An improved descriptor of cluster stability: application to small carbon clusters

Producción CientíficaThe mass spectra of gas-phase clusters in cluster beams have a rich structure where the relative heights of the peaks compared to peaks corresponding to clusters of neighbor sizes reveal the stability of the clusters as a function of the size N. In an analysis of the published mass spectrum of carbon clusters cations C + N with N ≤16 we have employed the most common descriptor of cluster stability, which is based on comparing the total energy of the cluster of size N with the averaged energies of clusters with sizes N+1 and N-1. Those energies have been obtained from density functional calculations. The comparison between the stability function and the mass spectrum leaves some experimental features unexplained; in particular, the correlation with the detailed variation of the height of the mass peaks as a function of size N is not satisfactory. We then propose a novel stability descriptor which improves matters substantially, in particular the correlation with the detailed variation of the height of the mass peaks. The new stability index is based on comparing the atom-evaporation energy of the cluster of size N with the averaged atom-evaporation energies of clusters with sizes N+1 and N-1. The substantial improvement achieved is attributed to the fact that evaporation energies are quantities directly connected with the processes controllig the cluster abundances in the beam.Ministerio de Economía, Industria y Competitividad ( Grants MAT2017-85089-C2-1-R / RYC-2015-17730)Junta de Castilla y Leon (Grant VA021G18)Universidad de Valladolid (Grupo de Física de Nanoestructuras), y Unión Europea a través de ERC-Synergy Program (Grant ERC-2013-SYG610256 NANOCOSMOS

Hydrogen quenches the size effects in carbon clusters

Producción CientíficaA characteristic fingerprint of atomic clusters is that their properties can vary in a non-smooth way with the cluster size N. This is illustrated herein by studying the cluster size dependence of several properties of neutral CN and cationic C+N carbon clusters: C–C bond lengths, cluster structure, intrinsic cluster stability, ionization energy, and spatial distribution of the reactivity index for charge exchange with electrophiles. Nonetheless, clusters can lose the size dependence of their properties by interaction with other chemical species, which is rationalized in this study by analyzing carbon clusters fully saturated with hydrogen to form linear alkanes, CNH2N+2. In all cases, the lowest energy structures are zigzagging linear chains, the variations of C–C bond lengths and Image ID:c9cp01114e-t1.gif angles with alkane size are very minor and smooth, the stability function shows practically no structure as a function of the alkane size, the ionization energies just decrease smoothly with alkane size, and the spatial distribution of the reactivity index is analogous and highly delocalized in all the alkanes. In summary, the interaction of carbon clusters with hydrogen to form alkanes quenches all the size-dependent features that the carbon clusters originally owned. The arrival at the quenching of the size effects follows an involved path. In each CNHn family with fixed N, the values of the properties of the molecules like the ionization potential, the electron affinity, and others show sizable oscillations as the number of hydrogen atoms grows from the pure carbon cluster to the alkane.Ministerio de Economía, Industria y Competitividad ( grants MAT2017-85089-C2-1-R / RYC-2015-17730)Comunidad de Madrid (FotoArt-CM Project P2018/NMT-4367)Junta de Castilla y León (Grant VA021G18)Universidad de Valladolid (GIR Grupo de Física de Nanoestructuras),y Unión Europea mediante el programa ERC-Synergy (Grant ERC-2013-SYG-610256 Nanocosmos

Water adsorption and dissociation on gold catalysts supported on anatase-TiO2(101)

Producción CientíficaThe presence of water can strongly a ect the reactivity of gold catalysts. For this reason, ab initio density functional simulations have been performed to study the adsorption and dissociation of water on the anatase-TiO2(101) surface, both clean and in the presence of a supported model gold nanocluster, Au4. When adsorbed not too close to the cluster, water is adsorbed and dissociated with roughly the same binding energies and dissociation barriers as in the catalystfree surface. If the molecule adsorbs at the Au/TiO2 perimeter interface, making contact with gold, we nd a slight stabilization of molecular water, whereas dissociated water becomes slightly less stable. The preferential mechanism for water dissociation is found to be a splitting of the H-OH bond at the TiO2 surface, with the gold cluster playing a minor role. Calculations of the relative stability of various water-related species show that the gold catalyst favours accumulation of excess hydroxyls around its perimeter.Ministerio de Economía, Industria y Competitividad y Fondo Europeo de Desarrollo ( grants MAT 2011-22781 / MAT 2014-54378-R )Consejería de Educación, Junta de Castilla y León (grants VA050U14 / VA021G18)Junta de Castilla y León (postdoctoral contract CIP13702

Ab initio studies of ethanol dehydrogenation at binary AuPd nanocatalysts

Producción CientíficaDensity Functional Theory (DFT) simulations have been performed to study ethanol anaerobic oxidation at 10-atom binary gold–palladium clusters with varying compositions, ranging from 0% to 100% gold content. For each case, we have studied the loss of two hydrogen atoms, the desorption of acetaldehyde, and recombination of hydrogen into gas-phase H2. The results show that alloying produces sizable changes in the reaction barriers and in the stability of the intermediate species, which are related to different intrinsic reactivities of palladium and gold.Ministerio de Economía, Industria y Competitividad (Project MAT2014-54378-R)Junta de Castilla y León (programa de apoyo a proyectos de investigación - Ref. A050U14

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

Elimination vs Substitution Reaction. A dicotomy between Bronsted-Lowry and Lewis basicity

Producción CientíficaThe Brønsted–Lowry and Lewis basicity dichotomy in the elimination vs substitution reaction competition is analyzed in terms of a novel Brønsted–Lowry–Lewis basicity ωp/e. This new index unifies the dichotomy and explains the competition between elimination and substitution mechanisms of alkyl centers with para-substituted phenols.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA050U14

The Diels-Adler cycloaddition reaction of substituted hemifullerenes with 1-3 butadiene: Effect of electron-donating and electron- withdrawing substituents

Producción CientíficaThe Diels-Alder (DA) reaction provides an attractive route to increase the number of six member rings in substituted Polycyclic Aromatic Hydrocarbons (PAHs). The density functional theory (DFT) B3LYP method has been used in this work to inquire if the substitution of H over the edge of triindenetriphenylene (pristine hemifullerene 1) and pentacyclopentacorannulene (pristine hemifullerene 2), could improve the DA cycloaddition reaction with 1,3-butadiene. The substituents tested include electron-donating (NH2, OMe, OH, Me, i-Pr) and electron-withdrawing groups(F, COOH, CF3, CHO, CN, NO2). The electronic, kinetic and thermodynamic parameters of the DA reactions of the substituted hemifullerenes with 1,3-butadiene have been analyzed. The most promising results were obtained for the NO2 substituent; the activation energy barriers for reactions using this substituent were lower than the barriers for the pristineJunta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA050U14

Mechanistic insight into the CO oxidation reaction at pure, Nb-doped and Mo-doped medium size Pt clusters

Producción CientíficaDensity Functional Theory (DFT) simulations have been performed to study the CO oxidation reaction on a pure Pt18 cluster, and on Nb- and Mo-doped NbPt17 and MoPt17 clusters. The results show that a specially stable conformation of the pure cluster causes a sizable reduction of CO adsorption energy. Substituting one Pt atom by Nb or Mo has noticeable effects, charge transfer to the surface Pt atoms and destabilization of the special Pt18 ground state conformation, which result in an enhacement of CO binding for the doped cluster. Finally, molecular oxygen binds strongly to pure and Nb- or Mo-doped clusters, and easily dissociates and reacts with co-adsorbed CO, with reaction barriers not exceeding 0.8 eV.Ministerio de Ciencia, Innovación y Universidades (project PID2019-104924RB-I00

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-

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