Importance of molecular shape in the adsorption of nitrogen,carbon dioxide and methane on surfaces and in confined spaces

The importance of shape in the adsorption of nitrogen, carbon dioxide and methane (common molecular probes for solid characterization) on surfaces and in confined spaces is investigated for its effects on the adsorption capacity and isosteric heat. We study the possibility of using an equivalent pseudo-sphere model to describe the potential energy of interaction of these molecular probes. On a flat open surface, we find that the equivalent pseudo-sphere model describes adsorption of these species sufficiently well. However, in the confined space of pores, especially pores that accommodate three layers or less, the pseudo-sphere model describes the adsorption badly because of the geometrical constraint on the molecular packing. It is recommended that to study adsorption properly in small pores, potential models that correctly describe molecular shape should be used. In characterization, pseudo-sphere models are commonly used to generate the kernels (local isotherms) for the determination of pore size distribution which can lead to misleading results. We illustrate this with an example to show that the wrong pore size distribution results if pseudo-sphere kernels are used.     

Adsorption of benzene on graphitized thermal carbon black: reduction of the quadrupole moment in the adsorbed phase

The performance of intermolecular potential models on the adsorption of benzene on graphitized thermal carbon black at various temperatures is investigated. Two models contain only dispersive sites, whereas the other two models account explicitly for the dispersive and electrostatic sites. Using numerous data in the literature on benzene adsorption on graphitized thermal carbon black at various temperatures, we have found that the effect of surface mediation on interaction between adsorbed benzene molecules must be accounted for to describe correctly the adsorption isotherm as well as the isosteric heat. Among the two models with partial charges tested, the WSKS model of Wick et al. that has only six dispersive sites and three discrete partial charges is better than the very expensive all-atom model of Jorgensen and Severance. Adsorbed benzene molecules on graphitized thermal carbon black have a complex orientation with respect to distance from the surface and also with respect to loading. At low loadings, they adopt the parallel configuration relative to the graphene surface, whereas at higher loadings (still less than monolayer coverage) some molecules adopt a slant orientation to maximize the fluid-fluid interaction. For loadings in the multilayer region, the orientation of molecules in the first layer is influenced by the presence of molecules in the second layer. The data that are used in this article come from the work of Isirikyan and Kiselev, Pierotti and Smallwood, Pierce and Ewing, Belyakova, Kiselev, and Kovaleva, and Carrott et al.     

Monte Carlo Simulation and Experimental Studies of CO2, CH4 and Their Mixture Capture in Porous Carbons

Adsorption of carbon dioxide and methane in porous activated carbon and carbon nanotube was studied experimentally and by Grand Canonical Monte Carlo (GCMC) simulation. A gravimetric analyzer was used to obtain the experimental data, while in the simulation we used graphitic slit pores of various pore size to model activated carbon and a bundle of graphitic cylinders arranged hexagonally to model carbon nanotube. Carbon dioxide was modeled as a 3-center-Lennard-Jones (LJ) molecule with three fixed partial charges, while methane was modeled as a single LJ molecule. We have shown that the behavior of adsorption for both activated carbon and carbon nanotube is sensitive to pore width and the crossing of isotherms is observed because of the molecular packing, which favors commensurate packing for some pore sizes. Using the adsorption data of pure methane or carbon dioxide on activated carbon, we derived its pore size distribution (PSD), which was found to be in good agreement with the PSD obtained from the analysis of nitrogen adsorption data at 77 K. This derived PSD was used to describe isotherms at other temperatures as well as isotherms of mixture of carbon dioxide and methane in activated carbon and carbon nanotube at 273 and 300 K. Good agreement between the computed and experimental isotherm data was observed, thus justifying the use of a simple adsorption model.     

Adsorption of argon from sub- to supercritical conditions on graphitized thermal carbon black and in graphitic slit pores: a grand canonical Monte Carlo simulation study

In this paper we consider the adsorption of argon on the surface of graphitized thermal carbon black and in slit pores at temperatures ranging from subcritical to supercritical conditions by the method of grand canonical Monte Carlo simulation. Attention is paid to the variation of the adsorbed density when the temperature crosses the critical point. The behavior of the adsorbed density versus pressure (bulk density) shows interesting behavior at temperatures in the vicinity of and those above the critical point and also at extremely high pressures. Isotherms at temperatures greater than the critical temperature exhibit a clear maximum, and near the critical temperature this maximum is a very sharp spike. Under the supercritical conditions and very high pressure the excess of adsorbed density decreases towards zero value for a graphite surface, while for slit pores negative excess density is possible at extremely high pressures. For imperfect pores (defined as pores that cannot accommodate an integral number of parallel layers under moderate conditions) the pressure at which the excess pore density becomes negative is less than that for perfect pores, and this is due to the packing effect in those imperfect pores. However, at extremely high pressure molecules can be packed in parallel layers once chemical potential is great enough to overcome the repulsions among adsorbed molecules.     

Adsorption of selected herbicides from aqueous solutions on activated carbon

The adsorption of MCPA and 2,4-D on the activated carbon Filtrasorb 300 was studied. The adsorption isotherms of herbicides from aqueous solutions were measured over a wide range of solute concentrations and at different temperatures. The experimental equilibrium data were analyzed by the Langmuir–Freundlich isotherm taking into account the energetic heterogeneity of adsorption system. The effect of temperature and herbicide properties on its uptake was discussed. The thermal analysis was applied in order to find the differences in herbicide interactions with carbon surface. The kinetic dependences were measured and the relations between solute properties and adsorption rate were discussed.     

Characteristic features of phase diagrams describing condensation of adsorbate in narrow pores

The phase diagrams describing condensation of adsorbate in micro- and mesoporous adsorbents having slit-shaped and cylindrical pores whose size varied from 1 to 20 monolayers were constructed. The study was performed using the lattice-gas model in the quasichemical approximation to take into account the intermolecular interactions. The phase diagrams for various values of the potential arising from different types of adsorbate--adsorbent interaction were analyzed for adsorption of helium, neon, methane, and carbon tetrachloride in graphite pores. Other adsorption systems are considered and the relationship between the pressure and temperature of adsorbate condensation is discussed. A nonmonotonic variation of the critical densities for pore widths from 3 to 10 molecular diameters was found. The pattern of this variation depends on the ratio of the energy of lateral interactions of the adsorbate molecules to the energy of interaction of the adsorbate molecules with pore walls. The critical temperature decreases monotonically with a decrease in the pore width. The stronger the adsorbate interaction with the pore walls, the greater the decrease in the critical temperature.     

Hypothetical high-surface-area carbons with exceptional hydrogen storage capacities: open carbon frameworks

A class of high-surface-area carbon hypothetical structures has been investigated that goes beyond the traditional model of parallel graphene sheets hosting layers of physisorbed hydrogen in slit-shaped pores of variable width. The investigation focuses on structures with locally planar units (unbounded or bounded fragments of graphene sheets), and variable ratios of in-plane to edge atoms. Adsorption of molecular hydrogen on these structures was studied by performing grand canonical Monte Carlo simulations with appropriately chosen adsorbent-adsorbate interaction potentials. The interaction models were tested by comparing simulated adsorption isotherms with experimental isotherms on a high-performance activated carbon with well-defined pore structure (approximately bimodal pore-size distribution), and remarkable agreement between computed and experimental isotherms was obtained, both for gravimetric excess adsorption and for gravimetric storage capacity. From this analysis and the simulations performed on the new structures, a rich spectrum of relationships between structural characteristics of carbons and ensuing hydrogen adsorption (structure-function relationships) emerges: (i) Storage capacities higher than in slit-shaped pores can be obtained by fragmentation/truncation of graphene sheets, which creates surface areas exceeding of 2600 m(2)/g, the maximum surface area for infinite graphene sheets, carried mainly by edge sites; we call the resulting structures open carbon frameworks (OCF). (ii) For OCFs with a ratio of in-plane to edge sites ≈1 and surface areas 3800-6500 m(2)/g, we found record maximum excess adsorption of 75-85 g of H(2)/kg of C at 77 K and record storage capacity of 100-260 g of H(2)/kg of C at 77 K and 100 bar. (iii) The adsorption in structures having large specific surface area built from small polycyclic aromatic hydrocarbons cannot be further increased because their energy of adsorption is low. (iv) Additional increase of hydrogen uptake could potentially be achieved by chemical substitution and/or intercalation of OCF structures, in order to increase the energy of adsorption. We conclude that OCF structures, if synthesized, will give hydrogen uptake at the level required for mobile applications. The conclusions define the physical limits of hydrogen adsorption in carbon-based porous structures.     

Predicting whether aromatic molecules would prefer to enter a carbon nanotube: A density functional theory study

The interaction of a carbon nanotube (CNT) with various aromatic molecules, such as aniline, benzophenone, and diphenylamine, was studied using density functional theory able to compute intermolecular weak interactions (B3LYP-D3). CNTs of varying lengths were used, such as 4-CNT, 6-CNT, and 8-CNT (the numbers denoting relative lengths), with the lengths being chosen appropriately to save computation times. All aromatic molecules were found to exhibit strong intermolecular binding energies with the inner surface of the CNT, rather than the outer surface. Hydrogen bonding between two aromatic molecules that include N and O atoms is shown to further stabilize the intermolecular adsorption process. Therefore, when benzophenone and diphenylamine were simultaneously allowed to interact with a CNT, the aromatic molecules were expected to preferably enter the CNT. Furthermore, additional calculations of the intermolecular adsorption energy for aniline adsorbed on a graphene surface showed that the concavity of graphene-like carbon sheet is in proportion to the intermolecular binding energy between the graphene-like carbon sheet and the aromatic molecule.     

Adsorption of chlorinated organic compounds from water with cerium oxide-activated carbon composite

Adsorptive removal of dichloromethane, chloroform, and carbon tetrachloride from aqueous solutions at 25 °C by activated carbon (AC) that was loaded with cerium oxide nanoparticles (CeO₂-NP/AC) was investigated. The developed adsorbent was characterized by scanning electron microscope (SEM), FTIR spectrophotometer, X-ray diffraction (XRD), and thermal gravimetric analysis (TGA). The effect of contact time, initial concentration, and the adsorbent dosage were also studied. The equilibrium and kinetics of adsorption were studied in a batch-type adsorption system, and the equilibrium experimental data were analyzed using Langmuir, Freundlich, and Temkin isotherm models. Freundlich adsorption isotherm showed the best fit for the equilibrium adsorption data. Three adsorption kinetic models, pseudo first- and second-order, and intraparticle diffusion models were applied to test the kinetic data. Kinetic characterization of the adsorption process onto CeO₂-NP/AC is well-described by the pseudo second-order model, and the adsorption best-fit by the intraparticle diffusion model. Our study shows that at optimum conditions, 82.72%, 99.40% and 89.42% of dichloromethane, chloroform, and tetrachloride, respectively, were removed by CeO₂-NP/AC, at concentration between 0.25 and 5.00 g/L.     

Effect of surface-perturbed intermolecular interaction on adsorption of simple gases on a graphitized carbon surface

In this paper, we investigate the effect of the solid surface on the fluid-fluid intermolecular potential energy. This modified fluid-fluid interaction energy due to the inducement of a solid surface is used in the grand canonical Monte Carlo (GCMC) simulation of various noble gases, nitrogen, and methane on graphitized thermal carbon black. This effect is such that the effective interaction potential energy between two particles close to surface is less than the potential energy if the solid substrate is not present. With this modification the GCMC simulation results agree extremely well with the experimental data over a wide range of pressures while the simulation results with the unmodified potential energy give rise to a shoulder near the neighborhood of monolayer coverage and the significant overprediction of the second and higher layer coverages. In particular the unmodified GCMC results exhibit very sharp change in those higher layers while the experimental data have a much gradual change in the uptake. We will illustrate this theory with adsorption data of argon, xenon, neon, nitrogen, and methane on graphitized thermal carbon black.     

Comparative analysis of methods for determination of structural characteristics of carbon adsorbents

Experimental data on the equilibrium adsorption of sulfur hexafluoride, methane, carbon dioxide, and benzene on carbon adsorbents of different porosity obtained in a wide pressure range at 298–408 K were analyzed. The adsorption volumes, surface areas, and sizes of slit-shaped pores of the carbons were determined using several independent methods. A method for determination of the adsorption volume from the experimental isotherm of excessive adsorption of gases and the total content equation was proposed. The resulting values are similar to the adsorption volumes calculated from the data for vapors. A new method for the calculation of the adsorbent surface area is described. The method is based on the dependence of the adsorption volume of adsorbent pores on the effective size of adsorbate molecules. A possibility to determine the average size of narrow slit-shaped carbon pores from the difference of the initial heats of adsorption of the gas under study on the carbon black and porous carbon adsorbent is considered. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2219–2227, October, 2005.     

Effects of confinement on the adsorption behavior of methane in high-silica zeolites

Adsorption isotherms of methane on high-silica zeolites at ambient temperature and up to high pressure were experimentally measured. The isotherms were analyzed on the basis of thermodynamics and statistical mechanics, and the relationship between microscopic properties and the macroscopic adsorption behavior was investigated. A comparison between the confined and unconfined phases revealed that molecular motion is restricted in the pores. As a result, the adsorbed phase is entropically destabilized, which cannot be neglected in comparison with the energetical stabilization that occurs as a result of the solid-molecule interaction. Our findings also indicate that the smaller slope (drho/d ln p) of the adsorption isotherms compared to that of the isotherm of the bulk at the same density is due to the smaller intermolecular interaction in the pores.The pore-size dependence is indicated not only in solid-molecule interactions but also in intermolecular interactions and molecular motion. Of these, the solid-molecule interactions strongly influence the adsorption behaviors in pores of different sizes. The origin of the restriction of molecular motion in the pores is well-explained by the one-dimensional transition and two-dimensional vibration (1D-trans, 2D-vib) model.     

氢在沸石上的吸附行为

应用基于Ono-Kondo格子理论得到的通用吸附等温方程, 通过分析氢在不同温度下, 在沸石NaX、CaA、NaA和ZSM-5上的吸附数据, 确定了氢的最大单层吸附容量. 并引入维里吸附方程, 由第二维里吸附系数和圆柱孔的Lennard-Jones(12-6)势模型计算了氢与沸石微孔壁面的作用势. 结果表明, 通用吸附等温方程可较好地描述氢在沸石上的超临界吸附行为, 拟合所得的氢在沸石上的最大单层吸附容量与吸附剂相关, 而与吸附温度无关. 圆柱孔作用势模型计算所得的氢分子在沸石上的吸附作用势与吸附热相近. 氢分子间的作用力表现为吸引力.     

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.     

Effect of surface functionalities on gas adsorption in microporous carbons: a grand canonical Monte Carlo study

In an attempt to offer a more realistic picture of adsorption in highly heterogeneous porous systems, such as oxygen functionalized porous carbons, we consider a series of carbon surfaces baring different amounts of oxygen functionalities (hydroxyl and epoxy). These surfaces are used to construct “oxidized” slit pores of varying width and functionality. With the aid of such inhomogeneous structures we study the interaction of Ar (87 K) inside “functionalized” pores and report grand canonical Monte Carlo adsorption simulations results. Based on our simulation data, we discuss the role of chemical heterogeneity on adsorbed/gas phase equilibrium properties such as density, heat of adsorption, and molecular packing within the pores. Comparisons are made with the case of the oxygen–free (completely homogeneous) slit pore models and conclusions on the suitability of Ar based pore size distributions for functionalized porous carbons are drawn.     

Some remarks on the calculation of the pore size distribution function of activated carbons

Different authors investigated the effects of geometric and energetic heterogeneities on adsorption and on carbon characterization methods. In most theoretical studies carbon structure is modeled as parallel infinite graphite walls that form ideal slit-shaped pores of the fixed widths. In the literature there is the lack of systematic studies showing the influence of pore structural and Lennard-Jones (LJ) potential parameters on the pore-size distribution functions. Moreover, the parameters characterizing the properties of the adsorbed phase and the heterogeneity of the adsorbent surface should be taken into account. The Nguyen and Do method with proposed by us ASA algorithm, were utilized for the assessment of the porosity from the series of almost few thousands numerically generated local adsorption isotherms. The values of the mentioned-above parameters are varied over the wide range (ca. +/-20%) of the reference ones. Different types of the theoretical and experimental adsorption isotherms (nitrogen at 77 K) were taken into account as the global ones. They were related to the mechanism of the primary, secondary or mixed micropore filling. The variations in some above-mentioned parameters have significant effects only for PSDs (and for average pore widths) corresponding to the primary micropore filling mechanism. On the other hand, for the process of the secondary micropore filling, the influence of these parameters (without the BET coefficient for adsorption on a "flat" surface, c(s,B)) is rather insignificant. Nevertheless the differences between local and global adsorption isotherms (in the whole range of relative pressures) the absence of micropores having pore half width equal to ca. 1 nm on PSDs was observed for studied adsorbate-adsorbent systems with exceptions of the strictly microporous adsorbents and/or the low values of c(s,B). Comparison of the experimental data with the generated theoretical isosteric enthalpy of adsorption indicates that the phenomenal uptake observed from experiment can be explained in terms of the reasonable solid-fluid interaction parameters. Therefore, we varied the heterogeneity of the adsorbent surface via the strength and the range of the solid-fluid potential and the parameter c(s,B) in order to reproduce the experimental data of enthalpy of adsorption. Note that similar procedure was applied by Wang and Johnson to reproduce some hydrogen adsorption data measured for carbon nanofibres. The analysis of the obtained results shows that the selection of the values of the parameters of the intermolecular interactions and the quantities characterizing the properties of the adsorbed phase and the heterogeneity of the adsorbent walls for molecular simulations should be made with care and the influence of possible errors should be considered.