We present numerical Monte Carlo studies of nitrogen multilayers adsorbed on the basal plane of graphite. The simulations have been carried out in canonical ensemble, varying surface coverage (from monolayer to four layers), the relative packing of the layers (hexagonal or cubic), and the temperature (from 5 K to 100 K). In all systems, two phase transitions have been observed: orientational disordering of the layers and melting. Strong heterogeneity has been observed between layers. Its direct manifestation is the variation of the structural properties from the herringbone in the first layer to pinwheel arrangement in the fourth one. The commensurate arrangement of molecules, stable within monolayer, becomes metastable in multilayer structure and evolves to stable one (incommensurate with graphite) when the temperature increases. As a consequence of structural heterogeneity the temperature and the mechanism of both phase transitions is also changing (with the surface coverage and between layers). In particular, melting of the multilayers is always preceded by compression of the contact layer. Heterogeneity has been also observed within layers as a consequence of the competition between intermolecular and N2-graphite interactions. This competition also leads to coexistence of different phases, the fact that was observed experimentally and is reproduced here for the first time in the numerical simulations.

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.

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.

We compare the self-assembly patterns of pentane (C5H12) and hexane (C6H14) adlayers physisorbed onto graphite at various coverages using the results of molecular dynamics simulations. Near monolayer coverage, the solid low temperature structure of the pentane film is nematic, and that of hexane-herringbone-like. At submonolayer coverages both systems exhibit three distinct topological regimes: vacancy patches at higher densities, percolating networks at intermediate densities and ultimately individual patches. The systems’ orientational behavior and melting dynamics is discussed with respect to its unique density-dependent topology. The simulations explicitly include hydrogens of pentane and hexane and the graphite is modeled as a six-layer all atom structure.

Hydrogen is the lightest molecule in nature, making both rotational and translational degrees of freedom eminently quantum mechanical (especially at low temperatures). For isolated molecules the first excited (degenerate) rotational states are at about 175 K above the (non-degenerate) ground state. When the hydrogen molecule is adsorbed, however, interaction with the substrate partially eliminates this degeneracy due to the different adsorption strengths of the different rotational states of the molecule. In this talk, we consider the adsorption of hydrogen in nanometer-size pores in carbon. We show that the rotation-vibration energy levels are strongly dependent on the pore structure (geometry and size). This dependence may be probed by inelastic neutron scattering as a local, non-destructive, probe intrinsic to the system, to characterize nanopores (in fact, using H2 as the probe makes sure that the pore structure probed is relevant for H2 adsorption). The rotation-vibration energy levels were also used as input for grand canonical Monte Carlo simulations of H2 adsorption, improving the accuracy of the simulations.

The low temperature structures and phase transitions in nitrogen multilayers physisorbed on graphite are analyzed using Monte Carlo simulations. The systems that differ in number of layers and their relative packing (hexagonal or cubic) are analyzed. The nitrogen molecules are simulated as rigid, interacting via site-site potential (Etters model). The interaction with graphite includes atomic corrugation (Steele's potential). We show that the temperature and the mechanism of both: orientational and melting phase transitions vary with the surface coverage and change from layer to layer. In particular, melting of the bilayer is preceded by compression of the first layer, which has not been observed before. The results are compared with simulations of two similar systems: (i) three nitrogen layers confined in slit graphite pore, and (ii) an adsorbed incommensurate structure that mimics low temperature alpha phase of bulk nitrogen.

Carbons are one of potentially promising groups of materials for hydrogen storage by adsorption. However, the heat of hydrogen physisorption in such materials is low, in the range of about 4-8 kJ/mol which limits the total amount of hydrogen adsorbed at P = 100 bar to ~2 wt% at room temperature and about ~10 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.