Mediated Electrochemical Reduction Research Articles

Comparative Evaluation of Mediated Electrochemical Reduction and Chemical Redox Titration for Quantifying the Electron Accepting Capacities of Soils and Redox-Active Soil Constituents.

The electron accepting capacity (EAC) of soil plays a pivotal role in the biogeochemical cycling of nutrients and transformation of redox-labile contaminants. Prior EAC studies of soils and soil constituents utilized different methods, reductants, and mediators, making cross-study comparison difficult. This study was conducted to quantify and compare the EACs of two soil constituents (hematite and Leonardite humic acid) and 12 soils of diverse composition, using chemical redox titration (CRT) with dithionite as the reductant and mediated electrochemical reduction (MER) with diquat as the mediator. The EACs of hematite and humic acid measured by CRT (EACCRT) and MER (EACMER) are similar and close to the theoretical/reported values. For soils, EACCRT and EACMER increased with iron and organic carbon (TOC) contents, suggesting iron and carbon were the main contributors to soil EAC. EACCRT > EACMER for all soils, and their difference (ΔEAC = EACCRT - EACMER) increased with TOC, presumably due to the longer contact time in CRT and thus more complete reduction of carbonaceous redox moieties. We propose an equation that relates EACCRT to EACMER (ΔEAC = 1796fTOC + 32) and another that predicts EACCRT from dithionite-reducible Fe and TOC (EACCRT = 2705 μmol e-/g C × fTOC + 17907 μmol e-/g Fe × fFedithionite-reducible). Our results suggest that at least 10-15% of soil organic carbon contributed to EACCRT.

Phosphorization of α-Fe2O3 Boosts Active Hydrogen Mediated Electrochemical Nitrate Reduction to Ammonia.

Inexpensive iron-based materials are considered promising electrocatalysts for nitrate (NO3 -) reduction, but their catalytic activity and spontaneous corrosion remain challenges. Here, the α-Fe2O3 active surface is reconstructed by gradient phosphorization to obtain FePx with higher electrochemical activity. FeP2.0 optimizes the adsorption energy of NO3 - and its reduction intermediates, meanwhile promote the generation of active hydrogen (*H) but inhibit its generation of H2. More importantly, Fe and P can serve as binding sites for NO3 - and *H, respectively, which improves the electron utilization of NO3 - deoxygenation and the efficiency of the subsequent hydrogenation for the selective synthesis of NH3. 91.7% NO3 - conversion rate is achieved for the reduction of 100mL 200mg L-1 NO3 --N, 99.3% ammonia (NH3 selectivity (yield of 1.79 mg h-1 cm-2), and 91.4% Faraday efficiency in 3 h. The high-purity solid NH4Cl is finally extracted by gas extraction and vacuum distillation (81.4% recovery). This study provides new insights and strategies for the conversion of NO3 - to NH3 products over iron-based electrocatalysts.

Amine-Functionalized Copper Catalysts: Hydrogen Bonding Mediated Electrochemical CO2 Reduction to C2 Products and Superior Rechargeable Zn-CO2 Battery Performance

Amine-Functionalized Copper Catalysts: Hydrogen Bonding Mediated Electrochemical CO2 Reduction to C2 Products and Superior Rechargeable Zn-CO2 Battery Performance

Fe distribution, redox state and electrochemical activity in Boom Clay

In the context of radioactive waste disposal in clay formations, the sorption of a number of redox sensitive radionuclides (e.g. U, Se, Tc) is controlled by the presence of Fe-containing minerals, mainly smectite (Guimaraes et al., 2016), illite (Bruggeman and Maes, 2010), and pyrite (Breynaert et al., 2010). In the repository lifetime, oxidizing conditions will prevail during the open drift phase enhanced by ventilation followed by recovery of reducing conditions after back-filling. Thus Fe-containing minerals will be subjected to redox cycles in the course of the repository operation. In order to evaluate the sensitivity of different Fe containing minerals in the Boom Clay to changing redox conditions, Fe2+/Fe3+ partitioning was determined in various size fractions (<0.2, 0.2–2, 2–8 μm) and bulk samples by complexation with phenanthroline and XANES spectroscopy. The integration of the Fe speciation with quantitative mineralogical data allowed for a calculation of Fe redox activities. These values were then compared with the empirical electron accepting capacity (EAC) and electron donating capacity (EDC) values deduced from the mediated electrochemical reduction (MER) and mediated electrochemical oxidation (MEO) experiments. The results indicate that Fe3+ present in the smectite has a major control over the measured EACs in the redox cycled (oxidized, reduced and re-oxidized) Boom Clay samples. Fe2+ in illite and chlorite explain well the measured EDC values in the various size fractions, whereas in bulk samples pyrite and organic matter are the major contributors to EDC. The theoretical electrochemical activities (both EACs and EDCs) are about factor two higher than the experimental values. It is therefore concluded that the electrochemical activities of pure minerals do not necessarily reflect their electrochemical activities in the natural redox cycled samples. The methodological approach presented here may serve as a basis for further electrochemical investigations of natural, redox cycled sediments.

Decreases in Iron Oxide Reducibility during Microbial Reductive Dissolution and Transformation of Ferrihydrite.

Ferrous iron formed during microbial ferric iron reduction induces phase transformations of poorly crystalline into more crystalline and thermodynamically more stable iron (oxyhydr)oxides. Yet, characterizing the resulting decreases in the reactivity of the remaining oxide ferric iron toward reduction (i.e., its reducibility) has been challenging. Here, we used the reduction of six-line ferrihydrite by Shewanella oneidensis MR-1 as a model system to demonstrate that mediated electrochemical reduction (MER) allows directly following decreases in oxide ferric iron reducibility during the transformation of ferrihydrite into goethite and magnetite which we characterized by X-ray diffraction analysis and transmission electron microscopy imaging. Ferrihydrite was fully reducible in MER at both pHMER of 5.0 and 7.5. Decreases in iron oxide reducibility associated with ferrihydrite transformation into magnetite were accessible at both pHMER because the formed magnetite was not reducible under either of these conditions. Conversely, decreases in iron oxide reducibility associated with goethite formation were apparent only at the highest tested pHMER of 7.5 and thus the thermodynamically least favorable conditions for iron oxide reductive dissolution. The unique capability to adjust the thermodynamic boundary conditions in MER to the specific reducibilities of individual iron (oxyhydr)oxides makes this electrochemical approach broadly applicable for studying changes in iron oxide reducibility in heterogeneous environmental samples such as soils and sediments.

Electrochemical Analysis of Changes in Iron Oxide Reducibility during Abiotic Ferrihydrite Transformation into Goethite and Magnetite.

Electron transfer to ferric iron in (oxyhydr-)oxides (hereafter iron oxides) is a critical step in many processes that are central to the biogeochemical cycling of elements and to pollutant dynamics. Understanding these processes requires analytical approaches that allow for characterizing the reactivity of iron oxides toward reduction under controlled thermodynamic boundary conditions. Here, we used mediated electrochemical reduction (MER) to follow changes in iron oxide reduction extents and rates during abiotic ferrous iron-induced transformation of six-line ferrihydrite. Transformation experiments (10 mM ferrihydrite-FeIII) were conducted over a range of solution conditions (pHtrans = 6.50 to 7.50 at 5 mM Fe2+ and for pHtrans = 7.00 also at 1 mM Fe2+) that resulted in the transformation of ferrihydrite into thermodynamically more stable goethite or magnetite. The changes in iron oxide mineralogy during the transformations were quantified using X-ray diffraction analysis. MER measurements on iron oxide suspension aliquots collected during the transformations were performed over a range of pHMER at constant applied reduction potential. The extents and rates of iron oxide reduction in MER decreased with decreasing reaction driving force resulting from both increasing pHMER and increasing transformation of ferrihydrite into thermodynamically more stable iron oxides. We show that the decreases in iron oxide reduction extents and rates during ferrihydrite transformations can be linked to the concurrent changes in iron oxide mineralogy.

Effect of Liquid Electron Mediators on Continuous N2O Gas Removal at Room Temperature: Electrochemical and Electroscrubbing Analysis

Many techniques used for N2O abatement involve high temperature catalytic removal with the synthesis of relatively non-valuable products. In the present attempt, N2O degradation was achieved at ambient temperature using different electron mediators during electroscrubbing to form a high value product, NH3. A combination of mediator precursors, such as Fe(III)[Ni(II)(CN)4]1+, Fe(III)[Co(II)(CN)5], and [Ni(II)(CN)4]2−•[Co(II)(CN)5]1−, were added to a 10 M KOH solution to generate the electron mediators. The oxidation/reduction potential (ORP) of the electrolyzed solutions of each complex and the UV-visible spectra confirmed the formation of Fe(II)[Ni(II)(CN)4], Fe(II)[Co(II)(CN)5]1−, and [Ni(I)(CN)4]3−•[Co(II)(CN)5]3− at the cathodic half-cell. N2O degradation was confirmed to occur via a mediated electrochemical reduction (MER) process from the change in reduction efficiency from 4% to 2% (Fe(II)-Ni) and 15% to 12.5% (Ni(I)-Co(II)). Online FTIR gas analysis confirmed that NH3 formed from the degradation of N2O. Although the reduction efficiency was lower for Fe(III)[Ni(II)(CN)4]1+, a high NH3 concentration formed (4.39 mg/hr.) when it was used as an electron mediator. The presence of the [Co(II)(CN)5]3− complex in the mixed complex reduced the level of NH3 formation. The developed method can degrade N2O to NH3 efficiently at ambient temperature.

Enhanced electro-reduction of NO to NH3 on Pt cathode at electro-scrubber.

Besides cheaper electrodes used in NH3 product formation during NO degradation by mediated electrochemical reduction (MER), a specific electrode that can perform direct electrochemical reduction (DER) and MER of NO is an added advantage. In the present study, a Pt electrode was used to examine NO degradation through NH3 formation during the electro-scrubbing process. Initially, the DER of NO was tested on a Pt electrode to determine if the DER of NO is possible. The NO degradation by only absorption, DER on Pt, and MER using electrogenerated [Ni(I)(CN)4]3- showed that a combination of DER and MER increased the NO degradation efficiency. In addition, the online FTIR spectra obtained under different conditions showed that the product formed was NH3, either from the DER or MER during electro-scrubbing. The feed gas flow rate and feed concentration results of NH3 formation revealed an additional chemical reaction that was influenced by the Pt electrode in addition to the DER and MER processes. Furthermore, the degradation efficiency of NO when using the Pt electrode increased to 90% compared to that of the Cu electrode (65%), which showed that Pt follows a combination of DER and MER processes. Based on the gas-phase FTIR results of NH3 formation during NO degradation, higher NH3 production (0.32mg/h) was obtained when using a Pt electrode than that using a Cu electrode (0.21mg/h), highlighting the specificity of the Pt electrode in NH3 formation during the degradation of NO gas.

Sustainable degradation of carbon tetrafluoride to non-corrosive useful products by incorporating reduced electron mediator within electro-scrubbing

The degradation of CF4 gas using existing technologies produces other types of greenhouse gas (CO2) and corrosive side products. The main aim of this study is to degrade CF4 gas at room temperature into useful products without producing corrosive side products by mediated electrochemical reduction (MER) process using an electrogenerated Cu1+[Ni2+(CN)4]1− mediator. Initial studies on the electrolytic reduction of the hetero-bimetallic complex in catholyte solution at anodized Ti cathode was monitored by oxidation/reduction potential (ORP) variation whether the Cu2+ or Ni2+ was reduced in the Cu2+[Ni2+(CN)4] and confirmed by electron spin resonance (ESR) spectroscopy the Cu1+[Ni2+(CN)4]1− formation. The concentration variation of Cu1+[Ni2+(CN)4]1− during CF4 injection demonstrated the degradation of CF4 followed the MER by electrogenerated Cu1+[Ni2+(CN)4]1−. Maximum removal efficiency of CF4 using electroscrubbing process was 96% at room temperature. Through the variation in gas phase parameters, the gas phase mass transfer coefficient was calculated that can facilitate scale up the developed process. Fourier transform infrared spectroscopy analysis in both the gas and solution phases showed that CH3CH2OH was the main product that formed during the removal of CF4 by electrogenerated Cu1+[Ni2+(CN)4]1− at electroscrubber along with a small amount of CF3CH3 intermediate. Importantly, this mechanism also avoided formation of the corrosive product HF.

Mediated Electrochemical Reduction of Iron (Oxyhydr-)Oxides under Defined Thermodynamic Boundary Conditions.

Iron (oxyhydr-)oxide reduction has been extensively studied because of its importance in pollutant redox dynamics and biogeochemical processes. Yet, experimental studies linking oxide reduction kinetics to thermodynamics remain scarce. Here, we used mediated electrochemical reduction (MER) to directly quantify the extents and rates of ferrihydrite, goethite, and hematite reduction over a range of negative reaction free energies, ΔrG, that were obtained by systematically varying pH (5.0 to 8.0), applied reduction potentials (-0.53 to -0.17 V vs SHE), and Fe2+ concentrations (up to 40 μM). Ferrihydrite reduction was complete and fast at all tested ΔrG values, consistent with its comparatively low thermodynamic stability. Reduction of the thermodynamically more stable goethite and hematite changed from complete and fast to incomplete and slow as ΔrG values became less negative. Reductions at intermediate ΔrG values showed negative linear correlations between the natural logarithm of the reduction rate constants and ΔrG. These correlations imply that thermodynamics controlled goethite and hematite reduction rates. Beyond allowing to study iron oxide reduction under defined thermodynamic conditions, MER can also be used to capture changes in iron oxide reducibility during phase transformations, as shown for Fe2+-facilitated transformation of ferrihydrite to goethite.

Microbial reduction of ferrihydrite-organic matter coprecipitates by Shewanella putrefaciens and Geobacter metallireducens in comparison to mediated electrochemical reduction

Despite numerous studies seeking to elucidate the effect of various specific organic compounds on the reactivity and stability of Fe oxyhydroxides in soil, studies examining the effect of natural organic matter (NOM) on the microbial reduction of Fe-organic matter (OM) coprecipitates are still rare. In this study, pure ferrihydrite (Fh) and Fe-OM coprecipitates were synthesized using three different types of NOM (extracellular polymeric substances extracted from Bacillus subtilis, OM extracted from the Oi horizon of a Cambisol, and OM extracted from the Oa horizon of a Podzol). These phases were characterized by N2 gas adsorption, nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), NOM desorption experiments, and mediated electrochemical reduction (MER). Iron(III) reduction under anaerobic conditions was monitored for 16days using two different strains of dissimilatory Fe(III)-reducing bacteria (Shewanella putrefaciens, Geobacter metallireducens). Mineral transformation during reduction was determined by XRD and FTIR of the solid post-incubation phases. Fe(III) reduction by Shewanella putrefaciens was influenced by the amount of available electron shuttling molecules provided by the NOM, whereas the Fe(III) reduction by Geobacter metallireducens as well as abiotic Fe(III) reduction was influenced by particle size and NOM-induced aggregation. The specific surface area proved to be a poor predictor of Fe reduction of Fe-OM coprecipitates. This study emphasizes that certain physicochemical properties of natural Fe oxyhydroxides (composition of sorbed NOM and aggregation state) impact the Fe reduction by distinct microorganisms to differing degrees. Understanding environmental Fe and C cycling, therefore, requires experimental approaches extending beyond the use of pure Fe oxyhydroxides and model organisms.

Synergistic Effect of Mediated Electrochemical Reduction and Mediated Electrochemical Oxidation on NO Removal by Electro-Scrubbing

A combined MER (mediated electrochemical reduction) and MEO (mediated electrochemical oxidation) approach was examined for the first time for the efficient removal of NO by electro-scrubbing. The generation of a mediator (Ni(I) from Ni(II)(CN)42− in 9 M KOH) by electrochemical reduction was identified by the changes in the ORP value and potentiometric titration. The Ni(I) formation found to be 9% (4.3 mM) in 2 h electrolysis, which is decreased to 1.83 mM (4%) during addition of NO demonstrates NO removal occurred. The MER of NO at the cathodic half-cell by electro-scrubbing revealed the formation of NH3 and N2. The reductive removal efficiency of NO was almost 100% up to a gas flow rate of 0.25 g min−1. At the anodic half-cell, the Co(III) mediator from Co(II)SO4 in 5 M H2SO4 was generated electrochemically with concomitant conversion of NO to NO3− in the solution phase with a removal efficiency of 28%. The combination of MEO and MER with electro-scrubbing demonstrates 97% removal efficiency of NO up to 0.5 g min−1. The combined removal approach is more suitable for high concentrated NO with energy efficiency of 0.99 g/kWh compared to the individual MEO (0.221 g/kWh) and MER (0.769 g/kWh) approaches.

Redox Properties of Structural Fe in Clay Minerals: 3. Relationships between Smectite Redox and Structural Properties

Structural Fe in clay minerals is an important redox-active species in many pristine and contaminated environments as well as in engineered systems. Understanding the extent and kinetics of redox reactions involving Fe-bearing clay minerals has been challenging due to the inability to relate structural Fe(2+)/Fe(total) fractions to fundamental redox properties, such as reduction potentials (EH). Here, we overcame this challenge by using mediated electrochemical reduction (MER) and oxidation (MEO) to characterize the fraction of redox-active structural Fe (Fe(2+)/Fe(total)) in smectites over a wide range of applied EH-values (-0.6 V to +0.6 V). We examined Fe(2+)/Fe(total )- EH relationships of four natural Fe-bearing smectites (SWy-2, SWa-1, NAu-1, NAu-2) in their native, reduced, and reoxidized states and compared our measurements with spectroscopic observations and a suite of mineralogical properties. All smectites exhibited unique Fe(2+)/Fe(total) - EH relationships, were redox active over wide EH ranges, and underwent irreversible electron transfer induced structural changes that were observable with X-ray absorption spectroscopy. Variations among the smectite Fe(2+)/Fe(total) - EH relationships correlated well with both bulk and molecular-scale properties, including Fe(total) content, layer charge, and quadrupole splitting values, suggesting that multiple structural parameters determined the redox properties of smectites. The Fe(2+)/Fe(total) - EH relationships developed for these four commonly studied clay minerals may be applied to future studies interested in relating the extent of structural Fe reduction or oxidation to EH-values.

Redox Properties of Structural Fe in Clay Minerals. 2. Electrochemical and Spectroscopic Characterization of Electron Transfer Irreversibility in Ferruginous Smectite, SWa-1

Structural Fe in clay minerals is an important, albeit poorly characterized, redox-active phase found in many natural and engineered environments. This work develops an experimental approach to directly assess the redox properties of a natural Fe-bearing smectite (ferruginous smectite, SWa-1, 12.6 wt % Fe) with mediated electrochemical reduction (MER) and oxidation (MEO). By utilizing a suite of one-electron-transfer mediating compounds to facilitate electron transfer between structural Fe in SWa-1 and a working electrode, we show that the Fe2+/Fe3+ couple in SWa-1 is redox-active over a large range of potentials (from E(H) = -0.63 V to +0.61 V vs SHE). Electrochemical and spectroscopic analyses of SWa-1 samples that were subject to reduction and re-oxidation cycling revealed both reversible and irreversible structural Fe rearrangements that altered the observed apparent standard reduction potential (E(H)(ø)) of structural Fe. E(H)(ø)-values vary by as much as 0.56 V between SWa-1 samples with different redox histories. The wide range of E(H)-values over which SWa-1 is redox-active and redox history-dependent E(H)(ø)-values underscore the importance of Fe-bearing clay minerals as redox-active phases in a wide range of redox regimes.

Novel Electrochemical Approach to Assess the Redox Properties of Humic Substances

Two electrochemical methods to assess the redox properties of humic substances (HS) are presented: direct electrochemical reduction (DER) on glassy carbon working electrodes (WE) and mediated electrochemical reduction (MER) and oxidation (MEO) using organic radicals to facilitate electron transfer between HS and the WE. DER allows for continuous monitoring of electron and proton transfer to HS by chronocoulometry and automated acid titration, respectively, and of changes in bulk HS redox potential E(h). Leonardite Humic Acid (LHA) showed an H(+)/e(-) ratio of unity and a decrease in potential from E(h) = +0.18 to -0.23 V upon transfer of 822 mumol(e-) g(LHA)(-1) at pH 7, consistent with quinones as major redox-active functional moieties in LHA. MER and MEO quantitatively detected electrons in LHA samples that were prereduced by DER to different extents. MER and MEO therefore accurately quantify the redox state of HS. Cyclic DER and O(2)-reoxidation revealed that electron transfer to LHA was largely reversible. However, LHA contained a small pool of moieties that were not reoxidized, likely due to endergonic first electron transfer to O(2). Electron accepting capacities of 13 different HS, determined by MER, strongly correlated with their C/H ratios and aromaticities and with previously published values, which, however, were a factor of 3 smaller due to methodological limitations.

Dechlorination of PCBs by electrochemical reduction with aromatic radical anion as mediator

Mediated electrochemical reduction was applied to the dechlorination of polychlorinated biphenyls (PCBs) in tetra- n-butylammonium perchlorate/dimethylformamide solution. Rapid and complete dechlorination was possible with biphenyl or naphthalene as the mediator, whereas the reaction was much slower with anthracene or 9,10-diphenylanthracene. The reaction rate was so high with naphthalene or biphenyl radical anion that differences in reactivity could not be observed between congeners. Side reactions, other than biphenyl formation, could occur depending upon the mediators and the substrates, but contributed less than 10% of the total products in the case of naphthalene-mediated dechlorination. Almost all chloride ion formed in the dechlorination remained in the cathode solution. The order of the reaction was determined to be 0.5 for the substrate and 1 for the mediator (naphthalene) in the dechlorination of 2-chlorobiphenyl; identical to results for the mediated dechlorination of 1-chloronaphthalene. The reaction rate in practical PCB dechlorination could be estimated with the use of the initial concentration of the mediator and chlorine content of the solution provided that the problem of the deactivation of the electrode surface could be solved.

Radical vs Anionic Pathway in Mediated Electrochemical Reduction of Benzyl Bromide in a Bicontinuous Microemulsion

Products derived from benzyl radicals or anions were obtained by reduction of benzyl bromide mediated by vitamin B12, Co(salen), and cobalt phthalocyaninetetrasulfonate in a bicontinuous microemuls...

Mediated electrochemical reduction of cytochrome c and tyrosinase at perfluorosulfonated ionomer coated electrodes

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMediated electrochemical reduction of cytochrome c and tyrosinase at perfluorosulfonated ionomer coated electrodesJames W. Furbee, C. Robin. Thomas, Richard S. Kelly, and Mitchell R. MalachowskiCite this: Anal. Chem. 1993, 65, 13, 1654–1657Publication Date (Print):July 1, 1993Publication History Published online1 May 2002Published inissue 1 July 1993https://doi.org/10.1021/ac00061a004RIGHTS & PERMISSIONSArticle Views200Altmetric-Citations44LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (511 KB) Get e-Alerts Get e-Alerts

Mediated electrochemical reduction of 1,1-dichlorocyclopropanes

Mediated electrochemical reduction of 1,1-dichlorocyclopropanes

Mediated electrochemical reduction of dibromonorcarane in an emulsified system using Cr(II)

High material yields were obtained in the indirect electrochemical reduction of 7,7-dibromobicyclo-[4-1-0]heptane (dibromonorcarane) to give the corresponding monobromo derivative in an ethylene chloride/water emulsion. The reducing agent was Cr(II) generated by current controlled electrolysis of aqueous chromic chloride at a silver cathode. The effect of the concentration of Cr(II) and the temperature was studied. At 0.1 M Cr(II) and at 40° C the best results were obtained (90% yield).