A model of the two-stage condensation mechanism of water adsorption on nonporous carbon adsorbents

The mechanism of adsorption of water molecules on nonporous carbon adsorbents has been suggested in terms of two different states of adsorbed water; stretched liquid water and water that occupies an intermediate state between the liquid and vapor. Two stages of adsorption were distinguished: condensation and pre-condensation that assumes the formation of molecular associates. The BET model was used to describe the pre-condensation stage. The equations of the adsorption isotherm for water vapor in the region of condensation process and the expression for the determination of the specific hydrophilic surface of adsorbents were found. Examination of the experimental data on adsorption of water vapor on nongraphitized samples of carbon adsorbents shows that in the region of polymolecular adsorption, all isotherms fall into a common curve determined by the equation of the stretched liquid film and can be calculated regardless of the properties of individual liquid water. The equation for adsorption of water vapor on the hydrophobic surface was obtained. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1933–1939, October, 1998.     

Does water condense in carbon pores?

Using grand canonical Monte Carlo (GCMC) simulations of molecular models, we investigate the nature of water adsorption and desorption in slit pores with graphitelike surfaces. Special emphasis is placed on the question of whether water exhibits capillary condensation (i.e., condensation when the external pressure is below the bulk vapor pressure). Three models of water have been considered. These are the SPC and SPC/E models and a model where the hydrogen bonding is described by tetrahedrally coordinated square-well association sites. The water-carbon interaction was described by the Steele 10-4-3 potential. In addition to determining adsorption/desorption isotherms, we also locate the states where vapor-liquid equilibrium occurs for both the bulk and confined states of the models. We find that for wider pores (widths >1 nm), condensation does not occur in the GCMC simulations until the pressure is higher than the bulk vapor pressure, P0. This is consistent with a physical picture where a lack of hydrogen bonding with the graphite surface destabilizes dense water phases relative to the bulk. For narrow pores where the slit width is comparable to the molecular diameter, strong dispersion interactions with both carbon surfaces can stabilize dense water phases relative to the bulk so that pore condensation can occur for P < P0 in some cases. For the narrowest pores studied--a pore width of 0.6 nm--pore condensation is again shifted to P > P0. The phase-equilibrium calculations indicate vapor-liquid coexistence in the slit pores for P < P0 for all but the narrowest pores. We discuss the implications of our results for interpreting water adsorption/desorption isotherms in porous carbons.     

Molecular simulation study on adsorption of methanol/water mixtures in mesoporous silicas modified pore surface silylation

Two types of molecular simulation techniques have been utilized to investigate adsorption of methanol/water mixtures in a mesoporous silica with a hydrophobic pore surface: the NVT-ensemble Molecular Dynamics method with the melt-quench algorithm for modeling a fully-silylated mesoporous silica and the μVT-ensemble Orientaional-Biased Monte Carlo method for calculating adsorption isotherms. Adsorption isotherms of methanol and water at 333 K are calculated for an equi-relative-pressure mixture (each component has the same relative pressure which is defined as the ratio of the partial pressure to the saturation pressure of the pure gas) together with pure gases. In the case of the pure gas, water hardly adsorb even at elevated pressures, while the adsorption isotherm for methanol shows the condensable adsorption. On the other hand, in the case of the mixture, water molecules are substantially adsorbed along with methanol molecules, showing an isotherm representing the condensation mechanism. In addition, it is found that the separation factor of methanol to water is the highest in the case of monolayer adsorption from a liquid mixture.     

Water vapor adsorption and microporous structure of carbon adsorbents. 17. Analysis of experimental isotherms for water vapor adsorption

Water vapor adsorption for various activated carbons with narrow and wide micropore volume distributions and mesopore surface areas between 40 and 300 m²/g have been investigated. For all the isotherms the point of inflection was determined, which can be taken as the point characterizing the formation of a water adsorption layer on the pore wall surface of carbon adsorbents. To do this the adsorption and desorption branches of the isotherms were approximated according to Weibull's distribution. A good correlation was obtained between values for the water monolayer capacity, calculated from the porous structure parameters of the carbons, and the adsorption values corresponding to the isotherm inflection pointsa _inf. For the group of carbons studied the values of relative pressure at the inflection point of the isotherms fell within the range 0.5–0.72.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 31–34, January, 1991.     

Water adsorption-desorption isotherms of two-dimensional hexagonal mesoporous silica around freezing point

Zr-doped mesoporous silica with a diameter of approximately 3.8 nm was synthesized via an evaporation-induced self-assembly process, and the adsorption-desorption isotherms of water vapor were measured in the temperature range of 263-298 K. The measured adsorption-desorption isotherms below 273 K indicated that water confined in the mesopores did not freeze at any relative pressure. All isotherms had a steep curve, resulting from capillary condensation/evaporation, and a pronounced hysteresis. The hysteresis loop, which is associated with a delayed adsorption process, increased with a decrease in temperature. Furthermore, the curvature radius where capillary evaporation/condensation occurs was evaluated by the combined Kelvin and Gibbs-Tolman-Koening-Buff (GTKB) equations for the modification of the interfacial tension due to the interfacial curvature. The thickness of the water adsorption layer for capillary condensation was slightly larger, whereas that for capillary evaporation was slightly smaller than 0.7 nm.     

Study of Water Adsorption on Activated Carbons with Different Degrees of Surface Oxidation

A carbon of wood origin was oxidized with different oxidizing agents (nitric acid, hydrogen peroxide, and ammonium persulfate). The microstructural properties of the starting material and the oxidized samples were characterized using sorption of nitrogen. The surface acidity was determined using Boehm titration and potentiometric titration. The changes in the surface chemistry were also studied by diffuse reflectance FTIR. Water adsorption isotherms were measured at three different temperatures close to ambient (relative pressure from 0.001 to 0.3). From the isotherms the heats of adsorption were calculated using a virial equation. The results indicated that the isosteric heats of water adsorption are affected by the surface heterogeneity only at low surface coverage. In all cases the limiting heat of adsorption was equal to the heat of water condensation (45 kJ/mol).     

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.     

Modeling of water vapor adsorption isotherms onto polyacrylic polymer

The adsorbed amounts of water vapor onto polyacrylic polymer (polymer ×10) were measured using a thermogravimetry method as a function of pressure at 298 and 313 K. The adsorption isotherms are categorized to type II isotherms by IUPAC classification leading to a hysteresis loop between adsorption and desorption branches. The current study was completed by the measurement of the adsorption heats at 298 K using a differential scanning calorimetry. The calorimetric curves showed two adsorption heats domains. These domains have been attributed to the adsorption of “equivalent monolayer” and the condensation of water between polymeric chains. The correlation of experimental data to some chosen theoretical models shows that the GAB model is the most adequate to describe water vapor sorption isotherms.     

Effect of temperature on the adsorption of water in porous carbons

We report experimental and simulation studies to investigate the effect of temperature on the adsorption isotherms for water in carbons. Adsorption isotherms are measured by a gravimetric technique in carbon-fiber monoliths at 378 and 423 K and studied by molecular simulation in ideal carbon pores in the temperature range 298-600 K. Experimental adsorption isotherms show a gradual water uptake, as the pressure increases, and narrow adsorption-desorption hysteresis loops. In contrast, simulated adsorption isotherms at room temperature are characterized by negligible uptake at low pressures, sudden and complete pore filling once a threshold pressure is reached, and wide adsorption-desorption hysteresis loops. As the temperature increases, the relative pressure at which pore filling occurs increases and the size of the hysteresis loop decreases. Experimental adsorption-desorption hysteresis loops are narrower than those from simulation. Discrepancies between simulation and experimental results are attributed to heterogeneities in chemical composition, pore connectivity, and nonuniform pore-size distribution, which are not accounted for in the simulation model. The hysteresis phase diagram for confined water is obtained by recording the pressure-density conditions that bound the simulated hysteresis loop at each temperature. We find that the hysteresis critical temperature, i.e., the lowest temperature at which no hysteresis is detected, can be hundreds of degrees lower than the vapor-liquid critical temperature for bulk model water. The properties of confined water are discussed with the aid of simulation snapshots and by analyzing the structure of the confined fluid.     

活性炭纤维的微孔结构水吸附

测定了两种活性炭纤维（ACF）的氮气、水吸附等温线和XPS,研究了ACF的微孔结构和表面性质,用αs图分析氮吸附等温线获得了ACF的比表面积、微孔容量和微孔径。XPS表明在ACF表面存在多种不同结合状态的氧。水在ACF上的吸附等温线呈V型,具有很大的脱附滞后环。水通过与ACF表面的氧形成氢键发生吸附。ACF表面的初始吸附点多,则在低、中压时的水吸附量就大。     

Adsorption of carbon dioxide on microporous carbon adsorbents

Adsorption isotherms of carbon dioxide on microporous carbon adsorbents prepared by activation with potassium sulfide in water vapor were measured. The measurements were carried out in the pressure interval from 1 Pa to 0.1 MPa at temperatures from 216.2 to 293.15 K. Based on the theory of volumetric filling of micropores, the main structural and energetic parameters of the microporous carbon adsorbents were calculated. The adsorption isosters of carbon dioxide were calculated from the adsorption isotherms in the same pressure and temperature ranges and approximated by linear dependences. The plots of the differential mole isosteric heats of adsorption vs amount adsorbed were constructed by using the adsorption isosters.     

Adsorption deformation of a microporous AR-V carbon adsorbent during the adsorption of <Emphasis Type="Italic">n</Emphasis>-hexane

Adsorption isotherms of n-hexane and adsorption deformation isotherms of an AR-V carbon adsorbent are measured in a pressure range of 1 Pa to 20 kPa at temperatures of 254.8–353 K. It is found that, except for the initial pressure range (p < 800 Pa), the relative linear deformation increases with increasing pressure and decreases with increasing temperature. At temperatures of 254.8, 273.2, and 293 K, the curves of adsorption and adsorption deformation exhibit hysteresis in the region close to the saturated vapor pressure. It is noted that, at pressures below 800 Pa, microporous AR-V carbon adsorbent undergoes contraction in the range of high temperatures. It is revealed that the contraction range of the adsorbent with respect to pressure gradually narrows with decreasing temperature and degenerates at 254.8 K.     

Probing the mechanism of water adsorption in carbon micropores with multitemperature isotherms and water preadsorption experiments

The phenomenon of water adsorption in carbon micropores is examined through the study of water adsorption equilibrium in molecular sieving carbon. Adsorption and desorption isotherms are obtained over a wide range of concentrations from less than 0.1% to beyond 80% of the vapor pressure. Evidence is provided in support of a proposed bimodal water adsorption mechanism that involves the interaction of water molecules with functional groups at low relative pressures and the adsorption of water molecules between graphene layers at higher pressures. Decomposition of the equilibrium isotherm data through application of the extended cooperative multimolecular sorption theory, together with favorable quantitative comparison, provides support for the proposed adsorption mechanism. Additional support is obtained from a multitemperature study of water equilibrium. Temperatures of 20, 50, and 60 degrees C were probed in this investigation in order to provide isosteric heat of adsorption data for water interaction with the carbon molecular sieve. At low loading, the derived isosteric heat of adsorption is estimated to be 69 kJ/mol. This value is indicative of the adsorption of water to functional groups. At higher loading, the isosteric heat of adsorption decreases with increasing loading and approaches the heat of condensation, indicative of adsorption between graphene layers. Further support for the proposed adsorption mechanism is derived from carbon dioxide adsorption experiments on carbon molecular sieve that is preadsorbed with various amounts of water. Significant exclusion of carbon dioxide occurs, and a quantitative analysis that is based on the proposed bimodal water adsorption mechanism is employed in this investigation.     

Naphthalene adsorption on activated carbons using solvents of different polarity

The hydrophobic-hydrophilic character of a series of microporous activated carbons was explored as a key factor in competitive adsorption of a non-polar compound from liquid phase. The selectivity of the carbon surface towards naphthalene was explored by performing the adsorption isotherms in water, cyclohexane and heptane. Solvent polarity and adsorbent hydrophobic character were found to strongly influence the adsorption capacity of naphthalene. In aqueous media, despite the non-polar character of the adsorbate, surface acidity lowered adsorption capacity. This is attributed to the competition of water from the adsorption sites, via H-bonding with surface functionalities and the formation of hydration clusters that reduce the accessibility and affinity of naphthalene to the inner pore structure. In organic media the uptake decreased due to competition of the hydrophobic solvent for the active sites of the carbon and to solvation effects. This competitive effect of the solvent is minimized in oxidized carbons as opposed to the trend obtained in aqueous solutions. The results confirmed that although adsorption of naphthalene strongly depends on the narrow microporosity of the adsorbent, competitive adsorption of the solvent for the active sites becomes important.     

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.     

纳米碳管储能的化学原理与储存容量研究

通过在大温度、压力范围内系统地测定氢在纳米碳管粉末与压片上的吸附等温线和对所得等温线的理论分析,计算出吸附热,并用超临界气体的吸附模型充分地描述了氢在纳米碳管上的吸附行为,证明纳米碳管储氢的原理是超临界吸附;比表面积和储气温度控制着储气容量.甲烷在干纳米碳管上的吸附机理与氢气相同,但在湿纳米碳管中的存储机理在于甲烷水合物的生成,因此孔容控制储气容量.单位质量多壁管的湿储容量是干储容量的5.1倍 ,单壁管可能产生更大的增强存储作用.     

Adsorption of Water Vapor on a Graphitized Carbon Black

Absorption isotherms for water vapor on a highly graphitized carbon black, Sterling FT-G (2700), have been determined at 280.15 and 295.15 K. Interparticle capillary condensation with extensive hysteresis is observed but capillary condensation (adsorption) occurs under metastable, supersaturation conditions. Contact angles for water adsorbed on this carbon black are calculated and two models for capillary condensation are discussed. Copyright 2000 Academic Press.     

Wetting and adsorption: II. Adsorption of Triton X-100 and Triton X-305 onto carbon black from water and cyclohexane solutions

The adsorption isotherms of nonionic surfactants Triton X-100 and Triton X-305 from water and cyclohexane on carbon black have been determined at 15 and 30°C. The Langmuir-type and BET-type isotherms are obtained for adsorption of Triton X-100 and Triton X-305 from water and cyclohexane respectively. Both the contact angles of water for graphite/water/air and graphite/water/cyclohexane decrease monotonously with increasing surfactant concentration. From these results, it is proposed that the adsorption of Triton X-100 and Triton X-305 on carbon black or graphite from water is monolayer. For the adsorption from cyclohexane solutions, the ethyleneoxide group of the surfactant molecules may be adsorbed onto the polar spot at the surface of carbon black, and the hydrophobic group of adsorbed molecules may direct toward the liquid phase or attaches to the nonpolar surface region around the polar spot. As the concentration increases, the ethylene oxide groups of the adsorbed molecules can be aggregated with each other via polar interactions to form hemi-reversed micelle.