One-point BET method

Specific surface areas were determined by N2 (Air Liquide, 30% N2 in He) adsorption at -196°C (one point BET method) with a Micromeritics Flow Sorb II. 

Specific surface areas of the catalytic materials were determined by the one-point BET method (Gemini III Micromeritics). 

One is obliged to conclude that this method, like those which derive the cumulative surface area from pore size calculations, can be regarded as no more than ancillary to the BET or Point B methods. The few cases where reasonable agreement with the BET area is obtained are probably to be explained by compensation of opposing effects. 

Data taken from the adsorption leg of the isotherm of Figure 17.11 are listed in the first two columns of the following table. Test the applicability of the following equilibrium theories (a) Langmuir (b) infinite BET and (c) Harkins and Jura. From (a) and (b) obtain estimates of the surface area of the adsorbent and compare the values with that obtained by the point B method. One molecule of nitrogen adsorbed on alumina occupies 0.162 nm2. 

The sparsity of data regarding type III isotherms, with C values of 2 or less, leaves open the question of the usefulness of the BET method for determining surface areas when type III isotherms are encountered. Often in this case it is possible to change the adsorbate to one with a higher C value thereby changing the isotherm shape. Brunauer, Copeland and Kantro, however, point to considerable success in calculating the surface area from type III isotherms as well as predicting the temperature coefficient of the same isotherms. 

The measurements of external and internal specific surface area have already been discussed in Chapter 1, Section 1.1.3. The principles and the isotherm equation of the BET method to measure external specific surface area, including macro- and mesopores, have been presented in Chapter 1, Section 1.3.4.1.5. The external specific surface area is usually determined by nitrogen gas adsorption at the temperature of liquid nitrogen. Both static (one-point) and dynamic (five-point) methods are applied. The calculations are made by Equation 1.75 (Chapter 1), using one or five different pressure values. The external specific surface area is calculated from the maximum number of surface sites, that is, monolayer and the cross-sectional area of nitrogen molecules. 

The measurement of pore size distributions is well established. However, the use of BET surface area measurements for zeolitic materials has been called into question due to potential multiple adsorption and nonconformity of monolayer adsorption implicite in the BET theory. The type of gas used, the method of data analysis, and even the use of the term surface area for a zeolitic material has been seriously questioned lately. On the other hand, most commercial manufacturers supply a surface area determined often by a three point or even a one point procedure that some researchers feel tells something about the material. 

Equation (3.12) can fit well in the usual region of the BET equation and has been used to estimate surface area with a single point [11]. This one point method has been incorporated into commercial equipment for rapid surface area determinations. 

Surface area measurements (BET) by low temperature - one point - nitrogen adsmption method by using a Micromeretics Flowsorb II apparatus (P/Po = 0.30). 

Microporous materials might be thought to be unsuitable for area determination by the BET method. However, Brunauer (35) pointed out that since small pores fill at a lower partial pressure than larger ones, there is a compensating effect so that by coincidence the method still gives a true value for the specific surface area provided there are no pores with radii smaller than about 12 A. 

Two types of commercial apparatus are available, one using a. vacuum system as in the original BET method (Micrbmeritics) and another the flow system (Quantachrome). An isotherm with 10-15 points can be determined in a few hours and surface area and pore size distribution quickly obtained (150,151). 

The use of methylene blue dye adsorption from aqueous solution has been used by Lirige and Tylerfor measuring the surface area of chalcopyrite. At a dye concentration of 10 m the plateau of the isotherm is apparently reached enabling a one-point method to be used. Comparison with BET-N2 areas led to an effective area of 2.4 0.1 (x mol m for methylene blue cf. p. 

The BET theory requires that a plot of [W Pq/P — 1)] versus P/Pq be linear with a finite intercept [cf. equation (4.38) and Fig. 4.1]. By reducing the experimental requirement to only one data point, the single-point method offers the advantages of simplicity and speed often with little loss in accuracy. 

However, Dubinin and co-workers do not accept the concept of monolayer formation in micropores and propose determining the microporous volume, Fq, on the basis of the thermodynamic theory of Polanyi adsorption. However, one can observe that the monolayer volume, Vm, when expressed in liquid nitrogen volume per unit mass, is very close to the Dubinin volume, Vo. The proportionality of the BET monolayer volume, Vm, and the so-called micropore volume, Va, (Vo 11 Vm) has been observed for many materials, as shown in different studies [2, 3]. This means that both variables are correlated, so determining one is equivalent to the determining the other. The discussion on the physicochemical meaning of these parameters may be interesting from a theoretical point of view but as far as practical characterization of porous materials is concerned, both methods can often be considered as equivalent. 

Of all existing GC techniques for determining the specific area of a solid, the heat desorption method is the one most often used. This method was developed by Nelson and Eggertsen and modified by a number of workers. In principle, the heat desorption method is based on the traditional Brunauer, Emmett, Teller (BET) technique in which the quantity of adsorbed gas (usually nitrogen) at a temperature near its boiling point is determined. By determining the adsorption at various pressures, it is possible, using the BET equation, to calculate the amount of adsorbate required for the formation of a monolayer. 

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