Hydrogen is widely considered an essential part of our energy future despite the substantial difficulties derived from its low volumetric energy density. The development of a suitable material for reversible storage of hydrogen still remains a 'grand challenge', in particular for vehicular applications. Although some materials are potentially attractive, even the most promising candidates have yet to meet the U.S. Department of Energy (DOE) 2010 targets (0.045 kg H2/kg system, 28 kg H2/m3 system at room temperature for light-duty vehicles).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 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. 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 of the adsorption energy (up to 10-13.5 kJ/mol) and storage capacity (~ 5 wt. % at 298 K, 100 bar) with a 97 % delivery rate. We analyze different possible mechanism of including boron atoms in the carbon structure: substitution and doping. We show how random distribution of boron modifies the energy landscape of interaction energy and mechanism of adsorption.

The results of extensive molecular dynamics simulations and theoretical considerations of the adsorption of methane-methyl mercaptan mixtures in slit-shaped carbon nanopores are presented. We observe significant mobility of both methane and mercaptan molecules within the pore volume, between pores, and between adsorbed and gas phases for a wide range of temperatures and pressures. Although mercaptans adsorb preferentially relative to methane, the process remains reversible, provided non-oxidizing conditions are maintained. A mercaptan/methane ratio of the order of 200 ppm in the adsorbed phase is sufficient for the gas phase to have a mercaptan concentration above the human threshold for detection. The reversibility of the adsorption process and low concentration of mercaptans makes it unlikely that these would be harmful for adsorbed natural gas storage systems.

Inelastic neutron scattering spectra are calculated for hydrogen molecules adsorbed on activated carbon. The slit-shaped pore model is used to calculate the adsorption potentials of a hydrogen molecule in pores of variable width. The motion of the hydrogen molecules is quantized perpendicular to the plane of the pore and both rotational and vibrational transition energies are found. The perturbation of adjacent hydrogen molecules on the transition energies is discussed. Form factors and Debye-Waller factors are calculated for momentum transfer parallel and perpendicular to the pore; this anisotropy is particularly pronounced in pores where there is nearly enough room for two adsorbed layers. A spectrum composed of a uniform distribution of pore sizes agrees only qualitatively with experimental results for activated carbon. Reasons for this disparity are discussed, including the possibility that transitions in the translational motion parallel to the adsorption plane contribute significantly to the spectra. In addition, the positioning of the center of gravity of the first rotational transition is discussed, with several factors contributing to shift it from the 14.7 meV of the gas phase. Our results suggest that inelastic neutron scattering can be valuable as a complementary sub-nanometer pore characterization technique.

Towards finite pore model

resume en piece jointe

Mechanism of adsorption in infinite slit-shaped pores has been studied for a long time and is now relatively well understood. On the contrary, little is known about the characteristics of adsorption in pores of finite (nanometric) wall surface or in large slit-like pores in which nanometric holes have been drilled in the pore walls. Meanwhile, many experimental attempts to increase the adsorption capacity of nanoporous materials consider pore walls fragmentation as an efficient method of increasing adsorption area (even by a factor of two with respect to homogeneous slits [1]) and the kinetics of pores’ access trough engineering openings in pore walls. In both types of systems the mechanism of adsorption is strongly influenced by the adsorption of the pore (holes) edges. In this work we present the numerical analysis of simple gazes (Ar, Kr, H2 and N2) adsorption in carbon slit pores in which the edge contribution is non-negligible. Both, small surface pores and large pores with holes are considered and discussed. The adsorption isotherms are simulated using Grand Canonical Monte Carlo method and the results are compared to those obtained using Density Functional Theory approach.

Mechanism of adsorption of nitrogen multilayers on the basal plane of graphite has been studied using Monte Carlo method. To account for multi-phase equilibrium large scale (~6000 molecules) simulations have been carried out in canonical ensemble, for surface coverage equivalent to more than four layers. To get more insight in the metastability aspect of the simulated situation and better understanding of the experimental phase diagram, the simulations were performed starting from liquid-like disordered configurations and progressively lowering the temperature from 100 K to 10 K. The analysis is focused on the system spatial heterogeneity and its influence on structures and phase transitions. An intricate phase situation is observed due to the competition between intermolecular and N2-graphite interactions. The multilayer structure is strongly spatially non-uniform, the molecular arrangement in each individual layer changing from herringbone in the first layer to pinwheel in the forth one. The commensurate multilayer structure is only metastable at low temperatures. The stable phase has still triangular symmetry within each layer, but with molecular packing that is 1.08 times denser than the commensurate one and is stabilized by the N2-N2 interactions.. 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 layer position with respect to graphite. We observed that layers formed on freezing had higher than commensurate density. Certainly, such an effect can be observed only if surface coverage is larger than monolayer: a single layer is stabilized by the strong N2-graphite interaction and is always commensurate with graphite: this structure seems to be the most stable one. However, in the multilayer systems the intermolecular interactions overcome the influence of graphite corrugation. The final structure results from the competition between the interaction with graphite (which tries to stabilize the herringbone structure) and the 3D intermolecular forces (which favor cubic -phase). In consequence, the pinwheel-like arrangement is observed in higher layers as a part of molecular order in the <111> crystallographic planes of cubic structure. These numerical results concord for the first time and explain quantitatively the multi-structure arrangements observed in the experiments.