Electroreduction of molecular oxygen on dispersed copper in an ion-exchange matrix

Electrochemical reduction of molecular oxygen was studied on a [dispersed copper]-[macroporous KU-23 15/100S sulfocation exchanger with various metal concentrations] composite electrode. It was found that a high proton concentration in the ion-exchange matrix causes a decrease in the oxygen reaction overvoltage. The nanostructured state of copper particles causes stabilization of the intermediate product, i.e., hydrogen peroxide. Using the rotating disk electrode method, it was detected that the process is limited by external diffusion of oxygen to composite grains. The oxygen reaction is mostly concentrated on the grain surface and surface layers; oxygen is reduced in the bulk due to dispersed copper oxidation.     

Reductive sorption of molecular oxygen from water by nanodisperse metals in an ion-exchanger matrix

It is shown that the reduction of oxygen dissolved in water by metal (Ag, Cu, Bi, and Ni) — KU-23 ion exchanger nanocomposite can be inhibited by either internal diffusion or kinetic mechanisms, depending on the nature of the metal and its particle size.     

Inner-sphere mechanism for molecular oxygen reduction catalyzed by copper amine oxidases

Copper and topaquinone (TPQ) containing amine oxidases utilize O2 for the metabolism of biogenic amines while concomitantly generating H2O2 for use by the cell. The mechanism of O2 reduction has been the subject of long-standing debate due to the obscuring influence of a proton-coupled electron transfer between the tyrosine-derived TPQ and copper, a rapidly established equilibrium precluding assignment of the enzyme in its reactive form. Here, we show that substrate-reduced pea seedling amine oxidase (PSAO) exists predominantly in the Cu(I), TPQ semiquinone state. A new mechanistic proposal for O2 reduction is advanced on the basis of thermodynamic considerations together with kinetic studies (at varying pH, temperature, and viscosity), the identification of steady-state intermediates, and the analysis of competitive oxygen kinetic isotope effects, (18)O KIEs, [kcat/KM((16,16)O2)]/[kcat/KM((16,18)O2)]. The (18)O KIE = 1.0136 +/- 0.0013 at pH 7.2 is independent of temperature from 5 degrees C to 47 degrees C and insignificantly changed to 1.0122 +/- 0.0020 upon raising the pH to 9, thus indicating the absence of kinetic complexity. Using density functional methods, the effect is found to be precisely in the range expected for reversible O2 binding to Cu(I) to afford a superoxide, [Cu(II)(eta(1)-O2)(-I)](+), intermediate. Electron transfer from the TPQ semiquinone follows in the first irreversible step to form a peroxide, Cu(II)(eta(1)-O2)(-II), intermediate driving the reduction of O2. The similar (18)O KIEs reported for copper amine oxidases from other sources raise the possibility that all enzymes react by related inner-sphere mechanisms although additional experiments are needed to test this proposal.     

Cooperative interactions of metal nanoparticles in the ion-exchange matrix with oxygen dissolved in water

The kinetics of the reduction of molecular oxygen dissolved in water with nanocomposites consisting of an ion-exchange matrix and copper nanoparticles deposited in it in various amounts was studied. As the metal content in the polymer increased, the amount of reduced oxygen initially increased and then reached the limiting value. At a certain metal content, ionization of individual particles with formation of metal counterions changes to the oxidation of particles assembly giving layers of oxide products. The mechanism changes at the percolation threshold of the electron conductivity of the nanocomposite and determines the maximum amount of absorbed oxygen.     

Kinetics of oxygen reduction by nanocomposite silver-ion exchanger

It was experimentally determined that the rate of reduction of molecular oxygen from water by the nanocomposite silver-ion exchanger KU-23 is substantially lower than by copper-containing composite and takes place with substantial kinetic inhibition. The kinetic parameters of the process were determined by solving the reverse kinetic problem; it was found that this was due to a reduction in the rate constant of the reaction of silver with oxygen at virtually constant internal diffusion coefficient of molecular oxygen. The transition from kinetic to diffusion control was determined to depend on the silver particle size.     

Stepwise oxidation of 1,2-diols resulting from molecular oxygen activation by copper

The oxidation of 1,2-diols through copper promoted activation of molecular oxygen was studied. The influence of the substituents and experimental conditions is discussed, and examples of catalytic applications are reported.     

The reduction of molecular oxygen by iron porphyrins

Molecular assemblies have been synthesised to reproduce the structure of the cytochrome c oxidase (CcO) active site and to explore the roles played by its different features. It was discovered that a single iron porphyrin, adsorbed at the surface of a graphite electrode, is a selective catalyst for the four-electron reduction of dioxygen to water, at pH 7. To cite this article: D. Ricard et al., C. R. Chimie 5 (2002) 33–36     

Kinetics of the reduction of water by activated aluminum powder

Experimental investigations of the kinetics of reduction of water when it is reacted with activated aluminum suggest that the mechanisms for the processes at low and high temperatures differ in the activation energy and the pre-exponential factor. This is explained on the basis of the theory we developed previously for electron transfers from the solid phase to antibonding orbitals of the H₂O via tunneling and activation mechanisms.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 31, No. 4, pp. 238–242, July–August, 1995.     

Kinetics of reduction of copper(II) by sodium tetrahydroborate

Summary The kinetics of reduction of copper(II) ion by sodium tetrahydroborate in buffered aqueous solution have been investigated. The rate of the reaction is first order in the concentrations of each of the reactants. The activation parameters were evaluated and a plausible mechanism for the reduction of copper(II) ion is proposed.     

Influence of carbon dioxide in the determination of oxygen in copper by reduction fusion

It has been reported that copper melted in a graphite crucible at high temperature will give off its oxygen content mainly as CO and partially as CO(2). Thus if oxygen in copper is determined by means of apparatus designed to measure only CO as the reaction product, the results are obviously liable to error. Methods of suppressing formation of CO(2) during the fusion process are proposed. When the oxygen is determined by gas chromatography, formation of CO(2) can be suppressed by adding a 0.5% Si-1.5% NiCu bath-alloy together with the copper sample or by inserting a spectrographically pure carbon rod into the graphite crucible used for the fusion. When the oxygen is determined by coulometry, formation of CO(2) can be suppressed by the addition of the SiNiCu bath-alloy or by appropriate modification of the graphite crucible to obtain an optimum working temperature. The results obtained by either method are in agreement with those obtained by a modified vacuum fusion method in which CO and CO(2) can both be measured. These methods have been validated by analysis of two standard reference materials.     

Atomic absorption spectrometric determination of trace copper in water by sorption on an ion-exchange resin and direct atomization of the resin

Traces of copper ions are sorbed on a cation-exchange resin, and a copper-loaded resin bead is inserted directly into a graphite furnace for atomic absorption spectrometry. The procedure is used for the determination of copper ( > 2 pg ml^?1) in tap and various distilled waters.     

Kinetics of the medium-temperature reduction of copper chromite with hydrogen

The static volumetric method was used to study the kinetics of copper chromite CuCr₂O₄ reduction with hydrogen at 50-80 kPa and 498, 523 and 580 K. The rate of copper chromite reduction is maximal initially and decreases monotonically with time. This observation suggests that the reduction is not a topochemical process. The apparent activation energy of the reaction equals initially 107 kJ/mole. The results obtained argue for a particular mechanism of the copper chromite reduction via redox substitution of hydrogen for copper in the chromite and, therefore, agree with earlier data obtained by various structural and adsorption methods. Specific features of the reduction mechanism are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Green synthesis of 1,2,4-thiadizoles from thioamides in water using molecular oxygen as an oxidant

We present here an efficient green process for the synthesis of 1,2,4-thiadiazoles via iodine-catalyzed,oxidative dimerization of thioamides in water using molecular oxygen as a terminal oxidant. Under the optimized reaction conditions, aryl thioamides produced 3,5-diaryl-1,2,4-thiadiazoles in good to excellent yields. Alkyl thioamides and substituted thioureas could also provide corresponding 1,2,4-thiadiazole products.     

Kinetics of azobenzene nitrene oxidation by molecular oxygen in glassy propylene carbonate

The kinetics of azobenzene nitrene oxidation by molecular oxygen dissolved in a matrix was studied in glassy propylene carbonate. The reaction was carried out in excess oxygen within its concentration range 0.008 to 0.048 M. The oxidation kinetics, controlled by oxygen diffusion, is not described by the exponential function. A specific reaction rate decreases in the course of the process. The higher the oxygen concentration in the matrix, the sharper is the decrease in the reaction rate. It is shown that at all concentrations, the oxidation kinetics is described in the framework of the model of heterogeneous matrix with a characteristic size of heterogeneities of 1.5 nm and a dispersion of the activation energy of oxygen molecule jump of 4 kJ/mol. The reaction radius is about 0.5 nm.     

Kinetics of hydroperoxide reduction by copper(I) O,O-dialkyldithiophosphates

The kinetics of hydroperoxide ROOH (R=H, t–Bu, Ph(CH₃)₂C) reduction by copper(I) O,O-dialkyldithiophosphates has been studied using the UV-spectrophotometric method. Rate constants for the redox reactions have been determined and some suggestions concerning their mechanisms have been made.

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An ion-exchange procedure for quantifying biologically active copper in sea water

An ion-exchange column method is described for determining the dissolved inorganic copper concentration and copper(II) ion activity in sea water. The method is based on the assumption that the amount of copper sorbed by a sulfonic-acid cation exchanger is related to the activities of the cationic forms of copper in solution. This assumption was tested in artificial sea water containing copper and the organic ligands EDTA, NTA, histidine and glutamic acid. A correlation was also observed between the inorganic copper fraction determined by the ion-exchange procedure and the toxic fraction of copper quantified by a diatom bioassay (Thalassiosira pseudonana). These results demonstrate the potential of such a technique for determining the concentration of biologically active trace metals in marine and estuarine environments.