Pore structure characterization

J. M. Hayes and P. Rossi-Doria, Principles and Applications of Pore Structural Characterization, J. W. Arrowsmith, Bristol, 1985. 

The development of microporosity during steam activation was examined by Burchell et al. [23] in their studies of CFCMS monoliths. A series of CFCMS cylinders, 2.5 cm in diameter and 7.5 cm in length, were machined from a 5- cm thick plate of CFCMS manufactured from P200 fibers. The axis of the cylinders was machined perpendicular to the molding direction ( to the fibers). The cylinders were activated to bum-offs ranging from 9 to 36 % and the BET surface area and micropore size and volume determined from the N2 adsorption isotherms measured at 77 K. Samples were taken from the top and bottom of each cylinder for pore structure characterization. 

Pore Structure Characterization of Catalyst Supports via Low-Field NMR Spectroscopy... 

R. Mann, A. Al-Lamy, and A. Holt, Visualized porosimetry for pore structure characterization of a Nickel/Alumina Reforming Catalyst, Trans, I, Chem, E. 73(A) 47 (1995). 

G.C. Frye, A.]. Ricco, S.J Martin and C.J. Brinker, Characterization of the surface area and porosity of sol-gel films using SAW devices. Mater. Res. Soc Symp. Proc., 121 (1988) 349. S.L. Hietala, D.M. Smith, V.M. Hietala, G.C. Frye and S.J. Martin, Pore structure characterization of thin films using a surface acoustic wave/volumetric adsorption technique. Langmuir, 9 (1993) 249. 

Characterization of the pore structure of amorphous adsorbents and disordered porous catalysts remains an important chemical engineering research problem. Pore structure characterization requires both an effective experimental probe of the porous solid and an appropriate theoretical or numerical model to interpret the experimental measurement. Gas adsorption porosimetry [1] is the principal experimental technique used to probe the structure of the porous material, although various experimental alternatives have been proposed including immersion calorimetry [2-4], positron... 

The molecular parameters of common adsorbates used in the pore structure characterization are listed in Table 11.1. It should be noted that these values are not unique as there are many other combinations of collision diameter and well depth of the interaction energy that have been determined in the literature [16]. Also noted in the table are the different sets of values that are used in DFT and in MC simulation. The difference is due to the mean field approximation assumed in the DFT analysis. 

The pore structure characterization of the activated carbons was studied by using N2 and CO2 as adsorbates. Figures 4 and 5 show the N2 adsorption isotherms for carbons of various burn-off values, activated at 800 and 900°C,... 

In the case of microporous materials, surface areas estimated from N2 adsorption usually suffer from activated diffusion of N2, because of the low temperature of adsorption. Therefore, CO2 adsorption at room temperature was also used for pore structure characterization of the activated carbons. Figures 7 and 8 show the CO2 isotherms of samples with the same burn-off as those used for the N2 adsorption isotherms (Figures 4 and 5). Adsorption isotherms show that the amount of CO2 adsorbed, is affected by the degree of activation and increases considerably for carbons produced from demineralized lignite. 

Pores, and especially mesopores (with sizes between 2 and 50 nm) and micropores (with sizes less than 2 nm), play an essential role in physical and chemical properties of industrially important materials like adsorbents, membranes, catalysts etc. In addition to pore structural characterization described above, the description of transport phenomena in porous materials has received attention due to its importance in many applications such as drying, moisture transport in building materials, filtration etc. Although widely different, these applications present many similarities since they all depend on the same type of transport phenomena occurring in a porous media environment. In particular, transport in mesoporous media and the associated phenomena of multilayer adsorption and capillary condensation have been investigated as a separation mechanism for gas mixtures [29]. 

Neimark AV, Ravikovitch PI (2001) Capillary condensation in MMS and pore structure characterization. Microporous Mesoporous Mater 44-45 697-707... 

We suggest a model of adsorption in pores with amorphous and microporous solid walls, named the quenched solid non-local density functional theory (QSNLDFT) model. We consider a multicomponent non-local density functional theory (NLDFT), in which the solid is treated as a quenched component with a fixed spatially distributed density. Drawing on several prominent examples, we show that QSNLDFT model produces smooth Isotherms of mono- and polymolecular adsorption, which resemble experimental isotherms on amorphous surfaces. The model reproduces typical behaviors of N2 isotherms on micro- mesoporous materials, such as SBA-15. QSNLDFT model offers a systematic approach to the account for the surface roughness/heterogeneity in pore structure characterization methods. 

Recent progress in the theory of adsorption on porous solids, in general, and in the adsorption methods of pore structure characterization, in particular, has been related, to a large extent, to the application of the density functional theory (DFT) of Inhomogeneous fluids [1]. DFT has helped qualitatively describe and classify the specifics of adsorption and capillary condensation in pores of different geometries [2-4]. Moreover, it has been shown that the non-local density functional theory (NLDFT) with suitably chosen parameters of fluid-fluid and fluid-solid interactions quantitatively predicts the positions of capillary condensation and desorption transitions of argon and nitrogen in cylindrical pores of ordered mesoporous molecular sieves of MCM-41 and SBA-15 types [5,6]. NLDFT methods have been already commercialized by the producers of adsorption equipment for the interpretation of experimental data and the calculation of pore size distributions from adsorption isotherms [7-9]. 

QSNLDFT has immediate implications for the practical problems of pore structure characterization. The NLDFT models, which are routinely employed to study various materials, can now be extended to provide an assessment of the surface roughness and microporosity. 

S. Bukowiecki, B. Straube, and K.K. Unger Pore structure analysis of close-packed silica spheres by means of nitrogen sorption and mercury porosimetry, in Principles and Applications of Pore Structural Characterization (Haynes J.M. Rossi-Doria P., eds) Arrowsmith,... 

Table 6.3 Pore structure characterizations derived from (77 K) and CO (194.5 K) adsorption isotherm. (Data taken from [15])...

For the detailed information, pore structure porosimetry techniques are used. These methods enable measurement of pore diameter, pore shape, pore volume, and pore distribution in the electrode catalyst and gas diffusion layers. However, for PEMFC, these layers have hydrophobic and hydrophilic pores and there is no suitable technique available for characterization of such complex pore structures. Combination of multiple porosimetry techniques are employed to characterize layers with both hydrophobic and hydrophilic pores. The pore structure characterization techniques include capillary flow porosimetry, water intrusion porosimetry, and mercury intrusion porosimetry (Jena and Gupta, 2002). In water... 

Gupta, K. and Jena, A. (2003) Techniques for pore structure characterization of fuel cell components containing hydrophobic and hydrophilic pores. Presented at the 2003 Fuel Cell Seminar, Miami Beach, FL, Abstract pp. 723-726, November 3-7, 2003. 

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