Surface Area Determinations

Surface Area Determination. - Methods based on both chemisorption and physical adsorption of radioactive species have been used for surface area determinations. Although, at first sight, chemisorption, which measures only the metal area, might appear to provide a better estimate of the active area of a supported catalyst this is not necessarily true. In the case of 

Elvidge, in Isotopes Essential Chemistry and Applications , ed., J. A. Elvidge and J. R. Jones, Royal Society of Chemistry, London, 1980, p. 123. 

The normal method for determining the total area of a solid is that due to Brunauer, Emmett, and Teller in which the physical adsorption of a non-reactive gas such as N2 is examined at low temperatures. The amount of gas adsorbed is measured by its disappearance from the gas phase. 

In the method described above radioactivity was used as a means of measuring the gas pressure in equilibrium with the solid. However, if the area per unit weight of material is small, the precision of the measurement is low because the amount adsorbed is then the difference between two large quantitites - the amount of gas actually observed in the gas phase, and the amount which would be expected on the basis of the gas laws if there was no adsorption. Clarke has described an alternative procedure which permits the measurement of areas as small as 0.001 m g The amount of adsorbed [85-Kr] was monitored directly by a scintillation counter placed as near the sample as possible, but exterior to the Dewar flask containing the refrigerant. Provided the amount of gas phase viewed by the detector is small in volume and can be corrected for, the direct method is obviously of great potential. 

Several papers have pointed out that difficulties may be encountered in determining absolute surface areas, as distinct from relative areas of different samples of the same solid, if Kr or Xe is used as adsorbate. 

The estimation of surface area from solution adsorption is subject to many of the same considerations as in the case of gas adsorption discussed in Chapter XVII, but with the added complication that larger molecules are involved. 

The use of adsorption from solution for the determination of surface areas has been discussed in earlier reports (c/. ref. 1, pp. 76—81 ref. 3, pp. 73—78). 

Kawahara, and Y. Ogino, High Pressure Sci. Technol., AIRAPT Conf. 6th, 1977 (Publ. 1979), 1, 593 (Chem. Abstr., 1979, 90, 153 760). 

Jankowska, A. Swiatkowski, and S. Zietek, Buil. Wojsk. Akad. Tech., 1978, 27, 75 (Chem. Abstr, 1978, 88, 19 830). 

and have reconsidered the problem of estimating adsorp- 

An alternative method of determining the surface phase capacity of heterogeneous surfaces has been proposed by Dabrowski and Jaroniec.  

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Perhaps the simplest case of reaction of a solid surface is that where the reaction product is continuously removed, as in the dissolving of a soluble salt in water or that of a metal or metal oxide in an acidic solution. This situation is discussed in Section XVII-2 in connection with surface area determination. 

J. F. Padday, Pure arul Applied Chemistry, Surface Area Determination, Butter-worths, London, 1969. 

It will be seen that each method for surface area determination involves the measurement of some property that is observed qualitatively to depend on the extent of surface development and that can be related by means of theory to the actual surface area. It is important to realize that the results obtained by different methods differ, and that one should in general expect them to differ. The problem is that the concept of surface area turns out to be a rather elusive one as soon as it is examined in detail. 

A number of methods have been described in earlier sections whereby the surface free energy or total energy could be estimated. Generally, it was necessary to assume that the surface area was known by some other means conversely, if some estimate of the specific thermodynamic quantity is available, the application may be reversed to give a surface area determination. This is true if the heat of solution of a powder (Section VII-5B), its heat of immersion (Section X-3A), or its solubility increase (Section X-2) are known. 

As pointed out in Section XVII-8, agreement of a theoretical isotherm equation with data at one temperature is a necessary but quite insufficient test of the validity of the premises on which it was derived. Quite differently based models may yield equations that are experimentally indistinguishable and even algebraically identical. In the multilayer region, it turns out that in a number of cases the isotherm shape is relatively independent of the nature of the solid and that any equation fitting it can be used to obtain essentially the same relative surface areas for different solids, so that consistency of surface area determination does not provide a sensitive criterion either. 

Brunauer (see Refs. 136-138) defended these defects as deliberate approximations needed to obtain a practical two-constant equation. The assumption of a constant heat of adsorption in the first layer represents a balance between the effects of surface heterogeneity and of lateral interaction, and the assumption of a constant instead of a decreasing heat of adsorption for the succeeding layers balances the overestimate of the entropy of adsorption. These comments do help to explain why the model works as well as it does. However, since these approximations are inherent in the treatment, one can see why the BET model does not lend itself readily to any detailed insight into the real physical nature of multilayers. In summary, the BET equation will undoubtedly maintain its usefulness in surface area determinations, and it does provide some physical information about the nature of the adsorbed film, but only at the level of approximation inherent in the model. Mainly, the c value provides an estimate of the first layer heat of adsorption, averaged over the region of fit. 

D. H. Everett and R. H. Ottewill, eds.. Surface Area Determination, Butterworths, London, 1970. 

S. Brunauer, in Surface Area Determination, Proc. Int. Symp., Bristol, 1969, But-terworths, Lxjndon. 

To obtain the monolayer capacity from the isotherm, it is necessary to interpret the (Type II) isotherm in quantitative terms. A number of theories have been advanced for this purpose from time to time, none with complete success. The best known of them, and perhaps the most useful in relation to surface area determination, is that of Brunauer, Emmett and Teller. Though based on a model which is admittedly over-simplified and open to criticism on a number of grounds, the theory leads to an expression—the BET equation —which, when applied with discrimination, has proved remarkably successful in evaluating the specific surface from a Type II isotherm. 

Molecular area, a (Ar) of argon at 77 K on graphitized carbon blacks (Argon BET plots constructed with p (liquid) surface areas determined by BET-nitrogen, with a (Na)... 

Several of the lower alkanes, from C2 to C7, have been used from time to time for surface area determination. They possess the virtue of chemical inertness towards the majority of adsorbents, and their saturation pressures... 

Benzene has enjoyed some popularity as an adsorbate for surface area determination over a number of years. It can be used conveniently at temperatures around ambient, but assignment of a value to its molecular... 

When other adsorptives, such as those detailed in Section 2.9, are employed for surface area determination, calibration against nitrogen or argon is strongly recommended, so long as the specific surface exceeds lm g . For areas below this figure the calibration becomes too inaccurate, and an alternative adsorptive, usually krypton, has to be used. 

The evaluation of pore size distribution by application of the Kelvin equation to Type IV isotherms has hitherto been almost entirely restricted to nitrogen as adsorptive. This is largely a reflection of the widespread use of nitrogen for surface area determination, which has meant that both the pore size distribution and the specific surface can be derived from the same isotherm. 

Striking confirmation of the conclusion that the BET area derived from a Type IV isotherm is indeed equal to the specific surface is afforded by a recent study of a mesoporous silica, Gasil I, undertaken by Havard and Wilson. This material, having been extensively characterized, had already been adopted as a standard adsorbent for surface area determination (cf. Section 2.12). The nitrogen isotherm was of Type IV with a well defined hysteresis loop, which closed at a point below saturation (cf. F, in Fig. 3.1). The BET area calculated from it was 290 5 0 9 m g , in excellent agreement with the value 291 m g obtained from the slope of the initial region of the plot (based on silica TK800 as reference cf. p. 93). 

Comparison of surface areas determined by mercury porosimetry and by nitrogen adsorption ... 

The incorporation of the new material without any increase in the overall length of the book has been achieved in part by extensive re-writing, with the compression of earlier material, and in part by restricting the scope to the physical adsorption of gases (apart from a section on mercury porosimetry). The topics of chemisorption and adsorption from solution, both of which were dealt with in some detail in the first edition, have been omitted chemisorption processes are obviously dependent on the chemical nature of the surface and therefore cannot be relied upon for the determination of the total surface area and methods based on adsorption from solution have not been developed, as was once hoped, into routine procedures for surface area determination. Likewise omitted, on grounds of... 

The heat-transfer surface area determined by the basic sizing or rating method described herein is considered the minimum required area. There are also additional surface area requirements in the final sizing of a heat exchanger. 

Surface Area Determination The surface-to-volume ratio is an important powder property since it governs the rate at which a powder interacts with its surroundings. Surface area may be determined from size-distribution data or measured directly by flow through a powder bed or the adsorption of gas molecules on the powder surface. Other methods such as gas diffusion, dye adsorption from solution, and heats of adsorption have also been used. It is emphasized that a powder does not have a unique surface, unless the surface is considered to be absolutely smooth, and the magnitude of the measured surface depends upon the level of scrutiny (e.g., the smaller the gas molecules used for gas adsorption measurement the larger the measured surface). 

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