Página 11 dos resultados de 2015 itens digitais encontrados em 0.008 segundos

## Identification of species formed after pyridine adsorption on iron, cobalt, nickel and silver electrodes by SERS and theoretical calculations

ANDRADE, Gustavo F. S.; TEMPERINI, Marcia L. A.
Fonte: JOHN WILEY & SONS LTD Publicador: JOHN WILEY & SONS LTD
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
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The adsorption of pyridine (py) on Fe, Co, Ni and Ag electrodes was studied using surface-enhanced Raman scattering (SERS) to gain insight into the nature of the adsorbed species. The wavenumber values and relative intensities of the SERS bands were compared to the normal Raman spectrum of the chemically prepared transition metal complexes. Raman spectra of model clusters M(4)(py) (four metal atoms bonded to one py moiety) and M(4)(alpha-pyridil) where M = Ag, Fe, Co or Ni were calculated by density functional theory (DFT) and used to interpret the experimental SERS results. The similarity of the calculated M(4)(py) spectra with the experimental SERS spectra confirm the molecular adsorption of py on the surface of the metallic electrodes. All these results exclude the formation of adsorbed alpha-pyridil species, as suggested previously. Copyright (C) 2009 John Wiley & Sons, Ltd.; FAPESP[03/00411-9]; Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); FAPESP[07/52262-8]; Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); CNPq

## Tetragonal to monoclinic phase transition observed during Zr anodisation

Trivinho-Strixino, Francisco; Da Silva, Donizete X.; Paiva-Santos, Carlos O.; Pereira, Ernesto C.
Tipo: Artigo de Revista Científica Formato: 191-199
ENG
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Plasma electrolytic oxidation (PEO) is a coating procedure that utilises anodic oxidation in aqueous electrolytes above the dielectric breakdown voltage to produce oxide coatings that have specific properties. These conditions facilitate oxide formation under localised high temperatures and pressures that originate from short-lived microdischarges at sites over the metal surface and have fast oxide volume expansion. Anodic ZrO2 films were prepared by subjecting metallic zirconium to PEO in acid solutions (H2C 2O4 and H3PO4) using a galvanostatic DC regime. The ZrO2 microstructure was investigated in films that were prepared at different charge densities. During the anodic breakdown, an important change in the amplitude of the voltage oscillations at a specific charge density was observed (i.e., the transition charge density (Q T)). We verified that this transition charge is a monotonic function of both the current density and temperature applied during the anodisation, which indicated that Q T is an intrinsic response of this system. The oxide morphology and microstructure were characterised using SEM and X-ray diffraction experiments (XRD) techniques. X-ray diffraction analysis revealed that the change in voltage oscillation was correlated with oxide microstructure changes during the breakdown process. © 2012 Springer-Verlag Berlin Heidelberg.

## A transition metal complex (Venus flytrap cluster) for radioimmunodetection and radioimmunotherapy.

Paxton, R J; Beatty, B G; Hawthorne, M F; Varadarajan, A; Williams, L E; Curtis, F L; Knobler, C B; Beatty, J D; Shively, J E
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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A novel transition metal complex, Venus flytrap cluster (VFC), is described for the preparation of radio-labeled antibodies. VFC contained 57Co, which was held tightly between the faces of two covalently bridged carborane ligands by cluster bonding of the metal with appropriate ligand orbitals. Anti-carcinoembryonic antigen monoclonal antibody T84.66 was conjugated to 57Co-VFC with full retention of immunological activity. Biodistribution studies in nude mice bearing carcinoembryonic antigen-producing tumors showed excellent tumor localization of 57Co-VFC-T84.66. The accumulation of radionuclide in normal liver was low and independent of dose, which may reflect the stability of the radionuclide complex. These results presage the use of VFC systems for binding transition metals that are clinically useful for radioimmunodiagnosis and radioimmunotherapy.

## Extension of the method of partial retention of diatomic differential overlap to second row atoms and transition metals

Marynick, Dennis S.; Lipscomb, William N.
Tipo: Artigo de Revista Científica
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The method of partial retention of diatomic differential overlap (PRDDO) has been extended to the elements through the first transition series. A minimum basis set of Slatertype orbitals is employed, with optional 3d functions on atoms in the second major row. The 3d shell for transition metal atoms is a fixed-contracted double-ζ combination of Slater orbitals. The method retains the basic n3 dependency inherent in PRDDO. Limited one-center parameterization and empirical two-center exchange integrals are included. The overall accuracy of the approach is comparable to that of the PRDDO method as previously applied to molecules containing first-row atoms only.

## Erythrocyte catalase inactivation (H2O2 production) by ascorbic acid and glucose in the presence of aminotriazole: role of transition metals and relevance to diabetes.

Ou, P; Wolff, S P
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
360.35027%
Erythrocytes exposed to ascorbic acid in the presence of aminotriazole undergo a dose- and time-dependent inactivation of endogenous catalase which is proportional to environmental hydrogen peroxide (H2O2) concentrations. The production of H2O2 seems to be dependent upon the availability of transition metal chelatable by o-phenanthroline (OPT), although the kinetics of catalase inactivation and H2O2 production by externally added copper ions in the presence of OPT is complex. Furthermore, although glucose is also able to undergo a transition-metal-catalysed oxidation yielding H2O2, the production of H2O2 by glucose seems to be a minor process by comparison with ascorbic acid oxidation. Indeed, on the basis of these data, transition-metal-catalysed ascorbic acid oxidation is likely to be a more important source of oxidative stress in the diabetic state than hyperglycaemia.

## Formation, Reactivity, and Properties of Nondative Late Transition Metal–Oxygen and–Nitrogen Bonds

FULTON, J. ROBIN; HOLLAND, ANDREW W.; FOX, DANIEL J.; BERGMAN*, ROBERT G.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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Complexes containing bonds between heteroatoms such as nitrogen and oxygen and “late” transition metals (i.e., those located on the right side of the transition series) have been implicated as reactive intermediates in numerous important catalytic systems. Despite this, our understanding of such M–X linkages still lags behind that of their M–H and M–C analogues. New synthetic strategies have now made possible the isolation and study of a variety of monomeric late-metal alkoxide, aryloxide, and amide complexes, including parent hydroxide and amide species. The heteroatoms in these materials form surprisingly strong bonds to their metal centers, and their bond energies do not necessarily correlate with the energies of the corresponding H–X bonds. The M–X complexes typically exhibit nucleophilic reactivity, in some cases form strong hydrogen bonds to proton donors, and even deprotonate relatively weak acids. These observations, as well as thermodynamic investigations, suggest that late metal–heteroatom bonds are strongly polarized and possess significant ionic character, properties that play an important role in their interactions with organic compounds.

## Gas-phase activation of methane by ligated transition-metal cations

Schröder, Detlef; Schwarz, Helmut
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
360.35027%
Motivated by the search for ways of a more efficient usage of the large, unexploited resources of methane, recent progress in the gas-phase activation of methane by ligated transition-metal ions is discussed. Mass spectrometric experiments demonstrate that the ligands can crucially influence both reactivity and selectivity of transition-metal cations in bond-activation processes, and the most reactive species derive from combinations of transition metals with the electronegative elements fluorine, oxygen, and chlorine. Furthermore, the collected knowledge about intramolecular kinetic isotope effects associated with the activation of C–H(D) bonds of methane can be used to distinguish the nature of the bond activation as a mere hydrogen-abstraction, a metal-assisted mechanism or more complex reactions such as formation of insertion intermediates or σ-bond metathesis.

## First-principles study on transition metal-doped anatase TiO2

Wang, Yaqin; Zhang, Ruirui; Li, Jianbao; Li, Liangliang; Lin, Shiwei
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
361.9101%
The electronic structures, formation energies, and band edge positions of anatase TiO2 doped with transition metals have been analyzed by ab initio band calculations based on the density functional theory with the planewave ultrasoft pseudopotential method. The model structures of transition metal-doped TiO2 were constructed by using the 24-atom 2 × 1 × 1 supercell of anatase TiO2 with one Ti atom replaced by a transition metal atom. The results indicate that most transition metal doping can narrow the band gap of TiO2, lead to the improvement in the photoreactivity of TiO2, and simultaneously maintain strong redox potential. Under O-rich growth condition, the preparation of Co-, Cr-, and Ni-doped TiO2 becomes relatively easy in the experiment due to their negative impurity formation energies, which suggests that these doping systems are easy to obtain and with good stability. The theoretical calculations could provide meaningful guides to develop more active photocatalysts with visible light response.

## The detection of nitric oxide and its reactivity with transition metal thiolate complexes; Detection of NO and its reactivity with transition metal thiolate complexes

Tennyson, Andrew Gregory
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 173 leaves
ENG
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Nitric oxide (NO) is a molecule that is essential for life and regulates both beneficial and harmful processes. Because this gaseous radical influences many aspects of health and disease, we wish to explore the relationship between NO and physiology/pathophysiology. To this end, we seek to create tools for the fluorescent imaging of NO in vivo. We have adapted an existing small molecule-based sensor for more biologically relevant applications by including it within a polymeric film. We have also developed turn-on fluorescent sensors for NO based on conjugated polymers, which demonstrated good selectivity and sensitivity for this analyte. In addition, we have prepared a related sensor that will detect nitroxyl (HNO) but not NO. These systems demonstrate the versatility and value of the conjugated polymer scaffold for sensing applications. Many targets of the diatomic radical NO contain redox active units, such as transition metals or thiolate ligands. To gain insight into how NO might regulate biological processes by interacting with these redox active species, we have initiated a fundamental study of the reactivity of NO with transition metal thiolate model complexes. Our explorations in this field have yielded unique nickel and cobalt nitrosyl species with atypical electronic and structural parameters. These studies have suggested intermediates for the more biologically relevant iron nitrosyl complexes that have not yet been observed may exist. Furthermore...

## Iran - Medium Term Framework for Transition : Converting Oil Wealth to Development

World Bank
Fonte: Washington, DC Publicador: Washington, DC
Tipo: Economic & Sector Work :: Country Economic Memorandum; Economic & Sector Work
ENGLISH; EN_US
Relevância na Pesquisa
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Iran is about 10 percent of Gross Domestic Product (GDP) off an economic equilibrium that will reduce unemployment and improve significantly people's welfare by converting oil wealth into sustainable development. 1) It is about 10 percent of GDP short in the additional savings and investment needed to attain growth that will reduce unemployment. 2) Optimal management of Iran's oil to provide the above needed savings and achieve an optimal balance between consumption and savings that will sustain the benefits from oil after it is exhausted, requires that it allocate about 10 percent of GDP more for savings and investment and less to consumption from its oil wealth. 3) That 10 percent adjustment can come from the reform of Iran's inefficient energy subsidy system, which also happens to average about 10 percent of the GDP a year. These expenditures can be transformed into budget surpluses that will provide ample credit to the private sector to grow. This adjustment -at the core of Iran's medium and long term fiscal strategy- is a main pillar of Iran's transition to a market economy led by the private sector. Hence...

## Transition to a Low Carbon Economy in Poland

World Bank
Fonte: Washington, DC Publicador: Washington, DC
Tipo: Publications & Research :: ESMAP Paper; Publications & Research
ENGLISH; EN_US
Relevância na Pesquisa
363.03664%
Transition to a low carbon economy in Poland is a study by the World Bank for the Polish Government, supported by the UK Department for International Development and donors to the Energy Sector Management Assistance Program (ESMAP). The study poses the question of how Poland, an European Union (EU) member state, an industrialized 'annex one' country for the purposes of international climate discussions, and an Organization for Economic Co-operation and Development (OECD) member, can successfully transition to a low carbon economy as successfully as it underwent transition to a market economy in the early 1990s. Transition to a low carbon economy in Poland provides a detailed assessment of many aspects of a low carbon growth strategy for Poland, developing insights via a suite of models that should provide ongoing assistance to policymakers. These policymakers may find reassuring the main message that Poland's transition to a low carbon economy, while not free or simple is affordable. However, capturing the full package of technologically feasible and economically sensible abatement measures requires coordinated and early action by the government.

## First-principles density functional theory study of sulfur oxide chemistry on transition metal surfaces; 1st-principles density functional theory study of sulfur oxide chemistry on transition metal surfaces

Lin, Xi, 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 310 p.; 9412825 bytes; 9412627 bytes; application/pdf; application/pdf
ENG
Relevância na Pesquisa
360.35027%
In this thesis, the chemistry of sulfur oxides on transition metals is studied extensively via first-principles density functional theory (DFT) computations, focusing on the chemical reactivity and selectivity in sulfur poisoning chemical processes that address environmental concerns. The systematic approach we establish can be extended to general computational studies of small gas-phase molecules interacting with extended surfaces or finite-size clusters. The thesis starts with a theoretical presentation of modem quantum many-body theory that brings together mean-field theory, DFT, and Green's function quantum Monte Carlo theory. The essence of chemical reactivity theory in the framework of DFT is emphasized. The thesis continues with an extensive survey of the current status of sulfur oxide chemistry and an overall presentation of our computational approaches towards a detailed understanding of chemical reactivity and selectivity. The basic guidelines in chemical reactivity are systematically constructed by computed comprehensive thermodynamic data of surface S, O, SO, SO2, SO3, and S04 species as a function of coverage at low and intermediate temperatures. Under these basic guidelines, experimentally measured surface spectra are interpreted...

## Density-functional theory study of the catalytic oxidation of CO over transition metal surfaces

Stampfl, C.; Scheffler, M.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
360.35027%
In recent years due to improvements in calculation methods and increased computer power, it has become possible to perform first-principles investigations for simple'' chemical reactions at surfaces. We have carried out such studies for the catalytic oxidation of CO at transition metal surfaces, in particular, at the ruthenium surface for which unusual behavior compared to other transition metal catalysts has been reported. High gas pressure catalytic reactor experiments have revealed that the reaction rate over Ru for oxidizing conditions is the highest of the transition metals considered -- in contrast, under ultra high vacuum conditions, the rate is by far the lowest. We find that important for understanding the pressure dependence of the reaction is the fact that Ru(0001) can support high concentrations of oxygen at the surface. Under these conditions, the O-metal bond is atypically weak compared to that at lower coverages. We have investigated a number of possible reaction pathways for CO oxidation for the conditions of high oxygen coverages, including scattering reactions of gas-phase CO at the oxygen covered surface (Eley-Rideal mechanism) as well as the Langmuir-Hinshelwood mechanism involving reaction between adsorbed CO molecules and O atoms.; Comment: 6 pages...

## Observation of a novel orbital selective Mott transition in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$

Neupane, M.; Richard, P.; Pan, Z. -H.; Xu, Y.; Jin, R.; Mandrus, D.; Dai, X.; Fang, Z.; Wang, Z.; Ding, H.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
360.35027%
Electrons in a simple correlated system behave either as itinerant charge carriers or as localized moments. However, there is growing evidence for the coexistence of itinerant electrons and local moments in transition metals with nearly degenerate $d$-orbitals. It demands one or more selective electron orbitals undergo the Mott transition while the others remain itinerant. Here we report the first observation of such an orbital selective Mott transition (OSMT) in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ by angle-resolved photoemission spectroscopy (ARPES). While we observed two sets of dispersing bands and Fermi surface associated with the doubly-degenerate $d_{yz}$ and $d_{zx}$ orbitals, the Fermi surface associated with the wider $d_{xy}$ band is missing, a consequence of selective Mott localization. Our theoretical calculations demonstrate that this novel OSMT is mainly driven by the combined effects of interorbital carrier transfer, superlattice potential, and orbital degeneracy, whereas the bandwidth difference plays a less important role.

## High pressures make Hg a transition metal in a thermodynamically stable solid

Wang, Xiaoli; Lin, Haiqing; Ma, Yanming; Miao, Mao-Sheng
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
360.35027%
The appropriateness of including Hg among the transition metals has been debated for a long time. Although the synthesis of HgF$_{4}$ molecules in gas phase was reported before, the molecules show strong instabilities and dissociate. Therefore, the transition metal propensity of Hg remains an open question. Here, we propose that high pressure provides a controllable method for preparing unusual oxidation states of matter. Using an advanced structure search method based on first-principles electronic structure calculations, we predict that under high pressures, Hg can transfer the electrons in its outmost $d$ shell to F atoms, thereby acting as a transition metal. Oxidation of Hg to the +4 state yielded thermodynamically stable molecular crystals consisting of HgF$_{4}$ planar molecules, a typical geometry for $d^{8}$ metal centers.; Comment: 4 pages, 4 figures

## Computational and experimental investigation of unreported transition metal selenides and sulphides

Narayan, Awadhesh; Bhutani, Ankita; Rubeck, Samantha; Eckstein, James N.; Shoemaker, Daniel P.; Wagner, Lucas K.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
360.35027%
Expanding the library of known inorganic materials with functional electronic or magnetic behavior is a longstanding goal in condensed matter physics and materials science. Recently, the transition metal chalchogenides including selenium and sulfur have been of interest because of their correlated-electron properties, as seen in the iron based superconductors and the transition metal dichalcogenides. However, the chalcogenide chemical space is less explored than that of oxides, and there is an open question of whether there may be new materials heretofore undiscovered. We perform a systematic combined theoretical and experimental search over ternary phase diagrams that are empty in the Inorganic Crystal Structure Database containing cations, transition metals, and one of selenium or sulfur. In these 27 ternary systems, we use a probabilistic model to reduce the likelihood of false negative predictions, which results in a list of 24 candidate materials. We then conduct a variety of synthesis experiments to check the candidate materials for stability. While the prediction method did obtain previously unknown compositions that are predicted stable within density functional theory, none of the candidate materials formed in our experiments. We come to the conclusion that these phase diagrams are "empty" in the case of bulk synthesis...

## Two-band second moment model for transition metals and alloys

Ackland, Graeme J.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
360.35027%
A semi-empirical formalism based on the second moment tight binding approach, considering two bands is presented for deriving interatomic potentials for magnetic d-band materials and transition metal alloys. It incorporates an empirical local exchange interaction, which accounts for magnetic effects without increasing the computing time required for force evaluation. The consequences of applying a two-band picture to transition metal alloys and transition metal impurities is examined, which combined with evidence from {\it ab initio calculations} leads to some surprisingly simplifying conclusions.

## Tunable Hydrogen Storage in Magnesium - Transition Metal Compounds

Er, Suleyman; Tiwari, Dhirendra; de Wijs, Gilles A.; Brocks, Geert
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
360.35027%
Magnesium dihydride ($\mgh$) stores 7.7 weight % hydrogen, but it suffers from a high thermodynamic stability and slow (de)hydrogenation kinetics. Alloying Mg with lightweight transition metals (TM = Sc, Ti, V, Cr) aims at improving the thermodynamic and kinetic properties. We study the structure and stability of Mg$_x$TM$_{1-x}$H$_2$ compounds, $x=[0$-1], by first-principles calculations at the level of density functional theory. We find that the experimentally observed sharp decrease in hydrogenation rates for $x\gtrsim0.8$ correlates with a phase transition of Mg$_x$TM$_{1-x}$H$_2$ from a fluorite to a rutile phase. The stability of these compounds decreases along the series Sc, Ti, V, Cr. Varying the transition metal (TM) and the composition $x$, the formation enthalpy of Mg$_x$TM$_{1-x}$H$_2$ can be tuned over the substantial range 0-2 eV/f.u. Assuming however that the alloy Mg$_x$TM$_{1-x}$ does not decompose upon dehydrogenation, the enthalpy associated with reversible hydrogenation of compounds with a high magnesium content ($x=0.75$) is close to that of pure Mg.; Comment: 8 pages, 5 figures

## Magnetization reversal of giant perpendicular magnetic anisotropy at the magnetic-phase transition in FeRh films on MgO

Odkhuu, Dorj; Park, Noejung
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
361.9101%
Phenomena originated from spin-orbit interaction, such as magnetic anisotropy (MA), Rashba-type interactions, or topological insulators, have drawn huge attention for its intriguing physics. In particular, the search for a novel antiferromagnetic material, with potentially large perpendicular MA (PMA), has been becoming very intensive for next-generation high density memory applications. Here, we propose that substitutions of transition metals Ru and Ir, neighboring and same group elements in the periodic table, for the Rh site in the vicinity of surface can induce a substantially large PMA, up to an order of magnitude of 20 erg/cm2, in FeRh films on MgO. The main driving mechanism for this huge PMA is the interplay between the dxy and dx2-y2 states of strong spin-orbit 4d and 5d orbitals. Further investigations demonstrate that magnetization direction of PMA undergoes a transition into an in-plane magnetization at the antiferromagnetic-to-ferromagnetic phase transition, which provides a viable route for achieving large and switchable PMA associated with the magnetic-phase transition in antiferromagnetic spintronics.; Comment: This paper has been withdrawn by the author due to the incompleteness of figure caption in Figure 2

## Superconducting transition temperatures of the elements related to elastic constants

Angilella, G. G. N.; March, N. H.; Pucci, R.