Thermodynamics of hydrogen adsorption in slit-like carbon nanopores at 77 K. Classical versus path-integral Monte Carlo simulations

Hydrogen in slit-like carbon nanopores at 77 K represents a quantum fluid in strong confinement. We have used path-integral grand canonical Monte Carlo and classical grand canonical Monte Carlo simulations for the investigation of the "quantumness" of hydrogen at 77 K adsorbed in slit-like carbon nanopores up to 1 MPa. We find that classical simulations overpredict the hydrogen uptake in carbon nanopores due to neglect of the quantum delocalization. Such disagreement of both simulation methods depends on the slit-like carbon pore size. However, the differences between the final uptakes of hydrogen computed from both classical and quantum simulations are not large due to a similar effective size of quantum/classical hydrogen molecules in carbon nanospaces. For both types of molecular simulations, the volumetric density of stored energy in optimal carbon nanopores exceeds 6.4 MJ dm(-3) (i.e., 45 kg m(-3); Department of Energy target for 2010). In contrast to the hydrogen adsorption isotherms, we found a large reduction of isosteric enthalpy of adsorption computed from the quantum Feynman's path-integral simulations in comparison to the classical values at 77 K and pressures up to 1 MPa. Depression of the quantum isosteric enthalpy of adsorption depends on the slit-like carbon pore size. For the narrow pores (pore width H in [0.59-0.7] nm), the reduction of the quantum isosteric enthalpy of adsorption at zero coverage is around 50% in comparison to the classical one. We observed new phenomena called, by us, the quantum confinement-inducing polymer shrinking. In carbon nanospaces, the quantum cyclic polymers shrink, in comparison to its bulk-phase counterpart, due to a strong confinement effect. At considered storage conditions, this complex phenomenon depends on the size of the slit-like carbon nanopore and the density of hydrogen volumetric energy. For the smallest nanopores and a low density of hydrogen volumetric energy, the reduction of the polymer effective size is the highest, whereas an increase of the pore size and the density of hydrogen volumetric energy causes the polymer swelling up to a value slightly below the one computed from the bulk phase. Quantum confinement-inducing polymer shrinking is of great importance for realizing the potential of quantum molecular sieves.     

Monte Carlo simulations of hydrogen adsorption in alkali-doped single-walled carbon nanotubes

Monte Carlo simulations and Widom's test particle insertion method have been used to calculate the solubility coefficients (S) and the adsorption equilibrium constants (K) in single-walled (10,10) armchair carbon nanotubes including single nanotubes, and nanotube bundles with various configurations with and without alkali dopants. The hydrogen adsorption isotherms at room temperature were predicted by following the Langmuir adsorption model using the calculated constants S and K. The simulation results were in good agreement with experimental data as well as the grand canonical Monte Carlo simulation results reported in the literature. The simulations of nanotube bundle configurations suggest that the gravimetric hydrogen adsorption increases with internanotube gap size. It may be attributed to favorable hydrogen-nanotube interactions outside the nanotubes. The effect of alkali doping on hydrogen adsorption was studied by incorporating K+ or Li+ ions into nanotube arrays using a Monte Carlo simulation. The results on hydrogen adsorption isotherms indicate hydrogen adsorption of 3.95 wt% for K-doping, and 4.21 wt% for Li-doping, in reasonable agreement with the experimental results obtained at 100 atm and room temperature.     

Molecular modeling of freezing of simple fluids confined within carbon nanotubes

We report Monte Carlo simulation results for freezing of Lennard-Jones carbon tetrachloride confined within model multiwalled carbon nanotubes of different diameters. The structure and thermodynamic stability of the confined phases, as well as the transition temperatures, were determined from parallel tempering grand canonical Monte Carlo simulations and free-energy calculations. The simulations show that the adsorbate forms concentric molecular layers that solidify into defective quasi-two-dimensional hexagonal crystals. Freezing in such concentric layers occurs via intermediate phases that show remnants of hexatic behavior, similar to the freezing mechanism observed for slit pores in previous works. The adsorbate molecules in the inner regions of the pore also exhibit changes in their properties upon reduction of temperature. The structural changes in the different regions of adsorbate occur at temperatures above or below the bulk freezing point, depending on pore diameter and distance of the adsorbate molecules from the pore wall. The simulations show evidence of a rich phase behavior in confinement; a number of phases, some of them inhomogeneous, were observed for the pore sizes considered. The multiple transition temperatures obtained from the simulations were found to be in good agreement with recent dielectric relaxation spectroscopy experiments for CCl(4) confined within multiwalled carbon nanotubes.     

Monte Carlo simulation of n-alkane adsorption isotherms in carbon slit pores

The single component adsorption of alkanes in carbon slit pores was studied using configurational-biased grand canonical Monte Carlo simulations. Wide ranges of temperature, pressure, alkane chain length, and slit height were studied to evaluate their effects on adsorption. Adsorption isotherms and density and orientation profiles were calculated. The behavior of long alkanes at high temperatures was found to be similar to short alkanes at lower temperatures. This suggests that the isotherms may be related through the Polanyi potential theory.     

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.     

Adsorption and selectivity of carbon dioxide with methane and nitrogen in slit-shaped carbonaceous micropores: Simulation and experiment

We have used the grand canonical Monte Carlo method to study the adsorption and selectivity of mixtures of carbon dioxide with methane and nitrogen at high (i.e., ambient) temperatures in model slit pores with graphitic surfaces. Experimental data, including new high pressure measurements for carbon dioxide and methane on a non-porous graphitic standard, were used to test the potential models. The mixture simulations predict that carbon dioxide is preferentially adsorbed in both systems. The results are discussed in terms of competing energetic and entropic effects and the underlying molecular mechanisms.     

Density functional theory of adsorption in pillared slit-like pores

We propose a density functional theory to describe adsorption of Lennard-Jones fluid in pillared slit like pores. Specifically, the pillars are built of chains that are bonded by their ends to the opposite pore walls. The approach we propose combines theory of quenched-annealed systems and theory of nonuniform fluids involving chain molecules. We compare the results of theoretical predictions with grand canonical ensemble Monte Carlo simulations and compute theoretical capillary condensation phase diagrams for several model systems.     

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.     

Effect of curvature and chirality for hydrogen storage in single-walled carbon nanotubes: a combined ab initio and Monte Carlo investigation

Combined ab initio and grand canonical Monte Carlo simulations have been performed to investigate the dependence of hydrogen storage in single-walled carbon nanotubes (SWCNTs) on both tube curvature and chirality. The ab initio calculations at the density functional level of theory can provide useful information about the nature of hydrogen adsorption in SWCNT selected sites and the binding under different curvatures and chiralities of the tube walls. Further to this, the grand canonical Monte Carlo atomistic simulation technique can model large-scale nanotube systems with different curvature and chiralities and reproduce their storage capacity by calculating the weight percentage of the adsorbed material (gravimetric density) under thermodynamic conditions of interest. The author's results have shown that with both computational techniques, the nanotube's curvature plays an important role in the storage process while the chirality of the tube plays none.     

巨正则系综Monte Carlo模拟方法确定活性炭的微孔尺寸

根据299K下甲烷在活性炭中的吸附实验数据,通过调节狭缝微孔的孔宽参数,利用巨正则系综MonteCarlo(GCEMC)方法得到不同孔宽下流体的微观结构以及吸附等温线.比较并拟合模拟结果和实验数据,确定了活性炭微孔的平均孔宽,为下一步求解微孔尺寸分布以及为预测吸附剂在不同温度下吸附不同吸附质分子时的吸附性能提供了基础与指导.模拟中,甲烷分子采用单点Lennard-Jones球型分子模型,活性炭用狭缝孔来近似表征,流体分子与单个狭缝墙的相互作用采用著名的Steele的10－4-3势能模型.模拟表明,此方法为考察介孔材料的微孔分布以及微孔平均孔宽提供了新的思路.     

Effect of chemical potential on the computer simulation of hydrogen storage in single walled carbon nanotubes

Hydrogen is a kind of clean, sustainable and renewable energy carrier. Of the problems to be solved for the utilization of hydrogen energy, how to store and transport hydrogen has been given high priority on the research agenda. Recently, carbon nanotubes (CNTs) were reported to be very promising candidates for hydrogen uptake[1], which may have possibility to satisfy the benchmark set by the US Department of Energy (DOE) Hydrogen Plan for fuel cell powered vehicles: a gravimetric density …     

Fluids in nanospaces: molecular simulation studies to find out key mechanisms for engineering

We have analyzed various phenomena that occur in nanopores, focusing on elucidating their key mechanisms, to advance the effective engineering use of nanoporous materials. As ideal experimental systems, molecular simulations can effectively provide information at the molecular level that leads to mechanistic insight. In this short review, several of our recent results are presented. The first topic is the critical point depression of Lennard-Jones fluid in silica slit pores due to finite size effects, studied by our original Monte Carlo (MC) technique. We demonstrate that the first layers of adsorbed molecules in contact with the pore walls act as a “fluid wall” and impose extra finite size effects on the fluid confined in the central portion of the pore. We next present a new kernel for pore size distribution (PSD) analysis, based entirely on molecular simulation, which consists of local isotherms for nitrogen adsorption in carbon slit pores at 77 K. The kernel is obtained by combining grand canonical Monte Carlo (GCMC) method and open pore cell MC method that was developed in the previous study. We show that overall trends of the PSDs of activated carbons calculated with our new kernel and with conventional kernel from non-local density functional theory are nearly the same; however, apparent difference can be seen between them. As the third topic, we apply a free energy analysis method with the aid of GCMC simulations to investigate the gating behavior observed in a porous coordination polymer, and propose a mechanism for the adsorption-induced structural transition based on both the theory of equilibrium and kinetics. Finally, we construct an atomistic silica pore model that mimics MCM-41, which has atomic-level surface roughness, and perform molecular simulations to understand the mechanism of capillary condensation with hysteresis. We calculate the work required for the gas–liquid transition from the simulation data, and show that the adsorption branch with hysteresis for MCM-41 arise from spontaneous capillary condensation from a metastable state.     

Capillary condensation and adsorption of binary mixtures

The adsorption of equimolar binary mixtures of hydrogen-carbon dioxide, hydrogen-methane, and methane-carbon dioxide in porous material models is determined by grand canonical Monte Carlo simulations. The material models have an adsorbent surface similar to that of nanofibers with a herringbone structure. Our main result, which is relevant for hydrogen purification and carbon dioxide capture, is that the adsorption selectivities calculated for the mixtures can differ significantly from those deduced from simulations of the adsorption of pure gases, in particular, when one of the adsorbed gases presents a capillary condensation induced by confinement within the pore network. A comparison of our data is also made with theoretical models used in the literature for predicting the properties of the mixture adsorption.     

Grand canonical Monte Carlo simulation study on the catenation effect on hydrogen adsorption onto the interpenetrating metal-organic frameworks

Among recently synthesized isoreticular metal-organic frameworks (IRMOFs), interpenetrating IRMOFs show high hydrogen adsorption capacities at low temperature and under ambient pressure. However, little is known about the molecular basis of their hydrogen binding properties. In this work, we performed grand canonical Monte Carlo (GCMC) simulations to investigate the effect of catenation on the interactions between hydrogen molecules and IRMOFs. We identified the adsorption sites and analyzed the adsorption energy distributions. The simulation results show that the small pores generated by catenation can play a role to confine the hydrogen molecules more densely, so that the capacity of the interpenetrating IRMOFs could be higher than that of the non-interpenetrating IRMOFs.     

Effects of confinement on the molar enthalpy of argon adsorption in graphitic cylindrical pores: a grand canonical Monte Carlo (GCMC) simulation study

Using a grand canonical Monte Carlo simulation, we study argon adsorption in graphitic cylindrical pores to investigate the differences between the isosteric heat and the integral molar enthalpy under subcritical and supercritical conditions and compare these results against those for a flat graphite surface to investigate the role of confinement on the enthalpy change of adsorption. The isosteric heat curve is finite under subcritical conditions, but for supercritical adsorption, it becomes infinite at the pressure where the excess concentration versus pressure is maximum. This can be circumvented using the integral molar enthalpy, which is a better variable to describe the energy change for supercritical adsorption. Finally, the effects of pore geometry (radius and length) on argon adsorption under subcritical and supercritical conditions are discussed.     

Computer simulation of the adsorption of methane, nitrogen, and mixtures thereof in pores of a lamellar carbon adsorbent

The Monte Carlo method in conjunction with the grand canonical ensemble was used to calculate the isotherms of adsorption of methane, nitrogen, and mixtures thereof in 1.34 × 3.02-nm rectangular-cross-section and 2.35 × 2.35-nm square pores in a lamellar carbon adsorbent. The phase diagrams of adsorbed methane were plotted, and the characteristics of the phases of the adsorbate were described. Modeling the adsorption of the binary mixture demonstrated that the square carbon pore is more selective with respect to methane.     

Cryogenic separation of hydrogen isotopes in single-walled carbon and boron-nitride nanotubes: insight into the mechanism of equilibrium quantum sieving in quasi-one-dimensional pores

Quasi-one-dimensional cylindrical pores of single-walled boron nitride and carbon nanotubes efficiently differentiate adsorbed hydrogen isotopes at 33 K. Extensive path integral Monte Carlo simulations revealed that the mechanisms of quantum sieving for both types of nanotubes are quantitatively similar; however, the stronger and heterogeneous external solid-fluid potential generated from single-walled boron nitride nanotubes enhanced the selectivity of deuterium over hydrogen both at zero coverage and at finite pressures. We showed that this enhancement of the D(2)/H(2) equilibrium selectivity results from larger localization of hydrogen isotopes in the interior space of single-walled boron nitride nanotubes in comparison to that of equivalent single-walled carbon nanotubes. The operating pressures for efficient quantum sieving of hydrogen isotopes are strongly depending on both the type as well as the size of the nanotube. For all investigated nanotubes, we predicted the occurrence of the minima of the D(2)/H(2) equilibrium selectivity at finite pressure. Moreover, we showed that those well-defined minima are gradually shifted upon increasing of the nanotube pore diameter. We related the nonmonotonic shape of the D(2)/H(2) equilibrium selectivity at finite pressures to the variation of the difference between the average kinetic energy computed from single-component adsorption isotherms of H(2) and D(2). In the interior space of both kinds of nanotubes hydrogen isotopes formed solid-like structures (plastic crystals) at 33 K and 10 Pa with densities above the compressed bulk para-hydrogen at 30 K and 30 MPa.