Graphitic Slit Pores Research Articles

Sulfur–Nitrogen Codoped Graphite Slit-Pore for Enhancing Selective Carbon Dioxide Adsorption: Insights from Molecular Simulations

Using Grand Canonical Monte Carlo calculations, sulfur and nitrogen codoped (S/N-) graphite slit-pores with different defects (Divacancy 5-8-5, NS-1, NS-2, Stone–Wales (SW) 5577) are constructed to study their selective CO2 adsorption from CO2/H2, CO2/N2, CO2/CH4 and CO2/H2O mixtures. Among all the defective S/N-graphite slit-pores, it is found that the doped sites of S and N atoms affect slightly on CO2 uptake of graphite slit-pore. More importantly, the increasing ratio of S/N enhances the selective CO2 adsorption. For example, at 300 K and 1 bar, the full N-graphite slit-pore with SW 5577 has a CO2 uptake of 79.77 mmol/mol with good CO2/H2 selectivity (∼356) whereas full S-graphite slit-pore with SW 5577 possesses a considerable CO2 uptake of 104.66 mmol/mol with excellent CO2/H2 selectivity (∼526). Furthermore, density functional theory calculations indicate this defective S/N-graphite slit-pore with higher ratio of S/N interacts more strongly with CO2 molecules compared with other gases, which demons...

The influence of the functional group on activated carbon for acetone adsorption property by molecular simulation study

In this paper, different types and amounts of oxygen functional groups are inserted in graphitic slit pores to investigate the individual effect of each factor, which aims to clarify the effects of functional groups in activated carbon on acetone adsorption based on molecular simulation. Adsorption isotherms of acetone are then calculated by using Grand Canonical Monte Carlo (GCMC) simulation. Simulation results show that acetone adsorption capacity is increased at low fugacity after inserting oxygen-containing functional groups, however, equilibrium adsorption capacity is decreased. Along with the increase of amounts of functional group, the acetone adsorption loading is not always increased due to the reduction of available adsorption sites on pore wall. Also, it is concluded that trace acetone (10ppmv) is adsorbed mainly in 1.0 nm pore, in which the oxygen content is 5 wt% and the functional group planted in is hydroxyl that is the most beneficial one to trace acetone adsorption.

Monte Carlo Simulation of Adsorption-Induced Deformation in Finite Graphitic Slit Pores

We present a grand canonical Monte Carlo simulation study of deformation in graphitic slit pores induced by argon adsorption at sub- and supercritical temperatures. We find that solvation pressure is the driving force for the deformation. This is analyzed by studying its spatial distribution across the pore in order to understand the effects of adsorbate location on the deformation. We find that (1) pore width affects the packing of the adsorbate molecules and note that the zero solvation pressure at saturation pressure could be used to distinguish between commensurate and incommensurate pores, and (2) thermal fluctuation increases with temperature meaning that molecular excursions are closer to the pore walls at high temperatures, resulting in greater repulsion compared to that at lower temperatures. Consequently, the pore deformation depends on an intricate interplay between packing and thermal fluctuation.

A novel algorithm to accelerate the convergence of grand canonical Monte Carlo simulation of non-uniform fluids

A novel Monte Carlo scheme, MTZ-GCMC, utilising the ‘mass transfer zone’ (MTZ) between the fluid phase and the adsorbed phase, is proposed as an effective method for simulating the approach of a non-uniform fluid to equilibrium. We have applied this procedure to study the adsorption of gases in a closed-end graphitic slit pore, paying special attention to the region where there is a very sharp (but not vertical) condensation transition within the pore. In this region, conventional Monte Carlo (MC) requires a much greater computational effort to achieve convergence and may lead to incorrect results. In the MTZ-GCMC scheme, the insertion/deletion trials during the course of a simulation are restricted to the MTZ, which is identified and characterised by the percentage of successful insertions (PSI). Since the MTZ changes during the equilibration stage, the PSI must be updated on the fly until equilibrium has been reached. The distinct advantage of this scheme is that unnecessary insertion and deletion trials in the dense adsorbed phase, and the gas-like region are avoided; since most of these trials would be rejected, this means that convergence to the true equilibrium can be achieved an order of magnitude faster.

Predicting the adsorption of n-perfluorohexane in BAM-P109 standard activated carbon by molecular simulation using SAFT-γ Mie coarse-grained force fields

This work is framed within the Eighth Industrial Fluid Properties Simulation Challenge, with the aim of assessing the capability of molecular simulation methods and force fields to accurately predict adsorption in porous media for systems of relevant practical interest. The current challenge focuses on predicting adsorption isotherms of n-perfluorohexane in the certified reference material BAM-P109 standard activated carbon. A temperature of [Formula: see text] K and pressures of [Formula: see text], 0.3, and 0.6 relative to the bulk saturation pressure p0 (as predicted by the model) are the conditions selected in this challenge. In our methodology we use coarse-grained intermolecular models and a top-down technique where an accurate equation of state is used to link the experimental macroscopic properties of a fluid to the force-field parameters. The state-of-the-art version of the statistical associating fluid theory (SAFT) for potentials of variable range as reformulated in the Mie group contribution incarnation (SAFT- γ Mie) is employed here. The parameters of the SAFT- γ Mie force field are estimated directly from the vapour pressure and saturated liquid density data of the pure fluids using the equation of state, and further validated by molecular dynamic simulations. The coarse-grained intermolecular potential models are then used to obtain the adsorption isotherm kernels for argon, carbon dioxide, and n-perfluorohexane in graphite slit pores of various widths using Grand Canonical Monte Carlo simulations. A unique and fluid-independent pore size distribution curve with total micropore volume of 0.5802 cm3/g is proposed for the BAM-P109. The pore size distribution is obtained by applying a non-linear regression procedure over the adsorption integral equation to minimise the quadratic error between the available experimental adsorption isotherms for argon and carbon dioxide and purpose-built Grand Canonical Monte Carlo kernels. The predicted adsorption levels of n-perfluorohexane at 273 K in BAM-P109 are 72.75 ± 0.01, 73.82 ± 0.01, and 75.44 ± 0.05 cm3/g at Standard Temperature and Pressure (STP) conditions for [Formula: see text], 0.3, and 0.6, respectively.

A Molecular Dynamics Simulation Study on Separation Selectivity of CO2/CH4 Mixture in Mesoporous Carbons

The effect of surface charged defects in carbon mesoporous materials on adsorption selectivity for a CO2/CH4 gas mixture relevant to natural gas was studied by means of classical molecular dynamics (MD) simulation. The mole fraction of CO2 was 0.1, T = 300K and the partial pressure of CO2 was up to 40 bars. A graphite slit pore of 3.0nm was taken as a model for mesoporous carbon. The localized charged defects within an electroneutral pore surface were found to increase the CO2/CH4 separation selectivity. In addition, such surface charges interact with CO2 molecules only. The selectivity reduces with increasing pressures. Our model with a defect charge of 0.2 electron/atom can reproduce the selectivity CO2/CH4 of the experimental data reported in the literature. A perfect charge graphite model underestimates the adsorption selectivity of CO2/CH4. The results show that a very high selectivity around 25 can be achieved with a charged defect of 0.45 electron/atom. This suggests that controlling the charged defects on surface of carbon mesoporous can be used to enhance the CO2/CH4 separation for natural gas.

On the orientation of N2 and CO2 molecules adsorbed in slit pore models with oxidised graphitic surface

We report grand canonical Monte Carlo studies of nitrogen (77 K) and carbon dioxide (253 K) adsorbed in graphitic slit pores with surface oxygen functionalities. The analysis is focused on the molecular orientation within the adsorption layers and the influence of surface heterogeneity on molecular density distributions. The oxygen-containing surface functionalities act like additional adsorption sites along the graphitic basal plane that interacts also electrostatically with the fluid. Our simulations reveal that the molecular orientation of both adsorbate particles changes greatly upon increasing surface heterogeneity, while the effect of the pore size on the adsorption behaviour is also discussed. The detailed microscopic description of the adsorbed layer on oxygen modified carbon surfaces can be used to highlight thermodynamic features of gas adsorption on real surfaces such as layering transitions and surface area calculations.

On the phase transition of argon adsorption in an open end slit pore—Effects of temperature and pore size

The phase evolution of argon adsorbed in open ended graphitic slit pores at temperatures below the 2D-critical temperature of the first layer was simulated using the grand canonical Monte Carlo (GCMC) and meso-canonical ensembles (MCE). In the latter the adsorption system is connected to a finite reservoir and the combined system is a canonical ensemble. Hysteresis loops and sigmoid van der Waals loops were found for the GCMC and MCE simulations respectively, corresponding to the observed 2D-transitions, which comprised vapor–solid, vapor–liquid and liquid–solid changes of state depending on the temperature. The MCE isotherms in large open end pores exhibited a sequential adsorption, not previously noted in the literature, where the monolayer filling on one wall is followed by monolayer filling on the opposite wall, giving rise to a double van der Waals loop. When the spacing between the pore walls is decreased, this double-vdW loop evolves to form a fused single loop, and the transition shifts from being predominantly a surface adsorption to pore volume filling.

Effects of Electric Field on the Vapor–Liquid Equilibria of Nanoconfined Methanol and Ethanol

The effects of the electric field on the vapor–liquid equilibria of methanol and ethanol confined in a graphitic slit pore of width 4 nm using molecular dynamics simulations are reported. The vapor–liquid critical temperature of methanol gets suppressed under confinement. The external electrical field further decreases the critical temperature with increasing electric field strength up to E = 1.5 V·nm–1. Surprisingly, a further increase in the electric field strength reverses the critical temperature behavior and is seen to increase with increasing electric field. The reversible behavior of the critical temperature with the electric field is also seen for nanoconfined ethanol at approximately 1.5 V·nm–1. The critical density, on the other hand, is found to continuously decrease with increasing electric field strength. Application of an external electric field results in the decrease in vapor and liquid densities. The coordination number in the liquid phase is found to decrease first with increasing electr...

Reconciliation of different simulation methods in the determination of the equilibrium branch for adsorption in pores

The recently proposed mid-density scheme [Liu Z, Herrera L, Nguyen VT, Do DD, Nicholson D. A Monte Carlo scheme based on mid-density in a hysteresis loop to determine equilibrium phase transition. Mol Simul. 2011; 37(11):932–939, Liu Z, Do DD, Nicholson D. A thermodynamic study of the mid-density scheme to determine the equilibrium phase transition in cylindrical pores. Mol Simul. 2012; 38(3):189–199] is tested against a method 2V-NVT (similar to the well-established gauge cell method) and the canonical ensemble (CE) method, using argon adsorption at 87 K in graphitic slit pores of infinite and finite length. In infinitely long pores, the equilibrium transition is vertical that is expected for an infinite system to have a first-order transition and this vertical transition was found to lie at the middle of the hysteresis loop and satisfies the well-known Maxwell rule of equal area. For pores of finite length, the equilibrium transitions are steep and are close to, but not exactly identical to, the desorption branch. This lends support to the conventional view that the desorption branch is nearest to equilibrium, although both adsorption and desorption branches are strictly speaking metastable; a view proposed originally by Everett [Everett DH. Capillary condensation and adsorption hysteresis. Berichte Der Bunsen-Gesellschaft [Phys Chem Chem Phys]. 1975; 79(9):732–734]. As a consequence, the Maxwell rule of equal area does not apply to finite systems. As the widely accepted CE and gauge cell methods do not falsify the mid-density scheme, this study lends strong support to the validity of this technique for the study of equilibria.

Molecular dynamics investigations of liquid-vapor interaction and adsorption of formaldehyde, oxocarbons, and water in graphitic slit pores.

Formaldehyde exposure has been associated with several human cancers, including leukemia and nasopharyngeal carcinoma, motivating the present investigation on the microscopic adsorption behaviors of formaldehyde in multi-component-mixture-filled micropores. Molecular dynamics (MD) simulation was used to investigate the liquid-vapor interaction and adsorption of formaldehyde, oxocarbons, and water in graphitic slit pores. The effects of the slit width, system temperature, concentration, and the constituent ratio of the mixture on the diffusion and adsorption properties are studied. As a result of interactions between the components, the z-directional self-diffusivity (D(z)) in the mixture substantially decreased by about one order of magnitude as compared with that of pure (single-constituent) adsorbates. When the concentration exceeds a certain threshold, the D(z) values dramatically decrease due to over-saturation inducing barriers to diffusion. The binding energy between the adsorbate and graphite at the first adsorption monolayer is calculated to be 3.99, 2.01, 3.49, and 2.67 kcal mol(-1) for CO2, CO, CH2O, and H2O, respectively. These values agree well with those calculated using the density functional theory coupled cluster method and experimental results. A low solubility of CO2 in water and water preferring to react with CH2O, forming hydrated methanediol clusters, are observed. Because the cohesion in a hydrated methanediol cluster is much higher than the adhesion between clusters and the graphitic surface, the hydrated methanediol clusters were hydrophobic, exhibiting a large contact angle on graphite.

The adsorption behaviour of CH4 on microporous carbons: effects of surface heterogeneity.

The effects of chemical and structural surface heterogeneity on the CH4 adsorption behaviour on microporous carbons have been investigated using a hybrid theoretical approach, including the use of density functional theory (DFT), molecular dynamics (MD), and grand canonical Monte Carlo (GCMC) simulations. Bader charge analysis is first performed to analyze the surface atomic partial charges. The CH4 adsorption densities in defective and functionalized graphite slit pores are lower than that in the perfect pore according to the MD simulations. Finally, the CH4 adsorption isotherms for the perfect, defective and functionalized slit pores are analyzed using the GCMC simulations in combination with the DFT and MD results. For pores with a defective surface, the adsorption capacities decrease; the embedded functional groups decrease the adsorption capacity at low pressure and enhance it at high pressure. Our results demonstrate the significant effects of chemical and structural surface heterogeneity on the CH4 adsorption and provide a systematic approach to understand the gas adsorption behaviour.

Molecular dynamics simulation of benzene in graphite and amorphous carbon slit pores

It is well known that confining a liquid into a pore strongly alters the liquid behavior. Investigations of the effect of confinement are of great importance for many scientific and technological applications. Here, we present a study of the behavior of benzene confined in carbon slit pores. Two types of pores are considered-graphite and amorphous carbon ones. We show that the effect of different pore structure is of crucial importance for the benzene behavior.

Grand-canonical quantized liquid density-functional theory in a Car-Parrinello implementation

Quantized Liquid Density-Functional Theory (QLDFT) [S. Patchkovskii and T. Heine, Phys. Rev. E 80, 031603 (2009)], a method developed to assess the adsorption of gas molecules in porous nanomaterials, is reformulated within the grand canonical ensemble. With the grand potential it is possible to compare directly external and internal thermodynamic quantities. In our new implementation, the grand potential is minimized utilizing the Car-Parrinello approach and gives, in particular for low temperature simulations, a significant computational advantage over the original canonical approaches. The method is validated against original QLDFT, and applied to model potentials and graphite slit pores.

A comparative study of critical temperature estimation of atomic fluid and chain molecules using fourth-order Binder cumulant and simplified scaling laws

Grand-canonical transition-matrix Monte Carlo simulation with histogram reweighting and finite-size scaling technique are used to calculate fourth-order Binder cumulant of order parameter along the vapour–liquid coexistence line to calculate the critical temperature of bulk and confined square-well fluid in slit pore of two pore sizes. Further, this approach is utilised to estimate the critical temperatures of relatively more complex fluids such as n-alkanes confined in graphite and mica slit pores of different slit widths. The estimated critical temperatures are compared with the critical temperature obtained for the same systems using simplified form of the scaling law. This investigation reveals that critical temperatures of simple and complex fluids in bulk state and under confinement, estimated using the scaling law, are within reasonable accuracy with that obtained using more accurate and rigorous approach of fourth-order Binder's cumulant.

A novel application of kinetic Monte Carlo method in the description of N2 vapour–liquid equilibria and adsorption

We present a novel application of kinetic Monte Carlo (kMC) to describe vapour–liquid equilibria (VLE), vapour–solid equilibria (VSE) and adsorption of nitrogen on a flat graphite surface and in graphitic slit pores. This method is applied, for the first time, to molecules having Lennard-Jones sites and fixed partial charges. For the bulk phase equilibria, we have found that all the thermodynamic properties agree well with experimental data and pre-existing simulations over a wide range of temperatures. A major advantage of the kMC method is its excellent performance under conditions where the Gibbs ensemble simulation fails to achieve convergence, especially in dense phases. Moreover, the calculation of the chemical potential is incorporated in the scheme, which avoids the need for tedious additional procedures, such as the Widom method, required in conventional methods, which add significant computational overheads. For adsorption on a graphite surface, the kMC results are superior to those from a conventional GCMC simulation under conditions where dense phases exist; this is attributed to efficient sampling of the kMC scheme.

Characterization of the PSD of activated carbons by a heterogeneous surface mixed model

In this study, a model is proposed to calculate the Pore Size Distribution (PSD) of microporous activated carbons taking into account the defects as heterogeneity particles in the inner surface of each graphitic slit pore. We particularly examined the effects of surface heterogeneity in the determination of the PSD for a controlled series of microporous carbons prepared from peach stones as the precursor material. In order to obtain the PSDs of the samples, the Heterogeneous Surface Mixed Model (HSMM) on the basis of Grand Canonical Monte Carlo (GCMC) was used to generate simulated isotherms for the adsorption of N2 at 77K and CO2 at 273K. These results are compared with different models, like the Quenched Solid Density Functional Theory (QSDFT) and the Non Local Density Functional Theory (NLDFT) in order to check the self-consistency and robustness of the proposed model. It was found that our model is very flexible, allowing for the possibility of changing both the kind of heterogeneity particle and its coverage on the surface.

On the hysteresis and equilibrium phase transition of argon and benzene adsorption in finite slit pores: Monte Carlo vs. Bin-Monte Carlo

Bin grand canonical Monte Carlo (Bin-GCMC) simulations are compared with conventional GCMC in a comprehensive investigation of the effects of simulation method for the description of the adsorption–desorption hysteresis of argon and benzene in graphitic slit pores. To study the equilibrium phase transition, we applied the Mid-Density scheme and show how pore width, length and adsorption temperature affect the equilibrium transition. It was found that the desorption branch becomes closer to the equilibrium transition as the pore length is increased, while its relative position between the adsorption and desorption segments of the hysteresis loop is rather insensitive to pore width and temperature. This is distinctly different from a previous study of cylindrical pores where the equilibrium transition was found to be closer to the adsorption branch of the hysteresis loop as the pore width is increased (Liu et al., 2012. Mol. Sim. 38 (3), 189-199).

Computer Simulation of Benzene–Water Mixture Adsorption in Graphitic Slit Pores

The adsorption of mixtures of benzene–water in graphitic slit pores has been studied using Monte Carlo simulation. It is shown that the adsorption of the mixture starts with the adsorption of benzene molecules on the graphite surface which then act as anchors for water molecules to adsorb. High loadings of water in a slit pore significantly affect the arrangement and orientation of benzene molecules. When the water density in the pore approaches its bulk liquid density, water molecules displace some benzene molecules from the surface, and the benzene sorption process switches from adsorption to an absorption mechanism. We have also studied the influence of the external vapor composition on the adsorption isotherms of benzene and water. It is observed that a high concentration of benzene in the vapor phase facilitates the adsorption of both benzene and water in such a manner that the condensation pressure of benzene and the onset pressure of water adsorption shift to lower values. At high pressures, the adsorption of water is dominant at the expense of benzene adsorption. This interesting finding calls for a review of the concept of graphite surface hydrophobicity when water is coadsorbed with hydrocarbons.

Affinity and Packing of Benzene, Toluene, and p-Xylene Adsorption on a Graphitic Surface and in Pores

A grand canonical Monte Carlo simulation has been carried out at ambient temperature to investigate the adsorption of benzene, toluene, and p-xylene (BTX) on a graphite surface and in a graphitic slit and cylindrical pores. Particular emphasis has been paid to the effects of the confined space on the affinity and packing density. Simulation results for adsorption on a graphite surface were tested against the experimental data to validate the potential models used in the description of adsorption. Our extensive simulation has shown that on an open graphite surface, where there is no restriction in the packing, p-xylene has the highest affinity and adsorbed amount at a given reduced pressure and benzene has the lowest values, due to the additional interaction of the methyl groups with the surface. In a confined space, the order of the affinity remains the same, but the packing (hence the amount adsorbed per unit physical pore volume) is affected by the geometry of the space. It was found that benzene has the highest packing density, whether it is expressed in terms of moles or mass