Search results for "visual_art"
showing 10 items of 2987 documents
Equilibrium and kinetic studies on complex formation and decomposition and the movement of Cu2+metal ions within polytopic receptors
2013
Potentiometric studies carried out on the interaction of two tritopic double-scorpiand receptors in which two equivalent 5-(2-aminoethyl)-2,5,8-triaza[9]-(2,6)-pyridinophane moieties are linked with 2,9-dimethylphenanthroline (L1) and 2,6-dimethylpyridine (L2) establish the formation of mono-, bi- and trinuclear Cu(2+) complexes. The values of the stability constants and paramagnetic (1)H NMR studies permit one to infer the most likely coordination modes of the various complexes formed. Kinetic studies on complex formation and decomposition have also been carried out. Complex formation occurs with polyphasic kinetics for both receptors, although a significant difference is found between bot…
Hydrogen-bonded 2-(2-pyridyl)imidazole (pyim) manganese(II) complexes as building blocks for molecular assembling: Syntheses and structural character…
2008
Abstract The preparation and crystal structures of three new manganese(II) complexes with 2-(2-pyridyl)imidazole (pyim) and pseudohalides as ligands, cis-[Mn(pyim)2(N3)2] (1), cis-[Mn(pyim)2(NCO)2] (2) and cis-[Mn(pyim)2(NCS)2] (3), are reported. The structural building block in each structure is a mononuclear, neutral complex unit. The metal ion in each case adopts a distorted octahedral coordination geometry with the two pseudohalide ligands in cis positions. The main differences between the molecular structures of the three compounds are associated with the orientation of the anionic ligands. Hydrogen bonds between imidazole N–H and the metal-coordinated nitrogen atom of the pseudohalide…
Hydrogen bonding and proton transfer involving the trihydride complexes Cp*M(dppe)H 3 (M = Mo, W) and fluorinated alcohols: the competitive role of t…
2003
International audience; The protonation of complexes Cp*M(dppe)H3 (dppe is ethylenebis(diphenylphosphine), M = Mo (1), W (2)) by a variety of fluorinated alcohols of different acid strength (FCH2CH2OH, CF3CH2OH, (CF3)2CHOH, and (CF3)3COH) was investigated experimentally by the variable temperature spectroscopic methods (IR, NMR) and stopped-flow technique (UV-Vis). The structures of the hydrogen-bonded and proton transfer products were studied by DFT calculations. In agreement with the calculation results, the IR data suggest that the initial hydrogen bond is established with a hydride site for complex 1 and with the metal site for complex 2. However, no intermediate dihydrogen complex foun…
Bio-inspired Ni dinuclear complexes as heterogeneous catalysts for hydrogen evolution
2021
Abstract A major challenge in the sustainable production of hydrogen is lowering the electrochemical overpotential and the activation energy of the hydrogen evolution reaction. Some enzymes have two metallic Ni centers and catalyze this reaction with high activity. Taking inspiration from them, we have developed two novel dinuclear Ni(II) complexes, Ni-PATIO and Ni-PACO, with promise as molecular catalysts. Ni-PACO exhibits a square-planar geometry for both Ni(II) and two-fold rotational symmetry both in solid and solution states, whereas Ni-PATIO displays one square-planar Ni(II) center and one octahedral Ni(II) configuration, which confer it magnetic properties. We show that both complexe…
The influence of external uniaxial stretching on the electromigration and diffusion of hydrogen in iron
2009
Electromigration and diffusion of hydrogen in iron, depending on unit elongation, were investigated. The resistivity method was applied in the investigations. It was found that the obtained experimental results cannot be described by means of a single flux of hydrogen, which diffuses in the electric field. In order to explain the obtained results, we assumed the existence of the following two fluxes of hydrogen in the stretched metal: migration, according to the interstitial mechanism, and migration according to the interstitial-defective mechanism. The dependence of effective valences and diffusion coefficients for both mechanisms were determined.
EQCM and EIS studies of Znaq2++2e−⇄Zn0 electrochemical reaction in moderated acid medium
2003
The zinc electrodissolution and electrodeposition have been studied by means of electrochemical quartz crystal microbalance and electrochemical impedance spectroscopy. Relevant information on the mechanism of these electrochemical processes is provided by the analysis of experimental data. The zinc electrodeposition takes place with an important participation of hydrogen ions while the zinc electrodissolution occurs, in these experimental conditions, by a mechanism in two steps. Besides, it is proved that the presence of molecular oxygen in the solution contributes to the formation of a passive layer on the metallic zinc deposit.
Study of the phase composition of iron-containing Langmuir-Blodgett layers before and after thermal and chemical treatment
1992
Iron-containing Langmuir–Blodgett mono- and multilayers were prepared by depositing ferric stearate onto silicon wafers. Particular care should be taken in order to avoid the presence of alkali ions, which may replace iron ions in the deposited film. By a subsequent heating in air, the fatty acid chains were thermodesorbed. This results in a rather homogeneous coating of the samples by an iron oxide, most probably in the form of crystalline and amorphous γ-FeOOH, as found by conversion electron Mossbauer, photoelectron and microprobe studies. By a subsequent heating in hydrogen gas or exposure to nascent hydrogen, a partial reduction of the iron oxide layer is possible up to the metallic st…
Aluminum Nanocrystals as a Plasmonic Photocatalyst for Hydrogen Dissociation
2016
Hydrogen dissociation is a critical step in many hydrogenation reactions central to industrial chemical production and pollutant removal. This step typically utilizes the favorable band structure of precious metal catalysts like platinum and palladium to achieve high efficiency under mild conditions. Here we demonstrate that aluminum nanocrystals (Al NCs), when illuminated, can be used as a photocatalyst for hydrogen dissociation at room temperature and atmospheric pressure, despite the high activation barrier toward hydrogen adsorption and dissociation. We show that hot electron transfer from Al NCs to the antibonding orbitals of hydrogen molecules facilitates their dissociation. Hot elect…
Budgeting soil erosion from floodplain sediments of the central Rhenish Slate Mountains (Westerwald), Germany
2011
The distribution, thickness and composition of the floodplain sediments in the valleys of the Gelbach (Lower Westerwald) and Große Nister (Upper Westerwald) depend on the occurrence of loessic periglacial cover beds in their catchments as well as on historical land use. The budget of floodplain sedimentation was derived from a study of the floodloams underlying the monastery garden of Marienstatt, a village in the Große Nister valley. From the study of two occupation levels, pottery shards and radiocarbon dating, 55% of the floodloam can be attributed to the Modern period (younger than c. AD 1450). From the radiocarbon age of a charcoal fragment extracted from the base of the floodloam at …
Metallic lead recovery from lead-acid battery paste by urea acetate dissolution and cementation on iron
2009
Abstract A suitable hydrometallurgical and environmentally friendly process was studied to replace the currently used practices for recycling lead-acid batteries via smelting. Metallic lead was recovered by cementation from industrial lead sludge solutions of urea acetate (200 to 500 g/L) using different types of metallic iron substrates (nails, shaving or powder) as reducing agents. Under specific operating conditions, up to 99.7% of lead acid battery paste, mainly composed of PbSO4, PbO2 and PbO·PbSO4 species, was converted to metallic lead.. The conversion of the metallic lead and rate of the cementation reaction were strictly dependent on the type of iron substrate used as the reductant…