Porous materials, characterization

Extensive experimental techniques have been developed for porous material characterization [1], including direct imaging [2-5] and bulk measurement techniques for the statistical properties of the pore space. NMR is one such bulk measurement that is both non-destructive and compatible with large samples. 

Temperature-programmed desorption (TPD) is a very useful methodology in porous materials characterization. A basic feature of the experiment is that, it is necessary to use an appropriate adsorbate, such as CO, NH3, or HzO [94-96] however, recently larger molecules have been applied as adsorbates [97],... 

In Sections 15.2 through 15.6, the terminologies macropores and micropores are based on that used in porous electrode theory, with the term macropores denoting the electrolyte-filled continuous interparticle space in between carbon particles, serving as transport pathways for ion transport across the electrode, whereas the term micropores is used for all the pore space within the carbon particles (intraparticle porosity see Figure 15.5b). In Section 15.7, the formal lUPAC terminology for porous material characterization is used where macro-, meso-, and micropores are distinguished on the basis of the pore sizes in a porous material. ... 

In this way MTPM distinguishes between transport properties of gases (gas viscosity, binary bulk-diffusion coefficients of all gas pairs) and textural properties of porous materials characterized by the set of transport parameters (, , ly). Transport parameters represent material properties of the porous solid, and, thus do not depend on temperature, pressure and the kind of used gases. The obtained transport characteristics have a wide practical use for simulation and prediction in many industrial processes (e.g. calculation of effective diffusion coefficients for any pairs of gases in automotive catalytic converter [11]). 

Thomas, Martin A., Ph.D. Powder Density Equations. Powder and Porous Materials Characterization. 2005. Quantachrome Instruments. July-Aug. 2006 . 

Increasing the surface-to-bulk ratio of the sample to be studied. This is easily done in the case of highly porous materials, and has been exploited for the characterization of supported catalysts, zeolites, sol-gels and porous silicon, to mention a few. 

Surface evaporation can be a limiting factor in the manufacture of many types of products. In the drying of paper, chrome leather, certain types of synthetic rubbers and similar materials, the sheets possess a finely fibrous structure which distributes the moisture through them by capillary action, thus securing very rapid diffusion of moisture from one point of the sheet to another. This means that it is almost impossible to remove moisture from the surface of the sheet without having it immediately replaced by capillary diffusion from the interior. The drying of sheetlike materials is essentially a process of surface evaporation. Note that with porous materials, evaporation may occur within the solid. In a porous material that is characterized by pores of diverse sizes, the movement of water may be controlled by capillarity, and not by concentration gradients. 

Improved characterization of the morphological/microstructural properties of porous solids, and the associated transport properties of fluids imbibed into these materials, is crucial to the development of new porous materials, such as ceramics. Of particular interest is the fabrication of so-called functionalized ceramics, which contain a pore structure tailored to a specific biomedical or industrial application (e.g., molecular filters, catalysts, gas storage cells, drug delivery devices, tissue scaffolds) [1-3]. Functionalization of ceramics can involve the use of graded or layered pore microstructure, morphology or chemical composition. 

The substantial literature on the bulk characterization of porous materials using conventional techniques provides a useful foundation as a starting point for overlapping the basic physics of traditional materials analysis with the parameters that can be measured using NMR. 

NMR signals are highly sensitive, via a number of different mechanisms, to the physical and chemical properties of porous materials. Using the set of NMR-based measurement methods that we have developed, it is possible to non-invasively and non-destructively characterize both the microstructural properties of the materials and relaxation properties of fluids imbibed into these materials. 

One must understand the physical mechanisms by which mass transfer takes place in catalyst pores to comprehend the development of mathematical models that can be used in engineering design calculations to estimate what fraction of the catalyst surface is effective in promoting reaction. There are several factors that complicate efforts to analyze mass transfer within such systems. They include the facts that (1) the pore geometry is extremely complex, and not subject to realistic modeling in terms of a small number of parameters, and that (2) different molecular phenomena are responsible for the mass transfer. Consequently, it is often useful to characterize the mass transfer process in terms of an effective diffusivity, i.e., a transport coefficient that pertains to a porous material in which the calculations are based on total area (void plus solid) normal to the direction of transport. For example, in a spherical catalyst pellet, the appropriate area to use in characterizing diffusion in the radial direction is 47ir2. 

David B. Graves and Cameron F. Abrams, Molecular Dynamics Simulations if Ion-Surface Interactions with Applications to Plasma Processing Christian M. Lastoskie and Keith E. Gubbins, Characterization of Porous Materials Using Molecular Theory and Simulation... 

Confinement effects may also be employed to characterize the nucleation and growth of porous materials [211]. The underlying mechanisms of self-assembly and crystallization of these complex heterogeneous systems may be traced by solid state NMR methods well before their detection by diffraction methods. 

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. 

Krypton Sorption. Volumetric adsorption using gases with low saturated vapor pressure has been found to be an effective technique to gain detailed structural information for small quantities of porous materials, especially using krypton (Kr).27 The substitution of nitrogen by Kr reduces significantly the amount of unadsorbed molecules in the dead volume, allows for the characterization of small surface areas, and is thus ideal for mesoporous... 

X-ray photoelectron spectroscopy is indeed quite informative, but requires the use of expensive instrumentation. Also, the detection of photoelectrons requires the use of ultrahigh vacuum, and therefore can mostly be used for ex situ characterization of catalytic samples (although new designs are now available for in situ studies [146,147]). Finally, XPS probes the upper 10 to 100 A of the solid sample, and is only sensitive to the outer surfaces of the catalysts. This may yield misleading results when analyzing porous materials. 

Transmission electron microscopy (TEM) can provide detailed stmcture of zeolites. I use the word characterize or characterization for stmctural study on a unit cell scale, such as various kind of stmctural defects and basic stmctural units, and determine or determination for obtaining atomic coordinates within the unit cell for all the atoms of a crystal. A simple text or reviews for stmctural characterization of porous materials can be found in a book or review articles [1-6]. Now, we are in a new era, that is, we can determine new stmctures of micro- and mesoporous materials only by electron microscopy(EM), an area called electron crystallography (EC) [7-11]. 

Another technique, capillary flow porometry has been developed by Porous Materials Inc. ° to characterize battery separators.The instrument can measure a number of characteristics of battery separators such as size of the pore at its most constricted part, the largest pore size, pore size distribution, permeability, and envelope surface... 

Czuryszkiewicz T, Rosenholm J, Kleitz F, Linden M (2002) Synthesis and characterization of mesoscopically ordered surfactant/cosurfactant templated metal oxides. Impact of Zeolites and Other Porous Materials on the New Technologies at the Beginning of the New Millennium, Book Series Studies in Surface Science and Catalysis, Pts A and B 142 1117-1124... 

Christian M. Lastoskie and Keith E, Gubbins, Characterization of Porous Materials Using Molecular Theory and Simulation... 

Barros, R.J., Wehtje, E., Garcia, F.A.P. and Adlercreutz, P (1998) Physical characterization of porous materials and correlation with the activity of immobilized enzyme in organic medium. Biocatalysis and Biotransformation, 16, 67-85. 

Since the discovery of the M41S materials with regular mesopore structure by Mobils scientists [1], many researchers have reported on the synthetic method, characterization, and formation mechanism. Especially, the new concept of supramolecular templating of molecular aggregates of surfactants, proposed as a key step in the formation mechanism of these materials, has expanded the possibility of the formation of various mesoporous structures and gives us new synthetic tools to engineer porous materials [2],... 

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