Porous texture adsorption

Gomez-Serrano, V. Acedo-Ramos, M., and Valenzuela-Calahorro, C., Treatment of activated carbon with hydrogen peroxide Effect on the porous texture, Adsorpt. Sci Technol., 15(2), 91-98 (1997). 

Nitrogen adsorption/desorption isotherms of all the activated carbons are of Type I, i.e. characteristic of basically microporous solids. There is a lack of adsorption/desorption hysteresis. More careful analysis permits to notice significant differences in the porous texture parameters depending on precursor origin. 

Porous texture characterization of all the samples was performed by physical adsorption of N2 at 77K. and CO2 at 273K, using an automatic adsorption system (Autosorb-6, Quantachrome). The micropore volume, Vpp (N2), was determined by application of Dubinin-Radushkevich equation to the N2 adsorption isotherm at 77K up to P/Po< 0.1. The volume of narrow micropores, Vnpp (DR,C02>, (mean pore size lower than 0.7 nm) was calculated from CO2 adsorption at 273 K. 

Characterization of porous texture, morphology and adsorption properties... 

Since the porosity of carbons is responsible for their adsorption properties, the analysis of the different types of pores (size and shape), as well as the PSD, is very important to foresee the behavior of these porous solids in final applications. We can state that the complete characterization of the porous carbons is complex and needs a combination of techniques, due to the heterogeneity in the chemistry and structure of these materials. There exist several techniques for the analysis of the porous texture, from which we can underline the physical adsorption of gases, mercury porosimetry, small angle scattering (SAS) (either neutrons—SANS or x-rays—SAXS), transmission and scanning electron microscopy (TEM and SEM), scanning tunnel microscopy, immersion calorimetry, etc. 

Fiber Diameter and Porous Texture Characterization by Physical Adsorption of N2 at 77 K and C02 at 273 K... 

Physical adsorption of gases and vapors is a powerful tool for characterizing the porosity of carbon materials. Each system (adsorbate-adsorbent temperature) gives one unique isotherm, which reflects the porous texture of the adsorbent. Many different theories have been developed for obtaining information about the solid under study (pore volume, surface area, adsorbent-adsorbate interaction energy, PSD, etc.) from the adsorption isotherms. When these theories and methods are applied, it is necessary to know their fundamentals, assumptions, and applicability range in order to obtain the correct information. For example, the BET method was developed for type II isotherms therefore, if the BET equation is applied to other types of isotherms, it will not report the surface area but the apparent surface area. 

The porous textural characterization of activated carbons is a very important subject due to the growing interest in the preparation of materials with well-defined pore structures and high adsorption capacities. Porosity characterization is an essential task to foresee their behavior in a given use and requires a combination of different techniques. Gas adsorption techniques constitute the most common approach to the characterization of the pore structure of porous materials. However these techniques have some limitations. 

A series of silica gels were synthesised from TEOS, selecting the experimental conditions (pH and water/TEOS ratio) in order to obtain samples with different porosity. Porous texture characterisation of these samples was done by gas adsorption (N2 and CO2 adsorption at 77 K and 273 K, respectively) (Quantachrome. Autosorb 6). The samples were degassed at 623 K under vacuum, until 10 torr. Water adsorption studies were carried out at 298 K in an automatic volumetric gas adsorption instrument (Belsorb 18). Experimental data was corrected for adsorption on inner wall of apparatus. Additionally, a blank experiment on all bulbs used showed that water adsorption on the inner surface of glass was negligible. Previous to water adsorption... 

This study consists in verifying the coherence of a few commonly used analysis methods of nitrogen adsorption-desorption isotherms. These methods were tested on model samples obtained by mechanically mixing two micro- and mesoporous solids respectively with known mass proportions. Although the individual analysis methods may lead to discrepancies in the interpretation of the isotherms, their systematic comparison allows drawing a coherent picture of the porous texture. 

The aim of this work is to test and to compare the performances of various nitrogen adsorption-desorption isotherms analysis methods. These models were applied to model samples obtained by mechanically mixing two micro- and mesoporous solids respectively in perfectly known proportions. The relevant morphological characteristics of the porous texture of the mixtures, such as the specific surface and volume, are physically additive. A criterion that allows determining the reliability of the analysis methods tested is thus to check the linearity of the relation between a given parameter and the weight percentage of the pure solids. 

Prepared iroin rayon several activated caiixm fibers (ACF) with difibient pore structure were used to remove Ci(VI) and/or Cr(lll) species fiom solutions. The a oiption experiments were carried out to determine the influence of ACF/solution contact time, pH, temperature, initial Cr(VI) and Ciflll) concentration on the efficiency of chromium ions removal by ACF. It was found that for ACF with total pore volume more than 0.4cm Vg the porous texture has no great importance for the amount of chromium retained. For all non-oxidized ACF the amounts of Cr species adsorbed and Cr(VI) reduced to Ciflll) after 48 h of ACF/solution contact are very close. At the beginning phase of ACF/solution contact the latent period of CtfVI) to Cr(III) reduction was observed. Oxidized ACF has lower adsorption capacity to Cr(VI) species and higher to Ctflll) ions in respect to non-oxidized ACF. The increase of initial CifVI) concentration increases the chromium species removal but the increase of pH and temperature decreases it. 

Thus, the factors which affect to Cr(VI) and Cr(lII) removal by ACF are not only temperature and initial concentration but also the oxii ion/reduction ability of fibers and theirs oxidation state before and during adsorption. The ACF used in the study can be useful to remove Cr(VI) and Cr(III) from aqueous solution. To ameliorate the rate of chromium removal it seems better to use at the same time the oxidized and non-oxidized ACF. The most usefid porous texture of ACF is when the total pore volume is more than... 

Therefore, these results indicate that Cr-K10 has, at least in part, a pillared structure. The results for Cr-PB indicate (Fig. 1 and Table 1) that this material has a micro-porous structure with some contribution of mesopores (shape of Nj adsorption-desorption isotherm) and a narrow pore volume distribution with a maximum at a pore radius of 2.1 nm. All Cr-PILC studied exhibit hysteresis loop of type H4 [11] which can be attributed to solids with a slit-shaped porous structure. Heat treatment results only in a small decrease of the BET surface area for both Cr-KIO and Cr-PB (Table 1). Sulfidation does not influence significantly the porous texture of both Cr-PILC as well [12]. 

Three carbon samples showing differences in pore structure are chosen to study the effect of porous texture on adsorption from liquid solutions. The benzene adsorption/desorption isotherms are applied to determine the properties of geometrical surface structure of investigated carbons. The liquid adsorption data are analyzed in terms of the theory of adsorption on heterogeneous solids. The relation between parameters of porous structure of the activated carbon samples and parameters of adsorption from the liquid phase is discussed. 

Porous texture characterisation of LTA, FAU and MFI type zeolites has been carried out by N2 adsorption at 77K and CO2 adsorption at 273K. These results have been compared to the data obtained from crystallographic estimations. In this way, the micropore volumes obtained by CO2 adsorption at 273K are, in all the cases, similar to the expected from the crystal framework of the zeolite. However, the micropore volumes obtained by N2 adsorption at 77K are significantly smaller. These results confirm the limitations of the N2 adsorption at 77K in measuring narrow microporosity and make evident the usefulness of CO2 adsorption at 273K to characterise porous solids with narrow microporosity. 

Characterisation of porous texture of solids is of relevance because their properties are determined, or at least, influenced by this characteristic [1-3]. A number of techniques exist to characterise the porous texture of solids. Among them, physical adsorption of gases is the most widely used due to its simplicity [1-11], N2 adsorption at 77K [3] is, undoubtedly, the most used. One of its main advantages is that it covers reduced pressures from 10 to 1, being sensitive to the whole range of porosity. However, N2 adsorption at 77K has some limitations when used to characterise solids containing ultramicroporosity (i.e., pore sizes lower than 0.7 nm). It can be influenced by diffusional limitations in this range of porosity [4]. 

To avoid the above problem and to achieve a correct assessment of the porosity, the use of other adsorptives and experimental conditions have been proposed, such as He adsorption at 4.2K[5,6] and CO2 adsorption at 273K or 298K [4,8-12]. He adsorption at 4.2 K also covers the whole range of partial pressures and accurately estimates the microporosity [6]. However, the experimental conditions required do not allow us to propose this technique as a routine procedure for the characterisation of the porous texture of solids. 

A confirmation, as well as a generalisation, of CO2 adsorption at 273K, as a useful technique for characterisation of the porous texture of solids, requires its application to noncarbon materials. In this sense, zeolites could be the most appropriate materials because of two main reasons. On one hand, they are crystalline solids with a well-defined and known stmcture [14-17]. On the other hand, the chemical composition of the zeolite determines its surface chemistry and, afterwards, its effect on CO2 adsorption can be analysed. Thus, this study focuses on the comparison between characterisation of porous texture of zeolites with... 

In order to establish a connection between the results obtained by PALS and the porosity of the sample, the results of this new technique have been compared with the surface area and the pore width determined from other techniques widely used in the characterization of porous texture (i.e., gas adsorption and SAXS)... 

Naono, H., Sonoda, I, Oka, K. and Hakuman, M. (1993) Evaluation of micro-porous texture of undecomposed and decomposed P-FeOOH fine particles by means of adsorption isotherms of nitrogen gas and water vapor. Proe. IVth Int. Conf. on Fimdamentals of Adsorption, Kyoto 1992, 467-474. 

The formation of controlled porous textures by removal of templates initially included in a solid matrix is a concept generally accepted by people working in the preparation of materials for adsorption, catalysis and, more recently, membranes. Recent works have shown the versatility of this process for membrane application. It can be adapted to different kinds of gel matrices in order to define uniform pore size in the macro-, meso- or micropore range. Nevertheless most of the results relate to silica materials and further work is needed to apply this concept to other sol-gel derived oxide materials. 

Naono, H., Hakuman, M., Tanaka, T., Tamura, N., and Nakai, K. 2000. Porous texture and surface character of dehydroxylated and rehydroxylated MCM-41 mesoporous sihcas— Analysis of adsorption isotherms of nitrogen gas and water vapor. Journal of Colloid and Interface Science 225, 411 25. 

Porous Texture Characterization from Gas-Solid Adsorption... 

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