L. M. Molina

Adsorption of O2 and oxidation of CO at Au nanoparticles supported by TiO2(110)

Density functional theory calculations are performed for the adsorption of O2, coadsorption of CO, and the CO+O2 reaction at the interfacial perimeter of nanoparticles supported by rutile TiO2(110). Both stoichiometric and reduced TiO2 surfaces are considered, with various relative arrangements of the supported Au particles with respect to the substrate vacancies. Rather stable binding configurations are found for the O2 adsorbed either at the trough Ti atoms or leaning against the Au particles. The presence of a supported Au particle strongly stabilizes the adsorption of O2. A sizable electronic charge transfer from the Au to the O2 is found together with a concomitant electronic polarization of the support meaning that the substrate is mediating the charge transfer. The O2 attains two different charge states, with either one or two surplus electrons depending on the precise O2 adsorption site at or in front of the Au particle. From the least charged state, the O2 can react with CO adsorbed at the edge sites of the Au particles leading to the formation of CO2 with very low (approximately 0.15 eV) energy barriers.

Adsorption, diffusion, and dissociation of molecular oxygen at defected TiO2(110): A density functional theory study

The properties of reduced rutile TiO2(110) surfaces, as well as the adsorption, diffusion, and dissociation of molecular oxygen are investigated by means of density functional theory. The O2 molecule is found to bind strongly to bridging oxygen vacancies, attaining a molecular state with an expanded O-O bond of 1.44 A. The molecular oxygen also binds (with somewhat shortened bond lengths) to the fivefold coordinated Ti atoms in the troughs between the bridging oxygen rows, but only when vacancies are present somewhere in the surface. In all cases, the magnetic moment of O2 is lost upon adsorption. The expanded bond lengths reveal together with inspection of electron density and electronic density of state plots that charging of the adsorbed molecular oxygen is of key importance in forming the adsorption bond. The processes of O2 diffusion from a vacancy to a trough and O2 dissociation at a vacancy are both hindered by relative large barriers. However, we find that the presence of neighboring vacancies can strongly affect the ability of O2 to dissociate. The implications of this in connection with diffusion of the bridging oxygen vacancies are discussed.

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 thermal properties of silicon-doped fullerenes

Extensive Molecular Dynamics simulations have been performed to investigate the structural and thermal properties of Si-doped fullerenes containing one and two silicon atoms. Both, a many-body potential and ab initio Density Functional Theory (DFT) have been used to investigate the structural features of the heterofullerenes. The competition between the exohedral and the substitutional types of doping, as a function of fullerene size (both even and odd heterofullerenes have been considered) and Si concentration, is analyzed. The DFT calculations confirm the main structural trends obtained with the many-body potential. The thermal stability and the structural transformations of the heterofullerenes have been also studied as a function of temperature (T=0–5000 K). The structural transformations include, local rearrangement of atoms, isomerization transitions, diffusion of atoms, eventual destruction of the cage, and sublimation of atoms. The isomerization transition between exohedral and substitutional isom...

Interaction of molecular and atomic hydrogen with single-wall carbon nanotubes

Density functional calculations are performed to study the interaction of molecular and atomic hydrogen with (5,5) and (6,6) single-wall carbon nanotubes. Molecular physisorption is predicted to be the most stable adsorption state, with the molecule at equilibrium at a distance of 5-6 a.u. from the nanotube wall. The physisorption energies outside the nanotubes are approximately 0.07 eV, and larger inside, reaching a value of 0.17 eV inside the (5,5) nanotube. Although these binding energies appear to be lower than the values required for an efficient adsorption/desorption operation at room temperature and normal pressures, the expectations are better for operation at lower temperatures and higher pressures, as found in many experimental studies. A chemisorption state with the molecule dissociated has also been found, with the H atoms much closer to the nanotube wall. However, this state is separated from the physisorption state by an activation barrier of 2 eV or more. The dissociative chemisorption weakens carbon-carbon bonds, and the concerted effect of many incoming molecules with sufficient kinetic energies can lead to the scission of the nanotube.

Conditions for the self-assembling of cluster materials

Clusters with closed electronic shells are highly stable, and this is a robust property with far-reaching implications for cluster abundance, cluster shapes, dynamical response to electric fields and cluster dissociation. Clusters with closed shells are also good candidates for the synthesis of novel materials. The density functional formalism is currently used to investigate the self-assembling of metallic clusters to yield cluster solids. A high intrinsic stability is not the only requirement for expecting a successful assembling, and we investigate additional conditions by performing computer simulations for selected examples. The study of the assembling of hydrogen-doped icosahedral aluminium clusters shows that an optimized relative orientation of each cluster unit with respect to all its neighbour clusters becomes a favourable condition. The analysis of the clustering effects observed in crystalline alloys formed by Pb and alkali metals (tetrahedral Pb4 clusters surrounded by the alkali cations form in those alloys) provides further insight: coating passivates the clusters and helps to stabilize the assembled material.