Evolution of methane density during melting in nanopores.

Phase properties of gases adsorbed in small nanopores are mainly determined by the pore size and shape as well as the structural heterogeneity of the adsorbate. Here we analyze the evolution of the melting mechanism that occurs in pores <3 nm in size. Melting in slit-shaped graphene pores is compared with melting in SURMOF channel pores with square cross-sections. We show how the melting transformation is related to the adsorption mechanism. We use a graphical representation of the evolution of molecular density as a function of temperature in the nanopores.

Heterogeneous melting of methane confined in nano-pores.

The melting transition of methane adsorbed in nanopores has been studied and compared in two types of structures: carbon slits pores and square shaped channels. We show that the nano-confinement not only modifies the temperatures of phase transformation but also induces strong space heterogeneity of the adsorbate. We emphasize the role of the structural heterogeneity on the mechanism of melting: in nanometric pores, each adsorbed layer exhibits different mechanisms of structural transformation and the notion of a unique transition temperature is not well defined.

Nanospace engineering of KOH activated carbon.

This paper demonstrates that nanospace engineering of KOH activated carbon is possible by controlling the degree of carbon consumption and metallic potassium intercalation into the carbon lattice during the activation process. High specific surface areas, porosities, sub-nanometer (<1 nm) and supra-nanometer (1-5 nm) pore volumes are quantitatively controlled by a combination of KOH concentration and activation temperature. The process typically leads to a bimodal pore size distribution, with a large, approximately constant number of sub-nanometer pores and a variable number of supra-nanometer pores. We show how to control the number of supra-nanometer pores in a manner not achieved previously by chemical activation. The chemical mechanism underlying this control is studied by following the evolution of elemental composition, specific surface area, porosity, and pore size distribution during KOH activation and preceding H(3)PO(4) activation. The oxygen, nitrogen, and hydrogen contents decrease during successive activation steps, creating a nanoporous carbon network with a porosity and surface area controllable for various applications, including gas storage. The formation of tunable sub-nanometer and supra-nanometer pores is validated by sub-critical nitrogen adsorption. Surface functional groups of KOH activated carbon are studied by microscopic infrared spectroscopy.

Enhanced hydrogen adsorption in boron substituted carbon nanospaces.

Activated carbons are one of promising groups of materials for reversible storage of hydrogen by physisorption. However, the heat of hydrogen adsorption in such materials is relatively low, in the range of about 4-8 kJ/mol, which limits the total amount of hydrogen adsorbed at P=100 bar to approximately 2 wt % at room temperature and approximately 8 wt % at 77 K. To improve the sorption characteristics the adsorbing surfaces must be modified either by substitution of some atoms in the all-carbon skeleton by other elements, or by doping/intercalation with other species. In this letter we present ab initio calculations and Monte Carlo simulations showing that substitution of 5%-10% of atoms in a nanoporous carbon by boron atoms results in significant increases in the adsorption energy (up to 10-13.5 kJ/mol) and storage capacity ( approximately 5 wt % at 298 K, 100 bar) with a 97% delivery rate.

Structural transformations of nitrogen adsorbed on graphite: Monte Carlo studies of spatial heterogeneity in multilayer system.

We present numerical Monte Carlo studies of nitrogen multilayers adsorbed on the basal plane of graphite. The analysis is focused on the system spatial heterogeneity and its influence on structures and phase transitions. The simulations have been carried out for surface coverage from monolayer to four layers, in canonical ensemble, in the temperature range from 5 to 100 K. An intricate phase situation is observed due to the competition between intermolecular and N(2)-graphite interactions. The commensurate monolayer is stabilized by the graphite corrugation. The multilayer commensurate structure is only metastable at low temperatures. Its stable structure is triangular, 1.08 times denser than the commensurate one stabilized by the N(2)-N(2) interactions. The multilayer structure is strongly spatially nonuniform, the individual layer structure changes from herringbone in the first layer to pinwheel arrangement in the fourth one. Two structural phase transitions, orientational order-disorder, and melting, are observed in each layer. Their mechanism and transition temperatures show strong variations depending on the position of the layer and the number of layers in the system.

Melting of hexane monolayers adsorbed on graphite: the role of domains and defect formation.

We present the first large-scale molecular dynamics simulations of hexane on graphite that completely reproduce all experimental features of the melting transition. The canonical ensemble simulations required and used the most realistic model of the system: (i) a fully atomistic representation of hexane; (ii) an explicit site-by-site interaction with carbon atoms in graphite; (iii) the CHARMM force field with carefully chosen adjustable parameters of nonbonded interaction, and (iv) numerous >or=100 ns runs, requiring a total computation time of ca. 10 CPU years. The exhaustive studies have allowed us to determine the mechanism of the transition: proliferation of small domains through molecular reorientation within lamellae and without perturbation of the overall adsorbed film structure. At temperatures greater than that of melting, the system exhibits dynamically reorienting domains whose orientations reflect the graphite substrate's symmetry and whose size decrease with increasing temperature.

Structural and phase properties of tetracosane (C24H50) monolayers adsorbed on graphite: an explicit hydrogen molecular dynamics study.

We discuss molecular dynamics (MD) computer simulations of a tetracosane (C24H50) monolayer physisorbed onto the basal plane of graphite. The adlayer molecules are simulated with explicit hydrogens, and the graphite substrate is represented as an all-atom structure having six graphene layers. The tetracosane dynamics modeled in the fully atomistic manner agree well with experiment. The low-temperature ordered solid organizes into a rectangularly centered structure that is not commensurate with underlying graphite. Above T=200 K, as the molecules start to lose their translational and orientational order via gauche defect formation a weak smectic mesophase (observed experimentally but never reproduced in united atom (UA) simulations) appears. The phase behavior of the adsorbed layer is critically sensitive to the way the electrostatic interactions are included in the model. If the electrostatic charges are set to zero (as for a UA force field), then the melting temperature increases by approximately 70 K with respect to the experimental value. When the nonbonded 1-4 interaction is not scaled, the melting temperature decreases by approximately 90 K. If the scaling factor is set to 0.5, then melting occurs at T=350 K, in very good agreement with experimental data.

Melting mechanism of monolayers adsorbed in cylindrical pores: the influence of the pore wall roughness.

We have analyzed the mechanism of melting of molecular layers adsorbed in porous materials with cylindrical pores and rough pore walls. The working example studied here is a monolayer of methane molecules adsorbed in MCM-41 pore of diameter 2R=4 nm. Both experimental (neutron scattering) and simulation (Monte Carlo) results demonstrate the strong influence of the wall roughness on the melting mechanism. In particular, the transformation between solidlike and liquidlike monolayer phases adsorbed on a rough surface is observed over a broad temperature range, and solidlike properties persist even above the bulk methane melting temperature.