Search results for "element"
showing 10 items of 13601 documents
Dual stereocontrolled alkylation of aldehydes with polystyrene-supported nickel complexes derived from α-amino amides
2015
Nickel(II) complexes derived from α-amino amide ligands anchored to gel-type and monolithic polymers act as efficient catalysts for the enantioselective addition of dialkylzinc reagents to aldehydes. Similar to the analogous homogeneous systems, dual stereocontrol in addition products can be achieved by controlling the stoichiometry of the immobilized nickel complex. Aromatic and aliphatic aldehydes were alkylated in good yields with enantioselectivities comparable to those obtained with the homogeneous analogues. These polymer-supported catalysts offer significant advantages as no metal leaching is observed and they can be easily recovered from the reaction mixture by simple filtration and…
A new application for nickel foam in alkaline fuel cells
2009
The use of nickel foam as an electrode substrate in alkaline fuel cells (AFCs) has been investigated for bi-polar cells incorporating an electrically conducting gas diffusion layer. This contribution focuses on the cathode, and draws comparisons between nickel foam and nickel mesh substrates. One of the principal electrocatalysts for the cathodic reduction of oxygen is silver, so an improvement in electrochemical performance was obtained by electroplating the nickel foam with silver. The electrodeposition process was optimised to maximise electrochemical performance with a minimum of silver deposited. Nickel foam, which is less expensive than the usual nickel mesh, appears to be a good subs…
Total oxidation of propane in vanadia-promoted platinum-alumina catalysts: Influence of the order of impregnation
2015
Differently prepared vanadium promoted Pt/alumina catalysts have been prepared, characterized and tested for propane total oxidation. V-promoted Pt/Al2O3 catalysts have shown remarkably higher catalytic activity than V-free Pt/Al2O3 catalyst. Among V-promoted Pt catalysts that prepared by coimpregnation gave the highest alkane conversions in the whole range of reaction temperatures studied. Factors such as Pt particle size or the oxidation state of platinum do not seem to be the responsible for the enhanced performance. Modification of the redox properties of the catalyst (i.e. high reducibility of vanadium species) likely provoked by the close contact between platinum particles and vanadiu…
Electrochemical characterization of cobalt cordierites attached to paraffin-impregnated graphite electrodes
2004
The electrochemistry of α, β and μ cobalt-containing cordierites (Co2Al4Si5O18) attached to paraffin-impregnated graphite electrodes has been studied by linear scan and cyclic voltammetries in HCl+NaCl and NaOH electrolytes. This electrochemistry is compared with that of vitreous cobalt cordierite, cobalt(II) oxide and cobalt spinel aluminate (CoAl2O4), the two last taken as reference materials. Electrochemical processes involve the site-characteristic reduction of Co(II) species to cobalt metal near to −0.5 V vs. SCE and their oxidative dissolution near +0.3 V, accompanied by solid state interconversion between Co(II) and Co(III) at potentials above +0.45 V. Cordierite-modified electrodes …
Impact of Organoaluminum Compounds on Phenoxyimine Ligands in Coordinative Olefin Polymerization. A Theoretical Study
2013
The reduction of the phenoxyimine moiety in three individual species—namely free ligand, aluminum complex, and titanium complex—with aluminum alkyls and aluminum hydride has been studied by means of DFT. It was demonstrated that the free phenoxyimine ligand in an equimolar mixture with trimethylaluminum does not undergo reduction. Instead, experimentally observed formation of the six-membered cyclic aluminum–phenoxyimine complex, useful in the ring-opening polymerization of lactones, takes place as the kinetically and thermodynamically favored process. However, it is anticipated that a 2-fold excess of the aluminum compound, especially aluminum hydride, acting on the resulting cyclic comple…
Cathodoluminescence and photoluminescence study of trap centers in amorphous silicon oxynitride
2002
Amorphous silicon oxynitride (a-SiO/sub x/N/sub y/) films with different compositions were prepared using low-pressure chemical vapor deposition (LPCVD) technique, The cathodoluminescence and photoluminescence of this samples were measured from the red band to the ultraviolet band to study the trap centers in silicon oxynitride. A 1.8-1.9 eV band was found and is attributed to the oxygen and nitrogen atoms with unpaired electrons whereas the 2.7 eV band is attributed to two-fold coordinated silicon atoms with two electrons. The 5.4 eV shoulder is due to the peroxy radicals and other ultraviolet bands are supposed to due due to the Si-Si bonds.
Anomalous and normal Hall effect in hydrogenated amorphous Si prepared by plasma enhanced chemical vapor deposition
2010
The double sign anomaly of the Hall coefficient has been studied in p -doped and n -doped hydrogenated amorphous silicon grown by plasma enhanced chemical vapor deposition and annealed up to 500 °C. Dark conductivity as a function of temperature has been measured, pointing out a conduction mechanism mostly through the extended states. Anomalous Hall effect has been observed only in the as-deposited n -doped film, disappearing after annealing at 500 °C, while p -doped samples exhibit normal Hall effect. When Hall anomaly is present, a larger optical band gap and a greater Raman peak associated with Si-H bond are measured in comparison with the cases of normal Hall effect. The Hall anomaly wi…
Structural Changes of Titanium Oxide Induced by Chromium Addition as Determined by an X-Ray Diffraction Study
1995
Abstract An X-ray diffraction study has been carried out on a series of polycrystalline TiO 2 (anatase) catalysts containing Cr(III) at different atomic concentrations (0.2-5.0 at. %), prepared by a co-precipitation method. Analysis of the variation of the unit cell parameters of chromium-doped anatase with respect to pure anatase has allowed us to estimate the solubility limit for the system at around 1.4 at. %. There is no evidence for the existence of a separate crystalline chromium oxide phase. Temperature effects on the cell parameter and on the anatase-rutile transformation rate were also analyzed. An increase in temperature produced release of the chromium from the solid solution. At…
Atomic Layer Deposition of Aluminum Oxide on TiO2 and Its Impact on N3 Dye Adsorption from First Principles
2011
The atomic layer deposition of aluminum oxide on an OH-terminated TiO2(101) anatase surface was studied employing density functional theory calculations. The formation of the Al2O3−TiO2 interface during the first atomic layer deposition cycle was modeled by studying the dissociative adsorption of an Al(CH3)3 precursor, followed with a H2O-pulse reaction step that changes the surface termination. Calculations provide evidence for the formation of a discontinuous, atomically rough aluminum oxide layer after the first cycle. To explore the role of the aluminum oxide layer on adsorption of a ruthenium-based N3 dye molecule, various adsorption geometries were investigated. Calculations show that…
Multi-channel receptors based on thiopyrylium functionalised with macrocyclic receptors for the recognition of transition metal cations and anions.
2010
We report herein the synthesis and characterization of a family of ligands containing different cation binding sites covalently connected to a thiopyrylium signalling reporter. The receptors L1–L6 are able to signal the presence of certain metal cations via three different channels; i.e. electrochemically, fluorogenically and chromogenically. An acetonitrile solution of L1–L6 shows a bright blue colour due to a charge-transfer band in the 575–585 nm region. The colour variation in acetonitrile of L1–L6 in the presence of the metal cations Ag+, Cd2+, Cu2+, Fe3+, Hg2+, Ni2+, Pb2+ and Zn2+ has been studied. A selective hypsochromic shift of the blue band was found for the systems L4-Pb2+ and L…