Showing papers on "Redox published in 1996"
TL;DR: 1. Advantages and disadvantages of Chemical Redox Agents, 2. Reversible vs Irreversible ET Reagents, 3. Categorization of Reagent Strength.
Abstract: 1. Advantages of Chemical Redox Agents 878 2. Disadvantages of Chemical Redox Agents 879 C. Potentials in Nonaqueous Solvents 879 D. Reversible vs Irreversible ET Reagents 879 E. Categorization of Reagent Strength 881 II. Oxidants 881 A. Inorganic 881 1. Metal and Metal Complex Oxidants 881 2. Main Group Oxidants 887 B. Organic 891 1. Radical Cations 891 2. Carbocations 893 3. Cyanocarbons and Related Electron-Rich Compounds 894
3,432 citations
TL;DR: The terminal heme/copper oxidases in respiratory electron-transfer chains illustrate a number of the thermodynamic and structural principles that have driven the development of respiration.
Abstract: Spatially well-organized electron-transfer reactions in a series of membrane-bound redox proteins form the basis for energy conservation in both photosynthesis and respiration. The membrane-bound nature of the electron-transfer processes is critical, as the free energy made available in exergonic redox chemistry is used to generate transmembrane proton concentration and electrostatic potential gradients. These gradients are subsequently used to drive ATP formation, which provides the immediate energy source for constructive cellular processes. The terminal heme/copper oxidases in respiratory electron-transfer chains illustrate a number of the thermodynamic and structural principles that have driven the development of respiration. This class of enzyme reduces dioxygen to water, thus clearing the respiratory system of low-energy electrons so that sustained electron transfer and free-energy transduction can occur. By using dioxygen as the oxidizing substrate, free-energy production per electron through the chain is substantial, owing to the high reduction potential of O{sub 2} (0.815 V at pH 7). 122 refs.
1,057 citations
TL;DR: Mn is well suited for the redox role with accessible oxidation states of II, III, and IV, and possibly V: oxidation states that have all been proposed to explain the mechanisms of the Mn redox enzymes.
Abstract: The essential involvement of manganese in photosynthetic water oxidation was implicit in the observation by Pirson in 1937 that plants and algae deprived of Mn in their growth medium lost the ability to evolve O{sub 2}. Addition of this essential element to the growth medium resulted in the restoration of water oxidation within 30 min. There is increased interest in the study of Mn in biological chemistry and dioxygen metabolism in the last two decades with the discovery of several Mn redox enzymes. The list of enzymes where Mn is required for redox activity includes a Mn superoxide dismutase, a binuclear Mn-containing catalase, a binuclear Mn-containing ribonucleotide reductase, a proposed binuclear Mn site in thiosulfate oxidase, a Mn peroxidase that is capable of oxidative degradation of lignin, and perhaps the most complex and important, the tetranuclear Mn-containing oxygen-evolving complex in photosystem II (Mn-OEC). Mn is well suited for the redox role with accessible oxidation states of II, III, and IV, and possibly V: oxidation states that have all been proposed to explain the mechanisms of the Mn redox enzymes.
974 citations
TL;DR: In this article, the reduction/oxidation behavior of high surface area CeO2 and Ce0.5Zr 0.5O2 mixed oxide is compared, and it is shown that the introduction of ZrO2 into the Ce2 framework with formation of a solid solution strongly modifies the reduction behaviour in comparison to that seen with CeO 2 alone, and that the expansion/contraction of the lattice parameter upon, respectively, reduction andoxidation, detected by X-ray diffraction, is responsible for the textural modification.
687 citations
TL;DR: In this article, various surface modification procedures were used on glassy carbon (GC) electrodes to yield surfaces with low oxide content or which lack specific oxide functional groups, and monolayers of several different adsorbates were formed on GC surfaces before electrochemical evaluation.
Abstract: Various well-established and novel surface modification procedures were used on glassy carbon (GC) electrodes to yield surfaces with low oxide content or which lack specific oxide functional groups. In addition, monolayers of several different adsorbates were formed on GC surfaces before electrochemical evaluation. Both the nonspecific monolayer adsorbates and reagents which chemisorb to specific functional groups were observed on the surface with Raman and photoelectron spectroscopy. The various GC surfaces were then evaluated for their electron transfer reactivity with nine redox systems in aqueous electrolyte, including Ru(NH3)62+/3+, Fe(CN6)3-/4-, ascorbic acid, and Feaq3+/2+. The nine systems were categorized according to their kinetic sensitivity to surface modification. Several, including Ru(NH3)62+/3+, are insensitive to surface modifications and are considered outer sphere. Feaq3+/2+, Vaq2+/3+, and Euaq2+/3+ are catalyzed by surface carbonyl groups and are very sensitive to the removal of surface...
669 citations
TL;DR: In this paper, a predictive model was developed for uniform carbon dioxide (CO2) corrosion, based on modeling of individual electrochemical reactions in a water-CO2 system, taking into account the reactions of hydrogen ion (H+) reduction, carbonic acid (H2CO3) reduction, direct water reduction, oxygen reduction, and anodic dissolution of iron.
Abstract: A predictive model was developed for uniform carbon dioxide (CO2) corrosion, based on modeling of individual electrochemical reactions in a water-CO2 system. The model takes into account the electrochemical reactions of hydrogen ion (H+) reduction, carbonic acid (H2CO3) reduction, direct water reduction, oxygen reduction, and anodic dissolution of iron. The required electrochemical parameters (e.g., exchange current densities and Tafel slopes) for different reactions were determined from experiments conducted in glass cells. The corrosion process was monitored using polarization resistance, potentiodynamic sweep, electrochemical impedance, and weight-loss measurements. The model was calibrated for two mild steels over a range of parameters: temperature (t) = 20°C to 80°C, pH = 3 to 6, partial pressure of CO2 (PCO2) = 0 bar to 1 bar (0 kPa to 100 kPa), and ω = 0 rpm to 5,000 rpm (vp = 0 m/s to 2.5 m/s). The model was applicable for uniform corrosion with no protective films present. Performance of...
468 citations
TL;DR: In this article, the authors used the partial equilibrium approach to explain the simultaneous occurrence of Fe(III) and sulfate reduction, observed in several field studies, and showed that, depending on the stability of the iron oxides, simultaneous reduction of Fe (III), sulfate, and methanogenesis is thermodynamically possible under a wide range of sedimentary conditions.
449 citations
TL;DR: In this paper, the effects of redox potential and pH values on metal solubility were investigated, and it was found that metal concentrations were slightly higher than those aerated with oxygen.
Abstract: To assess the mobilities of Pb, Cd, and Zn from a contaminated soil, the effects of redox potential and pH value on metal solubilities were investigated. Both redox potential and pH were found to greatly affect heavy metal solubility in the soil. Results showed that the soil suspension under continuous oxygen aeration for 21 days resulted in increases of redox potential from 290 to 440 mV and pH value from 6.9 to 7.0, respectively. Soluble concentrations of Pb, Cd, and Zn varied with time, and were all lower than 1 mg kg−1. When the soil suspension was aerated with nitrogen, final redox potential was −140 mV and pH value of 7.1. The soluble metal concentrations were slightly higher than those aerated with oxygen. The equilibrium solubility experiments were conducted under three different pH values (3.3, 5.0, 8.0) and three redox potential (325, 0, −100 mV). Results showed that metals were sparingly soluble under alkaline conditions (pH = 8.0). Metal solubilities were higher when under slightly acidic conditions (pH = 5.0), and increased drastically when pH was kept at 3.3. When solubilities were compared under same pH values, it was observed that metal solubilities increased as redox potential decreased. Generally speaking, acidic and reducing conditions were most favorable for metal solubilization, and the effect of pH was more significant than that of redox potential. It was proposed that heavy metals were mostly adsorbed onto Fe-Mn oxyhydroxides. The pH-dependent metal adsorption reaction and the dissolution of Fe-Mn oxyhydroxides under reducing conditions was the mechanism controlling the release of heavy metals from soils.
445 citations
TL;DR: In this paper, the authors focused on the microporous crystalline titanium silicates and their catalytic properties in H2O2 oxidation reactions and attributed the unusual properties of titanium silicalites have been attributed to the presence of TiIV in framework positions of the SiO2 lattice.
Abstract: Publisher Summary This chapter focuses on the microporous crystalline titanium silicates. Titanium containing materials have been investigated for various reactions, but selective oxidations with H2O2 as the oxidant have attracted the most interest. For these reactions, the formation of surface titanium peroxo compounds with H2O2 and the subsequent transfer of the peroxidic oxygen to the organic reactants have been proposed to explain the mechanism by which titanium participates in the catalytic cycle. H2O2 has the advantage of giving environmentally benign water as its by-product. Furthermore, H2O2 ranks first in terms of available oxygen in the list of oxygen donors.The unusual properties of titanium silicalites have been attributed to the presence of TiIV in framework positions of the SiO2 lattice. It is important to realize that there is a limit to the extent of substitution; the exact value is still under discussion, but is certainly not more than a few percent. Very likely, the structure of crystalline silica is not stable at higher degrees of substitution. The discovery of the new titanium silicates and of their catalytic properties in H2O2 oxidation reactions has had a major impact in catalytic science and its industrial applications.
388 citations
TL;DR: In the absence of iron, equimolar fluxes of NO and O interact to yield potent oxidants such as ONOO/ONOOH, which oxidize organic compounds.
358 citations
TL;DR: In this article, the role of Fenton's reaction [oxidation of Fe(II) by hydrogen peroxide] in the catalytic cycle was examined, and a number of competing processes were observed in model systems.
Abstract: Iron redox cycling can catalyze the oxidation of humic substances and increase the rate of oxygen consumption in surface waters rich in iron and organic carbon. This study examines the role of Fenton`s reaction [oxidation of Fe(II) by hydrogen peroxide] in this catalytic cycle. A number of competing processes were observed in model systems containing dissolved Fe, hydrogen peroxide, and Suwannee River fulvic acid. First, the effective rate constant of Fenton`s reaction increased with increasing fulvic acid concentration, indicating the formation Fe(II)-fulvate complexes that react more rapidly with hydrogen peroxide than Fe(II)-aquo complexes. This effect was significant at pH 5 but negligible at pH 3. A second effect was scavenging of the HO{sup .} radical produced in Fenton`s reaction by fulvic acid, forming an organic radical. The organic radical reduced oxygen to HO{sub 2}{sup .}/O{sub 2}{sup .-}, which then regenerated hydrogen peroxide by reaction with Fe(II). Finally, Fe(III) was reduced by a dark reaction with fulvic acid, characterized by an initially fast reduction followed by slower processes. The behavior of Fe(II) and hydrogen peroxide over time in the presence of fulvic acid and oxygen could be described by a kinetic model taking all of these reactions into account. The netmore » result was an iron redox cycle in which hydrogen peroxide as well as oxygen were consumed (even though direct oxidation of Fe(II) by oxygen was not significant), and the oxidation of fulvic acid was accelerated. 56 refs., 7 figs., 1 tab.« less
TL;DR: In this paper, the authors analyzed the literature data reported on the partial oxidation of propane to organic compounds (acrolein, acrylic acid and acrylonitrile) over mixed metal oxides, mainly magnesium vanadates, vanadia bismuth molybdates and vanadia antimony.
Abstract: The present review analyses the literature data reported on the partial oxidation of propane to organic compounds (acrolein, acrylic acid and acrylonitrile) over mixed metal oxides, mainly magnesium vanadates, vanadia bismuth molybdates and vanadia antimony. The data were compared to those reported on the partial oxidation of propylene over bismuth molybdate and antimony—tin multicomponent oxides and over cuprous simple oxide. For both reactions, we analyzed the involved reaction mechanisms, intermediate species, active phases and active sites. The role of water produced during the reaction and that of the Bronsted acid sites were shown to be important in the determination of the selectivity of the expected products. The main conclusion of our study is that a good mix of acid-base and redox properties of the oxide surface should permit a controlled orientation of the reaction towards selective products.
TL;DR: In this paper, the authors synthesized and electrochemically characterized the framework compounds M2(SO4)3 with M = (Ti Fe), (V Fe), Fe, Fe and LixM2(PO4) 3 with Lix
TL;DR: Peroxynitrite ion (ONO2-) reacted rapidly with CO2 to form a short-lived intermediate provisionally identified as the ONO2CO2- adduct, which produced 3-nitrotyrosine and 3,3'-dityrosine as the major oxidation products in tyrosine oxidation.
Abstract: Peroxynitrite ion (ONO2-) reacted rapidly with CO2 to form a short-lived intermediate provisionally identified as the ONO2CO2- adduct. This adduct was more reactive in tyrosine oxidation than ONO2- itself and produced 3-nitrotyrosine and 3,3'-dityrosine as the major oxidation products. With tyrosine in excess, the rate of 3-nitrotyrosine formation was independent of the tyrosine concentration and was determined by the rate of formation of the ONO2CO2- adduct. The overall yield of oxidation products was also independent of the concentration of tyrosine and medium acidity; approximately 19% of the added ONO2- was converted to products under all reaction conditions. However, the 3-nitrotyrosine/3,3'-dityrosine product ratio depended upon the pH, tyrosine concentration, and absolute reaction rate. These data are in quantitative agreement with a reaction mechanism in which the one-electron oxidation of tyrosine by ONO2CO2- generates tyrosyl and NO2 radicals as intermediary species, but are inconsistent with mechanisms that invoke direct electrophilic attack on the tyrosine aromatic ring by the adduct. Based upon its reactivity characteristics, ONO2CO2- has a lifetime shorter than 3 ms and a redox potential in excess of 1 V, and oxidizes tyrosine with a bimolecular rate constant greater than 2 x 10(5) M-1 s-1. In comparison, in CO2-free solutions, oxidation of tyrosine by peroxynitrite was much slower and gave significantly lower yields (approximately 8%) of the same products. When tyrosine was the limiting reactant, 3,5-dinitrotyrosine was found among the reaction products of the CO2-catalyzed reaction, but this compound was not detected in the uncatalyzed reaction.
TL;DR: The use of IR spectroscopic techniques to provide information on the mechanisms of catalytic oxidation over metal oxide catalysts is briefly discussed in this paper, where generalized mechanisms of these complex oxidation reactions are proposed and the catalyst features affecting selectivities in these reactions are discussed.
TL;DR: Experimental studies demonstrate that structural Fe(II) in magnetite and ilmenite heterogeneously reduce aqueous ferric, cupric, vanadate, and chromate ions at the oxide surfaces over a pH range of 1-7 at 25°C.
TL;DR: In this paper, the performance data of a symmetric supercapacitor based on p-doped poly(pyrrole), an unsymmetric supercameras based on poly(3-methylthiophene) and poly(dithieno[3,4-b : 3',4'-d]thiophenes) are compared.
TL;DR: In this article, a series of supported vanadia catalysts were investigated for selective catalytic reduction of NO with NH3 to obtain additional insight into this important industrial reaction, and the influence of surface vanadia coverage, promoters (surface tungsten oxide, niobium oxide, and sulfate species), and specific oxide support (TiO2, A l 2 O 3, and SiO2) was examined.
TL;DR: This mixed-valence trinuclear copper cluster, which contains copper in the highly oxidized trivalent oxidation state, exhibits O2 bond scission and intriguing structural, spectroscopic, and redox properties.
Abstract: The reaction of metal complexes with dioxygen (O2) generally proceeds in 1:1, 2:1, or 4:1 (metal:O2) stoichiometry. A discrete, structurally characterized 3:1 product is presented. This mixed-valence trinuclear copper cluster, which contains copper in the highly oxidized trivalent oxidation state, exhibits O2 bond scission and intriguing structural, spectroscopic, and redox properties. The relevance of this synthetic complex to the reduction of O2 at the trinuclear active sites of multicopper oxidases is discussed.
TL;DR: In this article, the redox chemistry of Cu in zeolite ZSM-5 has been studied using FTIR, TPR, EPR, and EXAFS.
Abstract: Motivated by the unique catalytic activity of Cu/ZSM-5 in the decomposition of NO to N2 + O2, the redox chemistry of Cu in zeolite ZSM-5 has been studied using FTIR, TPR, EPR, and EXAFS. Isolated ions, Cu2+, oxocations, [Cu−O−Cu]2+, and oxide particles have been identified. Their relative abundances depend on the overall Cu loading, the pH during ion exchange, and the gas atmosphere. Oxocations are only detected at Cu exchange levels exceeding 40%; their concentration is higher in high-pH preparations favoring hydrolysis. Oxocations are reduced by CO and NO at room temperature; they act as catalytic sites for the disproportionation of NO into N2O + NO2. CuO particles are detected in all samples; at elevated temperature they decompose in He to Cu2O. In FTIR Cu+ is detected using CO or NO as a probe. Flowing H2 reduces Cu2+ ions; the first detectable product is Cu+ because Cu0 is thermodynamically unstable in the presence of Cu2+. After all Cu2+ is used up Cu0 is detected. In CO only oxide particles and oxo...
TL;DR: In this paper, an intrinsic rate expression for nitrate disappearance was proposed based on the conventional Langmuir-Hinshelwood kinetic approach, considering both equilibrium nitrate as well as dissociative hydrogen adsorption processes to different types of active sites, and assuming an irreversible bimolecular surface reaction between adsorbed reactant species to be the rate-controlling step.
Abstract: Liquid-phase reduction using a solid Pd/Cu bimetallic catalyst provides a potential technique for the removal of nitrates from waters. Kinetic measurements were performed for a wide range of reactant concentrations and reaction conditions in an isothermal semi-batch slurry reactor operating at atmospheric pressure. The effects of catalyst loading and initial nitrate concentration on the reaction rate were also investigated. The proposed intrinsic rate expression for nitrate disappearance is based on the conventional Langmuir-Hinshelwood kinetic approach, considering both equilibrium nitrate as well as dissociative hydrogen adsorption processes to different types of active sites, and assuming an irreversible bimolecular surface reaction between adsorbed reactant species to be the rate-controlling step. The apparent activation energy for catalytic liquid-phase nitrate reduction and the heat of nitrate adsorption, in the temperature range 280.5–293 K, were found to be 47 and 22 kJ/mol, respectively. It is confirmed that the process of catalytic liquid-phase hydrogenation of aqueous nitrate solutions undergoes a redox mechanism.
TL;DR: In this paper, a kinetic model based on a previously proposed reaction scheme was used to describe reaction kinetics measurements for the selective catalytic reduction of nitric oxide by ammonia over a 6 wt% vanadia/titania catalyst.
TL;DR: Advances include the ligand's role in the O–O bond cleavage reaction, stabilization of the Fe(IV)=O intermediate and control of the heme iron redox potential.
Abstract: Several new heme enzyme crystal structures have revealed similarities and differences in the local environment of the proximal heme ligand. This information, together with protein engineering studies, has provided important advances in understanding the role of the proximal ligand in heme enzyme catalysis. These advances include the ligand's role in the O–O bond cleavage reaction, stabilization of the Fe(IV)=O intermediate and control of the heme iron redox potential.
TL;DR: In this article, two ceria samples with differing specific surface areas and porosities have been investigated by IR spectroscopy, and one-, two-and three-coordinated cerium oxyhydroxide impurities within the ceria pores were observed.
Abstract: Two ceria samples with differing specific surface areas and porosities have been investigated by IR spectroscopy. A ν(OH) band at ca. 3510 cm–1 was found to be due to cerium oxyhydroxide impurities within the ceria pores. One-, two- and three-coordinated OH species were observed. Doubly bridging OH species in the vicinity of surface oxygen vacancies were found to be very reactive towards D2 dissociation at 423 K, when the reduction is surface limited. At 673 K, doubly bridging OH species and hydrogen-bonded ones were formed when the already reduced ceria surface was exposed to H2. These OH species were not stable upon evacuation and are proposed to be responsible for the so-called reversible H2-reduction of ceria.
TL;DR: In this article, the treatment of a diesel-contaminated soil with hydrogen peroxide catalyzed by six iron compounds and by naturally-occurring minerals in which the pH is stabilized by phosphate was investigated using central composite rotatable experimental designs.
TL;DR: The mechanism of the proton-coupled electron transfer reaction, + H+ → QA(QBH)- was studied in reaction centers (RCs) from the photosynthetic bacterium Rb. sphaeroides by substituting quinones with different redox potentials into the QA site as discussed by the authors.
Abstract: The mechanism of the proton-coupled electron transfer reaction, + H+ → QA(QBH)- was studied in reaction centers (RCs) from the photosynthetic bacterium Rb. sphaeroides by substituting quinones with different redox potentials into the QA site. These substitutions change the driving force for electron transfer without affecting proton transfer rates or proton binding equilibria around the QB site. The measured rate constants, increased with increasing electron driving force (by a factor of 10 per 160 meV change in redox free energy). The proton-coupled electron transfer was modeled by (i) four possible two-step mechanisms in which electron transfer can precede or follow proton transfer and can be either the rate determining or fast step in the overall reaction and (ii) a one-step mechanism involving the concerted transfer of an electron and a proton. The free energy dependencies of these possible mechanisms were predicted using Marcus theory and were compared to the observed dependence. The two stepwise mec...
Patent•
03 May 1996
TL;DR: In this paper, a method for preparing a high energy density (HED) electrolyte solution for use in an all-vanadium redox cells was presented, and a process for the production of electricity from a charged redox battery, comprising the HED electrolyte, a redox cell, and in particular a charged allvanadium battery.
Abstract: Disclosed is a method for preparing a high energy density (HED) electrolyte solution for use in an all-vanadium redox cells, a high energy density electrolyte solution, in particular an all-vanadium high energy density electrolyte solution, a redox cell, in particular an all-vanadium redox cell, comprising the high energy density electrolyte solution, a redox battery, in particular an all-vanadium redox battery, comprising the HED electrolyte solution, a process for recharging a discharged or partially discharged redox battery, in particular an all-vanadium redox battery, comprising the HED electrolyte solution, a process for the production of electricity from a charged redox battery, and in particular a charged all-vanadium redox battery, comprising the HED electrolyte, a redox battery/fuel cell and a process for the production of electricity from a redox battery/fuel cell. A method for stabilising an electrolyte solution for use in a redox cell, in particular for stabilising an electrolyte solution for use in an all-vanadium redox cell, a stabilised electrolyte solution, in particular an all-vanadium stabilised electrolyte solution, a redox cell, in particular an all-vanadium redox cell, comprising the stabilised electrolyte solution, a redox battery, in particular an all-vanadium redox battery, comprising the stabilised electrolyte solution, a process for recharging a discharged or partially discharged redox battery, in particular an all-vanadium redox battery, comprising the stabilised electrolyte solution, and a process for the production of electricity from a charged redox battery, and in particular a charged all-vanadium redox battery, comprising the stabilised electrolyte solution are disclosed. Also disclosed are a redox battery/fuel cell and a process for the production of electricity from a redox battery/fuel cell.
TL;DR: In this paper, the mixed oxides LaNiO3, La0.1Sr0.9, La2NiO4 and LaSrNiO 4 were used as catalysts for the direct decomposition of NO.
Abstract: The mixed oxides LaNiO3, La0.1Sr0.9NiO3, La2NiO4 and LaSrNiO4 were prepared and used as catalysts for the direct decomposition of NO. The catalysts were characterized by means of XRD, XPS, O-2-TPD, NO-TPD and chemical analysis. By comparing the physico-chemical properties and catalytic activity for NO decomposition, a conclusion could be drawn as follows. The direct decomposition of NO over perovskite and related mixed oxide catalysts follows a redox mechanism. The lower valent metal ions Ni2+ and disordered oxygen vacancies seem to be the active sites in the redox process. The oxygen vacancy plays an important role favorable for the adsorption and activation of NO molecules on one hand and on the other hand for increasing the mobility of lattice oxygen which is beneficial to the reproduction of active sites. The presence of oxygen vacancies is one of the indispensable factors to give the mixed oxides a steady activity for NO decomposition.
TL;DR: In this article, the water-gas shift reaction (WGSR) was studied on Auα-Fe2O3 catalyst and the results obtained can be explained on the basis of the associative mechanism of the WGSR.
Abstract: The water-gas shift reaction (WGSR) has been studied on Auα-Fe2O3 catalyst. The structure of the samples has been investigated by chemical and physical methods—TEM, X-ray, DTA, FTIR. A high dispersion degree of the gold particles and an increased concentration of the hydroxyl groups on Auα-Fe2O3 has been established in comparison to the pure α-Fe2O3. The results obtained can be explained on the basis of the associative mechanism of the WGSR. The essential aspects are the dissociative adsorption of water on ultrafine gold particles, followed by spillover of active hydroxyl groups onto adjacent sites of the ferric oxide. The formation and decomposition of intermediate species is accompanied by redox transfer Fe3+ Fe2+ in Fe3O4.