During the last two decades a lot of effort has been devoted to develop a material that could store an applicable amount of hydrogen by physisorption. All these attempts have failed. Therefore, computer simulations have been used to guide the experiment and to determine in advance the potential storage capacity of a particular structure.Usually, to simplify the interaction model and to spare the computation time, the simulations of hydrogen adsorption in nanoporous materials use the superatom representation of H2 molecule with semi-empirical values of interaction model. This approach totally neglects the non-spherical shape of the molecule. However, this information may be crucial for the precise evaluation of the amount stored and the structure of the adsorbed layers, as packing of the spherical and elongated molecules is not the same. Therefore in the present work we compare the structure and storage of H2 in slit-shaped, infinite carbon pores of nanometric width (from 0.6 nm to 2.5 nm), modeled using united atom (UA) and all atom (AA) representation of H2 molecule.We used Grand Canonical Monte Carlo technique to simulate H2 adsorption isotherms at T = 77 K, either within Material Studio software (for AA model) or home-made code (for UA model). We shows that in both models the calculated amount of stored hydrogen is similar. This results confirm the validity of previous UA model-based estimations of storage capacity reported in the literature. Moreover, our simulation shows that UA model slightly overestimates the stored amount in narrowest pores (0.6 – 0.8 nm) and underestimates it in pores of width of 1.0 -1.2 nm. For pores larger than 1.5nm both models give the same results, at least at the adsorption pressure range studied here (1 – 700 bar). In particular, these observations do not depend on pressure.Both models shows that the H2 layer directly in contact with the pore wall is dense, with density largely exceeding the bulk density of liquid hydrogen at 33 K. This results confirm the recent experimental observations of hydrogen densification under confinement in carbon-based nanospaces [1, 2].

The isotherms of molecular hydrogen adsorption in slit pores have been calculated at room temperature ( T = 298 K) for various pore sizes, from 0.6 nm to 2.5 nm. The pressure has been varied from 0 to 120 bar (12 MPa). The wall surface has been characterized by different values of the adsorption energy, from 3 to 25 kJ/mol. The provided raw data give the number of molecules adsorbed per nm 2 of the adsorbing wall, and can be used for fast storage capacity screening of new porous adsorbents with known

! abstract The mechanisms of methane adsorption in (i) homogeneous carbon slit pores of widths between 1 nm and 2 nm and (ii) heterogeneous MOF pores of similar unit cell sizes have been compared. We discuss the mechanism of layering transition in subcritical conditions, for temperatures between 80 K and 180 K. The layer formation is strongly temperature-dependent. In slit pores it varies from a sharp adsorption at low temperatures to a more continuous uptake at higher temperatures. The pore size defines the number of adsorbed layers: the 1 nm pore allows adsorption of 2 layers while the 2 nm pore allows adsorption of 5 layers of methane molecules. We compare this behavior with the mechanism of adsorption in two MOFs, IRMOF-1 and IRMOF-16, with strongly heterogeneous walls (both structurally and energetically). This comparison allows us to discuss separately the influence of wall topology and intermolecular interactions on the mechanism of layering.

The melting transition of methane adsorbed in nanopores has been studied and compared in two types of structures: carbon slits pores and SURMOF 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 mechanism of structural transformations and the notion of a unique transition temperature is not well defined.

We present the results of extensive fully atomistic molecular dynamics (MD) simulations of tetracosane (C24H50) bilayer and trilayer systems adsorbed onto the basal plane of graphite. At low temperature, both layers of the bilayer exist in well-defined solid phases. With increasing temperature, the system exhibits separated smectic phases that eventually lead to melting. During this process, we observed a strong interlayer translational correlation and mobility between layers; however, the upper layer presents more intra- (chain) and intermolecular disorder because of a lack of confinement and a greater distance to the graphite substrate. Simulations of the perpendicular trilayer patch show that gauche defects provide the main mechanism for spreading of the bottom and outer perimeter of the patch in the solid, leading to the ultimate collapse of the patch with increasing temperature and formation of a flat (parallel) trilayer that melts at a higher temperature than the bilayer structure. The wide variety of structural order parameters, thermodynamic functions, and probability distributions we employed provide a clear picture of the roles of gauche defects, confinement, and interlayer correlation in the phases and phase transitions exhibited by these confined organic layers.