J. A. Alonso

Density functional study of adsorption of molecular hydrogen on graphene layers

Density functional theory has been used to study the adsorption of molecular H2 on a graphene layer. Different adsorption sites on top of atoms, bonds and the center of carbon hexagons have been considered and compared. We conclude that the most stable configuration of H2 is physisorbed above the center of a hexagon. Barriers for classical diffusion are, however, very small.

Selective propene epoxidation on immobilized au(6-10) clusters: the effect of hydrogen and water on activity and selectivity.

Epoxidation made easy: Subnanometer gold clusters immobilized on amorphous alumina result in a highly active and selective catalyst for propene epoxidation. The highest selectivity is found for gas mixtures involving oxygen and water, thus avoiding the use of hydrogen. Ab initio DFT calculations are used to identify key reaction intermediates and reaction pathways. The results confirm the high catalyst activity owing to the formation of propene oxide metallacycles. Al green, Au yellow, O red, and C gray.

Interaction of molecular and atomic hydrogen with (5,5) and (6,6) single-wall carbon nanotubes

Density functional theory has been used to study the interaction of molecular and atomic hydrogen with (5,5) and (6,6) single-wall carbon nanotubes. Static calculations allowing for different degrees of structural relaxation are performed, in addition to dynamical simulations. Molecular physisorption inside and outside the nanotube walls is predicted to be the most stable state of those systems. The binding energies for physisorption of the H2 molecule outside the nanotube are in the range 0.04–0.07 eV. This means that uptake and release of molecular hydrogen from nanotubes is a relatively easy process, as many experiments have proved. A chemisorption state, with the molecule dissociated and the two hydrogen atoms bonded to neighbor carbon atoms, has also been found. However, reaching this dissociative chemisorption state for an incoming molecule, or starting from the physisorbed molecule, is difficult because of the existence of a substantial activation barrier. The dissociative chemisorption deforms the...

Structural and dynamical properties of Cu–Au bimetallic clusters

The effect of alloying on the structural and thermal properties of Cun−xAux (n=13,14) clusters is investigated by constant energy Molecular Dynamics simulations. The interactions between the atoms in the clusters are mimicked by a many‐body (Gupta‐like) potential based on the second moment approximation to the tight‐binding model. The minimum energy structures and the lowest‐lying isomers of the pure and mixed clusters are obtained by thermal quenching. We find icosahedral‐like ground state structures for the 13‐ and 14‐atom clusters and for all the concentrations, the only exception being Au14 which has C6v symmetry. Mixed structures are preferred over the segregated ones. The lowest‐lying isomers of the binary clusters are the permutational ones, i.e., isomers having the same underlying geometry as the ground state structure and different relative arrangement of the unlike atoms in the atomic positions of the geometry. However, presence of these low lying permutational isomers does not affect the gross ...

Ab initio study of B32 clusters: competition between spherical, quasiplanar and tubular isomers

Abstract Using ab initio quantum-chemical methods, different novel structures of B 32 clusters have been investigated. The most stable isomers have quasiplanar or tubular structures often containing dove-tailed hexagonal pyramids. In contrast, hollow spheres are less stable. The stability can be understood as a competition between a curvature strain (favoring quasiplanar clusters) and elimination of dangling bonds (favoring tubular and cage structures). Atomic coordination is larger than in carbon clusters.

Density functional calculations of hydrogen adsorption on boron nanotubes and boron sheets

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 0.8 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.

Analysis of the bonding and reactivity of H and the Al13 cluster using density functional concepts

The bonding of hydrogen in the Al13H aggregate is analyzed in the framework of density functional theory using the local density approximation. The interaction between the H-1s orbital and only certain molecular orbitals of Al13 is responsible for the binding. Different measures of the charge transfer give consistent results and predict the stabilization of a sizable amount of electronic charge, about two electrons, around the proton site. The state of the H atom can be described as a negatively charged impurity screened by the surrounding electron gas, similarly to a H impurity embedded in a vacancy in metallic aluminum. Friedel-type oscillations can be appreciated in the screening charge. Local Fukui functions and condensed Fukui indexes associated to the ground state of the cluster Al13 are used as indicators of molecular reactivity. Those indices allow to predict and understand the equilibrium location of H found in the total energy calculations for Al13H.

Orbital-free molecular dynamics simulations of melting in Na8 and Na20: Melting in steps

The melting-like transitions of Na8 and Na20 are investigated by ab initio constant energy molecular dynamics simulations using a variant of the Car–Parrinello method which employs an explicit electronic kinetic energy functional of the density, thus avoiding the use of one-particle orbitals. Several melting indicators are evaluated in order to determine the nature of the various transitions, and are compared with other simulations. Both Na8 and Na20 melt over a wide temperature range. For Na8, a transition is observed to begin at ∼110 K, between a rigid phase and a phase involving isomerizations among the different permutational isomers of the ground state structure. The “liquid” phase is completely established at ∼220 K. For Na20, two transitions are observed: the first, at ∼110 K, is associated with isomerization transitions among those permutational isomers of the ground state structure which are obtained by interchanging the positions of the surface-like atoms; the second, at ∼160 K, involves a struc...

Structure and bonding in small neutral alkali halide clusters

restricted structural relaxation criterion. A trend of competition between rocksalt and hexagonal ringlike isomers is found and discussed in terms of the relative ionic sizes. The main conclusion is that an approximate value of r C /r A50.5 ~where r C and r A are the cationic and anionic radii! separates the hexagonal from the rocksalt structures. The classical electrostatic part of the total energy at the equilibrium geometry is enough to explain these trends. The magic numbers in the size range studied are n54, 6, and 9, and these are universal since they occur for all alkali halides and do not depend on the specific ground-state geometry. Instead those numbers allow for the formation of compact clusters. Full geometrical relaxations are considered for (LiF) n (n53 ‐ 7) and (AX)3 clusters, and the effect of Coulomb correlation is studied in a few selected cases. These two effects preserve the general conclusions achieved thus far. @S0163-1829~97!01848-1#

Electronic and atomic structure of Na, Mg, Al and Pb clusters

Density functional theory is used to study the electronic and atomic structure of small clusters of Na, Mg, Al and Pb. We study the quantityEN−1–EN, which has relevance to the processes of cluster growth and evaporation (EN is the total energy of the cluster withN atoms). By comparing the results of the jellium model with those of a more realistic model (although still simple) we are able to appreciate “structural” effects beyond the “electronic-shell effects” which form the essence of the predictions of the jellium model. The calculations predict formation of atomic shells and appreciable reconstruction as the cluster grows.