Piotr Kowalczyk

Adsorption-Induced Deformation of Microporous Carbons: Pore Size Distribution Effect

We present a thermodynamic model of adsorption-induced deformation of microporous carbons. The model represents the carbon structure as a macroscopically isotropic disordered three-dimensional medium composed of stacks of slit-shaped pores of different sizes embedded in an incompressible amorphous matrix. Adsorption stress in pores is calculated by means of Monte Carlo simulations. The proposed model reproduces qualitatively the experimental nonmonotonic dilatometric deformation curve for argon adsorption on carbide-derived activated carbon at 243 K and pressure up to 1.2 MPa. The elastic deformation (contraction at low pressures and swelling at higher pressures) results from the adsorption stress that depends strongly on the pore size. The pore size distribution determines the shape of the deformation curve, whereas the bulk modulus controls the extent of the sample deformation.

Hyper-parallel tempering Monte Carlo simulations of Ar adsorption in new models of microporous non-graphitizing activated carbon: effect of microporosity

The adsorption of gases on microporous carbons is still poorly understood, partly because the structure of these carbons is not well known. Here, a model of microporous carbons based on fullerene-like fragments is used as the basis for a theoretical study of Ar adsorption on carbon. First, a simulation box was constructed, containing a plausible arrangement of carbon fragments. Next, using a new Monte Carlo simulation algorithm, two types of carbon fragments were gradually placed into the initial structure to increase its microporosity. Thirty six different microporous carbon structures were generated in this way. Using the method proposed recently by Bhattacharya and Gubbins (BG), the micropore size distributions of the obtained carbon models and the average micropore diameters were calculated. For ten chosen structures, Ar adsorption isotherms (87xa0K) were simulated via the hyper-parallel tempering Monte Carlo simulation method. The isotherms obtained in this way were described by widely applied methods of microporous carbon characterisation, i.e.xa0Nguyen and Do, Horvath-Kawazoe, high-resolution α(s) plots, adsorption potential distributions and the Dubinin-Astakhov (DA) equation. From simulated isotherms described by the DA equation, the average micropore diameters were calculated using empirical relationships proposed by different authors and they were compared with those from the BG method.

Can carbon surface oxidation shift the pore size distribution curve calculated from Ar, N2 and CO2 adsorption isotherms? Simulation results for a realistic carbon model

Using the virtual porous carbon model proposed by Harris et al, we study the effect of carbon surface oxidation on the pore size distribution (PSD) curve determined from simulated Ar, N(2) and CO(2) isotherms. It is assumed that surface oxidation is not destructive for the carbon skeleton, and that all pores are accessible for studied molecules (i.e., only the effect of the change of surface chemical composition is studied). The results obtained show two important things, i.e., oxidation of the carbon surface very slightly changes the absolute porosity (calculated from the geometric method of Bhattacharya and Gubbins (BG)); however, PSD curves calculated from simulated isotherms are to a greater or lesser extent affected by the presence of surface oxides. The most reliable results are obtained from Ar adsorption data. Not only is adsorption of this adsorbate practically independent from the presence of surface oxides, but, more importantly, for this molecule one can apply the slit-like model of pores as the first approach to recover the average pore diameter of a real carbon structure. For nitrogen, the effect of carbon surface chemical composition is observed due to the quadrupole moment of this molecule, and this effect shifts the PSD curves compared to Ar. The largest differences are seen for CO(2), and it is clearly demonstrated that the PSD curves obtained from adsorption isotherms of this molecule contain artificial peaks and the average pore diameter is strongly influenced by the presence of electrostatic adsorbate-adsorbate as well as adsorbate-adsorbent interactions.

The influence of carbon surface oxygen groups on Dubinin–Astakhov equation parameters calculated from CO2 adsorption isotherm

We present the results of a systematic study of the influence of carbon surface oxidation on Dubinin-Astakhov isotherm parameters obtained from the fitting of CO2 adsorption data. Using GCMC simulations of adsorption on realistic VPC models differing in porosity and containing the most frequently occurring carbon surface functionalities (carboxyls, hydroxyls and carbonyls) and their mixtures, it is concluded that the maximum adsorption calculated from the DA model is not strongly affected by the presence of oxygen groups. Unfortunately, the same cannot be said of the remaining two parameters of this model i.e.xa0the heterogeneity parameter (n) and the characteristic energy of adsorption (E0). Since from the latter the pore diameters of carbons are usually calculated, by inverse-type relationships, it is concluded that they are questionable for carbons containing surface oxides, especially carboxyls.

Fullerene-Intercalated Graphene Nano-Containers — Mechanism of Argon Adsorption and High-Pressure CH4 and CO2 Storage Capacities

Using GCMC simulations, we discuss the mechanism of argon adsorption onto intercalated graphene nano-containers (NanoBuds). The mechanism is related to the shapes of the high-resolution αS-plots. Next, we have tested the applicability of these materials to the storage of methane and carbon dioxide. We show that intercalation improves the storage, especially in the range of low pressures where the effect of volume does not dominate. The results obtained may be of interest in the design of new carbon materials.

Ar, CCl4 and C6H6 adsorption outside and inside of the bundles of multi-walled carbon nanotubes—simulation study

This is the first paper reporting the results of systematic study of the adsorption of Ar, C(6)H(6) and CCl(4) on the bundles of closed and opened multi-walled carbon nanotubes. Using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations, we also study the effect of the introducing defects in the external and internal walls of osculating and separated nanotubes on Ar diffusion and on adsorption of all three adsorbates. The Ar diffusion coefficients obtained are very sensitive to the presence of defects. Simulated isotherms are discussed to show the relation between the shapes of the high resolution alpha(s)-plots and the mechanisms of adsorption. From obtained data, as well as from geometric considerations, from the VEGA ZZ package, and from simulations (ASA), the values of surface areas of all nanotubes are calculated and compared with those obtained using the most popular adsorption methods (BET, alpha(s) and the A,B,C-points). We show that the adsorption value for the C-point of the isotherm should be taken for the calculation of the specific surface area of carbon nanotubes to obtain a value which approaches the absolute geometric surface area. A fully packed monolayer is not created at the A-, B- or C-points of the isotherm; however, the number of molecules adsorbed at the latter point is closest to the number of molecules in the monolayer as calculated via the ASA method, the VEGA ZZ package or from geometric considerations.

Impact of the carbon pore size and topology on the equilibrium quantum sieving of hydrogen isotopes at zero coverage and finite pressures

Carbonaceous slit-shaped and square-shaped pores efficiently differentiate adsorbed hydrogen isotopes at 77 and 33xa0K. Extensive path integral Monte Carlo simulations revealed that the square-shaped carbon pores enhanced the selectivity of deuterium over hydrogen in comparison to equivalent slit-shaped carbon pores at zero coverage as well as at finite pressures (i.e.xa0quantum sieving of hydrogen isotopes is pore-topology-dependent). We show that this enhancement of the D(2)/H(2) equilibrium selectivity results from larger localization of hydrogen isotopes in square-shaped pores. The operating pressures for efficient quantum sieving of hydrogen isotopes are strongly dependent on the topology as well as on the size of the carbon pores. However, for both considered carbon pore topologies the highest D(2)/H(2) separation factor is observed at zero-coverage limit. Depending on carbon pore size and topology we predicted monotonic decreasing and non-monotonic shape of the D(2)/H(2) equilibrium selectivity at finite pressures. For both kinds of carbonaceous pores of molecular sizes we predict high compression of hydrogen isotopes at 77 and 33xa0K (for example, the pore density of compressed hydrogen isotopes at 77xa0K and 0.25xa0MPa in a square-shaped carbon pore of size 2.6xa0Å exceeds 60xa0mmolxa0cm(-3); for comparison, the liquid density of para-H(2) at 30xa0K and 30 MPa is 42xa0mmolxa0cm(-3)). Finally, by direct comparison of simulation results with experimental data it is explained why ordinary carbonaceous materials are not efficient quantum sieves.

Molecular dynamics of zigzag single walled carbon nanotube immersion in water

The results of enthalpy of immersion in water for finite single-walled carbon nanotubes are reported. Using molecular dynamics simulation, we discuss the relation between the value of this enthalpy and tube diameters showing that the obtained plot can be divided into three regions. The structure of water inside tubes in all three regions is discussed and it is shown that the existence of the strong maximum of enthalpy observed for tube diameter ca. 1.17 nm is due to freezing of water under confinement. The calculations of hydrogen bond statistics and water density profiles inside tubes are additionally reported to confirm the obtained results. Next, we show the results of calculation for the same tubes but containing surface carbonyl oxygen groups at pore entrances. A remarkable rise in the value of enthalpy of immersion in comparison to the initial tubes is observed. We also discuss the influence of charge distribution between oxygen and carbon atom forming surface carbonyls on the structure of confined water. It is concluded for the first time that the presence of surface oxygen atoms at the pore entrances remarkably influences the structure and stability of ice created inside nanotubes, and surface carbonyls appear to be chaotropic (i.e. structure breaking) for confined water. This effect is explained by the pore blocking leading to a decrease (compared to initial structure) in the number of confined water molecules after introduction of surface oxygen groups at pore entrances.

Role of short-range directional interactions in coarse-graining of protic/aprotic liquids.

We study the role of short-range directional interactions in coarse-graining (CG) of protic (i.e., acetamide, methanol, ethanol, and water) and aprotic (i.e., acetone, benzene, and toluene) liquids at normal conditions. For this purpose, we introduce a new CG method in which the average interactions between atomistic molecules and CG beads measured in an N,P,T ensemble are preserved. We show that the spherically symmetric effective CG potential constructed according to our scheme is able to reproduce structural/thermodynamic properties of aprotic liquids; the heat of vaporization and total bonding energy profile for monomer are reproduced with good accuracy, while the density and radial distribution function are reproduced with fair accuracy within the proposed method. In contrast, the isobaric heat capacity is underestimated in the CG simulation because some of the fluctuations have been washed out from atomistic aprotic liquids. For protic liquids, spherically symmetric effective CG potential produces more structure, enhanced packing of beads, and underestimated isobaric heat capacity of CG liquids. This fundamental difference between protic and aprotic liquids can be explained by the presence of short-range directional interactions in the former liquids. We conclude that some information during the CG into spherically symmetric interaction potentials of protic liquids has to be lost. However, understanding how short-range directional interactions influence the structural and thermodynamic properties of the CG liquids seems to be the key for improving the CG methods.

The system of carbon tetrachloride and closed carbon nanotubes analyzed by a combination of molecular simulations, analytical modeling, and adsorption calorimetry.

Using the combined techniques of molecular simulation, simple analytical modeling, and adsorption calorimetry, we propose new models describing adsorption onto closed carbon nanotubes. The models are capable of describing the adsorption isotherms and calorimetric enthalpy of carbon tetrachloride adsorption measured on three different closed carbon nanotubes. It is shown that the assumption of the presence of two types of surface centers (high- and low-energy centers) on external tube surfaces is sufficient to describe experimental adsorption and calorimetric enthalpy data.