Foley adsorption pore-size distribution

Figure 6.4 Features of beta zeolite after Fenton treatment, (a) Saito-Foley adsorption pore-size distribution from Ar-physisorption for (O) parent zeolite containing the template (no porosity) ( ) Fenton-detemplated and (V) commercial NH4-form BEA.

Figure 3. (Left) Saito-Foley adsorption pore-size distribution based on Ar-physisorption ( ) parent BEA zeolite, ( ) one-pot catalyst, ( ) calcined zeolite at 923 K (H-form), (O) BEA zeolite NH4-form. The pore size distribution of BEA is schematically illustrated. (Right) N2O decomposition performance. Conditions 4.5 mbar N2O in He (pressure, 3 bara) and W/F N20= 900 kgxsxmoF. Two reference catalysts are included for comparison, based on NH4-form commercial samples prepared by conventional Fe ion-exchange.

To confirm the observations in Table 2, micropore size distributions were measured on selected materials using low-pressure N2 adsorption. The Saito-Foley (SF) pore size distribution of the selected materials is displayed in Fig. 3. Micropore size distributions as calculated by classical models like SF are also affected by monolayer adsorption on the external surface [7]. For this reason the obtained pore size distributions are corrected for mesoporous content. 

Since Eq. (3.42) was derived for a slit-like pore, its application to other geometries, such as cylindrical pores, requires further consideration. Saito and Foley [31] followed the same procedure as that used by Horvath and Kawazoe to derive an equation for cylindrical pores with specific applications to the determination of pore size distribution in zeolites. In addition to using a cylindrical potential energy function, they also made the following assumptions (1) a perfect cylindrical pore with infinite length (2) The formation of the inside wall of the cylinder by a single layer of atoms (oxide ions in the case of zeolites) and (3) adsorption taking place only on the inside wall of the cylinder and due, only, to the adsorbate and adsorbent interactions. The final equations derived by Saito and Foley are... 

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. 

Low-pressure adsorption isotherms of N2 at 77 K were measured on an ASAP 2010 from Micromeritics, equipped with a 0.1 kPa pressure transducer. The micropore size distribution has been deconvoluted using the Saito-Foley (SF-) method incorporating cylindrical pore shape geometry. This particular pore shape geometry is used since the micropores in the... 

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