[show abstract][hide abstract] ABSTRACT: Storage is the main problem to use hydrogen as a fuel in the car industry. Porous carbons are promising storage materials. We have performed computer simulations to investigate carbide-derived porous carbons, showing that these materials exhibit a structure of connected pores with graphitic walls. We then apply a thermodynamic model to evaluate the hydrogen storage. The model accounts for the quantum effects of the motion of the molecules in the pores. The pore widths optimizing the storage depend on pore shape, temperature, and pressure. At 300 K and 10 MPa, the optimal widths lie in the range 6-10 angstrom. The predictions are consistent with experiment. The calculated storage capacities fall below the targets proposed by the U.S. Department of Energy. This is a consequence of the weak interaction between hydrogen and the pore walls. Metallic doping enhances the binding energy of hydrogen to the walls, which has promising consequences for hydrogen storage.
Journal of Materials Research 02/2013; 28(4):589-604. · 1.71 Impact Factor
[show abstract][hide abstract] ABSTRACT: The dissociation and adsorption of molecular hydrogen on the edges of graphene nanoribbons of widths of 1.14 and 1.36 nm, is investigated within the density functional formalism. Here, graphene nanoribbons are used as models for the pore walls of some nanoporous carbons (carbide-derived carbons among others) which have been shown to be formed by one-atom thick graphene layers interconnected among them and exhibiting exposed edges (Lopez et al. in J Chem Phys 135: 104706, 2011). The aim of this study is to shed some light on the contribution of the edges of the pore walls to the hydrogen storage capacity of nanoporous carbons. Nanoribbons with zigzag and armchair edge terminations have been considered. Molecular hydrogen dissociates and adsorbs atomically at the nanoribbon edges with no or small activation barrier. The adsorption energies per hydrogen molecule are quite large, 2.5 and 5.7 eV for armchair and zigzag edges, respectively. This indicates that the graphene edges are very reactive and will be saturated with hydrogen whenever available. However, under mild conditions of pressure and temperature hydrogen cannot be desorbed from the edges and, therefore, the edges do not contribute to the reversible storage capacity of the material. The magnetic properties of saturated and unsaturated ribbons are also discussed.
Journal of Nanoparticle Research 12/2012; 14(12). · 2.18 Impact Factor
[show abstract][hide abstract] ABSTRACT: In silica (SiO2) and in most silicates, atomic associations exist with composition SiO4 and a structure with four O atoms in tetrahedral coordination around the Si atom. A similar feature is observed in germania (GeO2) and some solids containing Ge instead of Si, although the number of phases containing GeO4 tetrahedra is smaller. In contrast, and in spite of the fact that C is in the same column of the periodic table as Si and Ge, CO2 is a molecular solid, and crystalline and amorphous phases of CO2 showing CO4 tetrahedra are only obtained under extremely high pressures. We have investigated the relation between free SiO4, GeO4 and CO4 clusters and the tetrahedral associations found in the solids mentioned above. The lowest energy structure of those three free clusters is planar, but they have near-tetrahedral and distorted-tetrahedral isomers. The promotion energy from the planar structure to the distorted tetrahedral is low in SiO4, large in CO4, and intermediate in GeO4. This correlates with the facility to form tetrahedral associations in the solids.
The European Physical Journal D 04/2012; 66(4). · 1.51 Impact Factor
[show abstract][hide abstract] ABSTRACT: An efficient storage of hydrogen is a crucial requirement for its use as a fuel in the cars of the future. Experimental and theoretical work has revealed that porous carbons are promising materials for storing molecular hydrogen, adsorbed on the surfaces of the pores. The microstructure of porous carbons is not well known, and we have investigated a class of porous carbons, the carbide-derived carbons, by computer simulation, showing that these materials exhibit a structure of connected pores of nanometric size, with graphitic-like walls. We then apply a thermodynamical model of hydrogen storage in planar and curved pores. The model accounts for the quantum effects of the motion of the molecules in the confining potential of the pores. The optimal pore sizes yielding the highest storage capacities depend mainly on the shape of the pore, and slightly on temperature and pressure. At 300 K and 10 MPa, the optimal widths of the pores lie in the range 6-10 angstrom. The theoretical predictions are consistent with experiments for activated carbons. The calculated storage capacities of those materials at room temperature fall below the targets. This is a consequence of an insufficiently strong attractive interaction between the hydrogen molecules and the walls of carbon pores. Recent work indicates the beneficial effect of metallic doping of the porous carbons in enhancing the binding energy of H-2 to the pore walls, and then the hydrogen storage.
Journal of the Mexican Chemical Society 01/2012; 56(3):261-269. · 0.28 Impact Factor
[show abstract][hide abstract] ABSTRACT: Doping of porous carbon materials with metallic atoms, clusters and nanoparticles is viewed as a way to enhance the hydrogen storage in those materials. Transition metals are dopants of interest. For this reason we present a theoretical study of the interaction of molecular hydrogen with small palladium clusters (Pd-n, n = 1-6) supported on a graphene layer. The adsorption of H-2 on those supported Pd clusters leads to two types of adsorption states. The simplest one is an activated state of the hydrogen molecule, with the H-H distance stretched and the H-H bond weakened, but not broken. Adsorption in the activated states occurs with no barriers and the binding energies are in a range of values of interest for achieving a favorable reversible hydrogen storage in the doped material. The second type is a dissociated and chemisorbed state, with the separated hydrogen atoms attached to the Pd cluster. The dissociative chemisorption states are more stable than the activated states. However, starting with Pd-4, there are barriers for the dissociative chemisorption of H-2, and the heights of those energy barriers are a few tenths of an electronvolt.
The Journal of Physical Chemistry C 01/2012; 116(40):21179-21189. · 4.81 Impact Factor
[show abstract][hide abstract] ABSTRACT: Nanoporous carbon refers to a broad class of materials characterized by nanometer-size pores, densities lower than water, large specific surface areas, and high porosities. These materials find applications in nanocatalysis and gas adsorption, among others. The porosity structure, that determines the properties and functionalities of these materials, is still not characterized in detail. Here, we reveal the detail porosity structure and the electronic properties of a type of nanoporous carbons, the so called carbide derived carbons (CDCs), through a simulation scheme that combines large simulation cells and long time scales at the empirical level with first-principles density functional calculations. We show that the carbon network consists in one layer thick nanographenes interconnected among them. The presence of specific defects in the carbon layers (heptagons and octagons) yields to open pores. These defects are not completely removed through annealing at high temperatures. We also suggest that, in contrast with graphene which is a zero-gap semiconductor, these materials would have a metallic character, since they develop an electronic band around the Fermi level. This band arises from the electronic states localized at the edges of the nanographene layers.
The Journal of chemical physics 09/2011; 135(10):104706. · 3.09 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have used a quantum-thermodynamical model to calculate the hydrogen storage capacities of nanoporous carbons at different temperatures and pressures as a function of the shape (planar, cylindrical and spherical) and size of the pores. Three different hydrogen-carbon surface interaction potentials, based on Lennard-Jones (LJ), Moller-Plesset (MP2), and Density Functional Theory (DFT) calculations, respectively, have been used with that model, in order to obtain sensible boundaries of the values of the storage capacities. We support the MP2 results as the most realistic of the three. The optimal pore sizes, which yield the highest storage capacities, depend mainly on the shape of the pore and on the interaction potential, and slightly on temperature and pressure. At 300 K and 10 MPa, the optimal width of the planar pores and the optimal radii of the cylindrical and spherical pores lie in the ranges of 5.8-7.5, 7.0-8.6 and 8.5-10.7 angstrom, respectively, for the three interaction potentials considered. These values are consistent with experiments for activated carbons and activated carbon fibers. The predictions of our simulations may be useful to design nanoporous carbons with optimal hydrogen storage capacities. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
International Journal of Hydrogen Energy 08/2011; 36(17):10748-10759. · 3.55 Impact Factor
[show abstract][hide abstract] ABSTRACT: Density functional calculations of palladium absorbed on graphene have been performed to study the first stages of Pd coating and/or Pd cluster formation on the graphene surface, a question of great relevance to many experiments. We have found that palladium atoms deposited on graphene have a strong tendency to form clusters. Three-dimensional clusters are more stable than planar clusters and the transition from planar to three-dimensional Pd clusters adsorbed on graphene occurs very early as a function of cluster size, at Pd4, as a consequence of the strong Pd-Pd interaction. Palladium might enhance hydrogen storage in porous materials by surface reactions. However, it is a heavy element and the formation of three-dimensional Pd clusters decreases the cluster surface and increases the cluster weight. Hence, a way should be found to prevent clusterization or to disperse finely the palladium atoms deposited on carbon materials. The adsorbed clusters are weakly magnetic, which may be of interest for some applications.
[show abstract][hide abstract] ABSTRACT: The interest in hydrogen goes across disciplines. Hydrogen is a molecular gas, but many interesting scientific aspects and most of the technological possibilities of this material come from condensed hydrogen phases, either as small or large molecular clusters, as a liquid, or adsorbed on the surface of materials. Recent work on these areas is reviewed. As H2 is a very stable molecule in which the two electrons form a closed electronic shell, (H2)N clusters have some similarities with the clusters of the inert gases. By irradiating molecular beams of large deuterium clusters with an intense femtosecond laser, the Coulomb explosion of the clusters has been induced, and collisions between the flying deuterium nuclei have achieved nuclear fusion. Here, we analyze the first stages of this interesting process, that is, the dynamical evolution of the deuterium clusters after absorbing energy from the laser. Fragmentation of the cluster, or ionization followed by Coulomb explosion can occur depending on the size of the cluster and the frequency and intensity of the laser field. Motivated by the firm expectations that hydrogen will be used in the near future as a fuel in electric cars, a strong effort is now dedicated in many laboratories to find good hydrogen storage materials. Theoretical work has focused on graphitic materials with a large specific surface area. The results of the simulations indicate that the hydrogen storage can be optimized using porous carbon materials with pore sizes of about 0.6nm and doped with impurities like lithium. Hydrogen can also be used to probe the local reactivity of clusters. This property is illustrated here for the family of planar gold clusters, highly relevant because of the catalytic activity shown by those clusters.
[show abstract][hide abstract] ABSTRACT: The characteristics and nature of atomic-scale defects produced on graphite surfaces by dielectric barrier discharge (DBD) plasma oxidation have been investigated, both experimentally and theoretically. Two main types of defect visualized by scanning tunneling microscopy (STM) were studied: protrusions 1−5 nm in diameter and smooth circular depressions 5−7 nm wide, the latter constituting a novel type of defect on carbon surfaces that was only very recently reported for the first time. STM and atomic force microscopy (AFM) experiments indicated that both the protrusions and the depressions are not associated to topographical features on the graphite surface and that their observation by STM should be related to electronic effects. The thermal behavior of the protrusions, which could only be removed at a temperature of 900 °C, as well as their reactivity toward molecular oxygen, allowed their identification as multiatomic vacancies. In comparison, the depressions displayed a higher thermal stability (they could be eliminated only at 1200 °C) and a lower reactivity toward oxidation. Density functional theory (DFT) calculations suggested that the depressions are associated with two-dimensional clusters of interstitial oxygen formed by the agglomeration of migrating oxygen atoms. Such clusters induce a lowering in the local density of electronic states on the graphite surface and are therefore detected as a depression by STM. Taken as a whole, the findings reported here provide a consistent picture of the basic mechanism underlying the modification of graphitic surfaces by this type of plasma, which is driven by physical processes (i.e., ion bombardment).
Journal of Physical Chemistry C - J PHYS CHEM C. 10/2009; 113(43).
[show abstract][hide abstract] ABSTRACT: The adsorption of molecular hydrogen on a metal-organic framework (MOF) material, MOF-5, has been studied using the density-functional formalism. The calculated potential-energy surface shows that there are two main adsorption regions: both near the OZn(4) oxide cores at the vertices of the cubic skeleton of MOF-5. The adsorption energies in those regions are between 100 and 130 meV/molecule. Those adsorption regions have the shape of long, wide, and deep connected trenches and passage of the molecule between regions needs to surpass small barriers of 30-50 meV. The shape of these regions, and not only the presence of metal atoms, explains the large storage capacity measured for MOF-5. The elongated shape explains why some authors have previously identified only one type of adsorption site, associated to the Zn oxide core, and others identified two or three sites. One should consider adsorption regions rather than adsorption sites. A third region of adsorption is near the benzenic rings of the MOF-5. We have also analyzed the possibility of dissociative chemisorption. The chemisorption energy with respect to two separated H atoms is 1.33 eV/H atom; but, since dissociating the free molecule costs 4.75 eV, the physisorbed H(2) molecule is more stable than the dissociated chemisorbed state by about 2 eV. Dissociation of the adsorbed molecule costs less energy, but the dissociation barrier is still high.
Physical Review B 11/2008; 78(20). · 3.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: Density functional calculations have been performed to investigate the destruction of narrow carbon nanotubes (CNTs) under the attack of nitronium tetrafluoroborate salts. The dissociation of these salts in a solvent produces nitronium and tetrafluoroborate ions which coadsorb on the external surface of the tubes. It is shown that the ions bind strongly to both metallic and semiconducting narrow nanotubes, although stronger to the metallic ones. The nitronium cations bind to the CNTs through a charge transfer mechanism, whereas the tetrafluoroborate anions remain negatively charged upon adsorption on the nanotubes. The surface of the nanotubes gets substantially deformed around the adsorption site of the nitronium ion, but it is hardly changed around the adsorption site of the tetrafluoroborate ion. These results are the theoretical basis to explain the destruction of the narrow CNTs found in the experiments and also to unravel, in agreement with the experimental interpretation, the distinct role played by the nitronium and the tetrafluoroborate ions. The tetrafluoroborate ions contribute to separate the CNTs from the bundles into individual tubes, without affecting the tubes. The nitronium ions, in contrast, modify the electronic and geometrical structures of the narrow tubes leading eventually to their destruction. The implications for the selective removal of intermediate diameter metallic CNTs found in the experiments are also discussed. The adsorption of the neutral nitrogen dioxide molecule is also studied, and the results show that the weak interactions of this molecule with both metallic and semiconducting tubes cannot be used as a model for the strong attack of the nitronium ions to the narrow tubes. The sensor effect of the nanotubes toward adsorption of nitrogen dioxide is also discussed.
The Journal of chemical physics 07/2008; 128(21):214703. · 3.09 Impact Factor
[show abstract][hide abstract] ABSTRACT: Density functional calculations are reported for the adsorption of molecular hydrogen on carbon nanopores. Two models for the pores have been considered: (i) The inner walls of (7,7) carbon nanotubes and (ii) the highly curved inner surface of nanotubes capped on one end. The effect of Li doping is investigated in all cases. The hydrogen physisorption energies increase due to the concavity effect inside the clean nanotubes and on the bottom of the capped nanotubes. Li doping also enhances the physisorption energies. The sum of those two effects leads to an increase by a factor of almost 3 with respect to the physisorption in the outer wall of undoped nanotubes and in flat graphene. Application of a quantum-thermodynamical model to clean cylindrical pores of diameter 9.5 A, the diameter of the (7,7) tube, indicates that cylindrical pores of this size can store enough hydrogen to reach the volumetric and gravimetric goals of the Department of Energy at 77 K and low pressures, although not at 300 K. The results are useful to explain the experiments on porous carbons. Optimizations of the pore size, concavity, and doping appear as promising alternatives for achieving the goals at room temperature.
The Journal of Chemical Physics 05/2008; 128(14):144704. · 3.16 Impact Factor
[show abstract][hide abstract] ABSTRACT: The hydrogen storage capacities of nanoporous carbons, simulated as flat graphene slit pores, have been calculated using a quantum-thermodynamical model. The model is applied for several interaction potentials between the hydrogen molecules and the graphitic walls that have been generated from density functional theory (DFT) and second-order Møller-Plesset (MP2) calculations. The hydrogen storage properties of the pores can be correlated with the features of the potential. It is shown that the storage capacity increases with the depth of the potential, De. Moreover, the optimal pore widths, yielding the maximum hydrogen storage capacities, are close to twice the equilibrium distance of the hydrogen molecule to one graphene layer. The experimental hydrogen storage capacities of several nanoporous carbons such as activated carbons (ACs) and carbide-derived carbons (CDCs) are well reproduced within the slit pore model considering pore widths of about 4.9–5.1 Å for the DFT potential and slightly larger pore widths (5.3–5.9 Å) for the MP2 potentials. The calculations predict that nanoporous carbons made of slit pores with average widths of 5.8–6.5 Å would yield the highest hydrogen storage capacities at 300 K and 10 MPa.
[show abstract][hide abstract] ABSTRACT: A thermodynamical model of hydrogen storage in slitpores is presented and applied to carbon and BN nanoporous materials. The model accounts for the quantum effects of the molecules in the confining potential of the slitpores. A feature of the model is a new equation of state (EOS) of hydrogen, valid over a range of pressures wider than any other known EOS, obtained using experimental data in the range 77-300 K and 0-1000 MPa, including data in the region of solid hydrogen. The model reproduces the experimental hydrogen storage properties of different samples of activated carbons and carbide-derived carbons at 77 and 298 K and at pressures between 0 and 20 MPa, for an average nanopore width of about 5 angstrom. The model predicts that in order to reach the US Department of Energy hydrogen storage targets for 2010, the nanopore widths should be equal to or larger than 5.6 angstrom for applications at low temperatures, 77 K, and any pressure, and about 6 angstrom for applications at 300 K and at least 10 MPa. (C) 2007 Elsevier Ltd. All rights reserved.
[show abstract][hide abstract] ABSTRACT: The electrostrictive response of small carbon clusters, hydrocarbon molecules, and carbon nanotubes is investigated using the density functional theory. For ringlike carbon clusters, one can get insight on the deformations induced by an electric field from a simple two-dimensional model in which the positive charge of the carbon ions is smeared out in a circular homogeneous line of charge and the electronic density is calculated for a constant applied electric field within a two-dimensional Thomas-Fermi method. According to the Hellmann-Feynman theorem, this model predicts, for fields of about 1 V∕Å, only a small elongation of the ring clusters in the direction of the electric field. Full three-dimensional density functional calculations with an external electric field show similar small deformations in the ring carbon clusters compared to the simple model. The saturated benzene and phenanthrene hydrocarbon molecules do not experience any deformation, even under the action of relatively intense (1 V∕Å) electric fields. In contrast, finite carbon nanotubes experience larger elongations (∼2.9%) induced by relatively weak (0.1 V∕Å) applied electric fields. Both C-C bond length elongation and the deformation of the honeycomb structure contribute equally to the nanotube elongation. The effect of the electric field in hydrogen terminated nanotubes is reduced with respect to the nanotubes with dangling bonds in the edges.
Physical Review A 12/2006; 74(6). · 3.04 Impact Factor
[show abstract][hide abstract] ABSTRACT: Density functional calculations of the adsorption of molecular hydrogen on the external surface of (5, 5), (6, 4), (8, 1) and (16, 2) carbon nanotubes have been carried out. Binding energies of single molecules have been studied as a function of orientation of the molecules and type of nanotube. We have found weak adsorption, with binding energies near 100 meV/molecule in the most stable configurations. The binding energies on metallic and semiconducting nanotubes are similar. When the nanotube surface is fully cov-ered with one molecule per graphitic hexagon, the binding energy per molecule decreases for some nanotubes due to repulsive inter-actions between neighbor molecules. For the same reason, direct adsorption of a single hydrogen layer with a coverage of more than one molecule per graphitic hexagon is not possible, even at low temperatures. However, adsorption of two layers (14.3 wt% hydro-gen adsorbed when all the surface is covered) leads to binding energies between 40 and 80 meV/molecule, although the molecules of the outer layer are more weakly bound compared to those of the inner one. All the small calculated binding energies indicate that substantial adsorption is only possible at very low temperatures.
[show abstract][hide abstract] ABSTRACT: Hydrogen adsorption on the recently discovered boron nanotubes, BNTs, and on boron sheets is investigated by density functional calculations. Both molecular physisorption and dissociative atomic chemisorption are considered. The geometric and electronic structures of BNTs and boron sheets have been elucidated. These two novel boron structures present buckled surfaces with alternating up and down rows of B atoms, with a large buckling height of about 0. A. The buckled structures are about 0.20 eV/atom more stable than the corresponding flat ones. However, the helicity of some BNTs does not allow for the formation of alternating up and down B rows in the surface and, therefore, these nanotubes have flat surfaces. The buckled and flat nanostructures have different geometric and bonding characteristics, but both are metallic. Molecular hydrogen physisorption energies are about 30–60 meV/molecule on boron sheets and nanotubes, actually lower than in graphene and in carbon nanotubes and far from the energies of 300–400 meV/molecule necessary for efficient hydrogen storage at room temperature and moderate pressures for onboard automotive applications. Chemisorption binding energies on BNTs are about 2.4–2.9 eV/H atom, similar to the ones obtained in CNTs. Finally, the energy barrier from molecular physisorption to dissociative chemisorption of hydrogen is about 1.0 eV/molecule. Therefore, the calculations predict physisorption as the leading adsorption mechanism of hydrogen at moderate temperatures and pressures. The expected hydrogen adsorption capacity of these novel B materials is even smaller than that of CNTs.