Carbon-based materials, due to their low cost and weight, have long been considered as suitable physisorption substrates for the reversible storage of hydrogen. Nanoporous carbons can be engineered to achieve exceptional storage capacities: gravimetric excess adsorption of 0.073 ± 0.003 kgH2/kg carbon, gravimetric storage capacity of 0.106 ± 0.005 kgH2/kg carbon, and volumetric storage capacity of 0.040 ± 0.002 kgH2/liter carbon, at 80 K and 50 bar. The nanopores generage high storage capacity by having a very high surface are, by generating a high H2-wall interaction potential, and by allowing multi-layer adsorption of H2 (at cryogenic temperatures). In this paper we show how the experimental adsorption isotherms can be understood from basic theoretical considerations and computational simulations of the adsorption in a bimodal distribution of narrow and wide pore spaces. We also analyze the possibility of multi-layer adsorption, and the effects of hypothetical larger adsorption energies. Finally, we present the results of coupled ab initio calculations and Monte Carlo simulations showing that partial substitution of carbon atoms in nanoporous matrix with boron results in significant increases of the adsorption energy and storage capacity.

Heterogeneity is an ubiquitous aspect of adsorption, often modifying substantially the observed behaviour of the adsorbate-adsorbent system. In this paper, the influence of heterogeneity is explicitly analyzed for the case of the adsorption of molecular hydrogen onto nanoporous carbon. Grand Canonical Monte Carlo simulations were used to study the mechanism of adsorption in the models of the adsorbate that include both energetic and structural modifications of graphene-based slit pores. In particular, a partial substitution of carbons by boron modifies both the symmetry of the energy landscape and the strength of hydrogen physisorption; which results in considerable increases of the amount of adsorbed gas without major modification of the mechanism of adsorption. Additional heterogeneity arises from structural modifications of the adsorbent by neutron irradiation of boron-doped samples, where the boron fission products generate additional surface area for adsorption. Simulations of adsorption in such pores show that hydrogen uptake is strongly dependent on the chemical nature of the modified pore walls. (C) 2009 Elsevier B.V. All rights reserved.

We review the current achievements in the numerical studies of adsorption of molecular hydrogen in boron substituted nanoporous carbons. We show that the enhanced attraction of H-2 by boron-substituted all-carbon structures may allow designing new porous materials with modulated capacity for hydrogen adsorption. Such new structures are characterized by modification of energy landscape of adsorbing surfaces extending beyond the vicinity of substituted atom over several graphene carbon sites, and show strong surface heterogeneity. Although the theoretical conception and description of boron-substituted carbons made a considerable progress during the last decade, the preparation of these materials involves tedious procedures and still needs to be improved.

The interplay between the torsional potential energy and the scaling of the 1-4 van der Waals and Coulomb interactions determines the stiffness of flexible molecules. In this paper we demonstrate for the first time that the precise value of the nonbond scaling factor (SF)—often a value assumed without justification—has a significant effect on the critical properties and mechanisms of systems undergoing a phase transition, and that, for accurate simulations, this scaling factor is highly dependent on the system under consideration. In particular, by analyzing the melting of n-alkanes (hexane C6, dodecane C12, tetracosane C24) on graphite, we show that the SF is not constant over varying alkane chain lengths when the structural correlated transformations are concerned. Instead, monotonic decrease of SF with the molecular length drives a cross-over between two distinct mechanisms for melting in such systems. In a broad sense we show that the choice for SF in any simulation containing adsorbed or correlated long molecules needs to be carefully considered.

The incorporation of eoordinatively unsaturated metal sites in microporous metal-organic frameworks (MOFs) has emerged as an important synthetic strategy for the development of potential room-temperature hydrogen storage materials, because the relatively strong, localized interaction of hydrogen with the metal centers induces an increase of the isosteric heat of hydrogen adsorption. Previous modeling studies have shown that these interactions are not adequately modeled when literature force-field parameters are used. Typical results of grand-canonical Monte Carlo (GCMC) simulations exhibit a pronounced underestimation of the hydrogen uptake at low pressures and low temperatures. In this study, it is shown that this shortcoming can be resolved by deriving a new set of potential parameters to represent the metal dihydrogen interaction from ab initio calculations for molecular model systems. The approach is computationally efficient and could be applied for any coordination environment of the metal center. The present work focuses on three MOFs with unsaturated copper centers. The newly derived Cu-H-2 potential model is combined with literature force-field parameters to model the dispersive interactions with other framework atoms. At cryogenic temperatures and pressures up to I bar, GCMC simulations using these parameters provide for a massively improved prediction of the hydrogen storage characteristics when compared to parameters from a literature force field. On the other hand, the unmodified literature parameters perform best in predicting the saturation uptake. At room temperature, the effect of the potential modification is much smaller, and the best agreement with experiment is obtained when the localized metal dihydrogen interaction is not accounted for in the simulations. This indicates that the metal dihydrogen interaction is too weak to permit a significant adsorption at the metal sites under these conditions. Calculations using an artificially enhanced potential model show that a drastic increase of the interaction strength could boost. the hydrogen storage capacity at room temperature, although the attainable uptake remains limited by the number of available metal sites. The implications of these results for the synthesis of new MOFs are critically discussed.

Carbon-based materials, due to their low cost and weight, have long been considered as suitable physisorption substrates for the reversible storage of hydrogen. Nanoporous carbons can be engineered to achieve exceptional storage capacities: gravimetric excess adsorption of 0.073 ± 0.003 kgH2/kg carbon, gravimetric storage capacity of 0.106 ± 0.005 kgH2/kg carbon, and volumetric storage capacity of 0.040 ± 0.002 kgH2/liter carbon, at 80 K and 50 bar. The nanopores generage high storage capacity by having a very high surface are, by generating a high H2-wall interaction potential, and by allowing multi-layer adsorption of H2 (at cryogenic temperatures). In this paper we show how the experimental adsorption isotherms can be understood from basic theoretical considerations and computational simulations of the adsorption in a bimodal distribution of narrow and wide pore spaces. We also analyze the possibility of multi-layer adsorption, and the effects of hypothetical larger adsorption energies. Finally, we present the results of coupled ab initio calculations and Monte Carlo simulations showing that partial substitution of carbon atoms in nanoporous matrix with boron results in significant increases of the adsorption energy and storage capacity.

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