BETs assessment methods

Surface Area and Permeability or Porosity. Gas or solute adsorption is typicaUy used to evaluate surface area (74,75), and mercury porosimetry is used, ia coajuactioa with at least oae other particle-size analysis, eg, electron microscopy, to assess permeabUity (76). Experimental techniques and theoretical models have been developed to elucidate the nature and quantity of pores (74,77). These iaclude the kinetic approach to gas adsorptioa of Bmaauer, Emmett, and TeUer (78), known as the BET method and which is based on Langmuir s adsorption model (79), the potential theory of Polanyi (25,80) for gas adsorption, the experimental aspects of solute adsorption (25,81), and the principles of mercury porosimetry, based on the Young-Duprn expression (24,25). 

Table 2.7 lists techniques used to characterise carbon-blacks. Analysis of CB in rubber vulcanisates requires recovery of CB by digestion of the matrix followed by filtration, or by nonoxidative pyrolysis. Dispersion of CB within rubber products is usually assessed by the Cabot dispersion test, or by means of TEM. Kruse [46] has reviewed rubber microscopy, including the determination of the microstructure of CB in rubber compounds and vulcanisates and their qualitative and quantitative determination. Analysis of free CB features measurements of (i) particulate and aggregate size (SEM, TEM, XRD, AFM, STM) (ii) total surface area according to the BET method (ISO 4652), iodine adsorption (ISO 1304) or cetyltrimethylammonium bromide (CTAB) adsorption (ASTM D 3765) and (iii) external surface area, according to the dibutylphthalate (DBP) test (ASTM D 2414). TGA is an excellent technique for the quantification of CB in rubbers. However, it is very limited in being able to distinguish the different types of... 

Measurement of Surface Area. The Teachability determined by these methods is usually reported as g/cm day. The total surface area of particulate material can be assessed 1) by assuming a particle shape e.g.spherical) and estimating the number of particles, or 2) by measurements using the Brunauer-Emmett-Teller (BET) nitrogen adsorption technique ( ). Unfortunately, the BET method measures the area of surfaces to which nitrogen has access this is not necessarily the same as the area to which a solution has access. Access by solutions requires much larger pore areas. 

The values of a(BET) in Table 9.1 are the BET-nitrogen areas, which were derived from the linear regions of the BET plots, with the molecular area assumed to be 0.162 nm2. The values of a(ext) in Table 9.1 are obviously much smaller than the corresponding values of a(BET). The question naturally arises does the BET method provide a reliable assessment of the total area (i.e. internal plus the external area) The non-linear character of the low-pressure region of each as-pk>t is a clear indication that the isotherm is distorted in the monolayer region and we may therefore conclude that a(BET) does not represent a real surface area. Additional support for this interpretation comes from the microcalorimetric data, which are discussed later in this section. More detailed discussion of some of the results in Table 9.1 is given in Chapter 12. 

The /-method of isotherm analysis adopted by Cases et al. (1992) is not entirely satisfactory and therefore the interpretation of the results is not altogether straightforward. However, the high BET C value is consistent with the conclusion that there was a small micropore filling contribution. To arrive at a more realistic quantitative assessment of the microporosity it would be desirable to obtain nitrogen isotherm data on a truly non-porous form of Na-montmorillonite. In practice, however, this may be difficult to accomplish and a more pragmatic approach would be to construct a series of comparison plots for the adsorption of N2 (and preferably also Ar) on pairs of samples of differing particle sizes and defect structures. In this way it should be possible to establish quantitative differences in the micropore capacities. 

Riccardo and coworkers [50, 51] reported the results of a statistical thermodynamic approach to study linear adsorbates on heterogeneous surfaces based on Eqns (3.33)—(3.35). In the first paper, they dealt with low dimensional systems (e.g., carbon nanotubes, pores of molecular dimensions, comers in steps found on flat surfaces). In the second paper, they presented an improved solution for multilayer adsorption they compared their results with the standard BET formalism and found that monolayer capacities could be up to 1.5 times larger than the one from the BET model. They argued that their model is simple and easy to apply in practice and leads to new values of surface area and adsorption heats. These advantages are a consequence of correctly assessing the configurational entropy of the adsorbed phase. Rzysko et al. [52] presented a theoretical description of adsorption in a templated porous material. Their method of solution uses expansions of size-dependent correlation functions into Fourier series. They tested... 

The catalysts were characterized by using various techniques. X-ray diffraction (XRD) patterns were recorded on a Siemens D 500 diffractometer using CuKa radiation. The specific surface areas of the solids were determined by using the BET method on a Micromeritics ASAP 2000 analyser. Acid and basic sites were quantified from the retention isotherms for two different titrants (cyclohexylamine and phenol, of p/Ta 10.6 and 9.9, and L ,ax 226 and 271.6 nm, respectively) dissolved in cyclohexane. By using the Langmuir equation, the amount of titrant adsorbed in monolayer form, Xm, was obtained as a measure of the concentration of acid and basic sites [11]. Also, acid properties were assessed by temperature-programmed desorption of two probe molecules, that is, pyridine (pKa= 5.25) and cyclohexylamine. The composition of the catalysts was determined by energy dispersive X-ray analysis (EDAX) on a Jeol JSM-5400 instrument equipped with a Link ISI analyser and a Pentafet detector (Oxford). 

The nitrogen adsorption-desorption isotherms for specific surface area and porosity assessment were recorded at -196 C in a Gemini instrument from Micromeritics. The specific surface areas were determined by the Brunauer-Emmett-Teller (BET) method. The pore size distributions were obtained from the desorption branch, and the micropore volume was determined by the t-plot method, using literature software [14]. 

Characterization of these materials normally includes both analysis of the mesoporous structure and assessment of nanoparticle parameters. It can be done by combination of methods used for mesoporous solid study (TEM, N2 BET adsorption, SAXS, etc.) and additional methods allowing estimation of particle size, particle size distribution, particle positioning, and interface interactions. 

This is fortunately offered by the calorimetric experiments which suggest that the BET monolayer content physically corresponds to an energetically strong retention. This quantity, provided by the BET equation, could therefore be called safely the BET strong retention capacity This quantity includes two parts, which are the micropore capacity and the monolayer content on the non-microporous portions of the surface. The latter, which provides the external (/.e. non-microporous) surface area is easily assessed by means of the as or t methods, without even requesting the very low part of the adsorption isotherm. The as method is to be preferred when one wishes to carry out a more detailed analysis of the micropores and when the low pressure range of the adsorption isotherm is available. Conversely, if one only wishes to assess a reliable external surface area, he will probably find it simpler to use the t method this can indeed easily be done in a software, after introducing the appropriate multilayer equation, like the Harkins and Jura t-curve equation [13]. The recommended succession of calculations is therefore ... 

Nitrogen adsorption at low temperature is a routine characterization technique of nanoporous materials. For instance, the specific surface of porous materials is usually assessed from adsorption experiments (prior to capillary condensation of the fluid) on the basis of the Brunauer, Emmett, and Teller (BET) method. The BET model corresponding to the N2 adsorption isotherm at 77 K in the atomistic model of MCM-41 materials fits very well the simulated data with a correlation coefficient = 0.999 (see [39] for the comparison). We found Sbet 1000 m /g (the latter value is obtained by considering as the surface area occupied by an adsorbed N2 molecule, A 2 = 0.162 nm ) and C = 100. The value obtained for C... 

Sing [92] states that there is general agreement that the BET method cannot be used to obtain a reliable assessment of an absorbent exhibiting molecular sieve properties [93,94] (Type lA isotherms) although the method may be useful for comparison purposes. 

Opinions differ on the applicability of the BET analysis to Type IB isotherms. Indications are that the method can be used to assess the... 

Adsorption from solution is sometimes used to assess the adsorptive capacity and hence the surface area of adsorbents. For instance, iodine (1,2), dyes (3,4), organic mixtures (5) or surfactants (6) have been used, but this never lead to a method as wide and generally accepted as those based on gas adsorption, like the BET, BJH, 0 j or Dubinin-Radushkevitch methods (7). The two main reasons are certainly that adsorption from solution is a competitive phenomenon between solvent and solute which is more difficult to interpret than the adsorption of a single gas and also that solute molecules are generally subject to more specific interactions with the surface than the gas molecules conventlonaly used (N, Ar. ..). 

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