Ab initio studies of propene epoxidation on oxidized silver surfaces

Producción CientíficaAb initio density functional simulations have been performed to study the adsorption of propene on partially oxidized silver surfaces and its interaction with surface oxygen. Two different adsorption conformations for propene are studied, with the molecule either intact or forming an Ag–C3H6–O oxymetallacycle (OMC) intermediate. Then, pathways for propene oxide, acrolein and propanone formation have been studied in detail, providing insight into the selectivity of the surfaces. We find that formation of acrolein must necessarily take place from OMC intermediates, requiring at least two neighbouring reactive surface oxygen anions. This suggests a strong relationship between the concentration of surface oxygen and the selectivity of these surfaces.Ministerio de Ciencia e Innovación (Proyect MAT2008-06483-C03-01 and MAT 2011-22781)Junta de Castilla y León (programa de apoyo a proyectos de investigación - Ref. A158A11-

Theoretical study of the adsorption of hydrogen on cobalt clusters

Producción CientíficaAdsorption and dissociation of molecular hydrogen on transition metal clusters are basic processes of broad technological application in fields such as catalysis, hydrogenation reactions, hydrogen fuel cells, hydrogen storage, etc. Here we focus on two cobalt clusters, Co6 and Co13, and use the density functional formalism to investigate: (i) the mechanisms for adsorption and dissociation of hydrogen, and (ii) the competition between the two processes as the amount of hydrogen increases towards cluster saturation. The dissociative adsorption of hydrogen is the preferred adsorption channel for low coverage. Each individual H atom binds to the cluster with an ionic type of bonding, similar to that in metal hydrides. The electronic levels of the H atoms hybridize with the deepest levels of the Co cluster, leading to the stabilization of the system. In contrast H2 binds to the cluster with a weak covalent type of bond and the electronic density of the molecule becomes polarized. The electronic levels of the molecule are deeper than those of the Co cluster and do not hybridize with them, which explains the weak bonding of the molecule to the cluster. Interestingly, the high magnetic moments of the Co clusters do not change when H2 is adsorbed in molecular form, but the magnetic moments decrease by two Bohr magnetons upon dissociative adsorption of the molecule. Adsorption and dissociation of H2 on Co6 and Co13 exhibit similar features, although the adsorption energies on Co13 are stronger. Saturation of Co6 with hydrogen has been also investigated. Co6 can adsorb up to four H2 molecules in the dissociated form. Additional hydrogen is adsorbed in molecular form leading to a saturated cluster with sixteen hydrogen molecules, four dissociated and twelve molecular. This limit corresponds to a content of 8.4 wt% of hydrogen in the Co cluster, which is promising for the purpose of hydrogen storage.2019-09-092019-09-09Ministerio de Economía, Industria y Competitividad (Project MAT2014-54378-R)Junta de Castilla y León (programa de apoyo a proyectos de investigación - Ref.VA021G18

Computer simulations of the structure of nanoporous carbons and higher density phases of carbon

The most stable form of solid carbon is graphite, a stacking of graphene 2 layers in which the carbon atoms show sp2 hybridization which leads to strong intra3 layer bonding. Diamond is a denser phase, obtained at high pressure. In diamond the 4 carbon atoms show sp3 hybridization. Metastable solid carbon phases can be pre5 pared also with lower density than graphite (in fact, densities lower than water); for 6 instance the carbide-derived carbons. These are porous materials with a quite disor7 dered structure. Atomistic computer simulations of carbide-derived carbons indicate 8 that the pore walls can be viewed as curved and planar nanographene ribbons with 9 numerous defects and open edges. Consequently, the hybridization of the carbon 10 atoms in the porous carbons is sp2. Because of the high porosity and large specific 11 surface area, nanoporous carbons find applications in gas adsorption, batteries and 12 nanocatalysis, among others. We have performed computer simulations, employing 13 large simulation cells and long simulation times, to reveal the details of the structure 14 of the nanoporous carbons. In the dynamical simulations the interactions between 15 the atoms are represented by empirical many-body potentials. We have also investi16 gated the effect of the density on the structure of the disordered carbons and on the 17 hybridization of the carbon atoms. At low densities, typical of the porous carbide18 derived carbons formed experimentally, the hybridization is sp2. On the other hand, 19 as the density of the disordered material increases, a growing fraction of atoms with 20 sp3 hybridization appear

Determination of the glass-forming concentration range in binary alloys from a semiempirical theory: Application to Zr-based alloys

Producción CientíficaA semiempirical theory is presented to calculate free-energy diagrams of glass-forming alloys. The theory is based on the enthalpy of formation of the solid solution expressed as a sum of two contributions: (a) a chemical contribution due to electronic redistribution in forming the alloy, and (b) an elastic contribution arising from the difference in size between solute and solvent atoms. The enthalpy of formation of the amorphous phase has only the chemical contribution. The theory is used to successfully describe the glass-forming concentration range of some Zr-based alloys, which have been experimentally analyzed by several distinct techniques

Palladium clusters anchored on graphene vacancies, and their effect on the reversible adsorption of hydrogen

Producción CientíficaThe hydrogen storage capacity of nanoporous carbons can be enhanced through metal doping, for instance doping with palladium. However, there are two problems that may limit the positive effect of metal doping on the reversible storage capacity. First, clustering of the metal atoms decreases its effectiveness, which is largest for maximum dispersion. A second problem that is often overlooked is that the desorption of metal–hydrogen complexes may compete favorably with the desorption of hydrogen molecules. Desorption of complexes would spoil the reversible storage of hydrogen in the material. Both problems can be avoided by firmly anchoring the metal atoms and clusters to defects of the carbon substrate, for instance vacancies. With this goal in mind, we have performed density functional calculations to investigate the desorption of hydrogen and of Pd–hydrogen complexes from Pd atoms and clusters supported on pristine graphene and on graphene layers with vacancies. We show that palladium atoms bind much stronger to graphene vacancies, binding energy Eb = 5.13 eV, than to pristine graphene, Eb = 1.09 eV. The Pd atoms tend to nucleate and form clusters around the vacancies and the small Pdn clusters (n = 2–6) also bind strongly, Eb ≈ 5 eV, to the vacancies. However, the Pd–Pd interaction is much smaller than the Pd–vacancy interaction, and therefore, the vacancies favor the dispersion of palladium on the graphene layer. For hydrogen adsobed on Pd atoms and clusters supported on pristine graphene, desorption of Pd–hydrogen complexes competes with desorption of molecular hydrogen. However, for hydrogen adsobed on a Pd atom anchored on a graphene vacancy, the desorption of the PdH2 complex costs 4.2 eV, and therefore, it does not compete with the desorption of molecular hydrogen, which takes place with an energy cost of only 0.2 eV. This shows the beneficial effect that anchoring Pd atoms and clusters to graphene vacancies has on the reversible adsorption/desorption of hydrogenMinisterio de Ciencia e Innovación (Proyect MAT2011-22781)Junta de Castilla y León (programa de apoyo a proyectos de investigación - Ref. VA158A11-2

Searching for DFT-based methods that include dispersion interactions to calculate the physisorption of H2 on benzene and graphene

Producción CientíficaSimulations of the hydrogen storage capacities of nanoporous carbons require an accurate treatment of the interaction of the hydrogen molecule with the graphite-like surfaces of the carbon pores, which is dominated by the dispersion forces. These interactions are described accurately by high level quantum chemistry methods, like the Coupled Cluster method with single and double excitations and a non-iterative correction for triple excitations (CCSD(T)), but those methods are computationally very expensive for large systems and for massive simulations. Density functional theory (DFT)-based methods that include dispersion interactions at different levels of complexity are less accurate, but computationally less expensive. In order to find DFT-methods that include dispersion interactions to calculate the physisorption of H2 on benzene and graphene, with a reasonable compromise between accuracy and computational cost, CCSD(T), Møller-Plesset second-order perturbation theory method, and several DFT-methods have been used to calculate the interaction energy curves of H2 on benzene and graphene. DFT calculations are compared with CCSD(T) calculations, in the case of H2 on benzene, and with experimental data, in the case of H2 on graphene. Among the DFT methods studied, the B97D, RVV10, and PBE+DCACP methods yield interaction energy curves of H2-benzene in remarkable agreement with the interaction energy curve obtained with the CCSD(T) method. With regards to graphene, the rev-vdW-DF2, PBE-XDM, PBE-D2, and RVV10 methods yield adsorption energies of the lowest level of H2 on graphene, very close to the experimental dataMinisterio de Economía, Industria y Competitividad (Project MAT2014-54378-R)Junta de Castilla y León (programa de apoyo a proyectos de investigación - Ref. VA050U14

Steric and chemical effects on the hydrogen adsorption and dissociation on free and graphene–supported palladium clusters

Palladium doping enhances the hydrogen storage capacity of nanoporous carbons. The purpose of this work is to asses the effect of the carbonaceous support on the adsorption of hydrogen on Pd clusters. Hydrogen adsorbs on Pd clusters following two channels: molecular adsorption and dissociative chemisorption. These two adsorption channels are investigated on free Pd clusters and Pd clusters supported on pristine and defective (with vacancies) graphene using the Density Functional Formalism. Pd6 is taken as case study. Free Pd6 can adsorb twelve hydrogen molecules in the molecular form, a number higher than the nine and eight molecules that can be adsorbed on the cluster supported on pristine graphene and on a graphene vacancy, respectively. However the most stable adsorption channel is, in all cases, the dissociative chemisorption of hydrogen. As the cluster is being loaded with hydrogen, there is a competition between the two adsorption channels. Pd6 supported on a graphene vacancy is able to dissociate three hydrogen molecules, whereas the free cluster can dissociate up to seven molecules. In both cases, six additional molecules can be adsorbed in the molecular form. The higher saturation limit obtained for the free clusters is explained in terms of the steric and chemical effects of the supporting layer. These effects are of primarily importance to asses the role of the Pd dopant on the adsorption and storage of hydrogen on nanoporous carbons.This work was supported by MICINN of Spain (Grant MAT2014- 54378-R)and Junta de Castilla y León (Grant VA050U14)

Chemical composition, antibacterial and repellent activities of Azorella trifurcata, Senecio pogonias, and Senecio oreophyton essential oils

The antibacterial and insect-repellent activities of the essential oils (EOs) from Argentinian medicinal plants Azorella trifurcata (Gaertn.) Pers., Senecio cfr. oreophyton J. Remy and Senecio cfr. pogonias Cabrera, were investigated. All EOs showed good repellent properties against Triatoma infestans Klug, the vector of the Chagas disease, with percent repellence values between 60% and 70% at 24 hours compared with positive control N-N diethyl-m-methylbenzamide (DEET) and moderate activity against the bacteria tested with minimum inhibitory concentrations (MICs) values between 31.2 and 2000 μg/ml. The Azorella trifurcata, Senecio pogonias and S. oreophyton EOs, obtained by hydrodistillation, were characterized by GC-FID and GC/MS analyses. Spathulenol (38.2%), myrtenyl acetate (8.4%) and α-terpineol (4.5%), limonene (9.8%) and α-thujene (5.4%) were the main constituents in the EO of Azorella trifurcata. The Senecio pogonias and S. oreophyton EOs are characterized by a high content of monoterpenes hydrocarbons (92% and 95.1%, respectively) with α-pinene, the main component in both oils. To our knowledge, the essential oil composition from Andean medicinal plants A. trifurcata, Senecio pogonias and S. oreophyton collected in central Andean slopes are reported for first time.Fil: López, Sandra Beatriz. Universidad Nacional de San Juan. Facultad de Ingeniería. Instituto de Biotecnología; ArgentinaFil: Lima, Beatriz Viviana. Universidad Nacional de San Juan. Facultad de Ingeniería. Instituto de Biotecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Agüero, María Belén. Universidad Nacional de San Juan. Facultad de Ingeniería. Instituto de Biotecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: López, María Liza. Universidad Nacional de San Juan. Facultad de Ingeniería. Instituto de Biotecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Hadad, Martín Ariel. Universidad Nacional de San Juan. Facultad de Ingeniería. Instituto de Biotecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Zygadlo, Julio Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto Multidisciplinario de Biología Vegetal. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Instituto Multidisciplinario de Biología Vegetal; ArgentinaFil: Caballero, Duilio. Provincia de San Juan. Hospital Marcial Quiroga; ArgentinaFil: Stariolo, Raúl Luis. Ministerio de Salud. Secretaría de Programas Sanitarios. Subsecretaría de Programas de Prevención y Promoción. Coordinación Nacional de Control de Vectores; ArgentinaFil: Suero, Eduardo. Universidad Nacional de San Juan. Facultad de Ingeniería. Instituto de Biotecnología; ArgentinaFil: Feresin, Gabriela Egly. Universidad Nacional de San Juan. Facultad de Ingeniería. Instituto de Biotecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Tapia, Alejandro. Universidad Nacional de San Juan. Facultad de Ingeniería. Instituto de Biotecnología; Argentin

Collective electronic excitations in metal-coated C_60

Producción CientíficaA two-shell jellium-on-jellium model has been used to study the collective electronic excitations of C_60 coated by an increasing number N of Na atoms. We predict a transition from the π-electron polarization of the fullerene to the metallic sodium polarization as a function of increasing N. For low coverages, the Na layer produces only a broadening and fragmentation of the π plasmon of C_60. However, if the coverage is large enough, a Na surface plasmon appears. This occurs only after the electron density of the cluster clearly shows two distinct regions, the outer one having an ‘‘average’’ density similar to that in pure Na metal or Na clusters. The induced density at the collective-mode frequency shows structure corresponding to two interfaces, C_(60-)Na and Na vacuum, the first peak becoming less pronounced as the coverage increases due to a transfer of oscillator strength to the metallic counterpart. The static polarizability per electron also shows this trend and tends slowly to the Na value from below after an initial sharp rise in the region N∼13–21

Structure and bonding in small neutral alkali halide clusters

Producción CientíficaThe structural and bonding properties of small neutral alkali halide clusters, (AX)_n with n<10, A=Li^+,Na^+,K^+,Rb^+, and X=F^-,Cl^-,Br^-,I^-, are studied using the ab initio perturbed ion (PI) model and a 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_A=0.5 (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 n=4, 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 (n=3 – 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