Area enthalpy, of immersion

Denoyel et al. (1993) proposed a method to assess the total area of microporous carbons by immersion calorimetry. It was based on the assumption of the existence of a direct relationship between the enthalpy of immersion and the total area of the solid accessible to the wetting molecules. They used a non-porous carbon black (Vulcan 3) as a reference material to obtain the area enthalpy of immersion, Afijmn, (J m ), of a carbonaceous surface into different liquids. In this way, and considering that the enthalpy of immersion is simply proportional to the surface area available to the immersion liquids, irrespective of its external or microporous character, and whatever the size and shape of the micropores, they obtained the surface areas available to the different liquids using Equation (4.11). 

Figure 4.49. Evolution with bum-off (wt%) of the area enthalpies of immersion (mJ m ) of activated carbons from olive stones into different liquids (Rodrfguez-Reinoso etai, 1985).

In a further study, a series of CMS was prepared from coconut shells by carbonization and activation with carbon dioxide (De Salazar et al., 2000). This series was characterized by carbon dioxide adsorption at 273 K and by immersion calorimetry using liquids with different molecular sizes, dichloromethane (0.33 nm), benzene (0.37 nm), cyclohexane (0.48 nm), 2,2-dimethylbutane (0.56 nm) and a-pinene (0.7 nm). Immersion data were analyzed following the two methods described above. A graphitized carbon black, V3G, with a BET surface area of 62 m g (N2,77 K), was used as a non-porous reference to obtain the area enthalpy of immersion of a carbonaceous surface into the different liquids. With these values, and the enthalpies of immersion of the CMS into the dilferent liquids, the surface areas accessible to the liquids were obtained. These are plotted in Figure 4.50 as a function of the molecular dimension samples are identified by a number that indicates their activation time (De Salazar et al., 2000). 

Immersion calorimetry using polar liquids gives another insight to the characterization of the solid surfaces. Here, specific interactions between the liquid molecules and active centers at the solid surface play a major role. The comparison between enthalpies of immersion of liquids with different polarity provides a refined picture of the surface properties of the solid. This point can be illustrated by the following example. Stoeckli et al. (1983, 1998) measured the enthalpies of immersion of two non-porous illites, with different BET surface areas (N2 77 K) into benzene and water. Whereas the area enthalpy of immersion into benzene was similar for both samples, about 73mJm , the area enthalpies of immersion into water were quite different, 371 and 782 mJ m respectively, Stoeckli et al. (1998). Clearly, the compositions of these illite surfaces differ. 

Figure 4.52. Evolution of the area enthalpies of immersion into benzene, methanol and water as functions of HTT for (a) activated carbons (Rodriguez-Reinoso et at., 1997) and (b) graphite (Barton and Harrison, 1975).

Immersion calorimetry is a very useful technique for the surface characterization of solids. It has been widely used with for the characterization of microporous solids, mainly microporous carbons [6]. The heat evolved when a given liquid wets a solid can be used to estimate the surface area available for the liquid molecules. Furthermore, specific interactions between the solid surface and the immersion liquid can also be analyzed. The appropriate selection of the immersion liquid can be used to characterize both the textural and the surface chemical properties of porous solids. Additionally, in the case zeolites, the enthalpy of immersion can also be related to the nature of the zeolite framework structure, the type, valence, chemistry and accessibility of the cation, and the extent of ion exchange. This information can be used, together with that provided by other techniques, to have a more complete knowledge of the textural and chemical properties of these materials. 

As described above, immersion calorimetry constitutes a powerful technique for the textural and chemical characterization of porous solids. In the absence of specific adsorbate-adsorbent interactions, heats of immersion can be related to the surface area available for the molecules of the liquid. However, the use of polar molecules or molecules with functional groups produces specific adsorbent-adsorbate interactions related to the surface chemical properties of the solid. An adequate selection of the immersion liquid can be used to study hydrophilicity, acid-base character, etc. Table 2 reports the enthalpies of immersion (J/g) into different lineal and branched hydrocarbons (n-hexane, 2-methyl-pentane and 2,2-dimethyl-butane) for Zn exchanged NaX zeolites. 

More polar adsorbents (such as most oxides) are not easily amenable to a similar procedure because in polar liquids they give rise to specific interactions contributing to the enthalpy of immersion and modifying - in an a-priori unknown manner -its relationship with the surface area. Thus specific interactions can not be the same with the two walls of a slit shaped micropore containing just one molecule. 

The prerequisite for determining meaningful areal enthalpies of immersion is of course an independent assessment of the surface area really wetted by the immersion liquid. This is why, in Table 5, we only give them for the two reference samples which are known to be neither micro- nor meso-porous, ie for which the BET surface area can be expected to be reliable. 

Liquid nitrogen and liquid argon provide a very similar areal enthalpy of immersion of carbons for instance, 165 and 160 mJ. m in nitrogen and argon, respectively, if the surface area of the reference material is measured by the BET method with nitrogen at 77 K. 

Now, the enthalpy of immersion of the silica samples into liquid nitrogen is systematically higher than into liquid argon but this should not influence the derivation of the immersion surface area provided the reference sample is correctly selected. 

Table 1 also reports the specific enthalpies of immersion (J g- ) of the different CMS into liquids with different molecular size dicholomethane (CH2CI2, 0.33 nm), benzene (CeHs, 0.37 nm), cyclohexane (CeHi2, 0.48 nm), 2,2-dimethylbutane (2,2-DMB, 0.56 nm) and a-pinene (0.70 nm). These values can be analysed in different ways to obtain the pore size distribution of the CMS. On one hand, the areal enthalpy of wetting (per square meter of surface) of a given liquid for a carbon surface can be obtained by using a nonporous carbon of well-known surface area as reference. Theoretical and experimental evidence has been given to support the assumption that the immersion enthalpy is simply proportional to the surface area available to the immersion liquid, irrespective of the micropore... 

The measurement of the heat of immersion of a "dry" material in different liquids can permit a rapid and accurate determination of the surface area and pore size distribution below 10 A. The enthalpy change is related to the extent of the solid surface, to the presence of micropores and to the chemical and structural nature of the surface. The technique has been mainly applied to carbons [64]. The immersion liquid is usually water for hydrophilic oxides like mineral oxides, or an organic liquid (benzene, n-hexane) for hydrophobic solids like carbons. One of the limitations of this technique is that the specific enthalpy of immersion of the open surface must be determined with a non-porous standard material of surface composition similar to the porous solid studied. The non-microporous part of the surface area can be determined by prefilling the micropores with an absorbate prior to immersion. Information on the size of micropores can be obtained from the kinetics and enthalpy of immersion into a set of liquids with increasing molecular size [5]. 

Figure 25.4 Correlation between the enthalpy of immersion into water, the total surface oxygen, the basic groups titrated with HCI, the micropore filling, and the wetting of the nonporous surface area. (Adapted from Ref. [23].)...

Experimental enthalpies of immersion (-AHinm) at 298 K into different liquids of Nomex-derived carhon fibers steam-activated to different bum-offs. Surface areas derived from them, and from N2 adsorption at 77 K Sbei)- Reproduced from Ref. 46, with permission from Elsevier. 

Caknimetric techniques have been used to measure the surface area and to study surface reaclivily of both gas/solid and lic]ukl/solid systems. It is well known that the measurement of foe enthalpy of adsorption can lead to very interesting information about foe mechanism of adsorption pnx ess as well as about foe energetic and structural heterogeneity of adsorbents. The enthalpy of immersion is related to foe formation of an adsorbed layer of foe molecules of Ite liquid on foe surface of foe solid. When non-specific interactions between foe surface of... 

Eq. (4.2-28) or (4.2-29) are directly applicable to strictly microporous solids. For solids having high mesopore surface area, the specific enthalpy of immersion must allow for the heat associated with the open surface, that is... 

FIGURE 2.1 Relationship between the specific surface area of nanooxides and the enthalpy of immersion of them in water. 

The structural features of mixed oxides with a mosaic surface of nanoparticles, including patches of different oxide phases or a solid solution of a lower concentrated oxide in a more concentrated oxide, can strongly affect the interfacial phenomena in any media. For instance, the enthalpy of immersion in water (Table 2.1 is greater for mixed nanooxides than nanosilica. However, surface nonuniformity and the differences in the properties of a variety of surface sites result in a significant scatter in the relationship between the and 5bex values (Figure 2.1), despite a tendency of a decrease in the AH value (calculated per surface area unit) with increasing value. 

If measurements are made over a range of temperature, then the differences (Awh—between the enthalpy of immersion of unit area of solid in solution and pure liquid 2 can be calculated ... 

Stoeckli and Centano (1997) showed that, for carbons of low external surface areas, the ratios between the limiting volumes filled by liquids of variable molecular dimension and the micropore volume accessible to a small molecule (when used as a reference) can be closely estimated from enthalpies of immersion. This approach describes the development of porosity during an activation process and the RSDs of the developed microporosity. As a wetting agent, Stoeckli and Centano (1997) used benzene because of the similarity of molecular size with n-butane which cannot be used as a liquid. The majority of the micropores were <0.8 nm in access dimension. 

Extent of the solid surface. For a given liquid-solid system, the enthalpy of immersion increases with increasing surface area of the carbon being studied. 

The most important feature of this approach is the assumption of simple proportionality between the surface area and the enthalpy of immersion, irrespective of the role played by micropores in the enhancement of the adsorption potential. It is established that, for slitshaped micropores in which only one molecule of the wetting liquid can be accommodated, there is a twofold increase of the adsorption potential as compared with that in an open surface, and this would be also reflected in the enthalpy of immersion. However, such a pore has two opposite walls and, therefore, the surface area interacting with the liquid molecule is also twice that of the open surface. It follows that, in the case of... 

On the other hand, the calculated enhancement of the adsorption potential in a cylindrical micropore acconunodating a spherical molecule of the same diameter (2r) is 3.679. This value is very close to 3.627, which is the ratio between the area of the pore covered by the molecule (Anr ) and its mean cross-sectional area when it is adsorbed in a hexagonal compact assembly on an open surface (1.10267rr ). Thus, also in this case, the enthalpy of immersion is expected to be practically proportional to the accessible surface area. 

Following this equation, the relation between the product nd the microporous surface area will not be always the same, but depends on the average pore width L. Thus, for a highly microporous activated carbon, or any other adsorbent with slit-shaped pores with a low external surface area, the enthalpy of immersion is proportional to the product EqWq but not to the microporous surface area 5. Anyway, Eq can be estimated from Equation (4.15) for a given liquid, this allowing for the calculation of micropore size distributions by using liquids with different molecular sizes, Stoeckli et al. (1990). 

The difficulty which 2,2-dimethylbutane and iso-octane have in accessing the whole microporosity of the less activated carbons is clearly evident from data shown in Figure 4.49 where the plotted data correspond to the enthalpies of immersion as in Figure 4.48 but divided by the BET surface area (N2, 77 K) of the different samples. The area enthalpies... 

In summary, immersion calorimetry offers rapid and reproducible methodologies for the measurement of enthalpies of immersion which can be converted to PSDs, assessments of surface functionality and to units of surface area, noting the limitations which apply to this concept. 

Table 3 reports the enthalpy of immersion (-AHi, J g ) of the carbons in hexane and in 1-hexene at 303 K, after out-gassing under vacuum at 523 and 773 K for 4 h. The areal enthalpies (mJ m ) calculated taking into account the sinface area obtained for N2 adsorption are also reported. For all the samples, the enthalpy of immersion in 1-hexene is higher than in hexane. Comparing the areal en alpies of immersion in the two liquids, the values increase with the degree of strength of oxidation. 

Denoyel et al [4] proposed a different approximation focused on the determination of the surface area of porous materials fi om immersion calorimetry measurements. It is based on geometrical considerations so that they conclude that the immersion enthalpy is proportional to the surface accessible to the liquid probe and that it is independent of the shape and size of the ptores. The areal enthalpy of immersion, hi (J.m ), for a non-porous carbon black into each wetting liquid is used as reference to determine the surface area wetted hy the liquid probe in a porous carbon. 

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