Nonideality of binary mixtures in the adsorbent micropores

Analysis of a binary equilibrium in a distinct micropore showed that the noouniformity of the adsorption field results in a negative deviation of the adsorption solution from ideal behavior. This deviation is due to a decrease in the entropy and internal energy caused by rearrangement of adsorbed molecules. Different interactions between identical and different molecules in distinct micropores also result in a negative deviation from ideal behavior. A criterion was formulated that allows one to elucidate how the deviation from ideality depends on the nonuniformity of the adsorption space and the character of the interaction between molecules in a micropore. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No.6, pp. 1011–1017, June, 2000.     

Nitrogen adsorption on fluorinated activated carbon fiber

Nitrogen adsorption isotherms for fluorinated activated carbon fiber (F-ACF) and fluorinated carbon black (F-CB) were measured at 77 K. Surface structures of F-ACF and F-CB were examined by _s -plot analysis using the adsorption data on the nonporous carbon black (CB) and F-CB. The surface energy of F-ACF was lower than that of ACF. The micropore structure of ACF was preserved even after fluorination, although the limiting adsorption amount and the micropore width decreased with fluorination.     

Extension of the Theory of Volume Filling of Micropores to Adsorption in Supermicropores

An enhancement in characteristic energy of adsorption in large micropores is analyzed. The effect is due to the overlapping of potential fields from opposite walls of pores and to reduction of the surface adsorption film on filling the micropore volume. The effects of both factors are comparable in magnitude and dependent on the micropore size. This work is devoted to memory of the professor W. Schirmer     

Equilibrium adsorption of <Emphasis Type="Italic">n</Emphasis>-hexane and carbon tetrachloride vapors on sodium and polyhydroxyaluminum montmorillonites

Changes in the sorption and energy characteristics of montmorillonite upon the substitution of polyhydroxyaluminum cations for exchange sodium ions are studied. Based on the temperature dependence of the equilibrium parameters of adsorption, it was found that the substitution of polyhydroxyaluminum cations for exchange ions results in an increase of micropore volume and the heat of adsorption. The curves of heats of adsorption for C₆H₁₄ and CCl₄ have extreme characters. The observed maxima are related to the interactions of adsorbate molecules with and between active sites due to their compaction during the filling of micropore volume.     

Diffusion Mechanism of Carbon Dioxide in Zeolite 4A and CaX Pellets

The adsorption kinetics and equilibria of CO₂ in commercial zeolite 4A and CaX pellets were theoretically and experimentally studied by a gravimetric method in the range of 273–313 K and 0.0–0.8 atm. The diffusion mechanism of an adsorbate into a pellet is composed of micropore and macropore diffusion due to the bidisperse structure of the pellet. When one diffusion mechanism played a more important role than the other in determining the overall diffusion rate, the diffusion rate was estimated by the nonisothermal monodisperse diffusion model (NMDM). However, when the combined effects of both mechanisms controlled the overall adsorption kinetics, the experimental uptake was analyzed by the nonisothermal bidisperse diffusion model (NBDM). The CO₂ diffusion in zeolite 4A pellets was controlled by micropore diffusion within the experimental pressure and temperature ranges. However, both macropore and micropore diffusion contributed to CO₂ diffusion in the zeolite CaX pellet. The overall CO₂ diffusion rate in zeolite CaX became faster as pressure increased mainly due to its highly favorable isotherm in the zeolite CaX. The micropore diffusion time constant of CO₂ in the zeolite CaX pellet was approximately one hundred times greater than that in the zeolite 4A pellet. In addition, the activation energy of micropore diffusion of CO₂ diffusion in the zeolite CaX pellet was smaller than that in the zeolite 4A pellet. In this study, the dimensionless parameter, , indicating the relative importance of macropore and micropore diffusion, was modified to consider non-zero coverage as an initial condition for each step in the gravimetric method. When is greater than 100, the overall adsorption rate is controlled by macropore diffusion. However, in cases where is less than 0.1, micropore diffusion is the dominant mechanism in the overall adsorption rate. In the case of a system with between these values, both macropore and micropore diffusion contributed to the overall diffusion rate.     

Adsorption Characteristics in Zeolite Nano-Pore Evaluated by use of Nitrogen Monoxide as a Probe Adsorbate

Adsorption isotherms of nitrogen monoxide (NO) and in situ EPR spectra of adsorbed NO on mordenite zeolites (MOR) of different cation types (HM, NaM and CaM) are measured at different temperatures to elucidate the effect of the strong adsorption promoted by the enhancement of potential field in micropore of MOR (micropore filling) as well as the electrostatic interaction in MOR on NO adsorption. The NO molecules adsorb irreversibly and fill up the micropore of MOR at 201 K, above the critical temperature of NO, regardless of the kind of cation species. The NO adsorption takes place even at 273 K. In the adsorption at 273 K, the strength of electrostatic field formed by cation sites affects the adsorptivity and the order of saturation amount of adsorption (V _s) corresponds to that of the electrostatic field strength. EPR results show that NO molecules strongly interact with cation sites in MOR and disproponation reaction of NO take place on CaM.     

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.     

Equilibrium adsorption of a cyclopentane—benzene vapor mixture on active carbon

The data on adsorption of cyclopentane, benzene, and their mixture on active carbon were analyzed by the statistical thermodynamic model. A method to describe the state of a binary mixture in single micropores was offered. A substantial negative deviation from Raoult's law predicted by the concept of an ideal adsorption solution is a consequence of a decrease in the excessive Helmholtz energy brought about by progressive filling when values of the excessive energy are positive over a wide range of adsorption. The excessive values found for the entropy and internal energy of a mixture of molecules in a single micropore are negative due to the heterogeneity of the adsorption field. The approach suggested adequately describes the experimental data and can be used for the determination of differential heats of adsorption. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1070–1076, June, 1999.     

Carbon dioxide adsorption on the microporous ACC carbon adsorbent

Adsorption isotherms of carbon dioxide on the microporous ACC carbon adsorbent and the adsorption deformation of the adsorbent were measured. The heats of adsorption at temperatures raising from 243 to 393 K and pressures from 1 to 5⋅10⁶ Pa were measured. In the low-temperature region (243 K), an increase in the amount adsorbed is accompanied by adsorbent contraction, and at high micropore fillings (a > 10 mmol g⁻¹) the ACC carbon adsorbent expands. At high temperatures, adsorbent expansion is observed in the whole region of micropore filling. At 243 K in the low filling region (a < 1 mmol g⁻¹), the heat of adsorption decreases smoothly from 27 to 24 kJ mol⁻¹. The heat of adsorption remains virtually unchanged in the interval 2 mmol g⁻¹ < a < 11 mmol g⁻¹ and then decreases to 8 kJ mol⁻¹ at a = 12 mmol g⁻¹. Taking into account the nonideal character of the gas phase and adsorbent deformation the heats of adsorption are strongly temperature-dependent in a region of high pressures. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1331–1335, June, 2005.     

Fine pore mouth structure of molecular sieve carbon with GCMC-assisted supercritical gas adsorption analysis

N₂ adsorption isotherms of molecular sieve carbon were measured at 77 K and 303 K. The Ar adsorption isotherms of molecular sieve carbon samples were also measured at 303 K. The grand canonical Monte Carlo (GCMC) simulation technique was applied to calculate the N₂ and Ar adsorption isotherms at 303 K using the ultramicropore volume determined by H₂O adsorption. The comparative method of experimental and simulated isotherms of supercritical N₂ and Ar at 303 K gave the width of the micropore mouth of the molecular sieve carbon, which can be applied to the ultramicropore width determination for other noncrystalline porous solids.     

Optimum isotherm equation and thermodynamic interpretation for aqueous 1,1,2-trichloroethene adsorption isotherms on three adsorbents

Aqueous 1,1,2-trichloroethene (TCE) adsorption isotherms were obtained on Ambersorb^1® 563 and 572 adsorbents and Filtrasorb^2® 400 granular activated carbon (GAC). The data for Ambersorb 563 adsorbent covers TCE concentrations from 0.0009 to 600 mg/L. The data for each adsorbent was fit to 15 isotherm equations to determine an optimum equation.The best equation for the TCE adsorption isotherms is the Dubinin-Astakov (DA) isotherm. The DA isotherm coefficients were used to estimate the TCE micropore volume and the adsorption potential distribution. For each adsorbent, the TCE micropore volume is equivalent to the N₂ porosimetry micropore volume. The mean adsorption potential is 18.8, 13.0, and 8.9 kJ/mol, with coefficients of variation of 0.37, 0.53, and 0.67, for Ambersorb 563 and 572 adsorbents and Filtrasorb 400 GAC, respectively. Thus, Ambersorb 563 adsorbent has the most energetic and most homogeneous adsorption volume, while Filtrasorb 400 GAC has the least energetic and most heterogeneous adsorption volume. For these reasons, Ambersorb 563 adsorbent has the highest TCE capacity at low concentrations, whereas Filtrasorb 400 GAC has the highest TCE capacity at high concentrations. The performance of Ambersorb 572 adsorbent is generally intermediate to the other two adsorbents.     

Electrochemical behavior of Pd–Rh alloys

Pd–Rh alloys were prepared by electrochemical codeposition. Bulk compositions of the alloys were determined by the energy dispersive X-ray analysis method, while surface compositions were determined from the potential of the surface oxide reduction peak. Cyclic voltammograms, recorded in 0.5 M H₂SO₄ for Pd–Rh alloys of different bulk and surface compositions, are intermediate between curves characteristic of Pd and Rh. The influence of potential cycling on electrochemical properties and surface morphologies of the alloys was studied. Due to electrochemical dissolution of metals, both alloy surface and bulk become enriched with Pd. Carbon oxides were adsorbed at a constant potential from the range of hydrogen adsorption. The presence of adsorbed CO₂ causes remarkable diminution of hydrogen adsorption but it does not significantly influence hydrogen insertion into the alloy bulk. On the other hand, in the presence of adsorbed CO, both hydrogen absorption and adsorption are strongly suppressed. Oxidative removal of the adsorbates results in a characteristic voltammetric peak, whose potential increases with the decrease in Rh surface content. Electron per site (eps) values calculated for the oxidation of the adsorbates change with alloy surface composition, more for CO₂ than CO adsorption, indicating the variation of the structure and composition of CO₂ and CO adsorption products. The course of the dependence of eps values on surface composition suggests that the products of CO₂ and CO adsorption on Pd–Rh alloys are similar but not totally identical.     

Micropore filling of NO,SO2, NH3, and CO2 onα-FeOOH dispersed activated carbon fibers

Adsorption isotherms of NO, SO₂, NH₃, and CO₂ on-FeOOH dispersed activated carbon fibers at 303 K were examined to determine the role of surface modification in micropore filling. The parameters on micropore structures were obtained from both nitrogen adsorption at 77 K and benzene adsorption at 303 K; both isotherms were of BDDTI type and gave the same micropore volume. The preoxidation conditions of ACF, prior to the deposition of-FeOOH against the NO adsorptivity were examined. The dispersion of-FeOOH on ACF was effective in enhancement of micropore filling, irrespective of the adsorbate molecule. We determined the degree of volume filling for each gas by the use of a DR plot. The modified DR plot for an NO gas, of which the critical temperature was much lower than 303 K, was proposed. The degree of volume filling for various gases was correlated with the deviation of each boiling point from 303 K and with the van der Waalsa constant.     

Adsorption of perfluoropropane on the PAC microporous carbon adsorbent

The adsorption of perfluoropropane (N₃F₈) on the PAC microporous carbon adsorbent, which is close in properties to monoporous adsorbents, was studied at temperatures of 216, 243, 295, 318, and 343 K in the pressure interval from 1 to 1·10⁵ Pa. The adsorption isosters are well approximated by straight lines in the studied interval of temperatures and pressures. The dependence of the isosteric heats of adsorption on filling is described by a curve with a maximum in the region of high fillings. Such a curve is characteristic of adsorbents with a narrow pore size distribution.     

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.     

Recrystallization-induced defect porous nature of C60 crystals

Defective state of C₆₀ crystals was controlled by the recrystallization and annealing. The defective structure was examined by the X-ray diffraction and N₂ adsorption at 77 K. Recrystallized C₆₀ crystals without annealing showed broad diffraction peaks and the N₂ adsorption isotherm had marked low pressure uptake and a hysteresis in the high pressure region, indicating presence of both micropores and mesopores. The average micropore width was 8 Å which is closed to the C₆₀ molecular size, while the average mesopore one was 50 Å. The mesopores disappeared by annealing up to 393 K. On the other hand micropores of 8 Å remained even by heating up to 673 K.     

Influence of activated carbon in zeolite X/activated carbon composites on CH<Subscript>4</Subscript>/N<Subscript>2</Subscript> adsorption separation ability

A series of zeolite X/activated carbon composites with different ratio of zeolite X and activated carbon were prepared, which were adjusted by adding solid pitch powder and silicon dioxide as additional carbonaceous and silica source, respectively. The corresponding modified samples were obtained by treatment with the ammonium chloride solution. CH₄ and N₂ adsorption isotherms on all composites were determined within the pressure of 0–100 kPa at 298 K, and fitted with Henry model and Freundlich model. The results showed the adsorption separation abilities for CH₄ and N₂ were strongly influenced by activated carbon content, micropore structure and surface properties. The increase of activated carbon content increased the BET surface area, micropore surface area and micropore volume, leading to an enhanced CH₄ adsorption capacity and CH₄/N₂ adsorption selectivity. Compared with the unmodified composites, the modified composites showed higher CH₄/N₂ adsorption selectivity, and CH₄ adsorption capacity decreased slightly, which can be attributed to the reduction of the micropore structure parameters, the surface basic amount and basic strength. Furthermore, the modified composite HAX-3 presented the highest CH₄/N₂ selectivity of 3.4, and high CH₄ adsorption capacities, which is favorable for application in pressure swing adsorption processes.     

An analytical method of micropore filling of a supercritical gas

The supercritical gas adsorbed in the micropore having a strong molecular field was presumed to transform into the quasi-vapor to be filled in the micropore (quasi-vaporization adsorption mechanism). The Dubinin-Radushkevitch (DR) equation for micropore filling of vapor was extended to the quasi-vaporized supercritical gas using the quasi-saturated vapor pressureP _{0 q} and the inherent micropore volumeW _L . The reason why the concepts ofP _0q andW _L were introduced was explained with the molecule-pore interaction potential theory which is based on the Lennard-Jones interaction. The extended DR equation was named the supercritical DR equation. TheW _L was evaluated by the Langmuir plot of the adsorption isotherm for a supercritical gas and both ofP _0q andW _L provided the single reduced adsorption isotherms of supercritical NO, N₂, and CH₄ on activated carbon fibers and high surface area carbons were analyzed by the supercritical DR plots. The wide applicability of the reduced adsorption isotherm to these adsorption data was explicity shown. The two phase model of the organized and confined fluids was proposed in order to improve the quasi-vaporization adsorption mechanism.     

Tailoring Properties of Resol Resin-Derived Spherical Carbons for Adsorption of Phenol from Aqueous Solution

The polycondensation of resorcinol and formaldehyde in a water–ethanol mixture using the adapted Stöber method was used to obtain resol resins. An optimization of synthesis conditions and the use of an appropriate stabilizer (e.g., poly(vinyl alcohol)) resulted in spherical grains. The resins were carbonized in the temperature range of 600–1050 °C and then chemically activated in an aqueous HNO₃ solution, gaseous ammonia, or by an oxidation–reduction cycle (soaking in a HNO₃ solution followed by treatment with NH₃). The obtained carbons were characterized by XRD, the low-temperature adsorption of nitrogen, SEM, TGA, and XPS in order to determine degree of graphitization, porosity, shape and size of particles, and surface composition, respectively. Finally, the materials were tested in phenol adsorption. The pseudo-second order model perfectly described the adsorption kinetics. A clear correlation between the micropore volume and the adsorption capacity was found. The content of graphite domains also had a positive effect on the adsorption properties. On the other hand, the presence of heteroatoms, especially oxygen groups, resulted in the clogging of the pores and a decrease in the amount of adsorbed phenol.     

A Density Functional Theory Study on the Effect of Zero‐Point Energy Corrections on the Methanation Profile on Fe(100)

The thermodynamics and kinetics of the surface hydrogenation of adsorbed atomic carbon to methane, following the reaction sequence C+4 H?CH+3 H?CH₂+2 H?CH₃+H?CH₄, are studied on Fe(100) by means of density functional theory. An assessment is made on whether the adsorption energies and overall energy profile are affected when zero‐point energy (ZPE) corrections are included. The C, CH and CH₂ species are most stable at the fourfold hollow site, while CH₃ prefers the twofold bridge site. Atomic hydrogen is adsorbed at both the twofold bridge and fourfold hollow sites. Methane is physisorbed on the surface and shows neither orientation nor site preference. It is easily desorbed to the gas phase once formed. The incorporation of ZPE corrections has a very slight, if any, effect on the adsorption energies and does not alter the trends with regards to the most stable adsorption sites. The successive addition of hydrogen to atomic carbon is endothermic up to the addition of the third hydrogen atom resulting in the methyl species, but exothermic in the final hydrogenation step, which leads to methane. The overall methanation reaction is endothermic when starting from atomic carbon and hydrogen on the surface. Zero‐point energy corrections are rarely provided in the literature. Since they are derived from C? H bonds with characteristic vibrations on the order of 2500–3000 cm^?1, the equivalent ZPE of 1/2 hν is on the order of 0.2–0.3 eV and its effect on adsorption energy can in principle be significant. Particularly in reactions between CH_x and H, the ZPE correction is expected to be significant, as additional C? H bonds are formed. In this instance, the methanation reaction energy of +0.77 eV increased to +1.45 eV with the inclusion of ZPE corrections, that is, less favourable. Therefore, it is crucial to include ZPE corrections when reporting reactions involving hydrogen‐containing species.