dense phase. Such isotherms have been reported for the adsorption of several subcritical vapours carbon nanotubes [8, 9]. As type VI isotherms are never observed for amorphous solids, even with so-called subcritical vapours [7], we propose to exploit this... 

At the end of this chapter are the / plots that correspond to the types 1-VI isotherms given in Figs. 3-8. In addition a second type VI plot is presented that differs from the one presented in Fig. 8, which has the / plot feature 5. When transformed, types II and in are identical and so are types IV and V. Thus, the x representation cuts down on the number of isotherms to consider and specifies exactly the physical feature that each / plot feature corresponds to. One of the possible type VI isotherms that shows feature 5 in Table 3 above can be distinguished from the pore-filUng feature 3 in the X plot, whereas in the isotherm this discernment is not possible. 

Fig. 20. Type VI isotherm expressed as a standard plot or x plot...

Fig. 21. A standard or x plot of an alternate type VI isotherm (VIA). This is the result of...

Fig. 22. The normal isotherm for the alternate type VI isotherm (VIA) where one can observe the steps due to different E s.

There are six representative adsorption isotherms that reflect the relationship between porous structure and sorption type. This lUPAC classification of adsorption isotherms is shown in Fig. 3 [41]. These adsorption isotherms are characteristic of adsorbents that are microporous (type 1), nonporous and macroporous (types 11, 111, and VI), and mesoporous (types IV and V). The differences between types II and in and between types fV and V arise from the relative strength of fluid-solid and fluid-fluid attractive interactions. When the fluid-solid attractive interaction is stronger than that of fluid-fluid, the adsorption isotherm will be of types II and IV, and the opposite simation leads to types III and V. The type VI isotherm represents adsorption on nonporous or macroporous solid surfaces where stepwise multiplayer adsorption occurs. 

Adsorption isotherms are currently classified in five classes (I - V) according to the Bmnauer, Deming, Deming, Teller (BDDT) original classification, [43] which is often referred to as the Bmnauer, Emmet, and Teller (BET), [44] or simply to as the Bmnauer [45] classification. An extra type of isotherm (the stepped Type VI isotherm, which is relatively rare) is also reported. Type IV and V isotherms typically exhibit a hysteresis loop, which is characteristic of porous systems, involving capillary condensation [32]. 

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