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Showing papers on "Redox published in 2007"


Journal ArticleDOI
TL;DR: Cyclic voltammetry studies are reported for two representative quinones, benzoquinone and 2-anthraquinonesulfonate, in buffered and unbuffered aqueous solution at different pH's to unify quinone electrochemistry by bridging the apparent gap between the redox chemistry of quinONES in water and that in aprotic organic solvents.
Abstract: Cyclic voltammetry studies are reported for two representative quinones, benzoquinone and 2-anthraquinonesulfonate, in buffered and unbuffered aqueous solution at different pH's. While the redox reaction of quinones in buffered water is well described as an overall 2 e-, 2 H+ reduction to make the hydroquinone, a much better description of the overall reaction in unbuffered water is as a 2 e- reduction to make the strongly hydrogen-bonded quinone dianion, which will exist in water as an equilibrium mixture of protonation states. This description helps to unify quinone electrochemistry by bridging the apparent gap between the redox chemistry of quinones in water and that in aprotic organic solvents, where quinones undergo two sequential 1 e- reductions to form the quinone dianion.

485 citations


Journal ArticleDOI
16 Mar 2007-Science
TL;DR: This study studies the selectivity of a functional model of cytochrome c oxidase's active site that mimics the coordination environment and relative locations of Fea3, CuB, and Tyr244 to support the hypothesis that, during steady-state turnover, the primary role of these redox centers is to rapidly provide all the electrons needed to reduce oxygen by four electrons.
Abstract: We studied the selectivity of a functional model of cytochrome c oxidase's active site that mimics the coordination environment and relative locations of Fea3, CuB, and Tyr244. To control electron flux, we covalently attached this model and analogs lacking copper and phenol onto self-assembled monolayer–coated gold electrodes. When the electron transfer rate was made rate limiting, both copper and phenol were required to enhance selective reduction of oxygen to water. This finding supports the hypothesis that, during steady-state turnover, the primary role of these redox centers is to rapidly provide all the electrons needed to reduce oxygen by four electrons, thus preventing the release of toxic partially reduced oxygen species.

445 citations


Journal ArticleDOI
TL;DR: The results with roGFP2 as an artificial GRX target further suggest that redox-triggered changes of biologic processes might be linked directly to the glutathione redox potential via GRX as the mediator.
Abstract: The cellular glutathione redox buffer is assumed to be part of signal transduction pathways transmitting environmental signals during biotic and abiotic stress, and thus is essential for regulation of metabolism and development. Ratiometric redox-sensitive GFP (roGFP) expressed in Arabidopsis thaliana reversibly responds to redox changes induced by incubation with H(2)O(2) or DTT. Kinetic analysis of these redox changes, combined with detailed characterization of roGFP2 in vitro, shows that roGFP2 expressed in the cytosol senses the redox potential of the cellular glutathione buffer via glutaredoxin (GRX) as a mediator of reversible electron flow between glutathione and roGFP2. The sensitivity of roGFP2 toward the glutathione redox potential was tested in vivo through manipulating the glutathione (GSH) content of wild-type plants, through expression of roGFP2 in the cytosol of low-GSH mutants and the endoplasmic reticulum (ER) of wild-type plants, as well as through wounding as an example for stress-induced redox changes. Provided the GSH concentration is known, roGFP2 facilitates the determination of the degree of oxidation of the GSH solution. Assuming sufficient glutathione reductase activity and non-limiting NADPH supply, the observed almost full reduction of roGFP2 in vivo suggests that a 2.5 mm cytosolic glutathione buffer would contain only 25 nm oxidized glutathione disulfide (GSSG). The high sensitivity of roGFP2 toward GSSG via GRX enables the use of roGFP2 for monitoring stress-induced redox changes in vivo in real time. The results with roGFP2 as an artificial GRX target further suggest that redox-triggered changes of biologic processes might be linked directly to the glutathione redox potential via GRX as the mediator.

436 citations


Journal ArticleDOI
TL;DR: Analysis of the four redox couples throughout Arabidopsis rosette development showed that redox states were relatively constant but that total pools of NAD, glutathione, and ascorbate were significantly modified by day length and developmental stage.

408 citations


Journal ArticleDOI
TL;DR: A number of more complex P450 reactions also occur, and these can be understood largely in the context of the basic chemical mechanisms and subsequent rearrangements.

401 citations


01 Dec 2007
TL;DR: It is found that samples indicating mixed redox processes provide information on redox heterogeneity that is useful for assessing common water quality issues and should be included in routine water quality monitoring programs whenever possible.
Abstract: Reduction/oxidation (redox) conditions in 15 principal aquifer (PA) systems of the United States, and their impact on several water quality issues, were assessed from a large data base collected by the National Water-Quality Assessment Program of the USGS. The logic of these assessments was based on the observed ecological succession of electron acceptors such as dissolved oxygen, nitrate, and sulfate and threshold concentrations of these substrates needed to support active microbial metabolism. Similarly, the utilization of solid-phase electron acceptors such as Mn(IV) and Fe(III) is indicated by the production of dissolved manganese and iron. An internally consistent set of threshold concentration criteria was developed and applied to a large data set of 1692 water samples from the PAs to assess ambient redox conditions. The indicated redox conditions then were related to the occurrence of selected natural (arsenic) and anthropogenic (nitrate and volatile organic compounds) contaminants in ground water. For the natural and anthropogenic contaminants assessed in this study, considering redox conditions as defined by this framework of redox indicator species and threshold concentrations explained many water quality trends observed at a regional scale. An important finding of this study was that samples indicating mixed redox processes provide information on redox heterogeneity that is useful for assessing common water quality issues. Given the interpretive power of the redox framework and given that it is relatively inexpensive and easy to measure the chemical parameters included in the framework, those parameters should be included in routine water quality monitoring programs whenever possible.

394 citations


Journal ArticleDOI
TL;DR: In this paper, a review of the literature relating to damage to SOFCs caused by redox cycling of Ni-based anodes is reviewed, and a simplified mechanical model of the stress and damage that are likely to be caused by anode expansion is proposed and applied to anode-supported, electrolyte-supported and inert substrate-supported cell configurations.
Abstract: The published literature relating to damage to SOFCs caused by redox cycling of Ni-based anodes is reviewed. The review covers the kinetics of Ni oxidation and NiO reduction (as single phases and as constituents of composites with yttria-stabilised zirconia, YSZ), the dimensional changes associated with redox cycling and the effect of this on the mechanical integrity and electrical performance of cells and stacks. A critical parameter is the expansion strain that is caused by oxidation. Several studies report that the first complete oxidation of a Ni/YSZ composite causes a linear expansion of the order of 1%, but the actual values vary substantially between different investigations. The oxidation strain is the result of microstructural irreversibility during the redox process and leads to strain accumulation over several redox cycles. This can cause mechanical disruption to an anode, anode support or other cell components attached to the anode. A simplified mechanical model of the stress and damage that are likely to be caused by anode expansion is proposed and applied to anode-supported, electrolyte-supported and inert substrate-supported cell configurations. This allows the maximum oxidation strain to avoid damage in each configuration to be estimated.

356 citations


Journal ArticleDOI
TL;DR: The rule set developed for oxygen reduction may be applied to a variety of other small molecule activation reactions of consequence to energy conversion.
Abstract: The selective reduction of oxygen to water requires four electrons and four protons. The design of catalysts that promote oxygen reduction therefore requires the management of both electron and proton inventories. Pacman and Hangman porphyrins provide a cleft for oxygen binding, a redox shuttle for oxygen reduction, and functionality for tuning the acid–base properties of bound oxygen and its intermediates. With proper control of the proton-coupled electron transfer events, O–O bond breaking of oxygen, and more generally oxygenated substrates, may be achieved with high efficiencies. The rule set developed for oxygen reduction may be applied to a variety of other small molecule activation reactions of consequence to energy conversion.

331 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the oxygen reduction reaction at the La 2 NiO 4+ δ /YSZ interface by impedance spectroscopy and found at least the presence of three processes involved in the reduction reaction.

318 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of degree of oxidation on physical properties of the oxidized derivatives of alginate gels was analyzed and new active functional groups were found on alginates, which is a polymer of interest as support in drug-controlled delivery systems.

309 citations


Journal ArticleDOI
TL;DR: While CO oxidation takes place at copper oxide support interfacial sites and its activity correlates with the degree of reduction achieved on the dispersed copper oxide particles, H2 oxidation apparently proceeds when a massive copper oxide reduction occurs.
Abstract: Catalysts based on combinations between copper and cerium oxides are analyzed with respect to their catalytic properties for preferential oxidation of CO in a H2-rich stream (CO-PROX) by means of DRIFTS, XANES, and Raman under Operando conditions. The results allow analyzing entities/species responsible for the CO and H2 oxidation reactions taking place during the CO-PROX process. While CO oxidation takes place at copper oxide support interfacial sites and its activity correlates with the degree of reduction achieved on the dispersed copper oxide particles, H2 oxidation apparently proceeds when a massive copper oxide reduction occurs. This opens the possibility to modulate the catalytic behavior of these types of catalysts by acting, respectively, on the interfacial redox properties and on the dispersed copper oxide redox properties.

Journal ArticleDOI
TL;DR: Results directly demonstrate that HA and FA are capable of acting as reversible electron-transfer agents using different functional groups, some of which may not be quinones.
Abstract: Cyclic oxidation and reduction reactions using oxygen and palladium with H2, respectively, of dissolved humic and fulvic acids (HA and FA) and model quinone compounds were used to structurally characterize and quantify the electron-carrying capacity (ECC) of reversible redox sites present in humic substances. This technique was used to examine 8 quinone compounds and 14 HA and FA samples and identified 3 redox sites as a function of their stability against the Pd-catalyzed hydrogenolysis process. Six highly aliphatic HA and FA isolated from landfill leachate did not contain redox sites under any conditions; however, the other HA and FA demonstrated reversible redox properties characterized by a combination of three redox sites. On the basis of the model compound results, it is proposed that one site consists of a non-quinone structure (NQ) and the other two sites have quinone structures. The two quinone sites differ in that one group (Q1) has electron-withdrawing groups adjacent to the quinone functional ...

Journal ArticleDOI
TL;DR: In this article, a comparison of direct and indirect electrochemical oxidation of a synthetic solution containing methylene blue was performed, which revealed the suitability of both electrochemical processes for completely removing the COD and effectively decolouring the solution.

Journal ArticleDOI
TL;DR: In this article, a unifying redox approach is proposed to interpret the overall NO/NO2−NH3 SCR chemistry over V-based catalysts, in which vanadium sites are reduced by the reaction between NO and NH3 and are reoxidized either by oxygen (standard SCR) or by nitrates (fast SCR), with the latter formed via NO2 disproportion over other nonreducible oxide catalyst components.

Journal ArticleDOI
TL;DR: Time‐dependence and aggregation studies suggest that an aggregation intermediate is responsible for this increased HO.
Abstract: Aggregation of the beta-amyloid peptide (Abeta) to amyloid plaques is a key event in Alzheimer's disease. According to the amyloid-cascade hypothesis, Abeta aggregates are toxic to neurons through the production of reactive oxygen species (ROS). Copper ions play an important role, because they are able to bind to Abeta and influence its aggregation properties. Moreover, Cu-Abeta is supposed to be directly involved in ROS production. To get a better understanding of these reactions, we measured the production of HO(.) and the redox potential of Cu-Abeta. The results were compared to other biological copper-peptide complexes in order to get an insight into the biological relevance. Cu-Abeta produced more HO(.) than the complex of copper with Asp-Ala-His-Lys (Cu-DAHK), but less than with Gly-His-Lys (Cu-GHK). Cyclic voltammetry revealed that the order for reduction potential is Cu-GHK>Cu-Abeta>Cu-DAHK, but for the oxidation potential the order is reversed. Thus, easier copper redox cycling correlated to higher HO(.) production. The copper complex of the form Abeta1-42 showed a HO(.) production five-times higher than that of the form Abeta1-40. Time-dependence and aggregation studies suggest that an aggregation intermediate is responsible for this increased HO(.) production.

Journal ArticleDOI
TL;DR: At the center of zinc/redox signaling are the zinc/thiolate clusters of metallothionein, which can transduce zinc and redox signals and thereby attenuate or amplify these signals.

Journal ArticleDOI
TL;DR: The results of this work demonstrate the utility of the dithiothreitol assay for quantitatively assessing the redox potential of airborne particulate matter from a wide range of sources.
Abstract: Although the mechanisms of airborne particulate matter (PM) related health effects remain incompletely understood, one emerging hypothesis is that these adverse effects derive from oxidative stress, initiated by the formation of reactive oxygen species (ROS) within affected cells. Typically, ROS are formed in cells through the reduction of oxygen by biological reducing agents, with the catalytic assistance of electron transfer enzymes and redox active chemical species such as redox active organic chemicals and metals. The purpose of this study was to relate the electron transfer ability, or redox activity, of the PM samples to their content in polycyclic aromatic hydrocarbons and various inorganic species. The redox activity of the samples has been shown to correlate with the induction of the stress protein, hemeoxygenase-1. Size-fractionated (i.e. < 0.15; < 2.5 and 2.5 – 10 μm in diameter) ambient PM samples were collected from four different locations in the period from June 2003 to July 2005, and were chemically analyzed for elemental and organic carbon, ions, elements and trace metals and polycyclic aromatic hydrocarbons. The redox activity of the samples was evaluated by means of the dithiothreitol activity assay and was related to their chemical speciation by means of correlation analysis. Our analysis indicated a higher redox activity on a per PM mass basis for ultrafine (< 0.15 μm) particles compared to those of larger sizes. The PM redox activity was highly correlated with the organic carbon (OC) content of PM as well as the mass fractions of species such as polycyclic aromatic hydrocarbons (PAH), and selected metals. The results of this work demonstrate the utility of the dithiothreitol assay for quantitatively assessing the redox potential of airborne particulate matter from a wide range of sources. Studies to characterize the redox activity of PM from various sources throughout the Los Angeles basin are currently underway.

Journal ArticleDOI
TL;DR: In this article, a number of redox systems have been investigated in this work with the aim of identifying electrolytes suitable for testing redox flow battery cell designs, the criteria for the selection of suitable systems were fast electrochemical kinetics and minimal cross-contamination of active electrolytes.

Journal ArticleDOI
TL;DR: The glucose biosensor shows fast amperometric response (3s) with the sensitivity of 446.2nA/mM, detection limit of 24 microM and wide concentration range of 30 microM to 5mM and exhibits good stability, reproducibility and long life time.

Journal ArticleDOI
TL;DR: A general chemical mechanism can be used to rationalize most of the oxidations and involves a perfenyl intermediate and odd‐electron chemistry, i.e. abstraction of a hydrogen atom or electron followed by oxygen rebound and sometimes rearrangement.
Abstract: Cytochrome P450 (P450) enzymes catalyze a variety of oxidation and some reduction reactions, collectively involving thousands of substrates. A general chemical mechanism can be used to rationalize most of the oxidations and involves a perfenyl intermediate (FeO3+) and odd-electron chemistry, i.e. abstraction of a hydrogen atom or electron followed by oxygen rebound and sometimes rearrangement. This general mechanism can explain carbon hydroxylation, heteroatom oxygenation and dealkylation, epoxidation, desaturation, heme destruction, and other reactions. Another approach to understanding catalysis involves analysis of the more general catalytic cycle, including substrate specificity, because complex patterns of cooperativity are observed with several P450s. Some of the complexity is due to slow conformational changes in the proteins that occur on the same timescale as other steps.

Journal ArticleDOI
TL;DR: It is concluded that Fe-ZSM-5 and Fe-silicalite are not the ideal samples conceived before and that many types of species are present, some active and some other silent from adsorptive and catalytic point of view.
Abstract: Fe-ZSM-5 and Fe-silicalite zeolites efficiently catalyse several oxidation reactions which find close analogues in the oxidation reactions catalyzed by homogeneous and enzymatic compounds. The iron centres are highly dispersed in the crystalline matrix and on highly diluted samples, mononuclear and dinuclear structures are expected to become predominant. The crystalline and robust character of the MFI framework has allowed to hypothesize that the catalytic sites are located in well defined crystallographic positions. For this reason these catalysts have been considered as the closest and best defined heterogeneous counterparts of heme and non heme iron complexes and of Fenton type Fe2+ homogeneous counterparts. On this basis, an analogy with the methane monooxygenase has been advanced several times. In this review we have examined the abundant literature on the subject and summarized the most widely accepted views on the structure, nuclearity and catalytic activity of the iron species. By comparing the results obtained with the various characterization techniques, we conclude that Fe-ZSM-5 and Fe-silicalite are not the ideal samples conceived before and that many types of species are present, some active and some other silent from adsorptive and catalytic point of view. The relative concentration of these species changes with thermal treatments, preparation procedures and loading. Only at lowest loadings the catalytically active species become the dominant fraction of the iron species. On the basis of the spectroscopic titration of the active sites by using NO as a probe, we conclude that the active species on very diluted samples are isolated and highly coordinatively unsaturated Fe2+ grafted to the crystalline matrix. Indication of the constant presence of a smaller fraction of Fe2+ presumably located on small clusters is also obtained. The nitrosyl species formed upon dosing NO from the gas phase on activated Fe-ZSM-5 and Fe-silicalite, have been analyzed in detail and the similarities and differences with the cationic, heme and non heme homogeneous counterparts have been evidenced. The same has been done for the oxygen species formed by N2O decomposition on isolated sites, whose properties are more similar to those of the (FeO)2+ in cationic complexes (included the [(H2O)5FeO]2+ “brown ring” complex active in Fenton reaction) than to those of ferryl groups in heme and non heme counterparts.

Journal ArticleDOI
TL;DR: In this article, the authors examined the redox reactions and other transformations of mercury (Hg) species in surface waters, and the factors determining the rates of these reactions, and found that both Hg oxidation and reduction were simultaneously occurring and were photochemically mediated in the waters investigated.

Journal ArticleDOI
TL;DR: It is shown, for the first time, that an electrochemical cell with a solid-state electrode polarized at -500 mV, in combination with a low-potential redox mediator (methyl viologen), can efficiently transfer electrochemical reducing equivalents to microorganisms which respire using chlorinated solvents.
Abstract: The ability to transfer electrons, via an extracellular path, to solid surfaces is typically exploited by microorganisms which use insoluble electron acceptors, such as iron-or manganese-oxides or inert electrodes in microbial fuel cells. The reverse process, i.e., the use of solid surfaces or electrodes as electron donors in microbial respirations, although largely unexplored, could potentially have important environmental applications, particularly for the removal of oxidized pollutants from contaminated groundwater or waste streams. Here we show, for the first time, that an electrochemical cell with a solid-state electrode polarized at -500 mV (vs standard hydrogen electrode), in combination with a low-potential redox mediator (methyl viologen), can efficiently transfer electrochemical reducing equivalents to microorganisms which respire using chlorinated solvents. By this approach, the reductive transformation of trichloroethene, a toxic yet common groundwater contaminant, to harmless end-products such as ethene and ethane could be performed. Furthermore, using a methyl-viologen-modified electrode we could even demonstrate that dechlorinating bacteria were able to accept reducing equivalents directly from the modified electrode surface. The innovative concept, based on the stimulation of dechlorination reactions through the use of solid-state electrodes (we propose for this process the acronym BEARD: Bio-Electrochemically Assisted Reductive Dechlorination), holds promise for in situ bioremediation of chlorinated-solvent-contaminated groundwater, and has several potential advantages over traditional approaches based on the subsurface injection of organic compounds. The results of this study raise the possibility that immobilization of selected redox mediators may be a general strategy for stimulating and controlling a range of microbial reactions using insoluble electrodes as electron donors.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the long-term kinetics of selenite ( Se(IV)O 3 - ) sorption to montmorillonite in the presence of Fe2+ under anoxic conditions.

Journal ArticleDOI
TL;DR: It is shown that depletion of reduced glutathione is an alternative trigger of synchronized mitochondrial oscillation in cardiomyocytes and that intermediate GSH/GSSG ratios cause reversible ΔΨm depolarization, although irreversible PTP activation is induced by extensive thiol oxidation.

Journal ArticleDOI
TL;DR: The binding stoichiometry between Cu(II) and the full-length β-amyloid Aβ(1−42), and the oxidation state of copper in the resultant complex were determined by electrospray ionization−Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) and cyclic voltammetry as mentioned in this paper.
Abstract: The binding stoichiometry between Cu(II) and the full-length β-amyloid Aβ(1−42) and the oxidation state of copper in the resultant complex were determined by electrospray ionization−Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) and cyclic voltammetry The same approach was extended to the copper complexes of Aβ(1−16) and Aβ(1−28) A stoichiometric ratio of 1:1 was directly observed, and the oxidation state of copper was deduced to be 2+ for all of the complexes, and residues tyrosine-10 and methionine-35 are not oxidized in the Aβ(1−42)−Cu(II) complex The stoichiometric ratio remains the same in the presence of more than a 10-fold excess of Cu(II) Redox potentials of the sole tyrosine residue and the Cu(II) center were determined to be ca 075 and 008 V vs Ag/AgCl [or 095 and 028 V vs normal hydrogen electrode (NHE)], respectively More importantly, for the first time, the Aβ−Cu(I) complex has been generated electrochemically and was found to catalyze the reduction of ox

Journal ArticleDOI
TL;DR: It was concluded that for this solid waste, desorption was the predominant mechanism in arsenic leaching and redox potential did not play a significant role in arsenic or iron solubilization.

Journal ArticleDOI
TL;DR: The redox properties of metallothionein and its apoprotein thionein are critical for buffering zinc ions and for controlling fluctuations in the range of picomolar concentrations of “free” zinc ions in cellular signaling.
Abstract: Zinc is involved in virtually all aspects of cellular and molecular biology as a catalytic, structural, and regulatory cofactor in over 1000 proteins. Zinc binding to proteins requires an adequate supply of zinc and intact molecular mechanisms for redistributing zinc ions to make them available at the right time and location. Several dozen gene products participate in this process, in which interactions between zinc and sulfur donors determine the mobility of zinc and establish coupling between cellular redox state and zinc availability. Specifically, the redox properties of metallothionein and its apoprotein thionein are critical for buffering zinc ions and for controlling fluctuations in the range of picomolar concentrations of “free” zinc ions in cellular signaling. Metallothionein and other proteins with sulfur coordination environments are sensitive to redox perturbations and can render cells susceptible to injury when oxidative stress compromises the cellular redox and zinc buffering capacity in chronic diseases. The implications of these fundamental principles for zinc metabolism in type 2 diabetes are briefly discussed.

Journal ArticleDOI
TL;DR: In this paper, a mesophase-derived activated carbon was further heat treated to 600 or 1000°C in nitrogen and the changes in texture and surface chemistry induced by the thermal treatment were deeply studied.

Journal ArticleDOI
TL;DR: A series of Cr-containing magnetites, Fe3−xCrxO4 (x = 0.00, 0.07,0.42 and 0.51) has been prepared using conventional co-precipitation method in this paper, which showed that the presence of Cr has a remarkable effect on the texturial properties of the material decreasing the pore diameter from meso-to micropore with a significant increase on the BET surface area.
Abstract: A series of Cr-containing magnetites, Fe3−xCrxO4 (x = 0.00, 0.07, 0.26, 0.42 and 0.51) has been prepared using conventional co-precipitation method. Mossbauer and powder X-ray diffraction measurements suggested the formation of the spinel crystalline phase with initial substitution of F e oct 3 + by Cr3+ and for higher Cr contents, chromium also replaces F e oct 2 + and F e tet 3 + in the crystalline structure. N2 adsorption/desorption revealed that the presence of Cr has a remarkable effect on the texturial properties of the material decreasing the pore diameter from meso- to micropore with a significant increase on the BET surface area. Thermal analyses (TG and DTA) coupled with XRD and Mossbauer showed that thermal treatment up to 270 °C with O2 leads to the oxidation of F e oct 2 + producing Cr-substituted maghemite but at 600 °C the cubic maghemite is converted to the hexagonal hematite and Cr is expelled from the iron oxide structure. The reactivity of Fe3−xCrxO4 was investigated using two H2O2 reactions, i.e. the decomposition to O2 and the oxidation of the dye methylene blue used as model contaminant. The obtained results showed that the presence of Cr directly promoted the H2O2 decomposition. On the other hand, the presence of small concentration of Cr, i.e. Cr0.07 and Cr0.21, caused a significant increase on the activity for the oxidation of the dyes with complete discoloration and high degree of mineralization. The higher activity of Fe2.93Cr0.07O4 was discussed in terms of a coupling of the redox pairs Fe3+/Fe2+ and Cr2+/Cr3+ producing a more efficient regeneration of the Fenton active specie Fe2+.