Porous texture isotherms

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. 

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. 

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. 

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. 

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. 

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. 

The porous texture of the dried gels and the pyrolyzed gels was characterised by the analysis of nitrogen adsorption-desorption isotherms, performed at 77 K. The analysis of the isotherms was performed according to the methodology proposed by Lecloux [19]. Samples bulk density was obtained by mercury pycnometry. Infrared and X-ray spectra analysis allowed to obtain data about the elementary composition of the samples and the aggregation state of the metals. 

The importance of Standard Isotherms in the Analysis of Adsorption Isotherms for Determining the Porous Texture of Solids, pp 265-281. 

Denoyel et al. [45] derived the pore size distributions of two sets of activated carbons (one activated in water vapor and the other activated with phosphoric acid) using immersion calorimetric data. They concluded that immersion calorimetry is a convenient technique to assess the total surface area available for a given molecule and the micropore size distribution. More recently, Villar-Rodil et al. [46] have followed this approach to characterize the porous texture of a series of NomexO-derived carbon fibers activated to various bum-offs using liquids with different molecular dimensions as well as N2 and CO2 adsorption Isotherms. Table 3 includes the immersion enthalpies and corresponding surface areas. Relative changes in surface area accessible to the different adsorbates were ascribed to... 

The characterization of the porous texture of the inhia] and oxidized activated carbon and the catalysts was achieved using nitrogen adsorption at 77.4 K in a standard volumetric apparatus. The nitrogen adsorption isotherms were used for detomining the follovnng texture parameters specific surface area (Sb t), by the BET method nucropore volume (Vj and specific surface area of the mesopores the t/F method mesopore vdume... 

The negligible differences in the porous textures of the supports AC and OAC reduce the differences in composition and character of incorporation into the support of the active phase of samples ACM, ACW, OACM and OACW as a result of the effect of two fiictors the media from which the active phase is deposited and the chemical nature of the carbon surface. Fig.l shows the N2 isotherm of sample OACM. The isotherms of the remaining catalysts are amilar to this type. 

Aerogels composed of 60% silica and 40% zirconia were calcined in air, at 400°C, 800°C and 1000°C. After thermal treatment, the porous texture of the samples have been analyzed by mercury porosimetry (Fig. 11-15) (Pirard, 1997c). The three samples are irreversibly densified by isostatic pressure in the whole pressure domain, from 0.01 to 200 MPa. The data analysis (Fig. 11-16) has been done using equation (11-7), with a constant k estimated at 48 nm MPa°-, by nitrogen adsorption-desorption isotherm analysis. The volume distributions versus pore size obtained show that the pore volume decreases for all pore sizes during aerogel calcination at increasing temperatures. This is... 

Porous texture has been characterized by (77 K) and COj (273 K) adsorption isotherms carried out in a conventional McBain silica spring balance and mercury porosimetry (Carlo Erba, series 200). 

Pig. 5 - redrawn from (ref. 5) - shows the benzene adsorption-desorption isotherms of benzene on two series of activated carbons prepared from almond shells but activated in CO, (Fig. 5A) and reacted with air (Fig. 5B), the burn-off of these series being very similar. Fig. 6 - redrawn from (ref. 6) - shows the corresponding Isotherms on the series prepared from olive stones in CO, for which the burn-off levels are also comparable. A detailed ccMq>arative analysis of the different porous texture of carbons prepared by CO, activation and by air-reaction has been published elsewhere (refs. 5,7). 

As mentioned above LPH loops and their evolution with increasing burn-off have been related to microcracks (ref. 3) or to more or less elastic distortion (or deformation) produced in the carbon texture during the adsorption process by induced pressure swelling (refs. 1,2). From a series of repeated adsorption-desorption cycles in a given air-reacted sample it was concluded that the induced pressure swelling did not produce any appreciable fracture or microcracks (ref. 4) the adsorption Isotherm being reversible after heat evacuation so that the porous texture deformation seems to be a more reliable explanation for the occurrence of LPH. 

The evolution of the PSZs porous texture during calcination between 450 C and 1000 C was investigated by the analysis of nitrogen adsorption-desorption isotherm following the method proposed by Lecloux (ref. 7-8). 

Nitrogen adsorption/desorption isotherms on Zeolite and V-Mo-zeolite are very similar and close to a type I characteristic of microporous materials, although the V-Mo-catalysts show small hysterisis loop at higher partial pressures, which reveals some intergranular mesoporosity. Table 1 shows that BET surface area, microporous and porous volumes, decrease after the introduction of Molybdenum and vanadium in zeolite indicating a textural alteration probably because of pore blocking by vanadium or molybdenum species either dispersed in the channels or deposited at the outer surface of the zeolite. The effect is far less important for the catalysts issued from ZSM-5. 

The nitrogen adsorption / desorption isotherms (Fig. 2) are typical of well-defined porous frameworks that are characteristic of either supermicroporosity (MSU-1) or a small mesoporosity (MSU-4) without any textural porosity [14]. In these two compounds, the silica walls (deduced from x-ray diffraction and nitrogen isotherms) are quite thick (< 20 A) [5],... 

For description of textural properties of carbonaceous adsorbents, adsorption/desorption isotherms of vapours and gases in static conditions as well as mercury porosimetry are used. The latter method often leads to destruction of porous structure of investigated materials while the usage of the former one is affected by the specific properties of molecular sieves described above. Taking into account these limitations, in this work the authors have made an attempt of determination of porous structure of carbon molecular sieves with the used of the pycnometric technique. 

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. 

The analysis of nitrogen adsorption-desorption isotherms is one of the most commonly used methods to assess the texture of porous materials. It has given rise to numerous theoretical studies and many mathematical models have been developed to analyze the results. These models establish a relationship between pressure and pore size on the basis of the real physicochemical adsorption mechanisms. However, the user is often bewildered by the diversity of the models, the disparity of basic hypothesis, the difficulty checking them and the apparent incoherence of the results. 

The shape of adsorption-desorption isotherms gives the first pieces of information on a porous solid texture. Existence of an inflexion point in the low-pressure region of the high-resolution isotherm is a sign of the microporous character of a solid [6]. NaY zeolite appears microporous (type I isotherm) regardless of probe molecule and temperature. Similarly, USY zeolite is mainly microporous,but a small hysteresis loop at high P/Po values reveals the presence of some mesopores created during dealumination of Y zeolite [8,9]. 

In Table 2 the textural properties of all the composites heat-treated at 150°, 500°C and 850°C are presented. The sample designation is the same as that used for the raw materials with the addition of the letter m to indicate that the results refer to monolith composites. The total pore volume is the sum of the micro- and mesopore volumes (0-2 nm and 2-50 nm) calculated from the corresponding nitrogen adsorption/desorption isotherms, and the macroporosity (50 nm - 100 pm) determined from MIP, respectively. The threshold diameter was that at which in the MIP analysis there was a sudden upswing in the cumulative volume curve where a large part of the porous network became filled. This pore size can be considered as that which controls any transport phenomena through the solid sample. 

The interpretation of adsorption-desorption isotherms provides a wealth of information on the texture of the adsorbent. The main parameters that can be assessed arc specific surface area, pore distribution, specific porous volume and information on the structure (pore shape, interconnection, etc.). The technique is highly suitable for the study of samples where the pore size is between approximately 2 and 50 nm, which corresponds to the mesoporous domain for which the adsorbed gas has liquid phase properties well described by thermodynamic models. 

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