Calorimetry immersion

Since the porosity of carbons is responsible for their adsorption properties, the analysis of the different types of pores (size and shape), as well as the PSD, is very important to foresee the behavior of these porous solids in final applications. We can state that the complete characterization of the porous carbons is complex and needs a combination of techniques, due to the heterogeneity in the chemistry and structure of these materials. There exist several techniques for the analysis of the porous texture, from which we can underline the physical adsorption of gases, mercury porosimetry, small angle scattering (SAS) (either neutrons—SANS or x-rays—SAXS), transmission and scanning electron microscopy (TEM and SEM), scanning tunnel microscopy, immersion calorimetry, etc. 

Guillot A, Stoeckli F, and Bauguil Y. The microporosity of activated carbon fibre KF1500 assessed by combined C02 adsorption and calorimetry techniques and by immersion calorimetry. Adsorpt. Sci. Technol., 2000 18(1) 1-14. 

Many attempts have been made to employ immersion calorimetry and solution adsorption measurements for the determination of the surface area of porous and non-porous materials (see Gregg and Sing, 1967), but in our view insufficient attention has... 

It is with this type of equation that, for instance, Micale el al. (1976) was able to check the consistency of the isosteric approach (from gas adsorption isotherms) with immersion calorimetry, for the water-microciystalline Ni(OH)2 system. 

If properly used, immersion calorimetry is a versatile, sensitive and accurate technique which has many advantages for the characterization of porous solids and powders. An indication of these possibilities is given in Figure 5.7. The major areas of application are outlined in this section and reference made to specific examples which are discussed in more detail in other chapters. 

It should be kept in mind that any change in surface area, surface chemistry, or microporosity will result in a change in the energy of immersion. Because immersion calorimetry is quantitative and sensitive, and because the technique is not too difficult to apply in its simplest form, it can be used for quality testing. The preliminary outgassing requires the same care as for a BET measurement, but, from an operational standpoint, energy of immersion measurements are probably less demanding than gas adsorption measurements. 

As stressed by Jaycock and Parfitt (1981), immersion calorimetry is thus a potential source of fundamental data and is certainly worth further use for this purpose. 

Lyklema (1995) pointed out that, in the absence of immersion calorimetry, the notion of surface hydrophilicity-hydrophobicity remains vague. Once the molar enthalpy of immersion in water is assessed it can readily be compared with the value 44 kJ molwhich corresponds to the enthalpy of condensation of water at room temperature. If it is higher, the surface is considered to be hydrophilic if lower, the surface is defined as hydrophobic. 

Figure 5.8 gives the main types of curve listed by Zettlemoyer and Narayan (1967) for this type of immersion calorimetry experiment with pre-coverage of the sample. Curve (a) is obtained with homogeneous surfaces with respect to the immersion liquid (e.g. chrysotile asbestos in water, Zettlemoyer et ai, 1953). Curve (b) is given by... 

Since the data provided by the above type of immersion calorimetry experiment can be directly related to the results obtained by gas adsorption calorimetry the question arises which type of experiment is to be preferred In fact, immersion calorimetry, although time-consuming, has certain advantages because of the difficulty in handling easily condensable vapours in adsorption manometry. 

Immersion calorimetry has an important and unique role in surface area determination. It can provide relative values, which are particularly useful in the study of... 

Immersion calorimetry was in use over 70 years ago for the characterization of activated charcoals and silica gels. The measurement of heat of wetting appeared to provide a relatively simple way of determining the surface area of a porous adsorbent (Brunauer, 1945). 

Immersion calorimetry provides a very useful means of assessing the total surface area of a microporous carbon (Denoyel et al., 1993). The basic principle of this method is that there is a direct relation between the energy of immersion and the total area of the microporous material. Indeed, for the two model cases of slit-shaped and cylindrical micropores, the predicted maximum enhancement of the adsorption potential (as compared with that of the flat surface of same nature) is 2.0 and 3.68, respectively (Everett and Powl, 1976). These values are remarkably similar to the increased surface area occupied by a molecule in the narrowest possible slit-shaped and cylindrical pores (i.e. 2.0 in a slit and 3.63 in a cylinder). To apply the method we... 

This is an indirect way of assessing the energetics of gas adsorption in micropores. The pre-adsorbed vapour can be that of the immersion liquid or it can be another adsorptive for instance, to study the water filling mechanism in microporous carbons, Stoeckli and Huguenin (1992) devised an experiment with water pre-adsorption prior to immersion calorimetry (in water or in benzene). 

Immersion calorimetry can be used to study either the surface chemistry or the texture of active carbons. A sensitive Tian-Calvet microcalorimeter is adaptable for either purpose, the main difference being in the choice of wetting liquids. 

As already described in Section 6.5.2, the surface area of active carbons can be directly assessed by immersion calorimetry with non-polar liquids (e.g. n-hexane). Satisfactory agreement with BET-nitrogen areas has been found with the supermi-croporous carbons. As expected, because of the unreliability of the BET areas, the ultramicroporous carbons gave poor agreement. However, we consider that this does not invalidate the use of immersion calorimetry. 

Immersion calorimetry has much to offer for the characterization of powders and porous solids or for the study of adsorption phenomena. The technique can provide both fundamental and technologically useful information, but for both purposes it is essential to undertake carefully designed experiments. Thus, it is no longer acceptable to make ill-defined heat of immersion measurements. To obtain thermodynamically valid energy, or enthalpy, or immersion data, it is necessary to employ a sensitive microcalorimeter (preferably of the heat-flow isothermal type) and adopt a technique which involves the use of sealed glass sample bulbs and allows ample time (usually one day) for outgassing and the subsequent temperature equilibration. 

Figure 2.20. DiiTerences between pairs of enthalpies of wetting, obtained from the temperature dependence of the adsorption (open circles) and from immersion calorimetry (closed circles), (a) hexane + hexadecane (b) pentane + decane. Adsorbent Graphon. (Source as in fig. 2.19.)...

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. 

Exchanged zeolites were characterized by N2 adsorption at 77K, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), immersion calorimetry and NHs-temperature programmed desorption (NH3-TPD). X-ray diffraction patterns (XRD) were obtained with a JSO Debye-Flex 2002 system, from Seifert, fitted with a Cu cathode and a Ni filter, using CuXa radiation (A,=1.5419) and 2°min of scanning rate. X-ray photoelectron spectroscopy (XPS) spectra were acquired with a VG-Microtech Multilab 3000 spectrometer equipped with a hemispherical electron analyzer and Mg Ka (1253.6 eV) 300W X-ray source. 

Immersion calorimetry measurements were carried out with a C80 calorimeter from SETARAM. Before the measurement, the samples were outgassed at 523 K during 4h to a final pressure of lO -lO" Torr. Calorimetric experiments were performed at 297.6 K. 

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. 

Water has been widely used as a probe molecule for the characterization of zeolites, especially of those with a high aluminium content [9]. Water adsorption on hydrophilic zeolites has been used to measure their pore volume, and it has been shown that the amount of water adsorbed is a linear function of the aluminium content [10]. Additionally, water adsorption is also highly sensitive to the nature, valence and accessibility of extra-framework cations [11]. Immersion calorimetry allows for the measurement of the degree of interaction between the zeolite and water, and this can be compared with the interaction between the zeolite and other molecules with different polarity. In this way, the polar character of the zeolite surface can be assessed. 

Zn(II) content would be responsible for the decrease in the acid character observed by immersion calorimetry into n-butylamine. This trend in the acid character has also been confirmed by the NH3-TPD experiments. The amount of NH3 desorbed increases from the parent NaX zeolite up to a maximum for the zeolite ZnNaX (60), and decreases thereafter for higher Zn loaxlings. This correlation between both techniques verifies the applicability of immersion calorimetry as a useful and fast technique for acid strength characterization, mainly if liquids with different basicity are used. 

Table 2 Results obtained with nitrogen (gas or liquid) at 77 K immersion calorimetry and gas adsorption...

Comparing BET areas with those derived from immersion calorimetry... 

It seems that immersion calorimetry into liquid nitrogen or liquid argon allows to go one step further in the determination of the internal surface area of micropores. These experiments requested a specially designed calorimeter operating at 77 or 87 K, with the special feature that the brittle end broken to start the immersion is located out of the calorimeter proper and therefore has no effeet on the calorimetric measurement. 

We think that the next step will be to associate the as method with this immersion calorimetry method to evaluate the size of the micropores. 

A considerable number of different techniques has been employed in the past to characterize the porosity and surface chemistry of porous carbon materials. These include gas adsorption (mostly N2 and CO2) [9-14], immersion calorimetry [9], small-angle X-ray [11,15] and neutron [14] scattering, inverse gas chromatography [12,13], differential thermal analysis [12], Fourier transform infrared [12], Raman [16] and X-ray photoelectron [17] spectroscopies and electron spin resonance [16]. It is worth mentioning that the information about the porous structure of the material provided by this array of techniques is only indirect... 

The porous structure of active carbons can be characterized by various techniques adsorption of gases (Ni, Ar, Kr, CO ) [5.39] or vapors (benzene, water) [5,39] by static (volumetric or gravimetric) or dynamic methods [39] adsorption from liquid solutions of solutes with a limited solubility and of solutes that are completely miscible with the solvent in all proportions [39] gas chromatography [40] immersion calorimetry [3,41J flow microcalorimetry [42] temperature-programmed desorption [43] mercury porosimetry [36,41] transmission electron microscopy (TEM) [44] and scanning electron microscopy (SEM) [44] small-angle x-ray scattering (SAXS) [44] x-ray diffraction (XRD) [44]. 

Rowe RC. A quantitative assessment of the reactivity of the fatty alcohols with cetrimide using immersion calorimetry. ] Pharm Pharmacol 1987 39 50-52. 

Use of immersion calorimetry to evaluate the separation ability of carbon molecular sieves... 

Two series of carbon molecular sieves have been prepared from coconut shells, with different pore size distribution. They have been characterised by carbon dioxide adsorption at 273 K and immersion calorimetry into liquids of different molecular sizes. The results have been related with the abihty of the CMS to separate the components of O2/N2, CO2/CH4 and n-C4H4/i-C4H4 gas mixtures. 

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