Showing papers on "Redox published in 2012"

P2-type Nax[Fe1/2Mn1/2]O2 made from earth-abundant elements for rechargeable Na batteries

Abstract: Although sodium is an abundant element that can be electrochemically and reversibly extracted from and inserted into layered materials, the resulting reversible capacity for storing energy remains low. A manganese–iron–sodium-based electrode is now shown to exhibit a reversible capacity of 190 mAh g−1 due to electrochemically active Fe3+/Fe4+ redox reactions.

Use of Metal Oxide Nanoparticle Band Gap To Develop a Predictive Paradigm for Oxidative Stress and Acute Pulmonary Inflammation

Abstract: We demonstrate for 24 metal oxide (MOx) nanoparticles that it is possible to use conduction band energy levels to delineate their toxicological potential at cellular and whole animal levels. Among the materials, the overlap of conduction band energy (Ec) levels with the cellular redox potential (−4.12 to −4.84 eV) was strongly correlated to the ability of Co3O4, Cr2O3, Ni2O3, Mn2O3, and CoO nanoparticles to induce oxygen radicals, oxidative stress, and inflammation. This outcome is premised on permissible electron transfers from the biological redox couples that maintain the cellular redox equilibrium to the conduction band of the semiconductor particles. Both single-parameter cytotoxic as well as multi-parameter oxidative stress assays in cells showed excellent correlation to the generation of acute neutrophilic inflammation and cytokine responses in the lungs of C57 BL/6 mice. Co3O4, Ni2O3, Mn2O3, and CoO nanoparticles could also oxidize cytochrome c as a representative redox couple involved in redox ho...

Overcoming the "oxidant problem": strategies to use O2 as the oxidant in organometallic C-H oxidation reactions catalyzed by Pd (and Cu).

Abstract: Oxidation reactions are key transformations in organic chemistry because they can increase chemical complexity and incorporate heteroatom substituents into carbon-based molecules. This principle is manifested in the conversion of petrochemical feedstocks into commodity chemicals and in the synthesis of fine chemicals, pharmaceuticals, and other complex organic molecules. The utility and function of these molecules correlate directly with the presence and specific placement of oxygen and nitrogen heteroatoms and other functional groups within the molecules.Methods for selective oxidation of C–H bonds have expanded significantly over the past decade, and their role in the synthesis of organic chemicals will continue to increase. Our group’s contributions to this field are linked to our broader interest in the development and mechanistic understanding of aerobic oxidation reactions. Molecular oxygen (O2) is the ideal oxidant. Its low cost and lack of toxic byproducts make it a highly appealing reagent that c...

A cobalt complex redox shuttle for dye-sensitized solar cells with high open-circuit potentials

Abstract: Dye-sensitized solar cells are a promising alternative to traditional inorganic semiconductor-based solar cells. Here we report an open-circuit voltage of over 1,000 mV in mesoscopic dye-sensitized solar cells incorporating a molecularly engineered cobalt complex as redox mediator. Cobalt complexes have negligible absorption in the visible region of the solar spectrum, and their redox properties can be tuned in a controlled fashion by selecting suitable donor/acceptor substituents on the ligand. This approach offers an attractive alternate to the traditional I(3)(-)/I(-) redox shuttle used in dye-sensitized solar cells. A cobalt complex using tridendate ligands [Co(bpy-pz)(2)](3+/2+)(PF(6))(3/2) as redox mediator in combination with a cyclopentadithiophene-bridged donor-acceptor dye (Y123), adsorbed on TiO(2), yielded a power conversion efficiency of over 10% at 100 mW cm(-2). This result indicates that the molecularly engineered cobalt redox shuttle is a legitimate alternative to the commonly used I(3)(-)/I(-) redox shuttle.

Antioxidant Properties of Humic Substances

Abstract: Humic substances (HS) are heterogeneous, redox-active organic macromolecules. While electron transfer to and from HS under reducing conditions is well investigated, comparatively little is known on the electron donating (i.e., antioxidant) properties of HS under oxic conditions. In this work, the electron donating capacities (EDCs) of terrestrial and aquatic HS were quantified by mediated electrochemical oxidation over a wide range of pH values and applied redox potentials (Eh) using 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) as an electron transfer mediator. Electrochemical oxidation of three model humic acids (HAs) was largely irreversible, and the EDCs of these HAs increased with increasing Eh and pH. These results suggest that HS contain a wide variety of moieties that are oxidized at different potentials and that, upon oxidation, release protons and undergo irreversible follow-up reactions. At a given pH and Eh, the EDCs of the HS correlated well with their titrated phenol contents suggest...

Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes

Abstract: Atomic layer deposition (ALD) was utilized to deposit uniform thin films of hematite (α-Fe2O3) on transparent conductive substrates for photocatalytic water oxidation studies. Comparison of the oxidation of water to the oxidation of a fast redox shuttle allowed for new insight in determining the rate limiting processes of water oxidation at hematite electrodes. It was found that an additional overpotential is needed to initiate water oxidation compared to the fast redox shuttle. A combination of electrochemical impedance spectroscopy, photoelectrochemical and electrochemical measurements were employed to determine the cause of the additional overpotential. It was found that photogenerated holes initially oxidize the electrode surface under water oxidation conditions, which is attributed to the first step in water oxidation. A critical number of these surface intermediates need to be generated in order for the subsequent hole-transfer steps to proceed. At higher applied potentials, the behavior of the electrode is virtually identical while oxidizing either water or the fast redox shuttle; the slight discrepancy is attributed to a shift in potential associated with Fermi level pinning by the surface states in the absence of a redox shuttle. A water oxidation mechanism is proposed to interpret these results.

Nitrogen-Doped Graphene Nanosheets as Metal-Free Catalysts for Aerobic Selective Oxidation of Benzylic Alcohols

Abstract: This work demonstrates the molecular engineering of active sites on a graphene scaffold. It was found that the N-doped graphene nanosheets prepared by a high-temperature nitridation procedure represent a novel chemical function of efficiently catalyzing aerobic alcohol oxidation. Among three types of nitrogen species doped into the graphene lattice—pyridinic N, pyrrolic N, and graphitic N—the graphitic sp2 N species were established to be catalytically active centers for the aerobic oxidation reaction based on good linear correlation with the activity results. Kinetic analysis showed that the N-doped graphene-catalyzed aerobic alcohol oxidation proceeds via a Langmuir–Hinshelwood pathway and has moderate activation energy (56.1 ± 3.5 kJ·mol–1 for the benzyl alcohol oxidation) close to that (51.4 kJ·mol–1) proceeding on the catalyst Ru/Al2O3 reported in literature. An adduct mechanism was proposed to be different remarkably from that occurring on the noble metal catalyst. The possible formation of a sp2 N–...

Nitrogen‐Doped Carbon Monolith for Alkaline Supercapacitors and Understanding Nitrogen‐Induced Redox Transitions

Abstract: A nitrogen-doped porous carbon monolith was synthesized as a pseudo-capacitive electrode for use in alkaline supercapacitors. Ammonia-assisted carbonization was used to dope the surface with nitrogen heteroatoms in a way that replaced carbon atoms but kept the oxygen content constant. Ammonia treatment expanded the micropore size-distributions and increased the specific surface area from 383 m2?g-1 to 679 m2?g-1. The nitrogen-containing porous carbon material showed a higher capacitance (246 F?g-1) in comparison with the nitrogen-free one (186 F?g-1). Ex situ electrochemical spectroscopy was used to investigate the evolution of the nitrogen-containing functional groups on the surface of the N-doped carbon electrodes in a three-electrode cell. In addition, first-principles calculations were explored regarding the electronic structures of different nitrogen groups to determine their relative redox potentials. We proposed possible redox reaction pathways based on the calculated redox affinity of different groups and surface analysis, which involved the reversible attachment/detachment of hydroxy groups between pyridone and pyridine. The oxidation of nitrogen atoms in pyridine was also suggested as a possible reaction pathway.

An All‐Organic Non‐aqueous Lithium‐Ion Redox Flow Battery

Abstract: A non-aqueous lithium-ion redox flow battery employing organic molecules is proposed and investigated. 2,5-Di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene and a variety of molecules derived from quinoxaline are employed as initial high-potential and low-potential active materials, respectively. Electrochemical measurements highlight that the choice of electrolyte and of substituent groups can have a significant impact on redox species performance. The charge-discharge characteristics are investigated in a modified coin-cell configuration. After an initial break-in period, coulombic and energy efficiencies for this unoptimized system are ∼70% and ∼37%, respectively, with major charge and discharge plateaus between 1.8-2.4 V and 1.7-1.3 V, respectively, for 30 cycles. Performance enhancements are expected with improvements in cell design and materials processing.

Redox Transformations of Iron at Extremely Low pH: Fundamental and Applied Aspects.

Abstract: Many different species of acidophilic prokaryotes, widely distributed within the domains Bacteria and Archaea, can catalyze the dissimilatory oxidation of ferrous iron or reduction of ferric iron, or can do both. Microbially-mediated cycling of iron in extremely acidic environments (pH <3) is strongly influenced by the enhanced chemical stability of ferrous iron and far greater solubility of ferric iron under such conditions. Cycling of iron has been demonstrated in vitro using both pure and mixed cultures of acidophiles, and there is considerable evidence that active cycling of iron occurs in acid mine drainage streams, pit lakes and iron-rich acidic rivers, such as the Rio Tinto. Measurements of specific rates of iron oxidation and reduction by acidophilic microorganisms show that different species vary in their capacities for iron oxido-reduction, and that this is influenced by the electron donor provided and growth conditions used. These measurements, and comparison with corresponding data for oxidation of reduced sulfur compounds, also help explain why ferrous iron is usually used preferentially as an electron donor by acidophiles that can oxidize both iron and sulfur, even though the energy yield from oxidizing iron is much smaller than that available from sulfur oxidation. Iron-oxidizing acidophiles have been used in biomining (a technology that harness their abilities to accelerate the oxidative dissolution of sulfidic minerals and thereby facilitate the extraction of precious and base metals) for several decades. More recently they have also been used to simultaneously remediate iron-contaminated surface and ground-waters and produce a useful mineral by-product (schwertmannite). Bioprocessing of oxidized mineral ores using acidophiles that bring about the reductive dissolution of ferric iron minerals such as goethite has also recently been demonstrated, and new biomining technologies based on this approach are being developed.

Titanium and zinc oxide nanoparticles are proton-coupled electron transfer agents.

Abstract: Oxidation/reduction reactions at metal oxide surfaces are important to emerging solar energy conversion processes, photocatalysis, and geochemical transformations. Here we show that the usual description of these reactions as electron transfers is incomplete. Reduced TiO(2) and ZnO nanoparticles in solution can transfer an electron and a proton to phenoxyl and nitroxyl radicals, indicating that e(-) and H(+) are coupled in this interfacial reaction. These proton-coupled electron transfer (PCET) reactions are rapid and quantitative. The identification of metal oxide surfaces as PCET reagents has implications for the understanding and development of chemical energy technologies, which will rely on e(-)/H(+) coupling.

Visible-Light-Mediated Utilization of α-Aminoalkyl Radicals: Addition to Electron-Deficient Alkenes Using Photoredox Catalysts

Abstract: Synthetic use of α-aminoalkyl radicals formed by single electron oxidation of amines is quite limited. Here we demonstrate addition of α-aminoalkyl radicals to electron-deficient alkenes by visible-light-mediated electron transfer using transition metal polypyridyl complexes as photocatalysts, via a sequential redox pathway.

Inhibition of Charge Disproportionation of MnO2 Electrocatalysts for Efficient Water Oxidation under Neutral Conditions

Abstract: The development of Mn-oxide electrocatalysts for the oxidation of H(2)O to O(2) has been the subject of intensive researches not only for their importance as components of artificial photosynthetic systems, but also as O(2)-evolving centers in photosystem II. However, limited knowledge of the mechanisms underlying this oxidation reaction hampers the ability to rationally design effective catalysts. Herein, using in situ spectroelectrochemical techniques, we demonstrate that the stabilization of surface-associated intermediate Mn(3+) species relative to charge disproportionation is an effective strategy to lower the overpotential for water oxidation by MnO(2). The formation of N-Mn bonds via the coordination of amine groups of poly(allylamine hydrochloride) to the surface Mn sites of MnO(2) electrodes effectively stabilized the Mn(3+) species, resulting in an ~500-mV negative shift of the onset potential for the O(2) evolution reaction at neutral pH.

Bimetallic redox synergy in oxidative palladium catalysis.

Abstract: Polynuclear transition metal complexes, which are embedded in the active sites of many metalloenzymes, are responsible for effecting a diverse array of oxidation reactions in nature. The range of chemical transformations remains unparalleled in the laboratory. With few noteworthy exceptions, chemists have primarily focused on mononuclear transition metal complexes in developing homogeneous catalysis. Our group is interested in the development of carbon–heteroatom bond-forming reactions, with a particular focus on identifying reactions that can be applied to the synthesis of complex molecules. In this context, we have hypothesized that bimetallic redox chemistry, in which two metals participate synergistically, may lower the activation barriers to redox transformations relevant to catalysis. In this Account, we discuss redox chemistry of binuclear Pd complexes and examine the role of binuclear intermediates in Pd-catalyzed oxidation reactions.Stoichiometric organometallic studies of the oxidation of binucl...

Hydrogen peroxide electrochemistry on platinum: towards understanding the oxygen reduction reaction mechanism

Abstract: Understanding the hydrogen peroxide electrochemistry on platinum can provide information about the oxygen reduction reaction mechanism, whether H2O2 participates as an intermediate or not. The H2O2 oxidation and reduction reaction on polycrystalline platinum is a diffusion-limited reaction in 0.1 M HClO4. The applied potential determines the Pt surface state, which is then decisive for the direction of the reaction: when H2O2 interacts with reduced surface sites it decomposes producing adsorbed OH species; when it interacts with oxidized Pt sites then H2O2 is oxidized to O2 by reducing the surface. Electronic structure calculations indicate that the activation energies of both processes are low at room temperature. The H2O2 reduction and oxidation reactions can therefore be utilized for monitoring the potential-dependent oxidation of the platinum surface. In particular, the potential at which the hydrogen peroxide reduction and oxidation reactions are equally likely to occur reflects the intrinsic affinity of the platinum surface for oxygenated species. This potential can be experimentally determined as the crossing-point of linear potential sweeps in the positive direction for different rotation rates, hereby defined as the “ORR-corrected mixed potential” (c-MP).

Redox-active ligands in catalysis.

Abstract: A review. Nature's use of redox-active moieties combined with 3d transition-metal ions is a powerful strategy to promote multi-electron catalytic reactions. The ability of these moieties to store redox equiv. aids metalloenzymes in promoting multi-electron reactions, avoiding high-energy intermediates. In a biomimetic spirit, chemists have recently developed approaches relying on redox-active moieties in the vicinity of metal centers to catalyze challenging transformations. This approach enables chemists to impart noble-metal character to less toxic, and cost effective 3d transitional metals, such as Fe or Cu, in multi-electron catalytic reactions.

Sulfur-tolerant redox-reversible anode material for direct hydrocarbon solid oxide fuel cells

Abstract: A novel composite anode material consisting of K(2) NiF(4) -type structured Pr(0.8) Sr(1.2) (Co,Fe)(0.8) Nb(0.2) O(4+δ) (K-PSCFN) matrix with homogenously dispersed nano-sized Co-Fe alloy (CFA) has been obtained by annealing perovskite Pr(0.4) Sr(0.6) Co(0.2) Fe(0.7) Nb(0.1) O(3-δ) (P-PSCFN) in H(2) at 900 °C. The K-PSCFN-CFA composite anode is redox-reversible and has demonstrated similar catalytic activity to Ni-based cermet anode, excellent sulfur tolerance, remarkable coking resistance and robust redox cyclability.

Long-range electron transport in Geobacter sulfurreducens biofilms is redox gradient-driven.

Abstract: Geobacter spp. can acquire energy by coupling intracellular oxidation of organic matter with extracellular electron transfer to an anode (an electrode poised at a metabolically oxidizing potential), forming a biofilm extending many cell lengths away from the anode surface. It has been proposed that long-range electron transport in such biofilms occurs through a network of bound redox cofactors, thought to involve extracellular matrix c-type cytochromes, as occurs for polymers containing discrete redox moieties. Here, we report measurements of electron transport in actively respiring Geobacter sulfurreducens wild type biofilms using interdigitated microelectrode arrays. Measurements when one electrode is used as an anode and the other electrode is used to monitor redox status of the biofilm 15 μm away indicate the presence of an intrabiofilm redox gradient, in which the concentration of electrons residing within the proposed redox cofactor network is higher farther from the anode surface. The magnitude of the redox gradient seems to correlate with current, which is consistent with electron transport from cells in the biofilm to the anode, where electrons effectively diffuse from areas of high to low concentration, hopping between redox cofactors. Comparison with gate measurements, when one electrode is used as an electron source and the other electrode is used as an electron drain, suggests that there are multiple types of redox cofactors in Geobacter biofilms spanning a range in oxidation potential that can engage in electron transport. The majority of these redox cofactors, however, seem to have oxidation potentials too negative to be involved in electron transport when acetate is the electron source.

NMR structure of a lytic polysaccharide monooxygenase provides insight into copper binding, protein dynamics, and substrate interactions.

Abstract: Lytic polysaccharide monooxygenases currently classified as carbohydrate binding module family 33 (CBM33) and glycoside hydrolase family 61 (GH61) are likely to play important roles in future biorefining. However, the molecular basis of their unprecedented catalytic activity remains largely unknown. We have used NMR techniques and isothermal titration calorimetry to address structural and functional aspects of CBP21, a chitin-active CBM33. NMR structural and relaxation studies showed that CBP21 is a compact and rigid molecule, and the only exception is the catalytic metal binding site. NMR data further showed that His28 and His114 in the catalytic center bind a variety of divalent metal ions with a clear preference for Cu2+ (Kd = 55 nM; from isothermal titration calorimetry) and higher preference for Cu1+ (Kd ∼ 1 nM; from the experimentally determined redox potential for CBP21-Cu2+ of 275 mV using a thermodynamic cycle). Strong binding of Cu1+ was also reflected in a reduction in the pKa values of the histidines by 3.6 and 2.2 pH units, respectively. Cyanide, a mimic of molecular oxygen, was found to bind to the metal ion only. These data support a model where copper is reduced on the enzyme by an externally provided electron and followed by oxygen binding and activation by internal electron transfer. Interactions of CBP21 with a crystalline substrate were mapped in a 2H/1H exchange experiment, which showed that substrate binding involves an extended planar binding surface, including the metal binding site. Such a planar catalytic surface seems well-suited to interact with crystalline substrates.

Superior radical polymer cathode material with a two-electron process redox reaction promoted by graphene

Abstract: Poly(2,2,6,6-tetramethyl-1-piperidinyloxy-4-yl methacrylate) (PTMA) displays a two–electron process redox reaction, high capacity of up to 222 mA h g−1, good rate performance and long cycle life, which is promoted by graphene as cathode material for lithium rechargeable batteries.

A Glucose Fuel Cell for Implantable Brain–Machine Interfaces

Abstract: We have developed an implantable fuel cell that generates power through glucose oxidation, producing steady-state power and up to peak power. The fuel cell is manufactured using a novel approach, employing semiconductor fabrication techniques, and is therefore well suited for manufacture together with integrated circuits on a single silicon wafer. Thus, it can help enable implantable microelectronic systems with long-lifetime power sources that harvest energy from their surrounds. The fuel reactions are mediated by robust, solid state catalysts. Glucose is oxidized at the nanostructured surface of an activated platinum anode. Oxygen is reduced to water at the surface of a self-assembled network of single-walled carbon nanotubes, embedded in a Nafion film that forms the cathode and is exposed to the biological environment. The catalytic electrodes are separated by a Nafion membrane. The availability of fuel cell reactants, oxygen and glucose, only as a mixture in the physiologic environment, has traditionally posed a design challenge: Net current production requires oxidation and reduction to occur separately and selectively at the anode and cathode, respectively, to prevent electrochemical short circuits. Our fuel cell is configured in a half-open geometry that shields the anode while exposing the cathode, resulting in an oxygen gradient that strongly favors oxygen reduction at the cathode. Glucose reaches the shielded anode by diffusing through the nanotube mesh, which does not catalyze glucose oxidation, and the Nafion layers, which are permeable to small neutral and cationic species. We demonstrate computationally that the natural recirculation of cerebrospinal fluid around the human brain theoretically permits glucose energy harvesting at a rate on the order of at least 1 mW with no adverse physiologic effects. Low-power brain–machine interfaces can thus potentially benefit from having their implanted units powered or recharged by glucose fuel cells.