Micropore surface area

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... 

Fractional Conversion of CaCOj m Average Micropore Radius, ym Micropore Surface Area Sg, cm2/g S S ... 

The second procedure directly relates the energy of immersion A, U to the micropore surface area a(mic), as described in Section 6.5.2. As we saw, very simply ... 

Immersion of dry samples in liquids of different molecular size This method is designed to take advantage of molecular sieving. The basic data are simply in the form of a curve of the specific energy of immersion versus the molecular size of the immersion liquid. This provides immediate information on the micropore size distribution. For room-temperature experiments one can use the liquids listed in Table 8.1, which are well suited for the study of carbons. Because of the various ways of expressing the critical dimension of a molecular probe or its molecular size , one must be careful to use a consistent set of data (hence the two separate lists in Table 8.1). Again, one can process the microcalori-metric data to compare either the micropore volumes accessible to the various molecules (see Stoeckli et a ., 1996), or the micropore surface areas, as illustrated in Figure 8.5. 

Table III shows XRD and porosimetry data for calcined USY and AFS zeolites. All samples show shrinkage of the unit cell to comparable values following calcination. As a result, calcined samples are compared at similar silica-alumina framework ratios. All calcined samples have well developed microporous structures and comparable total pore volumes. These porosimetry data confirm that the hydrothermally dealuminated materials contain a significant fraction of mesopores relative to chemically dealuminated materials. The extensive washing given to AFS-1 results in higher micropore surface area and volume compared to AFS-2 and suggest that AFS-2 contains occluded fluoroaluminate and fluorosilicate compounds within the microporous structure.

USY zeolites reduce to comparable values. Steam treatment decreases micropore surface area and increases mesopore volume. With the exception of AFS-1, all samples have comparable micropore volumes and surface areas. AFS-1 shows a large decrease in surface area relative to the other samples. 

The ZSM-5 catalyst shows quite high selectivity in the formation of paraffins and olefins and branched hydrocarbons, while the yield of gases is also high. Both high yields of gases and lighter liquids are the consequence of the large microporous surface area. 

Micropore Surface Area with respect to Fe Content of Pillared Clays, Ali-xFexOy. 

Table 2. Porous properties of the kaolin, metakaolins and the acid leached samples BET surface area (from BET method), external surface area, micropore surface area and micropore volume (from t-method).

It has to be mentioned that the micropore surface area values, calculated by Stoeckli-Ballerini equation [9], present a maximum value with the burn-off degree of the samples, either from the N2 or CO2 adsorption isotherms data. This is due to the increase in the pore size (L values), and the assumptions made in the equations. Thus, as the pore size -L- increases, the micropore surface area for a determined micropore volume -Wo- is smaller. 

Hodson M. E. (1999) Micropore surface area variation with grain size in unweathered alkali feldspars implications for surface roughness and dissolution studies. Geochim. Cosmochim. Acta 62(21-22), 3429-3435. 

The main porous structure characteristics (Table 2) were determined on the basis of benzene vapor adsorption isotherms using McBain-Baker sorption balances at 20°C (293 K), i.e., the specific BET surface area (5bht) [39], the surface area of mesopores (5 ,e), and the parameters of the Dubinin-Radushkevich equation (the volumes of the micropores and supermicropores. Woi and W 2, and the characteristic energies of adsorption, E, and o ) 136,37). In addition, the total micropore volume (ZVT, ) and geometric micropore surface area (5J 1168] were calcu-... 

Figure 2 Evolution of a) the total surface area (BET), b) the microporous surface area (t-plot) and c) the pore diameter as a function of the number of graftings (Table 1).

Double-layer properties of porous carbon materials have been widely investigated in relation to the development of the electrochemical capacitors. For detailed information the reader should consult specialized literature. For porous carbons materials, the double-layer capacitance depends on their specific snrface area [82,83], pore stmcture (notably, the pore size distribntion) [84-87], and their crystalline stmctnre and snrface chemistry [83,88,89], Shi [84] measnred the dc capacitance of varions carbons in a KOH electrolyte and noticed that the overall capacitance may reasonably be described as a sum of the capacitance of micro- and mesopores. Assuming that the electrical double layer propagates into micropores accessible for N2 adsorption, the author estimated the differential donble-layer capacitance per unit of micropore surface area as 15 to 20 p,F/cm. Lower values were reported by Vilinskaya... 

Figure 4. Effect of CO2 pressure on on morphology of macroporous crosslinked polymer monolith, (a) BET surface area (continuous line = total surface area, hed line = micropore surface area) (b) Percent micropore volume (c) Median pore diameter (d) Intrusion volume (macropore volume).

To increase the micropore volume, the above carbon aerogels were activated with CO2 at 1173 K for 1-7 h [146]. Activation for 5 h increased both the microporosity and mesoporosity the micropore volume and microporous surface area are 0.68 cm g and 1750 m g respectively, and the mesopore volume and mesoporous surface area are 2.04 cm g" ... 

BET surface and pore volume data of the four carbon supports are shown in Table 1. Pecan-based AC has the largest surface area and the largest total pore size. Wood-based AC has the highest fraction of microporous surface area and the highest fraction of micropore volume, but the largest average pore... 

Increase in Total Available Surface Area For conventional zeolites, having crystal sizes in the micrometer range, the proportion of external surface area is usually negligible, that is, the Brunauer-Emmett-TeUer (BET) surface area corresponds almost completely with the surface area associated to the micropores. However, in the case of hierarchical zeolites, the presence of mesoporosity implies that a great part of the surface area is related to the latter, while a reduction is usually observed in the micropore surface area compared with the standard zeolites. 

Alloy Etching time [min] BET surface area [mVg] Micropore surface area [mVg] Micro- pore volume [cmVg] Total volume of pores (micro-and mesopores <100 nm) [cmVg] Average pore size [nm]... 

Fig. n.3 Control over pore size for porous polymers prepared in SCCO2 can be achieved by adjusting the solvent density. The graph shows the variation in total BET surface area (closed circles) and micropore surface area (open circles) for a cross-linked poly(meth-acrylate) as a function of CO2 pressue. (Reproduced from [38]). 

All of the above seven organically modified siloxane resins were subject to pyrolysis at 450 °C for 2 h. Porosity measurements were performed on the pyrolyzed resins using the nitrogen sorption method (Table 1). The surface areas, calculated using the BET equation [6], were considered to give the total internal and external surface area of the material. The micropore surface areas and micropore volume were calculated using the de Boer r-method. 

---