Micropore surface

Once the micropore surfaces of activated carbon are saturated with target material, the spent carbon must be replaced or regenerated. Granular activated carbon (GAC) is favored over powder activated carbon (PAC) in most cases, because the former is considered to be capable of regeneration and sustainable to flow, although the costs of both carbons and the cost of regeneration are high. 

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... 

Angley et al., 1992) and the retarded rate of desorption and diffusion from the soil micropore surfaces to the macropore water. In the case of PCBs, the retarded rate of desorption can affect both lower- and higher-chlorinated PCB congeners however, the effect is likely to be more severe with higher chlorinated congeners. 

Ma et al. [104] attributed a decrease in diffusivity with an increase in initial concentration to pore diffusion effects. Because zeolites are bi-dispersed sorbents, both surface and pore diffusions may dominate different regions. In micropores, surface diffusion may be dominant, while pore diffusion may be dominant in macropores. This, therefore, supports the use of a lumped parameter (De). To explore further the relative importance of external mass transfer vis-a-vis internal diffusion, Biot number (NBl — kf r0/De) was considered. Table 9 summarizes the NBi values for the four initial concentrations. The NBi values are significantly larger than 100 indicating that film diffusion resistance was negligible. 

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. 

The BET surface areas of the zeolite samples were determined by N2 adsorption-desorption at -196 C in a Micromeritics ASAP 2010 equipment. Prior to the determination of the adsorption isotherm, the calcined sample (0.5 g) was outgassed at 400 C under a residual pressure of 1 Pa in order to remove moisture. The adsorption data were treated with the full BET equation. The t-plot method using the universal t-curve was applied in order to obtain an estimation of the micropore volume, microporous surface and external surface area [7]. 

Concerning the values of Vmicro and Sext, it was deduced that mild dealumination induced a slight decrease in the micropore volume for the samples H-Y(d32 / ) and H-Y(d5o / ). This effect could be ascribed to the formation of mesopores during dealumination, which could be accompanied by the destruction or blocking of part of the micropores [23]. For these samples, the contribution of the microporous surface to the total surface area was similar to that of the parent material (around 90-95%). As said before, the extent of this structural degradation appeared to be not very severe for dealuminated samples with a Si/Al ratio lower than 6.2 as evidenced from the results in Table 2. 

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. 

FIGURE 38.4 Composite membrane with a dense skin coated on the microporous surface. 

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-... 

After the oxidation a considerable part of the inner micropore surface contributes to the double layer. In order to estimate the "created inner surface area the capacitance of micropore per unit surface has to be known. According to Probstle et al. [5] this capacitance is 6 nFjcm. By disregarding pseudo capacitance, a specific inner micropore surface accessible for 5 04 -ions can be estimated now from the measured specific capacitance. The maximum specific capacitance of 12.-5 F/g yield from impedance spectroscopy at G.l Hz gives an inner surface of 208 m jg. 

---