Immersion calorimetry activated carbons

Stoeckli and Kraehenbuehl [42] discussed the derivation of an exact expression for the enthalpy of immersion of activated carbons using Dubinin s theory as a starting point They tested this expression with experimental data for 10 different carbons immersed in benzene and -heptane. In a subsequent paper, Kraehenbuehl et al. [43] reported the use of immersion calorimetry to determine the micropore size distribution of carbons in the course of then-activation. Later on, Stoeckli and Centeno [44] pointed out that immersion calorimetiy is a useful tool for characterizing solid surfaces in general, but in the case of microporous solids it usually requires complementary information obtained from the adsorption isotherms. They also discussed the limitations and possibilities of the technique and recommended that at least one adsorption isotherm from the vapor phase (e.g., CH2CI2 or CsH ) be determined to remove all the uncertainties. 

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. 

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. 

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]. 

Immersion calorimetry can be apply successfully to the characterisation of CMS to evaluate their pore size distribution and, in this way, their ability to separate gas mixtures as a function of their molecular size. On the other hand, carbon molecular sieves can be prepared from coconut shells by activation with C02. These materials can be used for the separation of gas mixtures such as O2/N2, CO2/CH4 and n-C4Hio/i-C4Hio. 

The porous structure of chars from a high volatile bituminous coal from mine Pumarabule in Spain, initial and preoxidized, then steam activated, was characterized by carbon dioxide and benzene adsorption measurements, as well as by immersion calorimetry molecular probes with increasing critical dimensions were used. The influence of preoxidation of the coal on the values of parameters describing the pore size distribution, with particular attention to micropores, evaluated according to each of the applied methods, is discussed. 

Adsorption calorimetry, based on the use of different adsorbates, is now employed to probe the effects of various types of surface modification treatments on the surface chemistry of sohds. The technique is employed in particular to investigate and characterize activated carbons. Recent studies show good agreement between the results from this technique and immersion calorimetry. 

Stoeckli, F., Moreno-Castilla, C., Carrasco-Marin, F., and L6pez-Ram6n, M.V. (2001). Distribution of surface oxygen complexes on activated carbons from immersion calorimetry, titration and temperature-programmed desorption techniques. Carbon, 39(14), 2235-7. 

The heat of immersion is a parameter that is measured directly in a calorimeter, while the surface energy of a solid is not easily measured. Indeed, the heat of immersion provides an indirect measure of the surface energy it also provides information on the surface heterogeneity of carbonaceous solids. F.arly work on this subject has been reviewed by Zettlemoyer and Narayan [39]. The use of immersion calorimetry to characterize the porous texture and also the surface chemistry of activated carbons has been reviewed by Rodriguez-Reinoso and coworkers [40,41]. 

Denoyel et al. [45] derived the pore size distributions of two sets of activated carbons (one activated in water vapor and the other activated with phosphoric acid) using immersion calorimetric data. They concluded that immersion calorimetry is a convenient technique to assess the total surface area available for a given molecule and the micropore size distribution. More recently, Villar-Rodil et al. [46] have followed this approach to characterize the porous texture of a series of NomexO-derived carbon fibers activated to various bum-offs using liquids with different molecular dimensions as well as N2 and CO2 adsorption Isotherms. Table 3 includes the immersion enthalpies and corresponding surface areas. Relative changes in surface area accessible to the different adsorbates were ascribed to... 

Also using immersion calorimetry, Bagreev and Tarasenko [312] showed the reversible character of halide sorption on activated carbon and the increase in the anion affinity of the activated carbon surface in the series F[Pg.208]

However, there is uncertainty about this method because of networking effects of some adsorbents including activated carbons and carbon nanostructures. Other experimental techniques that usually implement for characterizing the pore stmcture of porous materials are mercury porosim-etry. X-ray diffiaction (XRD) or small angle X-iay scattering (SAXS), and immersion calorimetry. 

The Microporosity of Activated Carbon Fibre KF 1500 Assessed by Combined CO2 Adsorption and Calorimetry Techniques and by Immersion Calorimetry,... 

The specific questions now required to be answered concern the dimensions of the effective porosity and how to measure them. The method of analysis of pore filling by nitrogen at 77 K, at ptp values >0.6 cannot be used, obviously. Centeno etal. (2003) indicate that immersion calorimetry provides the means to measure micropore size distributions within an activated carbon (Section 4.7). They use a rearrangement of Equation (4.3) to calculate the micropore volume (Wo(Lc)) filled by a liquid of critical molecular dimensions, L, using ... 

Characterization of microporous adsorbents by immersion calorimetry is not as straightforward as for non-porous adsorbents. Atkinson et al. (1982) measured the heats of immersion of a microporous carbon cloth and a microporous activated carbon in a series of organic liquids and, for a given solid, obtained a significant dependence of the heat of immersion with the liquid used. They concluded that the heat of immersion is a measure of the volume of pores accessible to the molecule of the immersing liquid, thus opening the possibility of using immersion calorimetry as a tool to obtain PSDs in microporous carbons. 

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). 

This confirms the convenience of determining the surface area of activated carbons using immersion calorimetry in benzene when a non-porous carbon is used as the reference, independently of the chemical nature of the carbon. 

Kraehenbuehl F, Stoeckli HE, Addoun A, Ehrburger P, Donnet JB. The use of immersion calorimetry in the determination of micropore distribution of carbons in the course of activation. Carbon 1986 24(4) 483 88. 

Carbon molecular sieves (CMS) have been known and used for some time before the year 2000 when G6mez-de-Salazar et al. (2000) (University of Alicante) reported on the preparation of CMS using a different approach, that of controlled oxidations, as well as providing an overview of this specialized form of activated carbon. The separation ability of these CMS was studied further using immersion calorimetry by Gomez-de-Salazar et al. (2000) (Section 4.7) followed by a study of the use of pyrolytic carbon deposition in the preparation of CMS materials by G6mez-de-Salazar et al. (2005). 

Gonzdlez MT, Sepdlveda-Escribano A, Molina-Sabio M, Rodriguez-Reinoso F. Correlation between surface areas and micropore volumes of activated carbons obtained from physical adsorption and immersion calorimetry. Langmuir 1995b l 1 2151-2155. 

The micropore size distribution of some activated carbons, as deduced from immersion calorimetry, is plotted in Figure 6.6. The volume deduced from the adsorption of CO2 at 273 K has also been included for a comparison. The carbons with up to 0.4cm g exhibit some molecular sieving effect for molecules above 0.56 nm because the access of 2,2-dimethylbutane is very restricted. This means that in this range, the main pore size developed is around 0.37nm. No molecular sieving effect is detected above 0.4cm g when (V N2) > (V ,iC02). 

This paper deals with the characterization of activated carbons obtained from Polyethylene Terphtalate (PET). This has been carried out by using several techniques. Among them immersion calorimetry of several organic vapours (n-hexane, benzene, cyclohexane and 2,2-DMB) and adsorption of the same vapours. Nice agreement is found between the textural characteristics determined by both techniques. 

The aim of this paper is to determine the textural charaeteristics of several activated carbons by using immersion calorimetry measurements and to relate these results with the information obtained fi om other experimental techniques. The textural characteristics of the adsorbents obtained fi om the immersion enthalpies by the application of the several approaches, already commented, are discussed. 

The results of dhuracterization of the activated carbons by immersion calorimetry are coincident with those obhnned by vapoum adsorption. The variation of the sur ce areas and of the micropore volumes obtmned ftom calorimetry data clearly re L the characteristics of the adsorbents. 

The micropore structure can be determined by several methods such as immersion calorimetry, small-angle X-ray scattering (SAXS) high resolution transmission electron microscopy (HRTEM) and s- and liquid-phase adsorption, among which the most widely us is gas adsorption[7]. The pore structure of activated carbon is usually characterised in terms of the pore size distribution (PSD), perhaps die most imporlant aspect of characterization of die structural heterogeneity of porous solids used in industrial applications. This PSD could be obtained as an arbitrarily chosen form such as, for instance, mma or C ssian distribution[8]. For a local isodierm one may choose traditional mmlels, statistical mechanical methods such as DFT, or, most accurate for micropores, methods based on Monte Carlo simulation. 

Immersion calorimetry has been used for many years as a convenient method for the semi-routine characterisation of activated carbons and some oxide adsorbents. It would appear to offer the possibility of determining the surface area by a single measurement -provided that Ah is known for the liquid-solid system. The technique appears to have the advantage that it is... 

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