Calculated micropore area

The benefits of the method are appreciated when the textural parameters are compared. Data derived from N2-physisorption isotherms show that Fenton detemplation leads to improved textural parameters, with BET areas around 945 m g for a pore volume of 1.33 cm g , while calcination leads to reduced textural parameters (667m g 0.96cm g ). T-plot analysis, strictly speaking, is not apphcable for these bi-modal materials but it gives a good estimate. It shows that the micropore volume is doubled, which corresponds to an increase in the calculated micropore area from about... 

Still, this theory is over-simplified, and holds only for a limited part of the sorption isotherm, which is usually the case for relative pressures between 0.05-0.30, and the presence of point B (Fig. 1.14). Thus, isotherms of Types II (macroporous polymer supports) and IV (mesoporous polymer supports), but not Type I and III, are those amenable to BET analysis [21, 80]. Attention should also be paid to the constant C, which is exponentially related to the enthalpy of adsorption of the first layer. A negative or high value of C exceeding 200-300, is likely to indicate the presence of micropores and the calculated surface area should be questioned since the BFT theory would not be applicable [79, 80]. 

Kiselev, using the above equation by graphical integration of the isotherm between the limits of saturation and hysteresis loop closure, was able to calculate surface areas for wide-pore samples in good agreement with BET measured areas. For micropores, the absence of hysteresis at the low-pressure end of the isotherm indicates that only adsorption and not condensation occurs, thereby rendering Kiselev s method inapplicable. 

As evidenced by curves on figures 4 and 5, changes in texture characteristics are quasi-linearly related to increasing carbon content. It also appears that calculated surface areas strongly depend on the physisorption conditions, while measured adsorbate volumes are less affected. This observation is in favor of erroneous assumptions on nitrogen molecule area for the calculations of specific surface area [10]. But, as mentioned above, nitrogen molecule could penetrate in smaller pores, increasing measured micropore volume and surface areas. 

The final resin bead structure of a macroreticular resin contains many hard microspheres interspersed with pores and channels. Because each resin bead is really made up of thousands of smaller beads (something like a popcorn ball), the surface area of macroporous resins is much higher than that of microporous resins. A gel resin has a (calculated) surface area of less than 1 m g". However, macroporous resin surface areas range from 25 to as much as 800 m g". ... 

This equation describes the additional amount of gas adsorbed into the pores due to capillary action. In this case, V is the molar volume of the gas, y its surface tension, R the gas constant, T absolute temperature and r the Kelvin radius. The distribution in the sizes of micropores may be detenninated using the Horvath-Kawazoe method [19]. If the sample has both micropores and mesopores, then the J-plot calculation may be used [20]. The J-plot is obtained by plotting the volume adsorbed against the statistical thickness of adsorbate. This thickness is derived from the surface area of a non-porous sample, and the volume of the liquified gas. 

The table convincingly demonstrates how the unsuspected presence of micropores can lead to an erroneous value of the specific surface calculated from a Type II isotherm by application of the standard BET procedure. According to the foregoing analysis, the external specific surface of the solid is 114m g" the micropore volume (from the vertical separation of isotherms A and E) is 105 mm g but since the average pore width is not precisely known, the area of the micropore walls cannot be calculated. Thus the BET figure of 360m g calculated from isotherm E represents merely an apparent and not a true surface area. 

Wynne-Jones and Marshfound somewhat similar results with a number of carbons made by pyrolysis of eight organic polymers at a series of temperatures. The isotherms of Nj at 77 K and of COj at 195 K were measured, and the apparent surface area calculated by the usual BET procedure. (Owing to the microporous nature of the solids, these figures for area will be roughly proportional to the uptake at saturation and therefore... 

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 chemical compositions of the samples were obtained by ICP in a Varian 715-ES ICP-Optical Emission Spectrometer. Powder X-ray diffraction was performed in a Philips X pert diffractometer using monochromatized CuKa. The crystallinity of the zeolites was obtained from the intensity of the most intense reflection at 23° 20 considering the parent HZ5 sample as 100% crystalline. Textural properties were obtained by nitrogen physisorption at -196°C in a Micromeritics ASAP 2000 equipment. Surface areas were calculated by the B.E.T. approach and the micropore volumes were derived from the corresponding /-plots. Prior to the adsorption measurements the samples were degassed at 400°C and vacuum overnight. 

Nitrogen adsorption was performed at -196 °C in a Micromeritics ASAP 2010 volumetric instrument. The samples were outgassed at 80 °C prior to the adsorption measurement until a 3.10 3 Torr static vacuum was reached. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method. Micropore volume and external surface area were evaluated by the alpha-S method using a standard isotherm measured on Aerosil 200 fumed silica [8]. Powder X-ray diffraction (XRD) patterns of samples dried at 80 °C were collected at room temperature on a Broker AXS D-8 diffractometer with Cu Ka radiation. Thermogravimetric analysis was carried out in air flow with heating rate 10 °C min"1 up to 900 °C in a Netzsch TG 209 C thermal balance. SEM micrographs were recorded on a Hitachi S4500 microscope. 

The pore size distribution based on BJH (Barrett-Joyner-Halenda) calculations, the micropore fraction (t-plot analysis), and the BET (Brunauer-Enunett-Teller) surface area of the catalysts were acquired by physisorption measurements of nitrogen at 77 K (Micrometries Gemini 2360). Prior to BET analysis the samples were evacuated at 373 K for at least 12 h. 

A number of models have been developed for the analysis of the adsorption data, including the most common Langmuir [49] and BET (Brunauer, Emmet, and Teller) [50] equations, and others such as t-plot [51], H-K (Horvath-Kawazoe) [52], and BJH (Barrett, Joyner, and Halenda) [53] methods. The BET model is often the method of choice, and is usually used for the measurement of total surface areas. In contrast, t-plots and the BJH method are best employed to calculate total micropore and mesopore volume, respectively [46], A combination of isothermal adsorption measurements can provide a fairly complete picture of the pore size distribution in sohd catalysts. Mary surface area analyzers and software based on this methodology are commercially available nowadays. 

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... 

Kaganer modified Dubinin s method in order to calculate the surface area within micropores. He assumed that the adsorption potential of the sites is distributed according to a Gaussian function such that... 

The assumption usually made is that the ratio Fu /Sbet has the same value at a given relative pressure independent of the solid. A plot therefore of t versus P/Pq should give the same curve for any non-porous solid (see Fig. 8.6). In fact, plots of the number of adsorbed layers versus P/Pq show some discrepancies which for the analysis of large pores is not significant. Therefore, the Halsey equation can be used for the statistical thickness in that application. However, for micropore analysis, a statistical thickness must be taken from a t versus P/Pq curve that has approximately the same BET C value as the test sample. The unavailability of t versus P/Pq plots on numerous surfaces with various C values would make the MP method of passing interest were it not for the fact that t can be calculated from equation (8.36). This implies that surface area can be accurately measured on microporous samples. Brunauer points out that in most instances the BET equation does correctly measure the micropore surface area. 

Both deBoer s t-method and Brunauer s MP method are based on the assumption that the BET measured surface area is valid for micropores. Shields and Lowell, using this same assumption, have proposed a method for the determination of the micropore surface area using mercury porosimetric data. The surface area of micropores is determined as the difference between the BET surface area and that obtained from mercury porosimetry (see Section 11.5). Since mercury porosimetry is capable of measuring pore sizes only as small as approximately 18 A radius, this technique affords a means of calculating the surface area of all... 

Microporous silica particles with a density of 2.2 g/mL and a diameter of 10 pm have a measured surface area of 300 m2/g. Calculate the surface area of the spherical silica as if it were simply solid particles. What does this calculation tell you about the shape or porosity of the particles ... 

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. 

Recently, the Horvath-Kawazoe (HK) method for slit-like pores [40] and its later modifications for cylindrical pores, such as the Saito-Foley (SF) method [41] have been applied in calculations of the mesopore size distributions. These methods are based on the condensation approximation (CA), that is on the assumption that as pressure is increased, the pores of a given size are completely empty until the condensation pressure corresponding to their size is reached and they become completely filled with the adsorbate. This is a poor approximation even in the micropore range [42], and is even worse for mesoporous solids, since it attributes adsorption on the pore surface to the presence of non-existent pores smaller than those actually present (see Fig. 2a) [43]. It is easy to verify that the area under the HK PSD peak corresponding to actually existing pores does not provide their correct volume, so the HK-based PSD is not only excessively broad, but also provides underestimated volume of the actual pores. This is a fundamental problem with the HK-based methods. An additional problem is that the HK method for slit-like pores provides better estimates of the pore size of MCM-41 with cylindrical pores than the SF method for cylindrical pores. This shows the lack of consistency [32,43]. Since the HK-based methods use CA, one can replace the HK or SF relations between the pore size and pore filling pressure by the properly calibrated ones, which would lead to dramatic improvement of accuracy of the pore size determination [43] (see Fig. 2a). However, this will not eliminate the problem of artificial tailing of PSDs, since the latter results from the very nature of HK-based methods. 

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