Brunauer-Emmett-Teller value

Selected OSC values are reported in Table 8.1 for ceria and cerium-zirconium mixed oxides. These results confirm that the isomorphous substitution of Ce4+ by Zr4+ ions clearly improves the catalyst stability. BET (Brunauer, Emmett, Teller) area of ceria treated at 900°C is close to 20m2g 1 while it amounts to 35 15 m2g 1 for most mixed... 

TABLE 9.2 Values of Asp and c as Determined in 10 Different Laboratories by the Brunauer-Emmett-Teller Method for the Same Silica Sample Shown in Figure 9.9... 

The data reported by Basila et al. (221,224) lead to a value of 1.6 03. Using the apparent integrated absorption intensities as given by Hughes and White (198), Ward and Hansford (226) estimated the limits of detection of Br nsted acidity to be of the order of magnitude of 10 2 meq g-1 for silica-aluminas with Brunauer-Emmett-Teller (BET) surface areas between 350 and 500 m2 /g. 

The heats of adsorption of nitrogen on evaporated metal films of nickel and iron have been reported to decrease from 10 to 5 kcal./mole as the surface coverage increased from about 0.1 to 1.0 monolayer. Beeck (150) states that nitrogen is unsuitable for the evaluation of surface areas of evaporated iron and nickel films by the Brunauer-Emmett-Teller method because of its high heat of adsorption at 78°K., which would yield a value for monolayer adsorption too high by 50%. The author feels that this objection does not apply to the nitrogen adsorption on reduced electropolished planar copper plates for the following reasons. 

In practice, the amount of solid molecules on the surface being exposed to the solution is difficult or even impossible to quantify. Instead, the solid surface area to solution volume ratio is often used to quantify the amount of solid reactant. Therefore, experimentally determined second-order rate constants for interfacial reactions have the unit m s h As the true surface area of the solid is very difficult to determine, the BET (Brunauer-Emmett-Teller) surface area is fte-quentiy used. The maximum diffusion-controlled rate constant for a particle suspension containing pm-sized particles is ca 10 m s and for mm-sized particle suspensions the corresponding value is I0 m s h Unfortunately, the discrepancy between the true surface area and the BET surface area and the non-spherical geometry of the solid particles makes it impossible to exactly determine the theoretical diffusion-controlled rate constant. 

The sigmoidal shape of the nitrogen isotherm of Figure 1 is quite amenable to analyses by the Brunauer-Emmett-Teller (BET) multilayer theory (8). The BET surface area calculated is 2.8 mVg. This value is quite consistent with the predominant l-to-10- xm size distribution observed on this sample with an optical microscope. Assuming cubic habits and density of about 2.3 g/cm, we find the range of specific surface area to be 0.26 to 2.6 m /g. Apparently the sorbed N2 does penetrate into the internal portion of the particles only to a limited extent. 

The most relevant characteristic of porous materials is the disposal of a high effective surface/volume relationship, usually expressed in terms of their specific surface area (area per mass unit), which can be determined from nitrogen adsorption/desorption data. Different methods are available for determining the specific surface area (Brunauer-Emmett-Teller, Langmuir, and Kaganer), micropore volume (f-plot, ttj, and Dubinin-Astakhov), and mesopore diameter (Barrett-Joyner-Halenda Leroux et al., 2006). Table 1.1 summarizes the values of specific surface area for selected porous materials. 

Figure 23.10 Capacitance of various activated carbons in 30 % potassium hydroxide (KOH) aqueous medium as a function of their Brunauer, Emmett, Teller (BET) specific surface area. Values taken from Refs [76, 78].

The Brunauer-Emmett-Teller (BET) and Guggenheim-Anderson-deBoer (GAB) sorption isotherm models were used to obtain experimental steady-state moisture contents in dry basis by linear regression analysis according to Kouassi and Roos (2002). These equations provide the value of monolayer water content, which is an important parameter in food deterioration studies. 

In their pioneering studies of silica sols, Alexander and Iler (4) employed low-temperature nitrogen adsorption to determine the surface areas of the colloidal particles after removal of the aqueous medium. The Brunauer-Emmett-Teller (BET) areas were found to be only slightly larger than the values obtained from the particle size distributions as determined by light scattering and electron microscopy. These remarkable measurements indicated little change in the particle size or shape after the stabilized silica sols were carefully dried. 

Colloid chemists commonly measure surface area by the adsorption of N2 gas. The adsorption is conducted in vacuum and at temperatures near the boiling point of liquid nitrogen (—196° C). The approach is based on the Brunauer-Emmett-Teller (BET) adsorption equation, and has been adapted to a commercially available instrument. Unfortunately, the technique does not give reliable values for expansible soil colloids such as vermiculite or montmorillonite. Nonpolar N2 molecules penetrate little of the interlayer regions between adjacent mineral platelets of expansible layer silicates where 80 to 90% of the total surface area is located. Several workers have used a similar approach with polar H2O vapor and have reported complete saturation of both internal (interlayer) and external surfaces. The approach, however, has not been popular as an experimental technique. 

Moisture content is a very important parameter influencing the stability of dehydrated foods. It has been suggested that the optimal amount of water for long-term storage corresponds in most dehydrated foods to the Brunauer-Emmett-Teller (BET) monolayer value. On the other hand, items such as freeze-dried spinach, cabbage, and orange juice were reported to be more stable at a zero moisture content, whereas items like potatoes and corn had maximum stability at the monomolecular moisture content. It appeared that optimal moisture content could not be predicted with precision on the basis of theoretical considerations. 

The adsorption of N2 gas is often used to evaluate the surface-accessible area and pore size distribution by the Brunauer-Emmett-Teller (BET) method (353). The accessible surface is generally that of the internal pores within the crystallites and the external surface between the crystallites. Correspondingly, the measured pores are those inside of and between the crystallites. Within most common organic and inorganic LDHs, the interlayers are full of the anions as well as water, and thus only the external surface of the crystallites contributes to the accessible surface area. Values normally range from 10 to 200 g (4,354),... 

In recent times, following on from the development of automatic adsorption equipment and the computerized analysis of adsorption data, there exists a tendency to underestimate the value and importance of the adsorption isotherm. The assessment and role of what is termed internal surface area are assumed to be dominant. The problem with the use of computerized equipment is that it is programmed to re-assemble adsorption data into the coordinates of the Brunauer-Emmett-Teller (BET) or Dubinin-Radushkevich (DR) adsorption equations (see below) and to provide the best straight line through data, and so to obtain a value for a surface area. The adsorption isotherm is often not printed out. There is a need to do this as the isotherm, as such, provides considerable information simply from a visual inspection. No account is taken by the computer that not all of the adsorption data are appropriate. As a result, computer-based values of surface area are obtained which can be far removed from those obtained by manual calculations which use the appropriate manual points and a knowledge of adsorption processes. 

The pore volume of the samples was divided in different pore sizes and values for the total, meso- and microporosity were obtained from the nitrogen data. The total pore volume of the sample was calculated from the N2 adsorbed at a relative pressure of 0.98, the mesoporosity volume was studied by the Barret-Joyner-Halenda (BJH) method and the microporosity volume was calculated from the data obtained by the GCMC. The overall pore surface area was obtained by the Brunauer-Emmett-Teller (BET) theory. The surface areas of the mesopores and the macropores were obtained by the BJH method and GCMC respectively. 

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