Specific Surface Area Analysis

3 SPECIFIC SURFACE AREA ANALYSIS 13.3.1 Gas Adsorption—BET Method 

For this technique to be effective, the adsorbing surface must be free from all impurities such as moisture, solvents, and other contaminants. Prior to measurement, the sample is outgased under either vacuum or high purity nitrogen, with or without 

Several different mathematical relationships (referred to as isotherms) have been developed to describe the relationship of fractional surface coverage with respect to the adsorbing species. Work by Langmuir, Freundlich, Tempkin, and others have attempted to describe the above mentioned pressure differential to fractional surface coverage. The most widely used and accepted isotherm is the BET equation, named after its orignators, Brunauer, Emmett, and Teller, and is as follows 29 

The BET isotherm, like the isotherm developed by Langmuir (the first person to develop a rigorous model for gas adsorption), assumes that the adsorbing surface is energetically uniform, and that only one molecule could adsorb at each surface site. The BET isotherm is a generalized form of the Langmuir equation to account for multilayer adsorption, and assumes that after the adsorption of the first layer, the heat of condensation is equal to the heat of evaporation, and that the rates of adsorption for the second adsorbed layer and beyond are the same.29-31 From a practical perspective, variables in the equation must have specific values for the BET model to be valid, namely the y-intercept and BET constant, C, must be positive. Several excellent reviews of surface area measurement and gas adsorption can be found in References.6,32 34 

In most applications, measurement of specific surface area is synonymous with the use of nitrogen and the BET isotherm. For most commercial surface area instruments, at least 1 m2 of surface area must be present in the sample cell for the measurement to be repeatable. This requires that a significant mass of material ( 1 g) must be present in the sample cell for low surface-area materials. When these conditions cannot be met due to material availability, an instrument that uses krypton as the adsorbate should be utilized. Krypton allows specific surface area measurement with as little as 0.1 m2 present in the sample. 

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The distillation fractions were also analysed for their caibon and hydrogen contents using a Leco CHN Determinator which was also used for similar analysis of die used catalysts. The hydrocracked liquid and the used catalysts were analysed for their sulphur contents using a Leco Suli iur Determinator. Some specific surface area analysis by nitrogen adsorption was carried out on the used catalysts using a Micromeritics instrument... 

Attention has also been given to possible sources of error in specific surface area analysis. Lobenstein (5) has noted the problems associated with the lack of additivity in BET theory as applied to mixtures and the "interaction" between two or more components of dental materials. He has also derived equations to estimate the extent of the "interaction" as well as to correct for it to give a more realistic surface area value. 

Certain precautions are necessary in the determination of specific surface area, such as obtaining a representative sample, choosing the appropriate technique, and selecting standard materials for specific surface area analysis. The analyst should choose the technique and conditions of analysis that most closely approximate the intended use of the sample. Furthermore, precision studies should be carried out to see how sample handling affects the surface area. 

The Fe-B nanocomposite was synthesized by the so-called pillaring technique using layered bentonite clay as the starting material. The detailed procedures were described in our previous study [4]. X-ray diffraction (XRD) analysis revealed that the Fe-B nanocomposite mainly consists of Fc203 (hematite) and Si02 (quartz). The bulk Fe concentration of the Fe-B nanocomposite measured by a JOEL X-ray Reflective Fluorescence spectrometer (Model JSX 3201Z) is 31.8%. The Fe surface atomic concentration of Fe-B nanocomposite determined by an X-ray photoelectron spectrometer (Model PHI5600) is 12.25 (at%). The BET specific surface area is 280 m /g. The particle size determined by a transmission electron microscope (JOEL 2010) is from 20 to 200 nm. 

The determination of the specific surface area of a powder by air permeability methods essentially involves the measurement of the pressure drop across a bed of the powder under carefully controlled flow conditions. The data obtained are substituted in the Kozeny-Carman equation to estimate the specific surface area. Permeability methods have certain advantages, one of them being that the equipment used for carrying out the measurements is cheap and robust. Another advantage is that sample problems are minimized because a large sample of powder is required to be used for analysis. 

Various techniques and equipment are available for the measurement of particle size, shape, and volume. These include for microscopy, sieve analysis, sedimentation methods, photon correlation spectroscopy, and the Coulter counter or other electrical sensing devices. The specific surface area of original drug powders can also be assessed using gas adsorption or gas permeability techniques. It should be noted that most particle size measurements are not truly direct. Because the type of equipment used yields different equivalent spherical diameter, which are based on totally different principles, the particle size obtained from one method may or may not be compared with those obtained from other methods. 

Alkaline earth oxides (AEO = MgO, CaO, and SrO) doped with 5 mol% Nd203 have been synthesised either by evaporation of nitrate solutions and decomposition, or by sol-gel method. The samples have been characterised by chemical analysis, specific surface area measurement, XRD, CO2-TPD, and FTIR spectroscopy. Their catalytic properties in propane oxidative dehydrogenation have been studied. According to detailed XRD analyses, solid solution formation took place, leading to structural defects which were agglomerated or dispersed, their relative amounts depending on the preparation procedure and on the alkaline-earth ion size match with Nd3+. Relationships between catalyst synthesis conditions, lattice defects, basicity of the solids and catalytic performance are discussed. 

Table 1. Studied samples and thermal treatment temperatures, Nd203 content determined by ICP analysis, specific surface area values measured by BET method and Tmax of the desorption peaks in C02 TPD.

Materials. Na-Kaolinite A homoionic sample of kaolinite was prepared from a well-crystallized sample purchased from Source Clays, University of Missouri, using a standardized technique (14) which involved repeated washing with distilled water and by treatment with NaCl solutions to remove exchangeable ions such as Ca, and freeze-drying of the final product. Nitrogen specific surface area of this kaolinite was estimated to be 9.4nr/g and X-ray analysis showed the characteristic pattern of kaolinite. 

Physical properties of calcined catalysts were investigated by N2 adsorption at 77 K with an AUTOSORB-l-C analyzer (Quantachrome Instruments). Before the measurements, the samples were degassed at 523 K for 5 h. Specific surface areas (,S BEX) of the samples were calculated by multiplot BET method. Total pore volume (Vtot) was calculated by the Barrett-Joyner-Halenda (BJH) method from the desorption isotherm. The average pore diameter (Dave) was then calculated by assuming cylindrical pore structure. Nonlocal density functional theory (NL-DFT) analysis was also carried out to evaluate the distribution of micro- and mesopores. 

NL-DFT analysis was then carried out to obtain more accurate information of the distribution of mesopores. It was shown that smaller mesopores with 2 to 12 nm diameter are formed only on calcined Co(20)/CyDTA/SiO2 catalyst, whereas mesopore diameter is distributed above 12 nm on other catalysts. In Table 6.2, cumulative pore volume (VJ and specific surface area (Sc) of mesopores are tabulated. Because of the presence of smaller mesopores, cumulative specific surface area of calcined Co(20)/CyDTA/SiO2 catalyst is larger than that of calcined Co(20)/SiO2 catalyst, whereas cumulative pore volume of the former is smaller. The formation... 

Gas adsorption (physisorption) is one of the most frequently used characterization methods for micro- and mesoporous materials. It provides information on the pore volume, the specific surface area, the pore size distribution, and heat of adsorption of a given material. The basic principle of the methods is simple interaction of molecules in a gas phase (adsorptive) with the surface of a sohd phase (adsorbent). Owing to van der Waals (London) forces, a film of adsorbed molecules (adsorbate) forms on the surface of the solid upon incremental increase of the partial pressure of the gas. The amount of gas molecules that are adsorbed by the solid is detected. This allows the analysis of surface and pore properties. Knowing the space occupied by one adsorbed molecule, Ag, and the number of gas molecules in the adsorbed layer next to the surface of the solid, (monolayer capacity of a given mass of adsorbent) allows for the calculation of the specific surface area, As, of the solid by simply multiplying the number of the adsorbed molecules per weight unit of solid with the space required by one gas molecule ... 

Although there are several methods for analysis of nitrogen physisorption data, the most commonly used is BET surface area. Because for microporous materials the boundary conditions for multilayer adsorption are not fulfilled, the calculated BET surface area has no physical meaning. Such data should be considered proportional to the total micropore volume rather than the specific surface area. The Tplot method can be used to calculate the micropore volume and the mesopore... 

Materials. Synthetic hematite was obtained from J. T. Baker Chemical Company, Phillipsburg, NJ. Particle size analysis using a HIAC instrument (Montclair, CA) indicated the particles to be 80 percent (number) finer than 2 microns. Using nitrogen as the adsorbate, the B.E.T. specific surface area was found to be 9 square meters per gram. The point of zero charge, as obtained from electrophoretic measurements in the presence of indifferent electrolytes, occurred at pH 8.3. 

By comparison with many other silicate minerals, isotope studies of natural clays are complicated by a number of special problems related to their small particle size and, hence, much larger specific surface area and the presence of interlayer water in certain clays. Surfaces of clays are characterized by 1 or 2 layers of adsorbed water. Savin and Epstein (1970a) demonstrated that adsorbed and interlayer water can exchange its isotopes with atmospheric water vapor in hours. Complete removal of interlayer water for analysis with the total absence of isotopic exchange between it and the hydroxyl group, may not be possible in all instances (Lawrence and Taylor 1971). 

Table 3 shows results of physical analysis in which our products had larger specific surface area than one of phosphate rock. It was recognized that they could be some supporting media for substantial substance such as hormone, pesticide and enzyme. 

Coupled columns packed with different stationary phases can be used to optimize the analysis time (71, 75). In this approach the different columns are connected in a series or in parallel. liie sample mixture is first fractioned on a relatively short column. Subsequently the fractions of the partially separated mixture are separated on other columns containing the same or other stationary phases in order to obtain the individual components. Columns differing in length (number of theoretical plates), adsorptive strength or phase ratio (magnitude of specific surface area), and selectivity (nature of the stationary phase) can be employed, whereas, the eluent composition remains unchanged. Identification of the individual sample components via coupled column technique requires a careful optimization of each column and precise control of each switching step. 

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