Javier Fernández Sanz

Highly active copper-ceria and copper-ceria-titania catalysts for methanol synthesis from CO2

Converting CO2 into methanol by catalysis By hydrogenating CO2, scientists can transform a greenhouse gas into methanol, a desirable fuel. Graciani et al. cast copper in the role of the highly active catalyst for this reaction by putting copper particles on cerium oxide. The interface between the cerium oxide and the copper enables the reverse water-gas shift reaction that converts CO2 into CO, which reacts more readily with hydrogen to make methanol. This result takes a step forward in innovating catalysts for this environmentally friendly process. Science, this issue p. 546 Synergy at a metal-oxide interface generates highly active catalysts for carbon dioxide hydrogenation to methanol. The transformation of CO2 into alcohols or other hydrocarbon compounds is challenging because of the difficulties associated with the chemical activation of CO2 by heterogeneous catalysts. Pure metals and bimetallic systems used for this task usually have low catalytic activity. Here we present experimental and theoretical evidence for a completely different type of site for CO2 activation: a copper-ceria interface that is highly efficient for the synthesis of methanol. The combination of metal and oxide sites in the copper-ceria interface affords complementary chemical properties that lead to special reaction pathways for the CO2→CH3OH conversion.

High catalytic activity of Au/CeOx/TiO2(110) controlled by the nature of the mixed-metal oxide at the nanometer level

Mixed-metal oxides play a very important role in many areas of chemistry, physics, materials science, and geochemistry. Recently, there has been a strong interest in understanding phenomena associated with the deposition of oxide nanoparticles on the surface of a second (host) oxide. Here, scanning tunneling microscopy, photoemission, and density-functional calculations are used to study the behavior of ceria nanoparticles deposited on a TiO2(110) surface. The titania substrate imposes nontypical coordination modes on the ceria nanoparticles. In the CeOx/TiO2(110) systems, the Ce cations adopt an structural geometry and an oxidation state (+3) that are quite different from those seen in bulk ceria or for ceria nanoparticles deposited on metal substrates. The increase in the stability of the Ce3+ oxidation state leads to an enhancement in the chemical and catalytic activity of the ceria nanoparticles. The codeposition of ceria and gold nanoparticles on a TiO2(110) substrate generates catalysts with an extremely high activity for the production of hydrogen through the water–gas shift reaction (H2O + CO → H2 + CO2) or for the oxidation of carbon monoxide (2CO + O2 → 2CO2). The enhanced stability of the Ce3+ state is an example of structural promotion in catalysis described here on the atomic level. The exploration of mixed-metal oxides at the nanometer level may open avenues for optimizing catalysts through stabilization of unconventional surface structures with special chemical activity.

Molecular dynamics studies of the structure of γ-alumina

Abstract Molecular dynamics (MD) studies of γ-Al 2 O 3 using a pairwise additive interaction potential of Paulings type are reported. MD simulations on a medium-sized cluster of γ-Al 2 O 3 (Al 576 O 864 ) show a picture of its structure which is in agreement with neutron diffraction results and lead to an interpretation of its main properties. MD simulations at several temperatures and velocity autocorrelation functions are also reported.

Compact model potentials for abinitio embedded cluster calculations. Part I. Basic formulation

In light of the theory of separability of many‐electron systems, a methodology to perform embedded cluster calculations in order to study localized phenomena in a large electronic system is reported. A general equation for the cluster spin orbitals under the assumptions of frozen environment and cluster‐environment strong orthogonality requirements is presented. The interactions between active and frozen electrons are adequately introduced through computationally convenient ab initio compact model potentials (CMPs) built up as a sum of long‐ and short‐range terms. The short‐range term, Coulomb and exchange interactions, are developed as a spectral representation in a nonorthogonal basis set. Cluster wave function, energy, and analytic geometrical derivatives calculations with CMPs have been implemented for several kinds of cluster wave functions. In order to evaluate the accuracy of the results obtained using this methodology, some simple test calculations for different hydrogen molecule‐cation complexes ...

Graphenes as Efficient Metal‐Free Fenton Catalysts

Reduced graphene oxide exhibits high activity as Fenton catalyst with HO(.) radical generation efficiency over 82 % and turnover numbers of 4540 and 15023 for phenol degradation and H2 O2 consumption, respectively. These values compare favorably with those achieved with transition metals, showing the potential of carbocatalysts for the Fenton reaction.

Surface models for γ-Al2O3 from molecular dynamics simulations

Molecular dynamics simulations for γ-Al2O3 and La3+-doped γ-Al2O3 crystals have been performed. From bulk simulations a description of the crystal-terminating layers in both doped and pure alumina solids is reported. The molecular dynamics simulations allow us to describe systematically for the first time the surface of the γ-Al2O3 crystal, taking into account the actual stoichiometry of the solid. This description results in a model in which the number of different surface sites is increased with respect to the previously reported models. The number of different surface sites may account for the IR spectra of hydroxy groups adsorbed on γ-Al2O3.

Sonogashira cross-coupling and homocoupling on a silver surface:chlorobenzene and phenylacetylene on Ag(100)

Scanning tunneling microscopy, temperature-programmed reaction, near-edge X-ray absorption fine structure spectroscopy, and density functional theory calculations were used to study the adsorption and reactions of phenylacetylene and chlorobenzene on Ag(100). In the absence of solvent molecules and additives, these molecules underwent homocoupling and Sonogashira cross-coupling in an unambiguously heterogeneous mode. Of particular interest is the use of silver, previously unexplored, and chlorobenzene-normally regarded as relatively inert in such reactions. Both molecules adopt an essentially flat-lying conformation for which the observed and calculated adsorption energies are in reasonable agreement. Their magnitudes indicate that in both cases adsorption is predominantly due to dispersion forces for which interaction nevertheless leads to chemical activation and reaction. Both adsorbates exhibited pronounced island formation, thought to limit chemical activity under the conditions used and posited to occur at island boundaries, as was indeed observed in the case of phenylacetylene. The implications of these findings for the development of practical catalytic systems are considered.

Onset of perovskite formation in the catalytic system La2O3/γ-Al2O3

Molecular dynamics simulations of model systems for the surface interaction of lanthanum oxide supported on γ-alumina have been carried out at 1500 K. The onset of formation of perovskite-like phases has been analysed in samples containing four different concentrations of lanthanum oxide. A mechanism of the formation of perovskite-like polyhedra is proposed. This mechanism essentially involves a displacement of an oxide ion associated to an octahedral aluminum by a lanthanum ion and appears to be independent of La2O3 loadings.

PARALLEL COMPUTATION OF SECOND DERIVATIVES OF RHF ENERGY ON DISTRIBUTED MEMORY COMPUTERS

A parallel implementation of the computation of RHF energy second derivatives with respect to the nuclear coordinates is described. The algorithm and organization of the code are described in detail on the most computationally demanding steps with special emphasis on the integral transformation code. Key features of the proposed algorithm are its large degree of concurrency, limited interprocessor communication, and critical memory needs distributed over the processors. The cpu times and computer and network resources used are reported and discussed for a few examples.

Ab initio group model potentials: Application to the study of intermolecular interactions

A method for the determination of ab initio group model potentials within the Hartree–Fock framework is reported. The theoretical formulation is based on the core model potential approach and allows the replacement of a group by a polycenter model potential having a local molecular symmetry. Model potentials for the Ne atom and the HF molecule are obtained and used in Hartree–Fock calculations of the NeHF and (HF)2 complexes using a triple‐zeta plus polarization (TZP) basis set. Radial and angular properties of these model potentials are analyzed and compared with all electron reference calculations. The method is free of basis set superposition error and gives binding energies in agreement with Hartree–Fock calculations. The suitability of the method for the study of intermolecular interactions of larger systems is discussed.