Mesopore filling

Nitrogen adsorption-desorption isotherms of MCM-41-IBU after 2 hours of immersion in SBF show the characteristic mesopore filling at p/p° below 0.25 (type IV isotherm). 

At 9 hours of immersion, instead, isotherms do not show the pore filling associated with mesopores, which in turn appears again between 25 and 26 hours. After 28 hours of soaking, no mesopore filling is observed (figure 3). The DFT pore size distributions also confirm the presence of mesopores (around 2.2 nm) only at 2 hours of immersion and between 25 and 26 hours. The peak at around 5 nm is probably due to the textural interparticles porosity (figure 3 inset). 

In spite of its semi-empirical origin, the FHH equation does provide a means of identifying the different effects of micropore and mesopore filling (Carrott et a ., 1982). Capillary condensation in mesopores necessarily restricts the range of linearity of the FHH plot and tends to reduce the value of s. Although micropore filling may not significantly affect the linearity of the FHH plot, it does lead to an increase in s. These aspects are discussed in relation to fractal analysis in a recent paper by Sahouli et al. (1997). 

Nitrogen adsorption has become generally accepted as the standard method for mesopore size analysis. The recommended experimental and computational procedures involved are described in various official publications (e.g. IUPAC Sing et al., 1985 and Rouquerol et al., 1994 British Standard 7591, Part 2, 1992). However, the underlying principles of mesopore Filling are still not fully understood and in... 

We turn now to the question of validity of the Kelvin equation. Although the thermodynamic basis of the Kelvin equation is well established (Defay and Prigogine 1966), its reliability for pore size analysis is questionable. In this context, there are three related questions (1) What is the exact relation between the meniscus curvature and the pore size and shape (2) Is the Kelvin equation applicable in the range of narrow mesopores (say >vp < 5 nm) (3) Does the surface tension vary with pore width The answers to these questions are still elusive, but recent theoretical work has improved our understanding of mesopore filling and the nature of the condensate. 

A third possibility is a Type I isotherm with a short plateau, which terminates at p/p°< 1. An upward deviation, as indicated in Figure 8.1c, occurs at high p/p° when the microporous adsorbent also contains some wide mesopores or narrow macropores. Since the wall area of such relatively wide pores is likely to be much smaller than the micropore area, the scale of multilayer development or mesopore filling may be quite small. 

The view is generally held that capillary condensation is responsible for mesopore and macropore filling (i.e. in pores of width 2 nm). Since the filling of macropores (to > 50 nm) occurs at very high pjp°, we are essentially concerned with mesopore filling. Capillary condensation can be regarded as a secondary process, which is always preceded by adsorption on the pore walls. 

To convert the micropore capacity into the micropore volume, it is usually assumed that the pores are filled with liquid adsorptive - as in the case of mesopore filling. This procedure does not allow for the dependency of molecular packing on both pore size and pore shape. For this reason, we recommend that the term apparent micropore volume should be adopted and that the gas and temperature should always be specified. 

Low temperature N2 adsorption isotherm gives a reliable information on the mesoporous texture of solids. The adsorption-desorption plot follow the type IV isotherm with hysterisis in the mesopore filling region, the pore size distribution obtained by BJH analysis of nitrogen adsorbed is shown in Fig.3. The pore size distribution is narrow and the maximum is centered around 30 A for all the samples indicating a uniform pore texture of the samples... 

For composites calcined at 350°, the total efficiency first declines as the AC particle size decreases increasing again for the finest particles (Fig. 9). The efficiency with respect to C5+ hydrocarbons is lower for the finest AC than that for the most coarse fiiaction. For this series of samples, selectivity to various hydrocarbon fiiactions varied slightly and irregularly. Specific catalytic activity in FTS is usually expected to grow with decreasing the AC particle size below 0.1 mm due to decreasing difihision limitations for the reactants (first of all, CO) transport in the catalyst micro-and mesopores filled with condensed hydrocarbons [20]. Hence, for composite catalysts, performance of even coarse fractions seems not to be affected by the mass-transfer limitations. 

On the other hand, an offset in the t-plot is indicating additional sorption sites filled at low relative pressures, i.e., micropores, and the height of the offset is interpreted as the micropore volume Vmic. At large layer thicknesses above 0.6 nm (corresponding to pjp > 0.4) an upward bend of the curve indicates the beginning of mesopore filling (in Figure 21.27 the upward bend does not start until t > 0.9 nm). 

It is assumed that the surface tension and the molar volume are independent of the radius of curvature associated with the porosity. Corrections or adjustments can be made to the above Kelvin equation because in an adsorption process, at the stage when mesopore filling occurs, already the walls of the pore contain adsorbed material such that the effective diameter of the pore is reduced and /"p = tk + (where t is the thickness of the adsorbed layer). Another correction is where the pore shape is not cylindrical but is perhaps slit-shaped when the meniscus is hemi-cylindrical and the effective pore width Wp = + 2t. However,... 

Neutron diffraction characterization MCM-41 structure and mesopore filling signatures. 

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