Showing papers on "Copper published in 1995"

Evidence for stripe correlations of spins and holes in copper oxide superconductors

Abstract: ONE of the long-standing mysteries associated with the high-temperature copper oxide superconductors concerns the anomalous suppression1 of superconductivity in La2-xBaxCuO4 (and certain related compounds) when the hole concentration x is near . Here we examine the possibility that this effect is related to dynamical two-dimensional spin correlations, incommensurate with the crystal lattice, that have been observed in La2-xSrxCuO4 by neutron scattering2–4. A possible explanation for the incommensurability involves a coupled, dynamical modulation of spin and charge in which antiferromagnetic 'stripes' of copper spins are separated by periodically spaced domain walls to which the holes segregate5–9. An ordered stripe phase of this type has recently been observed in hole-doped La2NiO4 (refs 10–12). We present evidence from neutron diffraction that in the copper oxide material La1.6-xNd0.4SrxCuO4, with x = 0.12, a static analogue of the dynamical stripe phase is present, and is associated with an anomalous suppression of superconductivity13,14. Our results thus provide an explanation of the ' ' conundrum, and also support the suggestion15 that spatial modulations of spin and charge density are related to superconductivity in the copper oxides.

The Menkes/Wilson disease gene homologue in yeast provides copper to a ceruloplasmin-like oxidase required for iron uptake.

Abstract: The CCC2 gene of the yeast Saccharomyces cerevisiae is homologous to the human genes defective in Wilson disease and Menkes disease. A biochemical hallmark of these diseases is a deficiency of copper in ceruloplasmin and other copper proteins found in extracytosolic compartments. Here we demonstrate that disruption of the yeast CCC2 gene results in defects in respiration and iron uptake. These defects could be reversed by supplementing cells with copper, suggesting that CCC2 mutant cells were copper deficient. However, cytosolic copper levels and copper uptake were normal. Instead, CCC2 mutant cells lacked a copper-dependent oxidase activity associated with the extracytosolic domain of the FET3-encoded protein, a ceruloplasmin homologue previously shown to be necessary for high-affinity iron uptake in yeast. Copper restored oxidase activity both in vitro and in vivo, paralleling the ability of copper to restore respiration and iron uptake. These results suggest that the CCC2-encoded protein is required for the export of copper from the cytosol into an extracytosolic compartment, supporting the proposal that intracellular copper transport is impaired in Wilson disease and Menkes disease.

Nature of active species in copper-based catalysts and their chemistry of transformation of nitrogen oxides

Abstract: Copper-based catalysts are active in a wide range reactions of transformation of nitrogen oxides and represent an useful model system to better understand the fundamental aspects of the chemistry and mechanism of reaction of catalytic transformation of these pollutants. After an introduction on the reactivity of copper-based catalysts (supported and unsupported copper oxide, Cu-zeolites, cuprates and other copper compounds) in various reactions of conversion of nitrogen oxides, four main sub-topics are discussed in detail: (i) nature of copper species, (ii) chemisorption and surface transformations of NO, (iii) relationship between copper species and activity in the conversion of nitrogen oxides and (iv) mechanism of reduction of nitrogen oxides to N 2 . Five reactions of transformation of nitrogen oxides are discussed in detail: (i) decomposition of NO, (ii) reduction of NO with ammonia in the presence or not of oxygen, (iii) reduction of NO with hydrocarbons in the presence of oxygen, (iv) reduction of NO with CO and (v) decomposition of N 2 O. The mechanism of reduction of nitrite and N 2 O by copper enzymes is also discussed, with a view to provide some useful insights on the chemistry of transformation. In this review particular attention is directed towards controversial points in the literature, underestimated questions, and hypothesis and theories which do not allow interpretation of all sets of experimental data. Discussion is also focused on the presence of multiple and competitive pathways of transformation, the relative roles of which depend on reaction conditions.

Self-Assembled Monolayers of Long-Chain Hydroxamic Acids on the Native Oxides of Metals

Abstract: Departments of Materials Science and Engineering and Chemistry, Uniuersity of lllinois,Urbana-Champaign, (Irbana, Illinois 61801Receiued September 79, 1994eLong-chain alkanehydroxamic acids adsorb on the native oxides of metals and formed oriented self-assembled monolayers (SAMs). This study examined SAMs of hydroxamic acids on the native oxides ofcopper, silver, titanium, aluminum, zirconium, and iron. These SAMs were characterized usingwettability,X-ray photoelectron spectroscopy (XPS), and polarized infrared external reflectance spectroscopy (PIERS).Alkanehydroxamic acids give better monolayers than the corresponding alkanecarboxylic acids on certainbasic metal oxides (especially copper(Il) oxide). On the native oxide of copper (which has an isoelectricpoint greater than the pK" of the hydroxamic acid), the ligand is bound to the surface predominantly asthe hydroxamate. The strength of the interaction between copper oxide and the hydroxamate allowsincorporation of polar tail groups into the monolayer. On acidic or neutral metal oxides (e.g., TiO2), thepredominant species bound to the surface is the hydroxamic acid. Alkanehvdroxamic acids on titaniumdioxide bind relatively weakly but. nonetheless. form SAlts that are more stable than those from carboxylicacids (although not as stable as those from alkanephosphonic acids r.

The Investigation of Cerium as a Cathodic Inhibitor for Aluminum‐Copper Alloys

Abstract: In situ current density mapping, scanning electron microscopy, and energy dispersive spectroscopy were used to study the effects of cerium as a corrosion inhibitor for an aluminum copper alloy (Al 2024-T4) in chloride containing solutions. It was found that cerium inhibits corrosion of this alloy by reducing the rate of the cathodic reaction. This was due to the formation of cerium-rich films over copper containing intermetallics which act as local cathodic sites. Results from tests carried out on an aluminum/copper galvanic couple, which was used to simulate the electrochemical behavior of the copper containing intermetallics, showed that corrosion inhibition was associated with the formation of a Ce-rich film over the copper in agreement with that observed for the alloy.

Green luminescence from copper doped zinc sulphide quantum particles

Abstract: Free‐standing powder of zinc sulphide quantum particles has been synthesized using a chemical route. X‐ray diffraction analysis shows that the diameter of the particles is ∼21±2 A which is smaller than the Bohr exciton diameter for zinc sulphide. UV absorption shows an excitonic peak centered at ∼300 nm corresponding to an energy gap of 4.1±0.1 eV. These particles show a luminescence band at ∼424 nm. The quantum particles could be doped with copper during synthesis without altering the UV absorption or x‐ray diffraction pattern. However, doping shifted the luminescence to 480 nm, green wavelength in the visible region.

Identification of copper(II) and copper(I) and their interconversion in Cu/ZSM-5 De-NOx catalysts

Abstract: A combined Fourier transform IR (FT-IR) and electron paramagnetic resonance (EPR) study shows that copper in ‘excessively exchanged’ Cu/ZSM-5 is initially present as OH bridged Cu 2+ dimers, besides isolated Cu 2+ ions. Upon heating, the dimers lose water and become oxygen bridged [Cu-O-Cu] 2+ complexes. These are ‘EPR-silent’, presumably as a consequence of antiferromagnetic coupling of the unpaired electrons in each Cu 2+ ; they are, however, detectable by their perturbation of the lattice vibrations, detected by a FT-IR band at 918–923 cm −1 . Reduction by hydrogen or carbon monoxide converts the [Cu-O-Cu] 2+ complexes to pairs of Cu + ions, while the color changes from green to grey. Reductive adsorption of nitrogen monoxide on Cu 2+ results in the formation of Cu + -NO + . Destructive thermal desorption of nitrogen monoxide at 100°C not only restores the Cu 2+ ions, but also appears to regenerate the [Cu-O-Cu] 2+ complex. The results suggest that pairs of copper ions are instrumental in the catalytic decomposition of nitrogen monoxide.

Activity and structure-sensitivity of the water-gas shift reaction over CuZnAl mixed oxide catalysts

Abstract: The activity and structure-sensitivity of the water-gas shift (WGS) reaction over Cu Zn Al mixed oxide catalysts were studied. Three sets of samples with different Cu/Zn and (Cu+Zn)/Al atomic ratios were prepared by coprecipitation. Depending on the cation ratio, the ternary hydroxycarbonate precursors contained hydrotalcite, aurichalcite and/or rosasite phases. The decomposed precursors contained CuO, ZnO, ZnAl 2 O 4 , and Al 2 O 3 . The relative proportion of these phases depended on both the chemical composition of the sample and the calcination temperature employed for decomposing the precursor. After activation with hydrogen, samples were tested for the WGS reaction at 503 K. The turnover frequency of the eighteen samples tested was essentially the same (0.2–0.3 s −1 ) irrespective of changing the copper metal surface area between 3 and 35 m 2 /g Cu and the metallic copper dispersion between 0.5 and 5.0%. This indicated that the WGS reaction is a structure-insensitive reaction, as the specific reaction rate r 0 (mol CO/h/g Cu) is always proportional to the copper metal surface area. Preparation of mixed oxides with a high copper dispersion is therefore required for obtaining more active catalysts. It was found that the value of the metallic copper dispersion is related to the amount of hydrotalcite contained in the hydroxycarbonate precursor: the higher the hydrotalcite content in the precursor, the higher the copper metal dispersion in the resulting catalyst and, as a consequence, the higher the catalyst activity. Ternary Cu/ZnO/Al 2 O 3 catalysts exhibited a substantially faster WGS activity than binary Cu/ZnO catalysts. The addition of aluminium, although inactive for the WGS reaction, is required for improving the catalyst performance.

Electronic structure of the reduced blue copper active site : contributions to reduction potentials and geometry

Abstract: A detailed electronic structure description of the reduced blue copper active site has now been developed. Photoelectron spectroscopy (PES) of imidazole, dimethyl sulfide, and methanethiolate bound to Cu(I) sites at single crystal surfaces has been used to define normal Cu(I) bonding to ligands relevant to the blue copper site. Variable photon energy PES has been used to assign valence band spectra, assess metal-ligand covalency, and probe specific orbital contributions to Cu(I) bonding. Self Consistent Field-X{alpha}-Scattered Wave (SCF-X{alpha}-SW) molecular orbital calculations calibrated to the photoelectron spectra have been performed to quantitatively complement the experimental bonding descriptions. These calculations have been extended to the reduced blue copper active site in plastocyanin, the prototypical blue copper protein, to detail the electronic structure changes that occur relative to normal Cu(I) bonding and upon oxidation. Ionization energies have been used to estimate the electronic structure contributions to the reduction potential. The long Cu-thioether axial bond present at the active site destabilizes the oxidized state and is therefore a key determining factor in the high reduction potentials generally observed for blue copper proteins. Linear coupling terms have been evaluated for the distortions of a blue copper site unconstrained by the protein backbone. 99 refs., 22 figs.,more » 12 tabs.« less

Preparation of Bulky Amorphous Zr–Al–Co–Ni–Cu Alloys by Copper Mold Casting and Their Thermal and Mechanical Properties

Abstract: Bulky amorphous alloys were found to form in Zr-Al-M (M=Co, Ni, Cu) systems by arc melting on a copper hearth. The largest thickness for glass formation is 6. 1 mm for Zr 60 Al 10 Co 3 Ni 9 Cu 18 , 6.8 mm for Zr 60 Al 15 Co 5 Ni 15 Cu 5 and 6.2 mm for Zr 55 Al 20 Co 17.5 Ni 2.5 Cu 5 . The optimum composition for glass-forming ability shifts from the Cu-rich side to the Co-rich side through the Ni-rich side with increasing Al content from 10 to 20%. The use of a metallic mold casting process enabled the formation of amorphous cylinders with the largest diameter of 7 mm for the three alloys. The compositional effect for the large glass-forming ability has also been discussed by taking the present data into consideration. The cast amorphous Zr 60 Al 10 Co 3 Ni 9 Cu 18 alloy subjected to tensile testing exhibits distinct serrated flow before final fracture. The generation of the serrated flow is noticed because the alloy has a ductile nature which enables the momentary stop of the shear sliding. The Young's modulus, tensile fracture strength and fracture elongation are 97 GPa, 1510 MPa and 2.0%, respectively. The fracture occurs along the maximum shear plane and the fracture surface consists of a well-developed vein pattern. The size of their veins is about 10 times as large as that for the melt-spun ribbon and hence the shear deformation region occurs in a much wider region for the cast alloy, indicating the necessity of a larger amount of energy up to final fracture. The finding of the amorphous alloys with the large glass-forming ability and the extremely ductile nature is important for the subsequent development of metallic glassy materials.