Surface area from mercury porosimetry

Nitrogen adsorption/condensation is used for the determination of specific surface areas (relative pressure < 0.3) and pore size distributions in the pore size range of 1 to 100 nm (relative pressure > 0.3). As with mercury porosimetry, surface area and PSD information are obtained from the same instrument. Typically, the desorption branch of the isotherm is used (which corresponds to the porosimetry intrusion curve). However, if the isotherm does not plateau at high relative pressure, the calculated PSD will be in error. For PSD s, nitrogen condensation suffers from many of the same disadvantages as porosimetry such as network/percolation effects and pore shape effects. In addition, adsorption/condensation analysis can be quite time consuming with analysis times greater than 1 day for PSD s with reasonable resolution. 

The different pore volumes of the carbons and their corresponding activated carbons are compiled in Table I the micropore volume has been deduced from Nj (77 K) and CO2 (273 K) adsorption isotherms by use of DR equation (ref. 5) and the meso and macropore volume from mercury porosimetry. Table 1 also shows the Nj surface area deduced from BET equation and the CO2 surface area from DRK equation (ref. 5). Impiortant features related with these data are ... 

The incorporation of the new material without any increase in the overall length of the book has been achieved in part by extensive re-writing, with the compression of earlier material, and in part by restricting the scope to the physical adsorption of gases (apart from a section on mercury porosimetry). The topics of chemisorption and adsorption from solution, both of which were dealt with in some detail in the first edition, have been omitted chemisorption processes are obviously dependent on the chemical nature of the surface and therefore cannot be relied upon for the determination of the total surface area and methods based on adsorption from solution have not been developed, as was once hoped, into routine procedures for surface area determination. Likewise omitted, on grounds of... 

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... 

Cases that lead to porosimetry-measured surface areas exceeding those from nitrogen adsorption can result from ink-bottle shaped pores having a narrow entrance with a wide inner body. Intrusion into the wide inner body will not occur until sufficient pressure is applied to force the mercury into the narrow entrance. It will, therefore, appear as if a large volume intruded into narrow pores, generating an excessively high calculated surface area. 

Besides the three methods introduced above, there are many other methods of surface area determination Any surface-dependent phenomenon can be used for such measurement [24], Some available methods (mercury porosimetry, adsorption from solution, adsorption of dyes, chemisorption, density methods, and secondary ion mass spectroscopy) are explained in more detail elsewhere [6,30,31,32],... 

The applied pressure is related to the desired pore size via the Washburn Equation [1] which implies a cylindrical pore shape assumption. Mercury porosimetry is widely applied for catalyst characterization in both QC and research applications for several reasons including rapid reproducible analysis, a wide pore size range ( 2 nm to >100 / m, depending on the pressure range of the instrument), and the ability to obtain specific surface area and pore size distribution information from the same measurement. Accuracy of the method suffers from several factors including contact angle and surface tension uncertainty, pore shape effects, and sample compression. However, the largest discrepancy between a mercury porosimetry-derived pore size distribution (PSD) and the actual PSD usually... 

The pore size distribution (see Fig. 3) can be obtained from the mercury porosimetry data and the t-plot from N2 adsorption isotherms, using an active carbon with a very low surface area as a reference [13]. It was observed that the volumes of mercury intruded were very small. As a consequence, the volumes of meso (the largest ones) and macropores are low. Thus, the samples studied are mainly microporous, as already mentioned in the N2 and CO2 adsorption isotherm results. 

For macroporous samples (pore size greater than 50 nm), the absence of any capillary condensation phenomenon means that only the specific surface area can be obtained from the adsorption isotherm using the BET equation. Mercury porosimetry (Paragr. 1.2) will then be necessary to obtain the pore size distribution. 

In the present work the meso- and macro-structural characteristics of the mesoporous adsorbent MCM-41 have been estimated with the help of various techniques. The structure is found to comprise four different length scales that of the mesopores, the crystaUites, the grains and of the particles. It was found that the surface area estimated by the use of small angle scattering techniques is higher, while that estimated by mercury porosimetry is much lower, than that obtained from gas adsorption methods. Based on the macropore characterization by mercury porosimetry, and the considerable macropore area determined, it is seen that the actual mesopore area of MCM-41 may be significantly lower than the BET area. TEM studies indicated that MCM-41 does not have an ideal mesopore structure however, it may still be treated as a model mesoporous material for gas adsorption studies because of the large radius of curvature of the channels. 

On the basis of the results of various characterization techniques, it was found that MCM-41 consists of 4 levels of structure mesopores, crystaUites, grains and particles. AU these levels have been successfuUy characterized. Estimates of surface area by SAXS and SANS are higher, while those from mercury porosimetry are much lower, than those estimates by BET methods the estimates obtained from geometrical consideration using variable waU thickness are close to the BET results. It was confirmed that mesopores in MCM-41 are curved rather than straight channels and, even though they do not have an ideal mesopore structure, they can be considered as model mesoporous materials for gas adsorption studies. 

A) Pressure-controlled mercury porosimetry procedure. It consists of recording the injected mercury volume in the sample each time the pressure increases in order to obtain a quasi steady-state of the mercury level as P,+i-Pi >dP>0 where Pj+i, Pi are two successive experimental capillary pressure in the curve of pressure P versus volume V and dP is the pressure threshold being strictly positive. According to this protocol it is possible to calculate several petrophysical parameters of porous medium such as total porosity, distribution of pore-throat size, specific surface area and its distribution. Several authors estimate the permeability from mercury injection capillary pressure data. Thompson applied percolation theory to calculate permeability from mercury-injection data. 

The GMA-EGDM copolymer was synthesised from a GMA EGDM molar feed ratio of 31.5 68.5. The crosslinking density of the copolymer beads was 128%. Particle size of copolymer was in the range of 250-420 pm and the surface area estimated by single point BET method was 152 M / g. The pore volume estimated by mercury porosimetry was 0.74 cm /g. The polymer bound 2-picolyl amine was generated by the reaction of 2-picolyl amine... 

In Table 1 the results obtained from the textural characterization of the supports and catalysts by nitrogen adsorption and mercury intrusion porosimetry are presented. In the table the values of surface area obtained from the gas adsorption results, using the BET method for which the linear portion was usually located in the relative pressure range of 0.05 to 0.3 Sbet [9], and those from the intrusion curve of the porosimetry analysis, using a nonintersecting cylindrical pore model Sng [10], are shown. The pore volume Vp is that recorded at the liighest intrusion pressure reached during the porosimetry analysis, and as such represents the pore volume of pores between ca. SOpm to 3mn pore radius. The pore radii were taken from the maxima of the curves of pore size distribution. 

Sometimes mercury porosimetry and adsorption ofp-nitrophenol from an aqueous solution [245, 246] are also used to determine the surface area. The disadvantages of mercury intrusion technique will be discussed below. Regarding the method of adsorption firom solution, its drawback is associated, first of all, with the uncertainty in the determination of the cross-sectional area of the p-nitrophenol molecules, in which the benzene rings may adsorb either transversely or parallel to the surface. Also some sorbents swell in water, and this may lead to a misrepresentation of the surface area of the adsorbing resin in dry state. 

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