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.     

Bimodal micropore size distribution in active carbons

The porous structure of active carbon was compared with that of the original mineral coal and its carbonization products. The parameters of the porous structure were calculated from the adsorption isotherms of CO₂ (298 K) and H₂O (293 K). It was shown that carbonization of the original coal at 1120 K causes changes in the chemical composition, consolidation of the part which is amorphous to X-rays, generation of an ordered defect-containing structure on its basis, an increase in the volume of the micropores, and a decrease in the mean diameter. Activation of the carbonized coal affords a microporous structure with a bimodal size distribution.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 473–475, March, 1993.     

Energetics of methane adsorption on microporous activated carbons

The influence of microporous carbon surface oxidation on energetics of methane adsorption at 308 K is discussed. Obtained adsorption heats and integral molar entropies of the adsorbate show that microporous carbon surface oxidation changes the methane adsorption process. This is probably resulted by the existence of an endothermic effect during adsorption in oxidized carbon micropores.     

Adsorption of organic acids on TiO2 nanoparticles: effects of pH, nanoparticle size, and nanoparticle aggregation

In this study, the adsorption of two organic acids, oxalic acid and adipic acid, on TiO2 nanoparticles was investigated at room temperature, 298 K. Solution-phase measurements were used to quantify the extent and reversibility of oxalic acid and adipic acid adsorption on anatase nanoparticles with primary particle sizes of 5 and 32 nm. At all pH values considered, there were minimal differences in measured Langmuir adsorption constants, K ads, or surface-area-normalized maximum adsorbate-surface coverages, Gamma max, between 5 and 32 nm particles. Although macroscopic differences in the reactivity of these organic acids as a function of nanoparticle size were not observed, ATR-FTIR spectroscopy showed some distinct differences in the absorption bands present for oxalic acid adsorbed on 5 nm particles compared to 32 nm particles, suggesting different adsorption sites or a different distribution of adsorption sites for oxalic acid on the 5 nm particles. These results illustrate that molecular-level differences in nanoparticle reactivity can still exist even when macroscopic differences are not observed from solution phase measurements. Our results also allowed the impact of nanoparticle aggregation on acid uptake to be assessed. It is clear that particle aggregation occurs at all pH values and that organic acids can destabilize nanoparticle suspensions. Furthermore, 5 nm particles can form larger aggregates compared to 32 nm particles under the same conditions of pH and solid concentrations. The relative reactivity of 5 and 32 nm particles as determined from Langmuir adsorption parameters did not appear to vary greatly despite differences that occur in nanoparticle aggregation for these two different size nanoparticles. Although this potentially suggests that aggregation does not impact organic acid uptake on anatase particles, these data clearly show that challenges remain in assessing the available surface area for adsorption in nanoparticle aqueous suspensions because of aggregation.     

Thermodynamic and kinetic investigations of PO3-4 adsorption on blast furnace slag

The kinetics of adsorption of PO(3-)(4) by blast furnace slag were found to be fast, reaching equilibrium in 20 min and following a pseudo-second-order rate equation. The adsorption behavior of PO(3-)(4) on blast furnace slag has been studied as a function of the solution agitation speed, pH, and temperature. Results have been analyzed by Freundlich, Langmuir, BET, and Dubinin-Radushkevich (D-R) adsorption isotherms. The mean energy of adsorption, 10.31 kJ mol(-1), was calculated from the D-R adsorption isotherm. The rate constants were calculated for 293, 298, 303, and 308 K using a pseudo-second-order rate equation and the activation energy (E(a)) was derived using the Arrhenius equation. Thermodynamic parameters such as DeltaH(0), DeltaS(0), and DeltaG(0) were calculated from the slope and intercept of linear plot of lnK(D) against 1/T. The DeltaH(0) and DeltaG(0) values of PO(3-)(4) adsorption on the blast furnace slag show endothermic heat of adsorption. But there is a negative free energy value, indicating that the process of PO(3-)(4) adsorption is favored at high temperatures.     

Characterization and adsorption properties of tetrabutylammonium montmorillonite (TBAM) clay: thermodynamic and kinetic calculations

The montmorillonite has been subjected to modification through ion-exchange reaction by tetrabutylammonium bromide (TBAB). The modified sample was studied by X-ray diffraction (XRD) technique, Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric analysis (TGA) methods. The basal spacing of modified montmorillonite was determined as 14.40 A. The IR spectra of modified montmorillonite showed CH vibrations. The characterization of tetrabutylammonium montmorillonite (TBAM) and the adsorption of p-chlorophenol (p-CP) on organomontmorillonite was studied as a function of the solution concentration and temperature. The observed adsorption rates were found to fit to the pseudo-second-order kinetics. The rate constants were calculated for temperatures ranging between 25.0-35.0 degrees C at constant concentration. The adsorption energy, E, and adsorption capacity, (q(m)), for phenolic compounds adsorbing on organomontmorillonite were estimated using the Dubinin-Radushkevich (D-R) equation. Thermodynamic parameters (delta g(a) = -11.063 and -11.802 kJ/mol, delta h(a) = -30.032 and -30.789 kJ/mol, delta s(a) = -0.0636 and -0.0637 kJ/mol K for 298 and 308 K, respectively) were calculated by a new approximation from the adsorption isotherms of p-CP on organomontmorillonite. These isotherms were modeled according to Freundlich and Dubinin-Radushkevich adsorption isotherms, through which the first-order and second-order coefficients (K(1ads) = 0.0152 and 0.0127 micromol/g min, K(2ads) = 0.0130 and 0.0108 L/min micromol, respectively) were obtained at 298 and 308 K.     

Comparative Studies of the Micropore Size Distributions of an Alumina Pillared Montmorillonite and a Molecular Sieve Carbon

The aim of this work is to applicate and to compare various analysis methods for the characterization of the microporous structure from nitrogen adsorption at 77 K of an alumina pillared montmorillonite and a molecular sieve carbon. The adsorption potential distribution (X(A)), the Horvath-Kawazoe (HK) method, the Jaroniec-Gadkare-Choma (JGC) one and a numerical algorithm for the reconstruction of the micropore size distribution (MPSD) from the adsorption equilibrium isotherm have been applied. Comparison of all distributions revealed that the molecular sieve carbon shows smaller micropores and smaller structural hetereogeneity than the alumina pillared montmorillonite.     

Adsorption of gas-water binary systems in carbon micropores: Computer simulation

The adsorption of gas-water mixture in micropores of carbon materials at 298 K has been studied using computer simulation. Methane, nitrogen, ammonia, carbon dioxide, and hydrogen sulfide were considered as gas components. In the grand canonical ensemble Monte-Carlo simulation of adsorption, the displacement of a gas component from a pore as a result of the formation of water microclusters was observed for all systems studied. Cluster growth conditions on graphite-like and activated surfaces differ significantly. The comparative stability of adsorbed gas-water mixtures has been determined for all gases.     

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.     

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.     

Negative thermal expansion of water in hydrophobic nanospaces

The density and intermolecular structure of water in carbon micropores (w = 1.36 nm) are investigated by small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) measurements between 20 K and 298 K. The SAXS results suggest that the density of the water in the micropores increased with increasing temperature over a wide temperature range (20-277 K). The density changed by 10%, which is comparable to the density change of 7% between bulk ice (I(c)) at 20 K and water at 277 K. The results of XRD at low temperatures (less than 200 K) show that the water forms the cubic ice (I(c)) structure, although its peak shape and radial distribution functions changed continuously to those of a liquid-like structure with increasing temperature. The SAXS and XRD results both showed that the water in the hydrophobic nanospaces had no phase transition point. The continuous structural change from ice I(c) to liquid with increasing temperature suggests that water shows negative thermal expansion over a wide temperature range in hydrophobic nanospaces. The combination of XRD and SAXS measurements makes it possible to describe confined systems in nanospaces with intermolecular structure and density of adsorbed molecular assemblies.     

Effects of active carbon pore size distributions on adsorption of toxic organic compounds

The use of active carbons for the removal of toxic organic compounds, for example from air or smoke, is of significant interest. In this paper, the equilibrium and dynamic adsorption characteristics of two active carbons are explored; one microporous coconut based and the other micro-mesoporous derived from a synthetic resin. Benzene, acetaldehyde and acrylonitrile were chosen as the probe toxicant vapours and adsorption was measured at a temperature of 298 K. The nitrogen equilibrium data (at 77 K), analysed using the BET, Dubinin-Radushkevich equations and DFT models, showed a higher overall adsorption capacity, more supermicroporosity and a higher proportion of pores wider than 2 nm for the synthetic resin based material. A micropore distribution biased toward the ultramicropore width-range was observed for the nutshell material. As a consequence, the characteristic adsorption energies in micropores are higher for the nutshell material than the resin based carbon. The effect of these different pore size characteristics on the adsorption kinetics, obtained by fitting the data to the linear driving force (LDF) model, is that the resulting adsorption rate constants are higher across much of the relative pressure range (p/p ^s ) studied for the resin based carbon compared to the nutshell material. Significantly, the wider pores of the resin-based carbon result in higher rates of adsorption in the micropore filling domain. When evaluated under dynamic conditions in cigarette smoke, improved toxicant removal was observed using the resin based carbon.     

ADSORPTION OF MACROPOROUS PHOSPHONIC ACID RESIN FOR INDIUM

The adsorption kinetics and mechanism of a nrovel chelate resin, macroporous phosphonic acid resin (PAR) for In(Ill) were investigated Tile statically saturated adsorption capacity is 216mg·g^-1resin at 298K in HAc-NaAc medium. Tire apparent adsorption rate constant is k298=4.84×10^-5 s^-1. Tile adsorption behavior of PAR for In(Ill) obeys the Freundlich isotherm. The thermodynamic adsorption parameters, enthalpy change △H, free energy change △G and entropychange △S of PAR for In(Ⅲ) are 11.5kmol, -12.6kJmol and 80.8Jmol.K, respectively. The apparent activation energy is Ea=3.5k.l/mol. Tire molar coordination ratio of the functional group of PAR to In(Ⅲ) is about 3:1.     

Thermodynamics of krypton adsorption on microporous carbon adsorbent at high pressures

The behavior of the thermodynamic functions for the adsorption system krypton—microporous carbon sorbent ACC is described. The dependences of the differential molar isosteric heat of adsorption, entropy, enthalpy, heat capacity, and differential molar energy of the adsorption system on the adsorption equilibrium parameters were studied over the temperature range from 178 to 393 K and at pressures ranging from 1 to 6?106 Pa. Consideration of the non-ideality of gas phase and non-inertness of the adsorbent leads to a temperature dependence of the thermodynamic functions of the studied adsorption system, especially in the range of high pressures of the adsorptive. The non-ideality of the gas phase and the energetics of the adsorbent—adsorbate system exert the most significant effect on the thermodynamic functions. The non-inertness of the adsorbent in the investigated range of parameters of the adsorption system has a weak effect on the thermodynamic functions of adsorption. In the region of high filling of ACC micropores, the entropy increases, indicating the existence of processes, which change the structure of the adsorbate in the micropores, in particular, to form associates.     

Heat of adsorption of water vapors on active carbons

The initial stage of adsorption of water vapors on active carbons is due to the reaction of the water molecules with inorganic and organic polar sites. For the lowest degrees of filling, the heats of adsorption of H₂O on active carbons evacuated at 500°C are overestimated due to the strong physical and chemical reaction with the primary active sites. Adsorption on secondary active sites (second coordination sphere of the active site) results in the formation of aqueous clusters with stressed hydrogen bonds and with heats below the heat of vapor condensation (). Filling of micropores containing water clusters and capillary condensation in mesopores simultaneously take place in the middle region and in the region of saturation. Micropores containing no water clusters do not fill with water even for P/P_s=1. In this region, q > by 3%. Desorption of water from the micropores is almost totally controlled by capillary evaporation of the liquid from the mesopores, and the sorption volume distribution curve by Kelvin radii reflects the distribution of the total volume of meso- and micropores by the radii of the cylindrical mesopores.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2183–2186, October, 1989.     

Influence of free-space calibration using He on the measurement of adsorption isotherms

It is well known that helium (He) molecules that remain inside micropores after free-space calibration at a low temperature (77.4 K) affect the shape of an adsorption isotherm, especially in a very low relative pressure region. This negative effect of the remaining He leads to a misunderstanding of the porous characteristics, such as micropore size distribution and surface properties. However, it is still believed that such erroneous interpretations are limited to narrow microporous materials such as activated carbon and measurements at low temperatures, namely the measurement of the adsorption of N₂ and Ar at their boiling points. Here we report a systematic investigation of the influence of free-space calibration using He on microporous, mesoporous and non-porous materials. Zeolite Y, mesoporous silica, carbon black and aerosil 200 were used for the measurements. N₂, H₂O and CO₂ adsorption isotherms were measured at 77.4, 298 and 298 K, respectively. Free-space calibration was carried out before and after the isotherm measurement for each sample. Although the influence of the He that remained in the sample was small for the non-porous sample, the shape of the isotherms for the other samples in a low relative pressure region was rather affected by the timing of the free-space calibration even for the mesoporous sample, and at an ambient temperature.     

Hydrogen storage in nanostructured carbons by spillover: bridge-building enhancement

The hydrogen storage capacity in nanostructured carbon materials can be increased by atomic hydrogen spillover from a supported catalyst. A simple and effective technique was developed to build carbon bridges that serve to improve contact between a spillover source and a secondary receptor. In this work, a supported catalyst (Pd-C) served as the source of hydrogen atoms via dissociation and primary spillover and AX-21 or single-walled carbon nanotubes (SWNTs) were secondary spillover receptors. By carbonizing a bridge-forming precursor in the presence of the components, the hydrogen adsorption amount was increased by a factor of 2.9 for the AX-21 receptor and 1.6 for the SWNT receptor at 298 K and 100 kPa. Similar results were obtained at 10 MPa, indicating that the enhancement factor is a weak function of pressure. The AX-21 receptor with carbon bridges had the highest absolute capacity of 1.8 wt % at 298 K and 10 MPa. Reversibility was demonstrated through desorption and readsorption at 298 K. The bridge-building process appears to be receptor specific, and optimization may yield even greater enhancement. Using this technique, enhancements in storage of up to 17-fold on other carbon-based materials have been observed and will be reported elsewhere shortly.     

Water adsorption in ion-bearing nanopores

Grand canonical Monte Carlo simulations are used to examine the adsorption of water into cylindrical nanopores containing single ions. The isotherms for water adsorbing into nanopores with radii of 0.44, 0.54, 0.64, and 0.74 nm and containing Na+, K+, Ca2+, Cl-, or F- at 298 K are computed. In all cases the nanopores are found to fill at reservoir chemical potentials below the chemical potential of saturated water vapor at 298 K. The threshold chemical potential is found to be sensitive to both the size of the channel and the ion species, with the anion-bearing pores filling at lower chemical potentials. Additionally, the filling threshold chemical potential is found to decrease as the radius of the pores is decreased. Pores with K+ and Cl- are compared, and the Cl- pores are found to exhibit higher water densities in the filled states and a more energetically favorable water structure while yielding lower per particle entropies. Sample simulation configurations are also examined and indicate that at low chemical potentials, the adsorbed water forms a cluster around the ion. Finally, the influence of the choice of water model on the adsorption isotherms is examined.     

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。