Showing papers on "Redox published in 1989"

Transition Metal Oxides: Surface Chemistry and Catalysis

Abstract: 1. Introduction. 2. Bulk and Surface Structure of Transition Metal Oxide. 3. Physical and Electronic Properties. 4. Surface Coordinative Unsaturation. 5. Surface Acidity. 6. Reduction of Oxides. 7. Oxygen on Oxides. 8. Preparation of Oxides. 9. Metathesis and Isomerization. 10. Decomposition, Hydrogenation and Related Reactions. 11. Selective Oxidation Reactions I. 12. Selective Oxidation Reactions II. 13. Catalytic Reaction Between Hydrogen and Carbon Oxides. 14. Photo-Assisted Surface Processes. Index.

Reduction potentials involving inorganic free radicals in aqueous solution

Abstract: Publisher Summary This chapter discusses the reduction potentials involving inorganic free radicals in aqueous solution. Because free radicals are usually transients, knowing their thermodynamic properties is primarily useful in mechanistic studies. Thus, the useful redox couples associated with a given free radical correspond to plausible elementary steps in reaction mechanisms. All potentials are expressed against the normal hydrogen electrode (NHE). Apart from the NHE, the standard state for all solutes is the unit molar solution at 25 °C. This violates the usual convention for species, such as O 2 that occur as gases, but because the rates of bimolecular reactions in solution are significant, the unit molar standard state is most convenient. The emphasis is on electron transfer reactions in which no bonds are formed or broken, electron transfer reactions in which concerted electron transfer and bond cleavage could occur, and certain atom transfer reactions. The chapter also presents a tabulation of ∆ f G ° values for all the radicals. A common approach in estimating the thermochemistry of aqueous free radicals is to use gas-phase data with approximations of solvation energies.

Effect of Coprecipitated Metal Ions on the Electrochemistry of Nickel Hydroxide Thin Films: Cyclic Voltammetry in 1M KOH

Abstract: Binary 9:1 composite hydroxides of nickel with 13 other metals were prepared by cathodic electrocoprecipitation from metal nitrate solutions and characterized by cyclic voltammetry in . Under these conditions, coprecipitated Ce, Fe, and La strongly catalyzed the oxygen evolution reaction. Conversely, Cd, Pb, and Zn poisoned this reaction. The hydrogen evolution reaction was less affected by the coprecipitated metals; Ag and Pb had a catalytic effect on this reaction. Co and Mn shifted the nickel hydroxide redox potentials to more cathodic values. In contrast, Cd, Ce, Cr, Fe, La, Y, and Zn each shifted these redox potentials anodically. The coulombic efficiency of the oxidation‐reduction process was substantially lowered by Ce, Fe, and La.

Dehydrogenases for the synthesis of chiral compounds

Abstract: Redox reactions are important steps in the metabolism and energy conversion of living cells. Besides the enzymes necessary, a number of coenzymes are involved in such reactions: ferrodoxins, lipoic acid, NAD(H), NADP(H), flavins and cytochromes. The coenzymes differ in their redox potential, in the binding constants and the mode of regeneration [1]. NADH/NADPH are the most frequently encountered coenzymes. In general they dissociate easily and need a second reaction with another metabolite for regeneration. These properties are one of the means by which nature directs the flow of intermediates in response to biosynthetic needs. Because of the spectral properties of the coenzyme moiety, dehydrogenases have been extensively studied in the past [2] and have found widespread applications in clinical and food analysis [3].

Electrocatalytic reduction of nitrite and nitric oxide to ammonia with iron-substituted polyoxotungstates

Abstract: Heteropolytungstates in which one of the positions normally occupied by a tungsten cation is occupied instead by an iron cation are shown to be catalysts for the electroreduction of nitrite to ammonia. The lacunary derivatives in which the empty tungsten site is unoccupied show no catalytic activity. The catalytic mechanism involves the intermediate formation of a nitrosyl complex of the Fe(II) form of the catalyst. The pH dependence of the rate of formation of the nitrosyl complex shows that nitrous acid is the reactive form of nitrite between pH 2 and 8. The catalyzed reduction does not produce hydroxylamine as an intermediate and appears to depend upon the ability of the multiply reduced heteropolytungstates to deliver electrons to the NO group bound to the iron center in a concerted, multiple-electron step. The iron-substituted heteropolytungstates are not degraded by repeated cycling between their oxidized and reduced states. A particularly valuable feature of the heteropolytungstate is the ease with which the formal potentials of the several redox couples they exhibit may be shifted by changing the identity of the central heteroatom. Exploitation of this feature provides diagnostic information that can be decisive in establishing the mechanism of electrocatalytic processes.

Activation of highly ordered pyrolytic graphite for heterogeneous electron transfer: relationship between electrochemical performance and carbon microstructure

Abstract: The electrochemical and vibrational spectroscopic properties of highly ordered pyrolytic graphite (HOPG) were determined before and after modification by anodization or pulsed laser irradiation. Both treatments greatly accelerated the heterogeneous electron-transfer-rate constants for the Fe(CN)6(3-/4-) and dopamine redox systems on HOPG by approximately six orders of magnitude. At intermediate electrochemical pretreatment (ECP) potentials, a spatially heterogeneous surface resulted, with surface regions exhibiting the 1360/cm band being separated by tens of microns. The results clearly indicate that graphitic edge plane is necessary for fast electron transfer, and that the pretreatment procedures accelerate k0 by generating edge-plane defects in the HOPG lattice. The mechanisms of defect generation for the two procedures appear very different, with ECP appearing to follow a nucleation process leading to a spatially heterogeneous surface, while the laser pulse appears to shatter the HOPG lattice, leading to a more uniform distribution of active sites. The results provide important conclusions about the relationship between carbon electrode microstructure and heterogeneous electron transfer activity. Of particular interest is the heterogeneous electron transfer rate between carbon electrodes and various well-known redox systems such as ascorbic acid, ferri/ferrocyanide, and the catecholamines. Not only are these systems of significant analytical interest, but they serve asmore » benchmarks for comparisons of electrode performance.« less

Electrochemical and Spectroscopic Evidence on the Participation of Quadrivalent Nickel in the Nickel Hydroxide Redox Reaction

Abstract: Electrochemical and iodometric measurements on anodically charged thin film nickel hydroxide electrodes indicated an average nickel valence of . In situ visible spectroscopy of nickel hydroxide deposited on optically transparent electrodes featured one strong and broad absorption band with an intensity proportional to the state of charge. The results are not consistent with the presence of separate trivalent and quadrivalent phases or the participation of peroxide radicals in the nickel hydroxide redox reaction, but are best explained by the formation of a single mixed valence phase such as which has a 3.67 average nickel valence. Full utilization of the proposed oxidized phase could increase the discharge capacity of nickel battery electrodes by 67% beyond the previously assumed one‐electron theoretical capacity. That the discharge is generally incomplete may be due to electronic isolation of portions of the oxidized material by a high resistivity layer of divalent nickel hydroxide produced at the surface during discharge.

Production of fluorocarbons

Abstract: A process for the production of a fluorocarbon by reducing a fluorocarbon which contains at least one atom of chlorine and/a bromine, in which the reduction is effected in the presence of a redox couple. The redox couple, which is oxidized in the process, may be reduced electrolytically for re-use in the process. A preferred redox couple is a chromous-chromic couple.

Pulse radiolytic measurement of redox potentials: the tyrosine and tryptophan radicals.

Abstract: With the technique of pulse radiolysis we have measured the redox midpoint potentials of the tryptophan side chain neutral indolyl radical (105 +/- 001 V vs NHE, pH 70 and 25 degrees C) and the tyrosine side chain neutral phenoxy radical (094 +/- 001 vs NHE, pH 70 and 25 degrees C) These potentials were obtained by using a variety of inorganic reference compounds in both kinetic and equilibrium protocols We compare these results with others already in the literature, and we also present data useful in establishing a pulse radiolysis redox reference scale over the range 042-128 V

Titanium gel made from metallic titanium and hydrogen peroxide

Abstract: Various titanium-peroxo intermediates may be formed in the Ti-H2O2 redox process. In a catalytically decomposed Ti-H2O2 system, an oxidizing transparent yellow-green gel with a final pH between 3 and 4 is typically formed within 48 h without addition of chelating agents. The redox potential of the gel is larger than that of the Fe(II)/Fe(III) system. The total Ti concentration in the studied gel was spectro-photometrically determined to be about 45 mM. The oxidizing component was determined to be about 15 mM with the use of iodometric titration. A model for the catalytic H2O2 decomposition is discussed, and an explanation for the existence of an ESR-active radical component in the gel is suggested. The possible relation of the gel formation to the known biocompatibility of titanium is speculated on.

Direct electron transfer of redox proteins at the bare glassy carbon electrode.

Abstract: A simple system is presented for the microscale, direct voltammetry of redox proteins, typically 25 μg, in the absence of mediators and/or modifiers. The sample consists of a droplet of anaerobic solution laid onto an oversized disc of nitric-cid-pretreated glassy carbon as the working electrode. Very reproducible, Nernstian responses are obtained with horse heart cytochrome c. The midpoint potential Em (pH 7.0) is dependent on the ionic strength, ranging from +293 mV in 1 mM potassium phosphate to +266 mV in 0.1 M phosphate. At fixed buffer and cytochrome concentrations the magnitude of the voltammetric response is found to be independent of pH over six pH units around neutrality. It is suggested that the response of the present system is not complicated by pH-dependent properties of the electrode surface around physiological pH and, therefore, that the use of this system is practical in biochemically oriented studies. Direct, quasi-reversible responses have also been obtained at pH 7.0 (5 mM phosphate) from Desulfovibrio vulgaris. Hildenborough strain, tetraheme cytochrome c3 (pI = 10.0 at 4°C; 3×Em=−0.32 mV, Em=−0.26 V), and cytochrome c553 (pI = 9.3; Em=+60 mV), and from Megasphaera elsdenu rubredoxin (pI ≃ 3; Em=−42 mV) and 2[4F–-4S] ferredoxin (pI ≃ 3; Em=−353 mV). The latter protein adsorbs onto the glassy carbon surface, thus forming a system with possible applications in the electrochemical study of ferredoxin-linked enzymes.

Electrochemical Properties of Organic Disulfide/Thiolate Redox Couples

Abstract: The redox behavior, kinetic reversibility, chemical reversibility, and stability, and the specific adsorption or chemisorption at electrode surfaces of a diverse group of organodisulfide cathode materials have been studied by potential-sweep and potential-step methods. The number of electrons involved in the redox reaction and the diffusion coefficients of the organodisulfide species in electrolyte solutions were determined with a rotating disk electrode in conjunction with chronocoulometry/chronoamperometry. Observations indicate that the overall, stoichiometric reaction of those redox couples is RSSR + {r reversible} where R represents an organic moiety. These reactions are chemically reversible, yet kinetically hindered, especially at ambient temperatures. The microscopic reversibility of the redox couples promises the possibility of constructing secondary energy conversion systems based on these materials. The slow electrode kinetics, however, indicates that the introduction of electrocatalysts to assist the electrode reaction may be effective in improving battery performance. The negligible adsorption of these materials at platinum electrodes, in addition, implies that the electrode kinetics can be formulated by simple electrodic equations without consideration of surface coverage.

An electrochemical method of measuring the oxidation rate of ferrous to ferric iron with oxygen in the presence of Thiobacillus ferrooxidans

Abstract: The oxidation of Fe(2+) with oxygen in sulfate solutions was studied in the presence of T. ferrooxidans. To measure the chemical activity of bacteria, and the oxidation rate of iron, the redox potentials of solutions were continuously monitored during the experiments. The redox potentials were simultaneously monitored on the platinum and pyrite indicator electrodes. The redox potential versus time curves were further used to calculate the basic kinetic parameters, such as the reaction orders, the activation energy, and the frequency factor. It was found that under atmospheric conditions, and at Fe(2+) < 0.001M, T < 25 degrees C, and at pH above 2.2, the oxidation of iron is governed by the following rate expression: -(d[Fe(2+)]) / dt = 1.62 x 10(11)C(bact) [H(+)] [Fe(2+)] p O(2)e(-(58.77/RT)). Below pH = 2.2, the oxidation rate is independent of H(+) Concentration.

Performance Characteristics of Carbon Plastic Electrodes in the All‐Vanadium Redox Cell

Abstract: La stabilite et les rendements energetiques d'une electrode de graphite modifiee par du polyethylene dans des solutions d'acide sulfurique contenant V(II)/V(III) et V(IV)/V(V) sont etudies a differentes temperatures

Effects of oxygen on the metabolism of nitroxide spin labels in cells.

Abstract: The products of the reduction of nitroxides in cells are the corresponding hydroxylamines, which cells can oxidize back to the nitroxides in the presence of oxygen. Both the reduction of nitroxides and the oxidation of hydroxylamines are enzyme-mediated processes. For lipid-soluble nitroxides, the rates of reduction are strongly dependent on the intracellular concentration of oxygen; severely hypoxic cells reduce nitroxides more rapidly than cells supplied with oxygen. In contrast, the rates of oxidation of hydroxylamines increase smoothly with increasing intracellular oxygen concentration up to 150 microM. In order to separate the effects on the rates of metabolism of nitroxides due directly to oxygen from effects due to the redox state of enzymes, we studied the cells under conditions in which each of these variables could be changed independently. Oxygen affects the metabolism of these nitroxides primarily by interacting with cytochrome c oxidase to change the redox state of the enzymes in the respiratory chain. Our results are consistent with the conclusions that in these cells reduction of lipophilic nitroxides occurs at the level of ubiquinone in the respiratory chain in mitochondria, and oxidation of the corresponding hydroxylamines occurs at the level of cytochrome c oxidase.

Multiple reversible electrochemical reduction of aromatic hydrocarbons in liquid alkylamines

Abstract: The thermodynamic reduction potentials of many aromatic hydrocarbons have been known since the work of Hoytink. Their values obtained either by polarographic or by voltammetric techniques have been reported several times in the literature. Recently, the further reduction of these species to their respective dianions or higher charged states has attracted increasing interest. The reason is that their electronic and spectroscopic behavior offers new insights into the unconventional properties of charged /pi/-systems. Moreover, highly charged aromatics are interesting model compounds for the study of thermodynamics of charge storage. Although chemical reduction of such systems is now easily achieved by the reaction with alkali metals, e.g., in dry THF/sup 3/, electrochemical reduction experiments leading to stable higher redox states still demand skill and a sophisticated technique. Therefore, up to now, relatively few reduction potentials for the formation of aromatic diions have been given in the literature. Many are the result of irreversible processes due to protonation reactions induced by impurities in the solvent or by reactivity of the supporting electrolyte. In these cases the desired thermodynamic information cannot be derived from the measured data. Therefore, the authors will report here on an easily accessible technique of measuring thermodynamic redox potentials ofmore » dianions and even higher charged anions of aromatic hydrocarbons using methylamine (MA) and dimethylamine (DMA) as solvents.« less

Ferrocene modified polypyrrole with immobilised glucose oxidase and its application in amperometric glucose microbiosensors.

Abstract: Gold microelectrodes were modified with electropolymerised polypyrrole in both organic and aqueous electrolytes. Cyclic voltammetry studies confirmed that electron conduction through the films in aqueous electrolytes is greatly reduced and is inefficient at mediating the oxidation/reduction of solution redox species. Films formed in aqueous electrolyte were also found to be more heterogenous in structure compared to those formed in acetonitrile. Aqueous polymerised films were used as an immobilisation matrix for glucose oxidase, and modified gold micro electrodes were used in the analysis of glucose using hydrogen peroxide detection. It was found that glucose oxidase was strongly absorbed into the films and that the sensors remained stable for several weeks. An improved linear range and oxygen insensitivity was achieved by incorporating the "water-insoluble" electron transfer mediator, dimethyl ferrocene, into the polypyrrole films by absorption. Good currents and response times to glucose addition were obtained from these electrodes, however, the electrodes possessed poor stability caused by the apparent loss of mediator from the films. Greater operational stability was achieved by covalently functionalising the films with the ferrocene derivative, ferrocenecarbonyl chloride. Covalently bound ferrocence was found to be an efficient oxidant of reduced glucose oxidase. Electrodes constructed in this manner possessed a good linear range, oxygen insensitivity and better stability. Electron transfer mechanisms are discussed.

Stabilization of Metal‐Metal Oxide Surfaces Using Electroactive Polymer Films

Abstract: Charge coupling of the catalyzed reduction of O/sub 2/ on platinized poly(3-methylthiophene) (P(3-MT)) films to the anodic dissolution of Ti is reported. Specifically, the redox polymer with Pt catalyst is used in these studies to mediate electrons generated by Ti dissolution and consumed by O/sub 2/ reduction. Due to the large redox polymer capacity ({ge}55 F/cm/sup 3/) of P(3-MT), these reactions occur at a stable potential approximately equal to the reversible oxidation potential of the polymer. Data presented support the conclusion that O/sub 2/ reduction on the polymer film can replenish polymer charge consumed by metal dissolution, thereby stabilizing the potential of Ti within the passive potential range and minimizing the rate of metal dissolution.