Adsorption of gases in carbon nanotubes: are defect interstitial sites important?

Molecular simulations are used to shed light on an ongoing controversy over where gases adsorb on single walled carbon nanotube bundles. We have performed simulations using models of carbon nanotube bundles composed of tubes of all the same diameter (homogeneous) and tubes of different diameters (heterogeneous). Simulation data are compared with experimental data in an effort to identify the best model for describing experimental data. Adsorption isotherms, isosteric heats of adsorption, and specific surface areas have been computed for Ar, CH 4, and Xe on closed, open, and partially opened homogeneous and heterogeneous nanotube bundles. Experimental data from nanotubes prepared from two different methods, electric arc and HiPco, were examined. Experimental adsorption isotherms and isosteric heats for nanotubes prepared by the electric arc method are in best agreement with simulations for heterogeneous bundles of closed nanotubes. Models including adsorption in defect interstitial channels are required to achieve good agreement with experiments. Experimental isosteric heats and specific surface areas on HiPco nanotubes are best described by a model consisting of heterogeneous bundles with approximately 11% of the nanotubes opened.     

Analysis of Adsorbate–Adsorbate and Adsorbate–Adsorbent Interactions to Decode Isosteric Heats of Gas Adsorption

A qualitative interpretation is proposed to interpret isosteric heats of adsorption by considering contributions from three general classes of interaction energy: fluid–fluid heat, fluid–solid heat, and fluid—high‐energy site (HES) heat. Multiple temperature adsorption isotherms are defined for nitrogen, T=(75, 77, 79) K, argon at T=(85, 87, 89) K, and for water and methanol at T=(278, 288, 298) K on a well‐characterized polymer‐based, activated carbon. Nitrogen and argon are subjected to isosteric heat analyses; their zero filling isosteric heats of adsorption are consistent with slit‐pore, adsorption energy enhancement modelling. Water adsorbs entirely via specific interactions, offering decreasing isosteric heat at low pore filling followed by a constant heat slightly in excess of water condensation enthalpy, demonstrating the effects of micropores. Methanol offers both specific adsorption via the alcohol group and non‐specific interactions via its methyl group; the isosteric heat increases at low pore filling, indicating the predominance of non‐specific interactions.     

Thermodynamics of CO2 adsorption on functionalized SBA-15 silica. NLDFT analysis of surface energetic heterogeneity

Siliceous SBA-15 mesoporous molecular sieves were functionalized with different amounts of 3-aminopropyl-trimethoxysilane. To obtain a more detailed insight into the material properties of the prepared samples, their textural parameters were combined with results of thermal analysis. Adsorption isotherms of carbon dioxide on parent and functionalized SBA-15 were measured in the temperature range from 273 to 333 K. From the temperature dependence of CO(2) isotherms the isosteric adsorption heats of CO(2) were determined and discussed. Information about the surface energetic heterogeneity caused by tethered 3-aminopropyl groups were obtained from CO(2) adsorption energy distributions calculated using the theoretical CO(2) adsorption isotherms derived from the non-local density functional theory. The values of isosteric heats and the energy distributions of CO(2) adsorption detect highly energetic sites and enabled quantification of their concentrations.     

Argon and nitrogen adsorption in disordered nanoporous carbons: simulation and experiment

We report experimental measurements of the isosteric heats of adsorption for argon and nitrogen in two microporous saccharose-based carbons, using a Tian-Calvet microcalorimeter. These data are used to test recently developed molecular models of these carbons, obtained by a constrained reverse Monte Carlo method. Grand canonical Monte Carlo simulation is used to calculate the adsorption isotherms and isosteric heats for these systems, and the results for the latter are compared to the experimental data. For argon, excellent quantitative agreement is obtained over the entire range of pore filling. In the case of nitrogen, very good agreement is obtained over the range of coverage 0.25 < or = gamma/gamma 0 < or = 0.85, but discrepancies are observed at lower and higher coverages. The discrepancy at low coverage may be due to the presence of oxygenated groups on the pore surfaces, which are not taken into account in the model. The differences at high coverage are believed to arise from the presence of a few mesopores, which again are not included in the model. Pair correlation functions (argon-carbon and argon-argon) are determined from the simulations and are discussed as a function of pore filling. Snapshots of the simulations are presented and provide a picture of the pore filling process.     

Air separation by single wall carbon nanotubes: thermodynamics and adsorptive selectivity

Separation of a nitrogen-oxygen mixture (air) by single wall carbon nanotubes has been studied using grand canonical Monte Carlo simulations at a range of nanotube diameters, temperatures, and pressures. It is demonstrated that depending on these operating parameters, the extent of adsorptive selectivity can vary significantly. Detailed calculations are also presented for the adsorption isotherms, energies, and isosteric heats of pure nitrogen, oxygen, and their mixture at 100 K in a carbon nanotube of 12.53-A diameter. In single-component simulations, it is found that near saturation loading nitrogen forms only an annular layer close to the nanotube wall, while smaller-sized oxygen also occupies the region near the center of the nanotube. In mixture adsorption, the energetically favored nitrogen is preferentially adsorbed at low loadings. However, at high loadings oxygen replaces nitrogen due to the dominant entropic effects, and therefore a high selectivity towards oxygen is observed close to the saturation loading. The effect of the entropic change on mixture adsorption is evident from the calculated isosteric heats of adsorption. The mixture isotherms obtained from simulations are found to be in good agreement with the predictions based only on the pure component simulation results.     

Adsorption, structure and dynamics of benzene in ordered and disordered porous carbons

Molecular simulations are used to study the adsorption, structure, and dynamics of benzene at 298 K in atomistic models of ordered and disordered nanoporous carbons. The ordered porous carbon is a regular slit pore made up of graphene sheets. The disordered porous carbon is a structural model that reproduces the morphological (pore shape) and topological (pore connectivity) disorder of saccharose-based porous carbons. As expected for pores of a regular geometry, the filling occurs at well-defined pressures which are an increasing function of the pore width H. In contrast, in qualitative agreement with experimental data for activated carbon fibers, the filling of the disordered carbon is continuous and spans over a large pressure range. The structure and dynamics of benzene in the disordered carbon also strongly depart from that for the slit pore geometry. While benzene in the slit graphite nanopores exhibits significant layering, benzene in the disordered porous carbon exhibits a liquid-like structure very close to its bulk counterpart. Both the ordering and self-diffusivity of benzene in the graphite nanopores depend in a complex manner on the pore width. The dynamics is either slower or faster than its bulk counterpart; our data show that the self-diffusivity decreases as the number of confined layers n divided by the pore width H increases (except for very small pore sizes for which benzene crystallizes and is necessarily slower than the liquid phase). The dynamics of benzene in the disordered porous carbon is isotropic and is much slower than that for the graphite slit nanopores (even with the smallest slit nanopore considered in this work). The results above show that the adsorption, structure, and dynamics of benzene confined in disordered porous carbons cannot be described in simple terms using an ideal model such as the slit pore geometry.     

Ethane/ethylene adsorption on carbon nanotubes: temperature and size effects on separation capacity

We present the results of Monte Carlo simulations of the adsorption of single-component ethane and ethylene and of equimolar mixtures of these two gases on bundles of closed, single-walled carbon nanotubes. Two types of nanotube bundles were used in the simulations: homogeneous (i.e., those in which all the nanotubes have identical diameters) and heterogeneous (those in which nanotubes of different diameters are allowed). We found that at the same pressure and temperature more ethane than ethylene adsorbs on the bundles over the entire range of pressures and temperatures explored. The simulation results for the equimolar mixtures show that the pressure at which maximum separation is attained is a very sensitive function of the diameter of the nanotubes present in the bundles. Simulations using heterogeneous bundles yield better agreement with single-component experimental data for isotherms and isosteric heats than those obtained from simulations using homogeneous bundles. Possible applications of nanotubes in gas separation are discussed. We explored the effect of the diameter of the nanotubes on the separation ability of these sorbents, both for the internal and for the external sites. We found that substrate selectivity is a decreasing function of temperature.     

碳纳米管内N2吸附行为及等量吸附热的GCMC分子模拟研究

碳纳米管及石墨烯具有高比表面积、高化学稳定性以及高耐蚀性等优点,被认为是一种理想的吸附材料。分子模拟技术的发展和应用丰富了人们对吸附机理研究的方式,而简单气体吸附体系的吸附机理研究对吸附理论的发展有着重要的推动作用。本文以单壁碳纳米管（SWCNT）-N₂吸附体系为研究对象,首先通过透射扫描电镜和氮气吸/脱附测试对所选用碳纳米管的微观孔形貌及吸/脱附等温线进行了表征,然后根据对应孔径参数采用巨正则蒙特卡罗方法对该体系的吸附过程进行了分子模拟,并详细研究了碳纳米管孔径和温度对该体系吸附行为的影响。结果显示,SWCNT孔径越小,吸附能力则越强;孔半径为0.746nm的SWCNT的吸附体系发生凝聚相变的临界温度为66K。通过对等量吸附热进行计算发现,孔半径0.746、1.15、1.56和1.83 nm的SWCNT-N₂吸附体系对应的初始固-液等量吸附热分别为10.9、9.2、8.6和8.4 kJ/mol。67.5K时,孔半径1.56和1.83 nm的吸附体系的等量吸附热有热峰出现。     

Determination of the average heat and characteristic energy from the adsorption isotherm

The isotherms of the total content, isosteric and average heats of adsorption, as well as characteristic energies of adsorption were determined from the isotherms of excess adsorption of carbon dioxide on six different carbon adsorbents at temperature ranging from 293 to 423 К at pressures up to 6 MPa. The average isosteric heats are in agreement with the average heats of adsorption, which were determined from the equation relating the heat of adsorption with the characteristic energy of adsorption.     

Pure and binary adsorption of CO2, H2, and N2 on activated carbon

A new developing field of application for pressure swing adsorption (PSA) processes is the capture of CO₂ to mitigate climate change, especially the separation of CO₂ and H₂ in a pre-combustion context. In this process scheme the conditions of the feed to the separation step, namely a pressure of 3.5 to 4.5 MPa and a CO₂ fraction of around 40% are favorable for an adsorption based separation process and make PSA a promising technology. Among the commercial adsorbent materials, activated carbon is most suitable for this application. To evaluate the potential, to benchmark new materials, and for process development a sound basis of the activated carbon thermodynamic data is required, namely equilibrium adsorption isotherms of the relevant pure components and mixtures, Henry’s constants and isosteric heats.     

Gibbs ensemble Monte Carlo simulation of supercritical CO2 adsorption on NaA and NaX zeolites

Adsorption of supercritical carbon dioxide on two kinds of zeolites with identical chemical composition but different pore structure (NaA and NaX) was studied using the Gibbs ensemble Monte Carlo simulation. The model frameworks for the two zeolites with SiAl ratio being unity have been chosen as the solid structures in the simulation. The adsorption behaviors of supercritical CO2 on the NaA and NaX zeolites, based on the adsorption isotherms and isosteric heats of adsorption, were discussed in detail and were compared with the available experimental results. A good agreement between the simulated and experimental results is obtained for both the adsorbed amount and the bulk phase density. The intermediate configurational snapshots and the radial distribution functions between zeolite and adsorbed CO2 molecules were collected in order to investigate the preferable adsorption locations and the confined structure behavior of CO2. The structure behaviors of the adsorbed CO2 molecules show various performances, as compared with the bulk phase, due to the confined effect in the zeolite pores.     

Preparation of highly porous carbon from fir wood by KOH etching and CO2 gasification for adsorption of dyes and phenols from water

Fir wood was first carbonized for 1.5 h at 450 degrees C, then soaked in a KOH solution KOH/char ratio of 1, and last activated for 1 h at 780 degrees C. During the last hour CO2 was poured in for further activation for 0, 15, 30, and 60 min, respectively. Carbonaceous adsorbents with controllable surface area and pore structure were chemically activated from carbonized fir wood (i.e., char) by KOH etching and CO2 gasification. The pore properties, including the BET surface area, pore volume, pore size distribution, and pore diameter, of these activated carbons were first characterized by the t-plot method based on N2 adsorption isotherms. Fir-wood carbon activated with CO2 gasification from 0 to 60 min exhibited a BET surface area ranging from 1371 to 2821 m2 g(-1), with a pore volume significantly increased from 0.81 to 1.73 m2 g(-1). Scanning electron microscopic (SEM) results showed that the surfaces of honeycombed holes in these carbons were significantly different from those of carbons without CO2 gasification. The adsorption of methylene blue, basic brown 1, acid blue 74, p-nitrophenol, p-chlorophenol, p-cresol, and phenol from water on all the carbons studied was examined to check their chemical characteristics. Adsorption kinetics was in agreement with the Elovich equation, and all equilibrium isotherms were in agreement with the Langmuir equation. These results were used to compare the Elovich parameter (1/b) and the adsorption quantity of the unit area (q(mon)/Sp) of activated carbons with different CO2 gasification durations. This work facilitated the preparation of activated carbon by effectively controlling pore structures and the adsorption performance of the activated carbon on adsorbates of different molecular forms.     

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 Carbon Dioxide on Activated Carbon

The adsorption of CO2 on a raw activated carbon A and three modified activated carbon samples B, C, and D at temperatures ranging from 303 to 333 K and the thermodynamics of adsorption have been investigated using a vacuum adsorption apparatus in order to obtain more information about the effect of CO2 on removal of organic sulfur-containing compounds in industrial gases. The active ingredients impregnated in the carbon samples show significant influence on the adsorption for CO2 and its volumes adsorbed on modified carbon samples B, C, and D are all larger than that on the raw carbon sample A. On the other hand, the physical parameters such as surface area, pore volume, and micropore volume of carbon samples show no influence on the adsorbed amount of CO2. The Dubinin-Radushkevich (D-R) equation was the best model for fitting the adsorption data on carbon samples A and B, while the Preundlich equation was the best fit for the adsorption on carbon samples C and D. The isosteric heats of adsorption on carbon samples A, B, C, and D derived from the adsorption isotherms using the Clapeyron equation decreased slightly increasing surface loading. The heat of adsorption lay between 10.5 and 28.4 kJ/mol, with the carbon sample D having the highest value at all surface coverages that were studied. The observed entropy change associated with the adsorption for the carbon samples A, B, and C (above the surface coverage of 7 ml/g) was lower than the theoretical value for mobile adsorption. However, it was higher than the theoretical value for mobile adsorption but lower than the theoretical value for localized adsorption for carbon sample D.     

Separation of CO2 and N2 by adsorption in C168 schwarzite: a combination of quantum mechanics and molecular simulation study

Using a hierarchical multiscale approach combining quantum mechanics and molecular simulation, we have investigated the adsorption of pure CO(2) and N(2) and their mixture at room temperature in C(168) schwarzite, as a model for nanoporous carbons. First, the adsorbate-adsorbent interaction potential is determined using ab initio quantum mechanics computations, and then the adsorption is predicted using full atomistic Monte Carlo simulations. The extents of adsorption, adsorption energies, and isosteric heats of pure CO(2) and N(2) simulated with the ab initio potential are found to be higher than those with the empirical Steele potential that had been developed from gas adsorption on planar graphite. The inclusion of the electric quadrupole moment of adsorbate in simulation has no discernible effect on N(2) adsorption but results in a larger extent of CO(2) adsorption at high coverages. The selectivity of CO(2) over N(2) in the C(168) schwarzite from a model flue gas is predicted to be significantly larger with the ab initio potential than with the Steele potential. This illustrates the importance of an accurate adsorbate-adsorbent interaction potential in determining gas adsorption and suggests that nanoporous carbons might be useful for the separation of flue gases. As a comparison, the adsorption and selectivity of CO(2) and N(2) in ZSM-5 zeolites are also simulated with the experimentally validated potential parameters. The selectivity in the C(168) schwarzite predicted with the ab initio or Steele potential is found to be larger than the selectivity in all-silica ZSM-5, but less than that in Na-exchanged ZSM-5 zeolites.     

Equilibrium isotherms and isosteric heat for CO<Subscript>2</Subscript> adsorption on nanoporous carbons from polymers

Four nanoporous carbons obtained from different polymers: polypyrrole, polyvinylidene fluoride, sulfonated styrene–divinylbenzene resin, and phenol–formaldehyde resin, were investigated as potential adsorbents for carbon dioxide. CO₂ adsorption isotherms measured at eight temperatures between 0 and 60 °C were used to study adsorption properties of these polymer-derived carbons, especially CO₂ uptakes at ambient pressure and different temperatures, working capacity, and isosteric heat of adsorption. The specific surface areas and the volumes of micropores and ultramicropores estimated for these materials by using the density functional theory-based software for pore size analysis ranged from 840 to 1990 m² g^?1, from 0.22 to 1.47 cm³ g^?1, and from 0.18 to 0.64 cm³ g^?1, respectively. The observed differences in the nanoporosity of these carbons had a pronounced effect on the CO₂ adsorption properties. The highest CO₂ uptakes, 6.92 mmol g^?1 (0 °C, 1 atm) and 1.89 mmol g^?1 (60 °C, 1 atm), were obtained for the polypyrrole-derived activated carbon prepared through a single carbonization-KOH activation step. The working capacity for this adsorbent was estimated to be 3.70 mmol g^?1. Depending on the adsorbent, the CO₂ isosteric heats of adsorption varied from 32.9 to 16.3 kJ mol^?1 in 0–2.5 mmol g^?1 range. Overall, the carbons studied showed well-developed microporosity and exceptional CO₂ adsorption, which make them viable candidates for CO₂ capture, and for other adsorption and environmental-related applications.     

Adsorption of C7 hydrocarbons on biporous SBA-15 mesoporous silica

In our recent studies (Vinh-Thang, H.; Huang, Q.; Eic, M.; Trong-On, D.; Kaliaguine, S. Langmuir 2005, 21, 2051-2057; Vinh-Thang, H.; Huang, Q.; Eic, M.; Trong-On, D.; Kaliaguine, S. Stud. Surf. Sci. Catal. 2005, in press), a series of synthesized SBA-15 materials were characterized using nitrogen adsorption/desorption isotherms at 77 K and SEM images. In the present paper, four of them (MMS-1-RT, MMS-1-60, MMS-1-80, and MMS-5-80) were further investigated with regard to their equilibrium characteristics using n-heptane and toluene as sorbates by the standard gravimetric technique. SBA-15 materials proved to have a broad pore size distribution within the micropore/small-mesopore range in the walls of their main mesoporous channels. The adsorption capacities for toluene were found to be higher than for n-heptane. The isosteric heats of adsorption, estimated by the Clausius-Clapeyron equation, are also higher for toluene compared to n-heptane. They were found to depend on framework microporosity of the relevant SBA-15 samples. The isosteric heats of adsorption for all sorbates decrease with increased loading and approach the heats of evaporation of the respective sorbate. The adsorption capacities of SBA-15 samples are significantly higher than those of silicalite, i.e., the MFI zeolite silica analogue. In contrast to that, the isosteric heats of adsorption in the mesopore channels of SBA-15 were found to be much smaller. This result also suggests that SBA-15 can potentially be a good candidate for separation of C(7) hydrocarbons.     

Influence of MgO template on carbon dioxide adsorption of cation exchange resin-based nanoporous carbon

In this work, porous carbons with well-developed pore structures were directly prepared from a weak acid cation exchange resin (CER) by the carbonization of a mixture with Mg acetate in different ratios. The effect of the Mg acetate-to-CER ratio on the pore structure and CO(2) adsorption capacities of the obtained porous carbons was studied. The textural properties and morphologies of the porous carbons were analyzed via N(2)/77K adsorption/desorption isotherms, SEM, and TEM, respectively. The CO(2) adsorption capacities of the prepared porous carbons were measured at 298 K and 1 bar and 30 bar. By dissolving the MgO template, the porous carbons exhibited high specific surface areas (326-1276 m(2)/g) and high pore volumes (0.258-0.687 cm(3)/g). The CO(2) adsorption capacities of the porous carbons were enhanced to 164.4 mg/g at 1 bar and 1045 mg/g at 30 bar by increasing the Mg acetate-to-CER ratio. This result indicates that CER was one of the carbon precursors to producing the porous structure, as well as for improving the CO(2) adsorption capacities of the carbon species.     

Prediction of high-pressure adsorption equilibrium of supercritical gases using density functional theory

In this paper, we present the results of the prediction of the high-pressure adsorption equilibrium of supercritical gases (Ar, N2, CH4, and CO2) on various activated carbons (BPL, PCB, and Norit R1 extra) at various temperatures using a density-functional-theory-based finite wall thickness (FWT) model. Pore size distribution results of the carbons are taken from our recent previous work,(1,2) using this approach for characterization. To validate the model, isotherms calculated from the density functional theory (DFT) approach are comprehensively verified against those determined by grand canonical Monte Carlo (GCMC) simulation, before the theoretical adsorption isotherms of these investigated carbons calculated by the model are compared with the experimental adsorption measurements of the carbons. We illustrate the accuracy and consistency of the FWT model for the prediction of adsorption isotherms of the all investigated gases. The pore network connectivity problem occurring in the examined carbons is also discussed, and on the basis of the success of the predictions assuming a similar pore size distribution for accessible and inaccessible regions, it is suggested that this is largely related to the disordered nature of the carbon.     

Adsorption equilibrium of binary methane/ethane mixtures in BPL activated carbon: isotherms and calorimetric heats of adsorption

The adsorption of pure methane and ethane in BPL activated carbon has been measured at temperatures between 264 and 373 K and at pressures up to 3.3 MPa with a bench-scale high-pressure open-flow apparatus. The same apparatus was used to measure the adsorption of binary methane/ethane mixtures in BPL at 301.4 K and at pressures up to 2.6 MPa. Thermodynamic consistency tests demonstrate that the data are thermodynamically consistent. In contrast to two sets of data previously published, we found that the adsorption of binary methane/ethane in BPL behaves ideally (in the sense of obeying ideal adsorbed solution theory, IAST) throughout the pressure and gas-phase composition range studied. A Tian-Calvet type microcalorimeter was used to measure low-pressure isotherms, the isosteric heats of adsorption of pure methane and ethane in BPL activated carbon, and the individual heats of adsorption in binary mixtures, at 297 K and at pressures up to 100 kPa. The mixture heats of adsorption were consistent with IAST.