Samples Calculated Specific Surface Area

In general, the BET equation fits adsorption data quite well over the relative pressure range 0.05-0.35, but it predicts considerably more adsorption at higher relative pressures than is experimentally observed. This is consistent with an assumption built into the BET derivation that an infinite number of layers are adsorbed at a relative pressure of unity. Application of the BET equation to nonpolar gas adsorption results is carried out quite frequently to obtain estimates of the specific surface area of solid samples. By assuming a cross-sectional area for the adsorbate molecule, one can use Wm to calculate specific surface area by the following relationship ... 

Simply calculating specific surface areas from the values in Tables 3-5 leads to apparent specific surface areas of approximately 400-500 m2/g [49,51], Specific surface areas obtained from similar analyses of nonpolar gas (nitrogen or krypton) adsorption studies, however, are typically in the range of 1 m2/g, independent of sample pretreatment. 

Table 9.2 Calculated specific surface areas E in m2/g for a batch of anatas, porous glass, a silica gel, and a special sample of the protein albumin [390].

Nitrogen isotherms were measured by using an ASAP (Micromeritics) at 77K. Prior to each analysis, the samples were outgassed at S73K for 10 - 12 h to obtain a residual pressure of less than 10 torr. The amount on nitrogen adsorbed was used to calculate specific surface area, and the micro pore volumes determined from the BET equation [14] and t-plot method [15], respectively. Also, the Horvath-Kawazoe model [16] was applied to the experimental nitrogen isotherms for pore size distribution. 

The adsorption capacities of the samples prepared via sol-gel method are noticeably larger than for samples obtained by impregnation, and consequently the specific surface areas of the FMT-SG samples is ca. twice that of the samples prepared by impregnation. A different behaviour can be observed, however, for sample FMT-SG3 in this case, the calculated specific surface area is of the same order as for the impregnated samples. Although the origin of this difference is not clear at all, it is also true that the nature of the precursor used to prepare this sample has an important influence on the type of the pores developed in the sample and also on its surface area. 

Specific Surface Area Constraint. An important requirement for model parameter estimation is that calculated sample scale specific surface area should be within 90% of independently measured surface area. A 90% limit was chosen based on the relatively large uncertainty in most standard methods for surface area measurements [e.g., EGME method (Carter et al., 1986)]. The sample scale expected value of specific surface area SAe is calculated according to ... 

Since PCBs are adsorbed on the particle surface, normally coated with a thin layer of organic matter such as humic acid, the concentration in sediment and soil samples is much more likely to be related to the particle surface area per volume unit than to the mass unit. ° For this reason, the concentration of each sample, expressed in pgg dry weight, is normalized by dividing it by the relevant calculated specific surface area, expressed in square meters of surface per cubic centimeter of dry sample (m cm ), as obtained by particle size analysis. Comparisons among concentration values of organic pollutants relevant to samples with different particle size distribution may lead to erroneous conclusions if these are expressed in a conventional way. °... 

Figure 94. Oxygen TPD spectra calculated for the case of very slow (a) and fast (b) oxygen chemical diffusion in the bulk for samples with specific surface area 10 mVg (1) and 1 m /g (2).

Harkins and Jura [21] found that a sample of Ti02 having a thick adsorbed layer of water on it gave a heat of inunersion in water of 0.600 cal/g. Calculate the specific surface area of the Ti02 in square centimeters per gram. 

The specific surface area of the fresh and used catalysts was measured by nitrogen adsorption method (Sorptometer 1900, Carlo Erba Instruments). The catalysts were outgassed at 473 K prior to the measurements and the Dubinin equation was used to calculate the specific surface area. The acidity of investigated samples was measured by infrared spectroscopy (ATI Mattson FTIR) by using pyridine (>99.5%, a.r.) as a probe molecule for qualitative and quantitative determination of both Bronstcd and Lewis acid sites (further denoted as BAS and LAS). The amounts of BAS and LAS were calculated from the intensities of corresponding spectral bands by using the molar extinction coefficients reported by Emeis (23). Full details of the acidity measurements are provided elsewhere (22). 

The following natural precursors have been selected for KOH activation coal (C), coal semi-coke (CS), pitch semi-coke (PS) and pitch mesophase (PM). An industrial activated carbon (AC) was also used. Activation was performed at 800°C in KOH with 4 1 (C KOH) weight ratio, for 5 hours, followed by a careful washing of the samples with 10% HC1 and distilled water. The activation process supplied highly microporous carbons with BET specific surface areas from 1900 to 3150 m2/g. The BET surface area together with the micro and the total pore volume of the KOH-activated carbons are presented in Table 1. The mean micropore width calculated from the Dubinin equation is designed as LD. 

The surface area of the catalysts was measured by nitrogen physisorption (Sorptometer 1900, Carlo Erba). The fresh and regenerated samples were outgassed at 150°C and the spent samples at 100°C for 3 hours. The specific surface area was calculated with the Dubinin equation. 

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. 

Three LaCoOs samples (1,11, and 111) with different specific surface areas were prepared by reactive grinding. In the case of LaCoOs (1), only one step of grinding was performed. This step allowed us to obtain a erystalline LaCoOs phase. LaCoOs (11) and LaCoOs (111) were prepared in two grinding steps a first step to obtain perovskite crystallization and a second step with additive to enhanee speeific surface area. The obtained compounds (perovskite + additive) were washed repeatedly (with water or solvent) to free samples from any traee of additive. The physical properties of the three catalysts are presented in Table 10. LaCoOs (1) was designed to present a very low specific surface area for comparison purposes. NaCl used as the additive in the case of LaCoOs (11) led to a lower surface area than ZnO used for LaCoOs (111), even if the crystallite size calculated with the Sherrer equation led to similar values for the three catalysts. The three catalysts prepared were perovskites having specific surface areas between 4.2, 10.9 and 17.2 m /g after calcination at 550 °C. A second milling step was performed in the presence of an additive, yielding an enhanced specific surface area. 

Figure 5.15 Comparison of the hydrogen adsorption in a slit and cylindrical pore [18].The amountofabsorbed hydrogen correlates with the specific surface area of the sample the maximum is at 0.6 mass% (p — 6 MPa, T— 300 K). No significant difference was found in the calculated amount of hydrogen between the slit and cylindrical pores. The calculation was verified experimentally with excellent agreement.

Chemical composition of fresh HTs was determined in a Perkin Elmer Mod. OPTIMA 3200 Dual Vision by inductively coupled plasma atomic emission spectrometry (ICP-AES). The crystalline structure of the solids was studied by X-ray diffraction (XRD) using a Siemens D-500 diffractometer equipped with a CuKa radiation source. The average crystal sizes were calculated from the (003) and (110) reflections employing the Debye-Scherrer equation. Textural properties of calcined HTs (at 500°C/4h) were analyzed by N2 adsorption-desorption isotherms on an AUTOSORB-I, prior to analysis the samples were outgassed in vacuum (10 Torr) at 300°C for 5 h. The specific surface areas were calculated by using the Brunauer-... 

Columns 11-13 are calculated from the data in the previous columns. The data in Column 13 is then plotted versus the corresponding relative pressures in Column 5. The slope s and intercept i are calculated and the value ofW is found as the reciprocal of their sum. Equation (4.13) is used to obtain the total sample surface area S and dividing by the sample weight yields the specific surface area, S. 

In many areas of powder technology the need to measure the powder volume or density often arises. For example, powder-bed porosities in permeametry, volume specific surface area, sample cell void volumes as well as numerous other calculated values all require accurately measured powder densities or specific volumes. It is appropriate, therefore, to introduce some discussion of powder density measurements. 

The nitrogen adsorption-desorption isotherms were obtained at 77K by AutoSorb-1 -C (Quantachrome). Prior to measurement, the samples were outgassed at 300°C for 3 h. The specific surface areas of the samples were determined from the linear portion of the BET plots. Pore size distribution was calculated from the desorption branch of N2 desorption isotherm using the conventional Barrett-Joyner-Halenda (BJH) method, as suggested by Tanev and Vlaev [15], because the desorption branch can provide more information about the degree of blocking than the adsorption branch. 

Specific surface areas and micropore volumes were obtained from nitrogen adsorption - desorption isotherms at -196°C using Micromeritics ASAP 2010. Prior to the measurements all powdered samples were degassed at 175 °C under vacuum 10 6 Torr for 6 hours. The total surface area was calculated using BET equation. The method of Horvath and Kawazoe was used to determine the pore size diameters of the product. 

The volumetric method is mainly used for the purpose of determining specific surface areas of solids from gas (particularly nitrogen) adsorption measurements (see page 134). The gas is contained in a gas burette, and its pressure is measured with a manometer (see Figure 5.4). All of the volumes in the apparatus are calibrated so that when the gas is admitted to the adsorbent sample the amount adsorbed can be calculated from the equilibrium pressure reading. The adsorption isotherm is obtained from a series of measurements at different pressures. 

Plot the adsorption isotherm and use the BET equation to calculate a specific surface area for the silica gel sample, taking the molecular area of nitrogen as 16.2 x 10-20 m2. 

Figure 2.2 BET plot for adsorption of N2 on shale samples at -195°C. Linear data array indicate conformity to Equation (2.11), by which indicated specific surface areas were calculated. Reproduced from Fanale and Camion (1971).

As pretreatment temperature is raised to 1073 K, the total loading is reduced. After pretreatment at this temperature the silica shows a decreased specific surface area. In order to test whether this decrease can account for the reduction in total loading, surface coverage (C) was calculated, by rationing the loading (1) to the specific surface area (SBET) of the sample. Multiplication by the Avogadro constant yields units of molecules/nm2. 

Parameters of the porous structure of titania samples (pores volume Vs, specific surface area Ssp) were calculated using BET theory [34] from the adsorption isotherms of methanol. The average pore diameter (Dp) values were estimated from the differential curves of pore size distribution. 

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