Entity

Time filter

Source Type


Serna P.,University of California at Davis | Serna P.,Technical University Valencia Consejo Superior Of Investigaciones Cientificas | Gates B.C.,University of California at Davis
Accounts of Chemical Research | Year: 2014

ConspectusRecent advances in the synthesis and characterization of small, essentially molecular metal complexes and metal clusters on support surfaces have brought new insights to catalysis and point the way to systematic catalyst design. We summarize recent work unraveling effects of key design variables of site-isolated catalysts: the metal, metal nuclearity, support, and other ligands on the metals, also considering catalysts with separate, complementary functions on supports. The catalysts were synthesized with the goal of structural simplicity and uniformity to facilitate incisive characterization. Thus, they are essentially molecular species bonded to porous supports chosen for their high degree of uniformity; the supports are crystalline aluminosilicates (zeolites) and MgO. The catalytic species are synthesized in reactions of organometallic precursors with the support surfaces; the precursors include M(L)2(acetylacetonate)1-2, with M = Ru, Rh, Ir, or Au and the ligands L = C2H4, CO, or CH3. Os3(CO)12 and Ir4(CO)12 are used as precursors of supported metal clusters, and some such catalysts are made by ship-in-A-bottle syntheses to trap the clusters in zeolite cages.The simplicity and uniformity of the supported catalysts facilitate precise structure determinations, even in reactive atmospheres and during catalysis. The methods of characterizing catalysts in reactive atmospheres include infrared (IR), extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), and nuclear magnetic resonance (NMR) spectroscopies, and complementary methods include density functional theory and atomic-resolution aberration-corrected scanning transmission electron microscopy for imaging of individual metal atoms. IR, NMR, XANES, and microscopy data demonstrate the high degrees of uniformity of well-prepared supported species. The characterizations determine the compositions of surface metal complexes and clusters, including the ligands and the metal-support bonding and structure, which identify the supports as ligands with electron-donor properties that influence reactivity and catalysis.Each of the catalyst design variables has been varied independently, illustrated by mononuclear and tetranuclear iridium on zeolite HY and on MgO and by isostructural rhodium and iridium (diethylene or dicarbonyl) complexes on these supports. The data provide examples resolving the roles of the catalyst design variables and place the catalysis science on a firm foundation of organometallic chemistry linked with surface science. Supported molecular catalysts offer the advantages of characterization in the absence of solvents and with surface-science methods that do not require ultrahigh vacuum.Families of supported metal complexes have been made by replacement of ligands with others from the gas phase. Spectroscopically identified catalytic reaction intermediates help to elucidate catalyst performance and guide design. The methods are illustrated for supported complexes and clusters of rhodium, iridium, osmium, and gold used to catalyze reactions of small molecules that facilitate identification of the ligands present during catalysis: alkene dimerization and hydrogenation, H-D exchange in the reaction of H2 with D2, and CO oxidation. The approach is illustrated with the discovery of a highly active and selective MgO-supported rhodium carbonyl dimer catalyst for hydrogenation of 1,3-butadiene to give butenes. © 2014 American Chemical Society. Source


Serna P.,Technical University Valencia Consejo Superior Of Investigaciones Cientificas | Serna P.,University of California at Davis | Gates B.C.,University of California at Davis
Journal of Catalysis | Year: 2013

Essentially molecular rhodium catalysts were made from Rh(C 2H4)2(acetylacetonate) on zeolite HY and on MgO and characterized by infrared and X-ray absorption spectroscopies. The supported rhodium species anchored to the zeolite, initially in the form of Rh(C2H4)2, selectively catalyzed ethene dimerization, typically at 298 K and 1 bar, but when the catalyst was poisoned by CO, or the support was changed to MgO or zeolite NaY, or the rhodium was converted into small clusters, the ethene underwent predominantly hydrogenation. The preciseness of the synthesis of the supported rhodium species facilitated determination of structure-catalyst performance relationships that led to a schematic representation of how the dimerization proceeds by a mechanism involving both the rhodium complexes and zeolite surface OH groups. The reaction is facilitated by H2 and proceeds as one ethene molecule is activated by an isolated rhodium complex and another by a weakly acidic Si-OH-Al group. © 2013 Elsevier Inc. All rights reserved. Source


Leyva-Perez A.,Technical University Valencia Consejo Superior Of Investigaciones Cientificas | Corma A.,Technical University Valencia Consejo Superior Of Investigaciones Cientificas
Angewandte Chemie - International Edition | Year: 2012

Relativistic effects in the valence shell of the elements reach a maximum in the triad Pt-Au-Hg and determine their catalytic activity in organic reactions. In this Review we examine the catalytic activity of Pt, Au, and Hg compounds for some representative reactions, and discuss the respective benefits and disadvantages along with other relevant properties, such as toxicity, price, and availability. For the reactions considered, gold catalysts are generally more active than mercury or platinum catalysts. Relatively related: Relativistic effects in the valence shell of the chemical elements reach a maximum in the triad Pt-Au-Hg and influence their catalytic activity in organic reactions. The catalytic activity for some representative reactions is examined together with other relevant properties, such as toxicity, price, and availability. For the reactions considered, gold is generally preferred to mercury or platinum catalysts. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source


De La Pena M.,Technical University Valencia Consejo Superior Of Investigaciones Cientificas | Garcia-Robles I.,Technical University Valencia Consejo Superior Of Investigaciones Cientificas
EMBO Reports | Year: 2010

Small ribozymes have been regarded as living fossils of a prebiotic RNA world that would have remained in the genomes of modern organisms. In this study, we report the ultraconserved occurrence of hammerhead ribozymes in Amniota genomes (reptiles, birds and mammals, including humans), similar to those described previously in amphibians and platyhelminth parasites. The ribozymes mapped to intronic regions of different genes, such as the tumour suppressor RECK in birds and mammals, a mammalian tumour antigen and the dystrobrevin beta in lizards and birds. In vitro characterization confirmed a high self-cleavage activity, whereas analysis of RECK-expressed sequence tags revealed fusion events between the in vivo self-cleaved intron and U5 or U6 small nuclear RNA fragments. Together, these results suggest a conserved role for these ribozymes in messenger RNA biogenesis. © 2010 European Molecular Biology Organization. Source


Corma A.,Technical University Valencia Consejo Superior Of Investigaciones Cientificas | Rodenas T.,Technical University Valencia Consejo Superior Of Investigaciones Cientificas | Sabater M.J.,Technical University Valencia Consejo Superior Of Investigaciones Cientificas
Chemical Science | Year: 2012

Thiols are smoothly and efficiently oxidized to disulfides (RSSR) with air in the presence of gold nanoparticles supported on CeO 2 in absence of solvent, as well as in aqueous solutions and neutral pH. It is shown that the reaction can occur through the coupling of two sulphur radicals on the metal surface. The sulphur radicals are formed from thiols by one-electron oxidation with the metal. This reaction mechanism strongly resembles that found for sulfhydryl oxidases, a class of enzymes which are involved in the oxidative protein folding through de novo formation of disulfides from thiols. © 2012 The Royal Society of Chemistry. Source

Discover hidden collaborations