Mesopore surface

Fig. 15. Mesopore surface area as a function of pore diameter obtained from mercury intrusion data for PAN derived carbon fiber porous monoliths [28].

The support and the catalysts were characterised by means of nitrogen adsorption, XPS, TPD and SEM. The nitrogen adsorption isotherms were determined at 77 K in a Coulter Omnisorp 1000 CX equipment, and were analysed by the BET equation (SBet), and by the t-plot for mesopore surface area (Smeso) and micropore and mesopore volume (Vmicr0, Vmeso), using the standard isotherm for carbon materials. The catalyst samples were previously outgassed at 120 °C. 

Figure. 1 Influence of NaY-zeolite content in Y-AI2O3 on specific surface area SBet and external + mesopore surface area St...

The investigated catalysts [9] were Pt-Pd/USY zeolite (SiO2/Al203 ratio 33.5, total- and mesopore surface areas 650 m2/g and 51 m2/g, total metal content 0,9%, Pd/Pt mass ratio 6 1 to 1 3, dispersion 0.41-0.55, acidity 0.20 mmol/g). Metal contents and Pd/Pt ratios are summarized in Table 1. As reference catalysts bimetallic Pt-Pd/Si02-Al203 (platinum-content 0.3%, palladium-content 0.6%, A1203 content 15%, surface area 292 m2/g, dispersion 0.41, acidity 0.18 mmol/g) and Pt-Pd/y-Al203 (platinum-content 0.3%, palladium-content 0.6%, surface area 182 m2/g, dispersion 0.35, acidity 0.12 mmol/g) were used. 

The deficiencies of measuring the surface areas by the BET method are overcome in comparative methods (CMs) [3,61,62], known in the literature by their modifications s method by Sing [53,63], t method by Lippens-de Boer [2,64], / method by Gregg [65], t/f method by Kadlec-Dubinin [66,67], micropoie analysis (MP) method by Brunauer [68], etc. All of these variants are based on comparison of a measured AI with the reference (or standard) AI (RAI). Obviously, both AI and RAI should be measured under similar conditions, that is, usually N2 at 77 K in some specific range of P/P0, which is determined by the nature of the studied material. There are numerous variants of RAI, measured on standards, but there are strong requirements to the choice of a standard it should have no micro- and mesopores, surface modification, and one should be able to measure its surface area by means of independent methods, at least by the BET method. 

Mesopore surface area mesopore volume " avarage mesopore diameter broad mesopore... 

Texture parameters from adsorption isotherms, i. e, total surface area (Stot), external surface area (Sext), mesopore surface area(Sme). mesopore volume (VTO) and mesopore diameter... 

The adsorption isotherm of N, on FSM-16 at 77 K had an explicit hysteresis. As to the adsorption hysteresis of N-, on regular mesoporous silica, the dependencies of adsorption hysteresis on the pore width and adsorbate were observed the adsorption hysteresis can be observed for pores of w 4.0nm. The reason has been studied by several approaches [5-8]. The adsorption isotherm of acetonitrile on FSM-16 at 303K is shown in Fig. 1. The adsorption isotherm has a clear hysteresis the adsorption and desorption branches close at PIP, = 0.38. The presence of the adsorption hysteresis coincides with the anticipation of the classical capillary condensation theory for the cylindrical pores whose both ends are open. The value of the BET monolayer capacity, nm, for acetonitrile was 3.9 mmol g. By assuming the surface area from the nitrogen isotherm to be available for the adsorption of acetonitrile, the apparent molecular area, am, of adsorbed acetonitrile can be obtained from nm. The value of am for adsorbed acetonitrile (0.35 nnr) was quite different from the value (0.22 nm2) from the liquid density under the assumption of the close packing. Acetonitrile molecules on the mesopore surface are packed more loosely than the close packing. The later IR data will show that acetonitrile molecules are adsorbed on the surface hydroxyls in... 

Zhang and co-workers reported partial conversion of a mesoporous starting material (SBA-15) into a mesoporous aluminosilicate with zeolitic characteristics in a so-called vapour phase transport method.[82] In this process, Al is firstly introduced onto the mesoporous surface, followed by a filling of the mesopores with a carbonaceous species, and finally a partial recrystallization of aluminosilicate in the vapour of the SDA is conducted. The advantage of this method, compared with the hydrothermal recrystallization method of Kloetstra et al., lies in the fact that the mesopore structure collapses to a lesser extent as the crystallization is limited to the surface of the mesoporous precursor. 

The classification of pores is also based on the difference in the types of molecular interactions controlling adsorption in the different groups. That is, in micropores, the overlapping surface forces of opposing pore walls are predominant in mesopores, surface forces and capillary forces are significant, while for the macropores, the contribution of the pore walls to surface forces is very small [15,19],... 

The deviations from linearity shown by the as-plots on gels B and C in Figure 10.11 confirm the respective mesoporous and microporous nature of these silica gels. Thus, the as-plot on gel B is linear up to pjp° = 0.4 (i.e. as = 1). This indicates that the argon monolayer has formed on the mesopore surface in the same manner as on the non-porous surface. The upward deviation is clearly due to the onset of capillary condensation. 

The parameters of the pore size distribution flmction given in Table I are related to the microporosity of the activated carbon. The total pore volume is 0.272 cm g. We compared these results to those provided by the classical characterization methods based on the treatment of a nitrogen isotherm at 77 K. (t-plot, D-R, H-K). The micropore volume provided by the t-plot method is 0.413 cm g. The macropore and mesopore surface area is 62 m g". The t-plot method shows that the main part of the microporosity is located in the very low pore diameter area (//<7.08 A). For such pore diameters, the t-plot method should be considered with care as it is valid for a layer thickness higher than 3.54 A. The H-K method as well as the D-R method provide a total micropore volume of 0.414 cm g. The maximum of the pore size distribution flmction provided by the H-K method is located at //=9.2 A. 

Figure 2. Specific surface area, Sbet (a), and estimated mesoporous surface area (b), StM o, Sbjh, and i Bda , as a function of the weight fraction of microporous material y.

For the five mixtures, a downward deviation from linearity is first observed and then an upward deviation corresponding to mesopores. It is sometimes possible to estimate the mesoporous surface area, Sext, from the slope of the second linear portion of the /-plot,... 

For the five mixtures, the cumulative mesoporous volume, Feds, and mesoporous surface area, S edB, and are both linear decreasing functions of the micropore content y (Figure 2b). The cumulative specific surface area SedB is definitely a better estimator of the mesoporous surface than the specific surface S xt computed Ifom the t-plot. The lUPAC classification states that mesopores are pores whose width is larger that 2 nm. In the case of the cylindrical pore model retained for the pore size distribution, this is equivalent to radii larger than 1 nm. It should however be stressed that the calculation of the cumulative surface and volume of the mesopores must not be continued at lower pressures than the closing of the hysteresis loop (gray zones of Figures 3a and 3b). If a black box analysis tool is used and if the calculation is systematically continued down to 1 nm, severe overestimation of the mesopores surface and volume may occur. 

BJH, Dollimore-Heal and BdB methods were compared for assessment of mesoporosity. Owing to the pore shape of the material used, the first two methods lead to an underestimation of pore size, and hence to an overestimation of total mesoporous surface. The last method is more general, and when the appropriate shape factor is used, a reliable estimation of the mesoporous texture is obtained. The S sdB estimator is almost a perfectly linear function of y, and vanishes in the X(IOO) sample. The external surface area, Xext, computed from the t-plot leads to an overestimation of the true mesoporous surface. 

The results indicate that the activity of carbons used to reduce electrochenii-cally generated oxygen species depends to a large extent on surface development, in particular on the contribution of the mesopore surface to the total specific surface area. This is reasonable for a basic supporting electrolyte. In an acidic environment, however, this process depends additionally on the previous chemical oxidation of the electrode material surface. These results may constitute a basis for the selection of active carbons for u.se as electrode materials. 

---