Estimation of micropore size distribution in active carbons from adsorption isotherms of water vapor

A possibility of estimation of the micropore size distribution in the carbon adsorbents with the developed micro-and mesoporous structure by analysis of the adsorption isotherms of water vapors was considered. At saturation water condenses in micropores in a form of a weakly compressed liquid. However, water molecules in micropores are packed not so closely as in the liquid because of steric hindrance. Therefore, the real density of water adsorbed in the micropores is lower than that of water adsorbed on an open surface and lower than the density of the normal liquid. An analysis of the adsorption isotherms of water vapors with account for the both opposite effects on the water density gives reliable data on the micropore sizes of the carbon adsorbents. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 40–43, January, 2007.     

Adsorption properties of iodine-doped activated carbon fiber

Iodine-doped activated carbon fibers (ACFs) were prepared by the iodine immersion method on pitch-based ACF. Then iodine-doped ACFs were heated in argon at 523 K for 4 h and at 673 K for 2 h. The iodine structure of the resultant iodine-doped ACFs was examined using X-ray photoelectron spectroscopy. The micropore structures were determined by N(2) adsorption at 77 K. The surface area and micropore volume of iodine-doped ACFs are less than those of pristine ACFs. However, the pore width does not change with the iodine doping. The effects of iodine doping on adsorption properties of ACFs for H(2)O and NO at 303 K were examined. The iodine doping affected remarkably the adsorptivities of ACFs for H(2)O and NO. In particular, iodine-doped ACFs treated at 673 K show enhanced adsorptivities for H(2)O and NO. This result suggests that iodine molecules doped on the micropores should be charged by heat treatment at 673 K.     

Density of He adsorbed in micropores at 4.2K

The density of He adsorbed in the cylindrical micropores of zeolites NaY and KL has been determined by He adsorption at 4.2K. He adsorption isotherms were then compared with N₂ adsorption isotherms at 77K. Crystallographic considerations of the micropore volumes gave the density of the He adsorbed layer, which is necessary for assessment of ultramicroporosity of less-crystalline microporous solids, such as activated carbons. The determined density of He adsorbed in the cylindrical micropores of the zeolite was in the range 0.22 to 0.26 gml^–1, greater than that of He adsorbed on a flat surface (0.202 gml^–1). A value for the density of He between 0.20 to 0.22 gml^–1 is recommended for evaluation of ultramicroporosity of a slit-shaped microporous system such as activated carbon.     

Influence of surface treatments on micropore structure and hydrogen adsorption behavior of nanoporous carbons

The scope of this work was to control the pore sizes of porous carbons by various surface treatments and to investigate the relation between pore structures and hydrogen adsorption capacity. The effects of various surface treatments (i.e., gas-phase ozone, anodic oxidation, fluorination, and oxygen plasma) on the micropore structures of porous carbons were investigated by N(2)/77 K isothermal adsorption. The hydrogen adsorption capacity was measured by H(2) isothermal adsorption at 77 K. In the result, the specific surface area and micropore volume of all of the treated samples were slightly decreased due to the micropore filling or pore collapsing behaviors. It was also found that in F(2)-treated carbons the center of the pore size distribution was shifted to left side, meaning that the average size of the micropores decreased. The F(2)- and plasma-treated samples showed higher hydrogen storage capacities than did the other samples, the F(2)-treated one being the best, indicating that the micropore size of the porous carbons played a key role in the hydrogen adsorption at 77 K.     

Prediction of the micropore structure parameters and adsorption properties of activated carbons

A regularity govering variations of volume and linear size of micropores in carbon adsorbents during their vapor-gas activation was found. A parameter was proposed that characterizes the degree of development of the micropore system in activating carbons and an initial carbonized material. The parameter is defined as the number (or surface area) of micropores in the volume unit of the micropore zones. This parameter allows one to rationalize the choice of carbonized materials for the preparation of activated carbons with specified adsorption properties and to establish the range of activation beyond which the structure of the micropores loses stability. Furthermore, the parameter serves to predict how activation affects micropore structure parameters and adsorption properties of carbons. This in turn indicates the optimal degrees of microporosity of carbons needed to attain required adsorption properties.     

Nonuniform microporous structures and adsorption properties of carbon adsorbents 11. Physical adsorption equations based on a normal distribution of micropore volume

The fundamental properties of three parameter equations for physical adsorption of vapors on nonuniform microporous structures based on the slit type model and having a normal micropore volume distribution have been considered. The widely applied two parameter Dubinin-Radushkevich equation provides for the special case of standard uniform microporous structures. Various threeparameter equations can be differentiated according to their energy coefficient k, which can be expressed as the product of the characteristic adsorption energy E_o and the maximum micropore dimension x_o, taken from the distribution curve. The coefficient k requires an independent determination.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2691–2699, December, 1990.The authors would like to thank M. V. Degtyarev for the experimental investigations.     

Features of the sorption of water vapor and nitrogen on cellulose

Molecular-kinetic parameters of adsorptives, i.e., water (at 300 K) and nitrogen (at 77 K) vapors, are calculated and compared at the initial steps of their adsorption by cellulose. The role of the dipole structure of water molecules is considered upon their interaction with active centers of cellulose, forming heterogeneous electric fields in its pores. The effect of the temperature of the adsorptive and the sizes of its molecules on activation penetration through narrowings of the micropores dominant in absolutely dry cellulose due to the mobility of its structure is determined. The development of a porous system upon water adsorption is demonstrated according to ¹H NMR. It is concluded that low-temperature nitrogen adsorption on cellulose yields rather limited information on its structure and adsorption properties.     

The effect of small micropores on methane adsorption of coals from Northern China

In this study, the effect of coal micropores on the adsorption properties, especially the Langmuir pressure (P _L ), was investigated by testing 11 coal samples from Northern China. The adsorption of CO₂ at 273 K was utilized to analyze the pore size distribution. The results of these coals show that micropore volume and micropore surface area are the major factors affecting the Langmuir volume (V _L ) but have weaker effects on P _L . Micropore filling theory considers that some smaller micropores with an obvious overlapping adsorption force cause volume filling adsorption. These micropores firstly reach saturated adsorption, controlling the adsorption volume at the low-pressure stage and thus have a great effect on P _L . Four times the methane molecular diameter, 1.5 nm, was assumed as the critical pore size with obvious overlapping adsorption force. The relationship between P _L and the proportion of the pore volume below 1.5 nm to the micropore volume was investigated, and it was found that the higher the volume proportion of these small micropores was, the smaller the P _L was, though two data points deviated from this trend. The reason for the anomalous coal samples could be the deviation from the assumed critical pore size of 1.5 nm for volume filling and the effects of the various micropore surface properties, which await further study. The micropore surface increases with increasing coal rank, as does V _L . The proportion of pore volume below 1.5 nm increases with coal rank, and P _L reverses. However, these relationships are discrete.     

Structure and diffusion characterization of SBA-15 materials

In situ formation of the micro- and mesoporous structures of SBA-15 materials was investigated. It was found that the structure is significantly different from that for cylindrical or hexagonal pores, which suggests that the SBA-15 is more complex than an array of hexagonally ordered channels. Nitrogen adsorption isotherms at 77 K provided evidence that large (primary) mesopores are accompanied by a certain amount of significantly smaller pores with a broad distribution in the micropore/small-mesopore range within the mesoporous walls of main channels. It was found that the microporosity can be controlled by the time of heating as well as the synthesis temperature. The diffusion properties of n-heptane as a probe molecule in four selected SBA-15 samples with different micropore volumes were studied by the standard zero length column technique and related to their structural characteristics. The results have shown that the diffusion process involving n-heptane at a low concentration level takes place inside the walls of main mesoporous channels and depends on the relative content of micropores. In the samples that have a relatively high content of micropores, n-heptane diffusivities are relatively low, their activation energies are high, and the process is similar to diffusion in typical microporous adsorbents, like zeolites. As the micropore content is decreased, diffusion becomes more and more controlled by secondary mesopores of the intrawall pore structure, rendering diffusion faster and activation energies lower.     

Hydrogen adsorption on model adsorbents from the viewpoint of the theory of volumetric filling of micropores

Hydrogen adsorption on model microporous adsorbents with slit-shaped pores was calculated on the basis of Dubinin’s theory of volumetric filling of micropores using the property of linearity of adsorption isosters. Model adsorbents with micropore widths of 0.5, 0.9, and 1.2 nm obtained by the successive exclusion of one, two, and three layers of hexagonal carbon in the crystalline lattice of graphite were used. Hydrogen adsorption was calculated in the structures with single-layer and two-layer carbon walls at temperatures 20, 33, 77, 200, 300, and 400 K and pressures up to 20 MPa. The maximal hydrogen desorption for the AU structure (1:3) with the pressure drop from 20 to 0.1 MPa was 8 wt.% at 200K. The parameters of the porous adsorbent structure were calculated.     

Guest-host encapsulation of microporous zeolites in ordered mesoporous materials by molecular simulations

The present work provides the first study of ordered mesoporous materials SBA-15 coated with microporous zeolites ZSM-5 using molecular simulations. Several model structures with characteristics such as periodic arrangement of mesopores, randomly arranged micropores, surface hydroxyls and bulk deformations of SBA-15 were used. Cartesian coordinates of ZSM-5 unit lattice were obtained from the literature and the 100 face of H-ZSM-5 unit cell was then placed on the surface of SBA-15 and the entire structure was equilibrated to obtain final configuration. The resulting structure was characterized using simulated small angle and wide angle X-ray diffraction, Connolly surface area (to compare BET area), accessible pore volume for nitrogen molecules (to compare with t-plot volume of micro and mesopores) and methane adsorption at 303 K. The orientation of ZSM-5 on the SBA-15 had no effect on the surface area, pore volume or adsorption capacity. In order to find out if the addition of microporous ZSM-5 should increase the total methane adsorption capacity due to addition of micropores, we studied adsorption on bare and coated SBA-15. However, total adsorption capacity was found to decrease, while the number of methane molecules adsorbed per unit cell of the SBA-15 structure increased. An existing experimental method (J. Am. Chem. Soc., 2004, 126, 14324) of the synthesizing hybrid ZSM-5/SBA-15 structure was studied using accessible micropore volume (by t-plot). It was found that the procedure made all the micropores inaccessible. A modification of the method or use of other host materials is suggested to use the benefits of narrow micropore distribution in ZSM-5.     

Hydrogen adsorption on a carbon adsorbent with slitlike micropores below and above the critical temperature

Hydrogen adsorption is calculated for model microporous adsorbents with slitlike micropore widths of 0.538, 0.878, and 1.218 nm obtained by the consecutive exclusion of one, two, and three layers of hexagonal carbon from graphite structure taken as a model cell. Calculations are performed using the basic concepts of the theory of volume filling of micropores, Dubinin-Radushkevich equation, and linear adsorption isosteres. For structures with one-and two-layer carbon walls, the calculation is carried out at temperatures of 20, 33, 77, 200, 300, and 400 K and pressures up to 20 MPa. For AC3:5 structure, the maximum hydrogen adsorption amounts to 7.9 wt % at 20 MPa and 300 K. The parameters of adsorbent porous structure are established. Hydrogen adsorption is shown to be governed by the capacity and the energy of adsorption.     

Differential heats of adsorption of water vapors on the lithium form of clinoptilolite

The maximum possible lithium form of clinoptilolite (K1) with an ion-exchange capacity of 2.3 mEq/g, corresponding to 85% Kl in nature, was prepared. The isotherm of adsorption of water vapors on LiKl at 25°C in the region of maximally low equilibrium pressures (10^–10 torr) was calculated by the adsorption-isostere method and permits determining the complete thermodynamic functions of the system. The adsorption isotherm is completely described by the two-term equation from the theory of volume filling of micropores (TVFM). The wavy-stepwise shape of the curve of the heat of adsorption permitted establishing the stoichiometric correlation between the adsorption values and concentration of Li⁺ in LiKl. The energy of the Li⁺-OH₂ adsorption complex is 97 kJ/mole for a zero degree of filling. The integral average molar entropy of adsorption is 20 J/(mole·K) less than the entropy of a normal liquid. The state of the H₂O molecules in LiKl is ice-like.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2186–2188, October, 1989.     

Hydrogen adsorption in transition metal carbon nano-structures

Templated microporous carbons were synthesized from metal impregnated zeolite Y templates. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were employed to characterize morphology and structure of the generated carbon materials. The surface area, micro- and meso-pore volumes, as well as the pore size distribution of all the carbon materials were determined by N₂ adsorption at 77 K and correlated to their hydrogen storage capacity. All the hydrogen adsorption isotherms were Type 1 and reversible, indicating physisorption at 77 K. Most templated carbons show good hydrogen storage with the best sample Rh-C having surface area 1817 m²/g and micropore volume 1.04 cm³/g, achieving the highest as 8.8 mmol/g hydrogen storage capacity at 77 K, 1 bar. Comparison between activated carbons and synthesized templated carbons revealed that the hydrogen adsorption in the latter carbon samples occurs mainly by pore filling and smaller pores of sizes around 6 Å to 8 Å are filled initially, followed by larger micropores. Overall, hydrogen adsorption was found to be dependent on the micropore volume as well as the pore-size, larger micropore volumes showing higher hydrogen adsorption capacity.     

On the mechanisms of phenol adsorption by carbons

The removal of phenol and related compounds from dilute aqueous solutions by activated carbons corresponds to the coating of the micropore walls and of the external surface by a monolayer. This process is described by an analog of the Dubinin—Radushkevich—Kaganer equation. On the other hand, as suggested by immersion calorimetry at 293 K, in the case of concentrated solutions, the mechanism corresponds to the volume filling of the micropores, as observed for the adsorption of phenol from the vapor phase. The equilibrium is described by the Dubinin—Astakhov equation. It follows that the removal of phenol from mixtures with water depends on the relative concentrations, and the limiting factor for adsorption is either the effective surface area of the carbon, or the micropore volume.     

Adsorption of Nitrogen,Oxygen, and Argon Vapors on Supermicroporous Oxides

The adsorption of N₂, O₂, and Ar vapors on a number of supermicroporous tin dioxide and zirconia xerogels at 77.4 K was studied. The micropore volumes calculated with the use of sorbate densities at the corresponding temperature were found to be in satisfactory agreement for all of the sorbates. At the same time, the volumes of larger pores measured using nitrogen were greater than the values found with other sorbates. The previously found behavior of oxygen and nitrogen molecules in the geometrically restricted space of supermicropores was substantiated. In particular, this behavior manifests itself in a change in the adsorption properties of a unit surface area of supermicropores as compared with the surface of mesopores. However, the effects of this kind were not found for the argon–nitrogen sorbate pair.     

Adsorption of benzene and perfluorobenzene vapor from a flow of moist air by a layer of equilibrium moistened active carbon

The equilibrium adsorption of benzene and perfluorobenzene vapor from a flow of moist air by a layer of active carbon (AC) hydrated to equilibrium were considered. It was ascertained that these organic substances partly displace the water from the adsorptive micropore volume of AC. The equilibrium adsorption values of benzene, perfluorobenzene, and water vapors on AC were calculated. The adsorption of benzene and perfluorobenzene from a flow of moist air by a layer of AC is characterized by volume filling of the micropore adsorption space. This work is part of the research devoted to the study of the dynamics of the adsorption of organic substances from a flow of moist air by a layer of AC hydrated to equilibrium.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1398–1401, August, 1994.     

多孔活性炭孔径分布的表征

总结了利用气体吸附法表征多孔活性炭中孔和微孔孔径分布的各种方法。BJH方法和MP模型忽略了微孔内势能叠加效应,仅适合描述中孔孔径分布;HK模型和以Dubinin填充理论为基础的各种方法,考虑了微观下势能叠加的效果,在一定程度上能很好地描述微孔孔径分布;最近围绕GAI(GeneralizedAdsorptionIsotherm)而展开的利用密度范函理论(DFT,densityfunctiontheory)和巨正则系综蒙特卡罗(GCMC,grandcanonicalensemblemontecarlo)模拟确定微孔孔径分布的方法较好地克服了Dubinin理论中存在的缺点,是较好的两种方法,但其有效性还需要更多的实验结果来证明。     

Cluster-Mediated Water Adsorption on Carbon Nanopores

The adsorption isotherms of water at 303 K and N2 at 77 K on various kinds of porous carbons were compared with each other. The saturated amounts of water adsorbed on carbons almost coincided with amounts of N2 adsorption in micropores. Although carbon aerogel samples have mesopores of the great pore volume, the saturated amount of adsorbed water was close to the micropore volume which is much small than the mesopore volume. These adsorption data on carbon aerogels indicated that the water molecules are not adsorbed in mesopores, but in micropores only. The adsorption isotherms of water on activated carbon having micropores of smaller than 0.7 nm in width had no clear adsorption hysteresis, while the water adsorption isotherms on micropores of greater than 0.7 nm had a remarkable adsorption hysteresis above P/P0 = 0.5. The disappearance of the clear hysteresis for smaller micropores suggested that the cluster of water molecules of about 0.7 nm in size gives rise to the water adsorption on the hydrophobic micropores; the formation and the structure of clusters of water molecules were associated with the adsorption mechanism. The cluster-mediated pore filling mechanism was proposed with a special relevance to the evidence on the formation of the ordered water molecular assembly in the carbon micropores by in situ X-ray diffraction.