BET analysis

PCNTs are marketed commercially by Hyperion Catalyst International Inc. (Cambridge, Mass. USA), based on a patent [22] Graphite Fibrils. The method of production appears to be essentially the same as that used for Endo PCNTs. The material consists of MWCNTs, 10-20 nm in diameter and 10-12 j,m long, with ca. 10 coaxial layers within each tube. The tubes have hollow cores of ca., 2 nm diameter. The Brunauer-Emmett-Teller (BET) analysis characteristically shows a surface area of 250 m /g, true density 2.0 g/cm- and bulk density of less than 0.1 g/cm (95% void vol). 

Brunauer, Emmett and Teller (BET) analysis of pristine magnesium and y-MPS modified... 

Sharma et al. [153] have devised a gentle accelerated corrosion test using a kinetic rate equation to establish appropriate acceleration factors due to relative humidity and thermal effects. Using an estimate for the thermal activation energy of 0.6 eV and determining the amount of adsorbed water by a BET analysis on Au, Cu and Ni, they obtain an acceleration factor of 154 at 65°C/80% RH with respect to 25 °C/35-40% RH. 

If one restricts the number of layers of adsorbate that may be stacked up, as would be the case in the very narrow capillaries of a porous catalyst, the BET analysis must be modified to allow for this. If n is the number of permissible layers, it can be shown that the adsorption isotherm becomes ... 

N2-BET analysis and porosity measurements were done on a Micrometries ASAP 2000 apparatus at liquid nitrogen temperature. Temperature-programmed desorption of ammonia (NH3-TPD) and temperature-programmed reduction by H2 (H2-TPR) were performed with a Micromeritics AutoChem 2910 apparatus. 

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. 

Table 3 Wm Values for Various Starches Obtained from BET Analysis of Moisture Uptake Isotherms...

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]. 

With nitrogen the data required for BET analysis or pore volumes lies above the thermal transpiration region. 

EXAMPLE 9.5 Calculating the Adsorption Energy from the Brunauer-Emmett-Teller Isotherm. The BET analysis uses p/p0 rather than p as a variable just as we used this pressure ratio to compare Langmuir adsorption at different temperatures in Example 9.3. What corrections, if any, are needed in the apparent adsorption energy of about 14 kJ mole-1 as calculated in Example 9.3 ... 

Samples of rutile Ti02 were outgassed at elevated temperatures in some instances this was followed by subsequent exposure to 02 at 150°C. The following results were obtained by the BET analysis of N2 adsorption after various preliminary treatments ... 

Brunauer-Emmett-Teller (BET) analysis was carried out on the ceramic aggregate with both nitrogen and krypton as adsorbates. The sample was crushed and dry sieved, and the fraction between 74 and 589 micrometer was retained for analysis. Results were repeatable, and consistent for the two gases. Over the range of firing and proportioning conditions studied, specific surface area of the ceramic aggregate was between 0.3 and 6 m Vg, which can be compared, for example, to a non-porous... 

Table 2 Primary particle size dp derived from Lll measurements and BET analysis...

The BET method is not exact for the determination of the surface area of MMS of pore widths less than about 4 nm [20], Given this, pore filling is observed at pressures very close to the pressure range where a monolayer-multilayer formation on the pore walls takes place, which may lead to an important overestimation of the monolayer capacity in case of a BET analysis [20]. However, the BET surface area is extensively taken as a reproducible parameter for the characterization... 

If the standard BET procedure is to be used, it should be established that monolayer-multilayer formation is operative and is not accompanied by micropore filling (Section 11.2.1.8.C), which is usually associated with an increase in the value of C (>200, say). It should be appreciated that the BET analysis does not take into account the possibility of micropore filling or penetration into cavities of molecular size. These effects can thus falsify the BET surface areas and in case of doubt their absence should be checked by means of an empirical method of isotherm analysis or by using surface area reference samples (see Section 11.2.1.6.B). 

In zeolites, cracking catalysts and in activated carbons we deal in many cases with a combination of microporosity, mesoporosity and macroporosity. Here the isotherms are type I + type IV. Strictly speaking, BET analysis is not allowed here nevertheless it is frequently applied. 

Many microporous substances also contain meso and macropores. In such cases, isotherms are found which are a hybrid of isotherms type I and IV. As mentioned before (see Section 12.7), BET analysis is not allowed here. 

Experimental techniques commonly used to measure pore size distribution, such as mercury porosimetry or BET analysis (Gregg and Sing, 1982), yield pore size distribution data that are not uniquely related to the pore space morphology. They are generated by interpreting mercury intrusion-extrusion or sorption hysteresis curves on the basis of an equivalent cylindrical pore assumption. To make direct comparison with digitally reconstructed porous media possible, morphology characterization methods based on simulated mercury porosimetry or simulated capillary condensation (Stepanek et al., 1999) should be used. 

Table 3. The results of BET analysis for fresh and used catalysts...

An unusual feature of the Kr isotherm in Figure 10.33 is the presence of the three small sub-steps (a b and c), which could be detected by the use of the continuous quasi-equilibrium procedure for the determination of the adsorption isotherm. At present, the significance of these sub-steps is not clear, but it seems more likely to be related to the adsorption on the different crystal faces rather than to any phase changes of the adsorbate. BET analysis of the adsorption isotherms of Kr, Ar and N2... 

Nitrogen adsorption/condensation measurements were performed using an Autosorb-1 analyzer to calculate sample surface area and pore size distribution. BET analysis at 77 K was applied for extracting the monolayer capacity from the adsorption isotherm and a N molecular cross-sectional area of 0.162 nm2 was used to relate tne monolayer capacity to surface area. PSD s were calculated from the desorption branches of the isotherms using a modified form of the BJH method [18]. Mercury intrusion measurements were performed using an Autoscan-33 continuous scanning mercury porosimeter (12-33000 psia) and a contact angle of 140°. 

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