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.

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 >= 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.

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 similar to 2 wt % at room temperature and similar to 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 (similar to 5 wt % at 298 K, 100 bar) with a 97% delivery rate. (C) 2009 American Institute of Physics. [doi:10.1063/1.3251788]

Carbons are one of several promising groups of materials for hydrogen storage by ad-sorption. Here it is shown how appropriately engineered nanoporous carbons provide materials for reversible hydrogen storage storage capacities ~ 80 g H2/kg carbon, ~ 50 g H2/liter carbon at 50 bar and 77 K. The nanopores generate high storage capacities by having very large surfaces for adsorption, and by hosting deep potential wells for narrow pores. Experimental studies are presented with surface areas as high as 3100 m2/g, in which 40% of all surface sites reside in pores of width ~ 0.7 nm and binding energy ~ 9 kJ/mol, and 60% of sites in pores of width > 1.0 nm and binding energy ~ 5 kJ/mol. The prevalence of two binding energies is also in agreement with results from computer simulations. We also compare experimental and theoretical calculations of distinct models for the adsorption where molecules adsorb to discrete sites (localized adsorption) and where molecules are able to freely move in the adsorption plane (mo-bile adsorption) and explore the regimes in which the quantum nature of H2’s rotational degrees of freedom plays an important role.

Hexane is the shortest representative of the family of alkanes (linear models for polymers and proteins having formula CH3-(CH2)n-2-CH3 whose flexibility has any considerable impact on its dynamics. When adsorbed on graphite, a monolayer of hexane melts from a herringbone solid to a liquid at a temperature of approximately 175 K. To understand the mechanisms that drive this transition we have performed large scale molecular dynamics simulations (several runs over 100 ns, requiring a total of ~10 cpu-years of computation), using the most realistic model of the system (a fully atomistic representation of hexane, explicit site-by-site interaction with graphite carbons and CHARMM force field with carefully chosen adjustable parameters of interactions). We show that the melting of the low temperature herringbone solid phase starts with the formation of gauche defects at the ends of neighboring molecules, followed by molecular reorientation within a lamellae, without perturbing the overall structure of the adsorbed film. The melted phase has a domain-type structure with domains’ orientation that reflects the 6–fold symmetry of graphite. The size of domains decreases progressively when the temperature increases and the deformation of molecules to quasi globular shape is driven by progressive formation of gauche defects. Work supported by the U.S. Department of Energy (DE-FG02-07ER46411) and the American Chemical Society Petroleum Research Fund (PRF43277-B5). Computational resources were provided by the University of Missouri Bioinformatics Consortium.

There are many theoretical attempts to propose a porous system which would store hydrogen at the minimum requirements fixed by DOE for mobile applications. One of the potentially promising groups of materials are porous structures based on carbon. However, the heat of hydrogen physisorption in such materials is low, in the range of about 4-8 kJ/mol. As a consequence, the total amount of hydrogen adsorbed at P=100 bar do not, and cannot exceed 2 wt% at room temperature and about 12 wt% at 77 K. To get better storage capacity, 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. Here we analyze the variation of interaction energy between a molecule of hydrogen and graphene-based sorbents prepared as hypothetical modifications of the graphene layer. In particular, we show that partial substitution of carbons (for example, by boron) modifies both the symmetry of the energy landscape and strength of hydrogen physisorption. The effect of substituent extends over several sites of graphene lattice making the surface more heterogeneous. The consequences of such substitution on the hydrogen uptake will be discussed.

One of the potentially promising porous structures for efficient hydrogen storage for mobile applications are carbon-based materials. However, the heat of hydrogen physisorption in pure porous carbons is low, in the range of about 4-8 kJ/mol. Consequently, the total amount of the adsorbed hydrogen cannot meet the DOE goals. To get better storage capacity, the adsorbing surfaces must be modified, either by substitution of a part of atoms in the all-carbon skeleton by other elements, or by doping/intercalation with other species. Such modifications lead to surfaces which are most probably, strongly heterogeneous. Here, we present Monte Carlo simulations of adsorption of molecular hydrogen in such modified carbon-based heterogeneous porous materials. We show that partial substitution of carbons (here, by boron) modifies the symmetry of the energy landscape and increases the strength of hydrogen physisorption on graphite. We discuss the consequences of such substitution on both the hydrogen uptake and adsorption mechanism. We extend our analysis on the influence of the size of slit-shape pores on the hydrogen uptake and show that, if carefully engineered, graphite-based sorbents can reach 2010 DOE requirements for hydrogen storage.