Material characterization methods mercury porosimetry

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. 

In gas adsorption for micro-, meso- and macropores, the pores are characterized by adsorbing gas, such as nitrogen at liquid-nitrogen temperature. This method is used for pores in the ranges of approximately <2 nm micropores), 2 to 50 nm (mesopores), and >50 nm macropores) (ISO/FDIS 15901-2, Pore Size Distribution and Porosity of Solid Materials—Evaluation by Mercury Porosimetry and Gas Adsorption, Part 2 Analysis of Meso-pores and Macro-pores by Gas Adsorption ISO/FDIS 15901-3, Part 3 Analysis of Micro-pores by Gas Adsorption). An isotherm is generated of the amount of gas adsorbed versus gas pressure, and the amount of gas required to form a monolayer is determined. 

Mercury porosimetry is the most suitable method for the characterization of the pore size distribution of porous materials in the macropore range that can as well be applied in the mesopore range [147-155], To obtain the theoretical foundation of mercury porosimetry, Washburn [147] applied the Young-Laplace equation... 

Physico-chemical techniques are widely used for characterization of catalysts and porous materials in general. Well-known methods based on physical adsorption of inert gases (N2 and CO2) and penetration of mercury at elevated pressures provide information on the total surface area, pore volume, and pore size distribution (PSD) of the sample [1,2]. Gas adsorption and mercury porosimetry are often compared since they generate data of similar nature in the pore size range 4 - 100 nm. 

The texture properties of the ultrathin porous glass membranes prepared in our laboratory were initially characterized by the equilibrium based methods nitrogen gas adsorption and mercury porosimetry. The nitrogen sorption isotherms of two membranes are shown in Fig. 1. The fully reversible isotherm of the membrane in Fig. 1 (A) can be classified as a type I isotherm according to the lUPAC nomenclature which is characteristic for microporous materials. The membrane in Fig. 1 (B) shows a typical type IV isotherm shape with hysteresis of type FIl (lUPAC classification). This indicates the presence of fairly uniform mesopores. The texture characteristics of selected porous glass membranes are summarized in Tab. 1. The variable texture demanded the application of various characterization techniques and methods of evaluation. 

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. 

The testing of battery separators and control of their pore characteristics are important requirements for proper functioning of batteries. Mercury porosimetry historically has been used to characterize the separators in terms of percentage porosity, mean pore size, and pore size distribution." In this method, the size and volume of pores in a material are measured by determining the quantity of mercury. 

The H NMR cryoporometry method can be used to measure broad pore sizes (Figure 1.239) if octamethylcyclotetrasiloxane is used as an adsorbate (Ono et al. 2009). Three very different methods such as the nitrogen adsorption, the H NMR cryoporometry, and the mercury porosimetry give the PSDs, which are mutually complementary. These results confirm that organic molecules are better probe compounds than water for the structural characterization of macroporous materials with NMR cryoporometry or relaxometry. 

The use of electron microscopy, nitrogen adsorption and mercury porosimetry for the characterization of porous materials have laeen treated extensively elsevhere and these methods will only briefly Ise reviewed here. Hcmever, the use of SEC for tlie determination of PSD has, until recently, been founded on enpirical ground only and therefore, this technique will be discussed in detail. The paragr ih is concluded with a ocnparison of the results obtained with the different methods. 

A well-known method for characterization of porous materials is mercury porosimetry. Mercury has a high surface tension and has to be forced into the pores of the material in order to fill the pores. From the uptake of mercury as a function of pressure one can calculate the pore size distribution based on volume in terms of equivalent cylindrical capillaries. The total uptake of mercury at the maximum applied pressure gives the porosity. 

Mercury porosimetry is a method currently used to characterize the texture of porous materials. It enables determining pore volume, specific surface area and also distributions of pore volume and surface area versus pore size. It can be applied to powders, as weU as to monolithic porous materials. The basic hypothesis usually accepted is that mercury penetrates into narrower and narrower cavities or pores as pressure increases. Data analysis is performed using the intrusion equation proposed by Washburn (1921) ... 

The determination of the buckling constant h by calibration from mercury intrusion porosimetry, or from nitrogen adsorption-desorption, can lead in some cases to different results. It is likely that, beyond the imprecision due to the method, the differences in pore sizes observed arise from a fundamental difference in the pore size concept. lUPAC proposed to define pore size as the distance between two opposite pore walls (Rouquerol, 1994). In the case of materials from the sol-gel process, this definition is not applicable, because pores are not included between walls, but are only delimited by interconnected filaments. In practice, it is considered that the sizes obtained from analysis methods of the texture of porous materials are characteristic pore sizes. Because the different analysis methods are based on different physical phenomena, it is not astonishing that they lead to slightly different characteristic pore sizes. Discrepancies resulting from using different characterization methods appear in several publications, often when the same material is analyzed by nitrogen adsorption-desorption and mercmy intrusion porosimetry (Smith, 1990, Brown, 1974, Milburn, 1988, Minihan, 1994). Me Enaney et al. noted that the distribution profiles obtained by different characterization techniques are often similar, but that differences, sometimes important, between the absolute values of characteristic pore sizes are almost unavoidable (Me Enaney et al., 1995). 

Mercury porosimetry (MP) is an extremely useful technique to characterize pore structure of materials (Giesche, 2006). This method measures an average diameter of open pores and its distribution, total volume of pores, specific surface, density, etc. Limitation of this method is that high pressures can distort the actual pore structure. Besides it does not give the actual size of pores or capillaries, but equivalent diameter of model cylindrical pores. Closed pores are inaccessible to mercury and cannot be studied. 

Mercury porosimetry is featured in many of the contributions to this volume. Indeed, it is now one of the most popular methods available for the characterization of a wide range of porous materials and the derived pore sizes are often quoted in the patent and technical literature. The method is based on the non-wetting nature of mercury and the application of the Washburn equation. The volume of mercury penetrating into a porous solid is determined as a function of the applied pressure, which is assumed to be directly related to the pore width. 

Although a number of methods are available to characterize the interstitial voids of a solid, the most useful of these is mercury intrusion porosimetry [52], This method is widely used to determine the pore-size distribution of a porous material, and the void size of tablets and compacts. The method is based on the capillary rise phenomenon, in which excess pressure is required to force a nonwetting liquid into a narrow volume. 

As new membranes are developed, methods for characterization of these new materials are needed. Sarada et al. (34) describe techniques for measuring the thickness of and characterizing the structure of thin microporous polypropylene films commonly used as liquid membrane supports. Methods for measuring pore size distribution, porosity, and pore shape were reviewed. The authors employed transmission and scanning electron microscopy to map the three-dimensional pore structure of polypropylene films produced by stretching extended polypropylene. Although Sarada et al. discuss only the application of these characterization techniques to polypropylene membranes, the methods could be extended to other microporous polymer films. Chaiko and Osseo-Asare (25) describe the measurement of pore size distributions for microporous polypropylene liquid membrane supports using mercury intrusion porosimetry. 

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