Valence Oxidation State Research Articles

Optimizing symmetric supercapacitor performance through rational design of Zr/Cu/Fe Oxide-Modified Poly(vinyl alcohol) hybrid nanofibers

Present work, ZrO2/CuO/PVA (ZC-PVA), ZrO2/Fe2O3/PVA (ZF-PVA) and ZrO2/CuO/Fe2O3/PVA (ZCF-PVA) nanofibers were synthesized by cost effective electrospinning method. The ZCF-PVA nanofiber prepared in this study were subjected to analysis using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier-transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) techniques, individually. XRD was employed to ascertain both the structure and crystallite size of the synthesized materials. SEM and FT-IR analysis were used to identify the morphology and functional groups present in the prepared ZC-PVA, ZF-PVA and ZCF-PVA nanofibers. XPS confirmed the surface elemental composition and oxidation states of Zr4+, Cu2+, and Fe3+ in ZCF-PVA nanofiber comprising ZrO2, CuO, and α-Fe2O3. In three electrode configuration ZCF-PVA nanofiber shows significantly higher specific capacitance of 824F/g at 1 A/g current density under 1 M KOH. The generation of synergic effect between multi valence oxidation states of mixed metal oxides and polymer (ZCF-PVA) enhances the capacitive behavior. The ZCF-PVA symmetric device exhibits 184F/g at 0.5 A/g current density with 91 % capacitance retention and 94 % of coulombic efficiency even after 5000 cycles at 3 A/g of current density.

Role of catalyst oxidation states in the selective detection of reducing gases: A comparative study of nanocomposites based on Au and chemically/thermally reduced graphene oxide

Catalyst-loaded reduced graphene oxide (rGO) is a promising material for developing high-performance gas sensors. In this work, surface functionalization is achieved by loading different quantities of nanogold (Au) on chemically and thermally reduced graphene oxide (CRGO and TRGO). These materials showed selective gas response toward reducing gases such as hydrogen (H2), carbon monoxide (CO), and methane (CH4) in the temperature range RT to 150 °C. The quantity of Au catalysts and their oxidation states in the films influenced the selectivity attributes of the studied sensors. The surface oxidation states of the Au catalyst are different for CRGO-Au and TRGO-Au films. It is apparent from the results that the difference in the Au oxidation states is due to the variation in surface oxygen functionalities of the base matrix CRGO and TRGO. The experimental results are tallied with the proposed gas selectivity mechanism of CRGO-Au and TRGO-Au films.Novelty of the Work:In reduced graphene oxide based gas sensors, the combined influence of the graphene oxide reduction method, gold (Au) catalyst quantity, and valence oxidation states (in the Au catalyst) on the gas selectivity is rarely reported. The available reports only focus on examining the metallic gold (Au0) phase of the catalyst. However, it is necessary to note that Au can also exist in various higher oxidation states (Au+1, Au+2, Au+3, etc.), which provide selective gas adsorption sites. Therefore, this work addresses the above issue by employing chemically reduced graphene oxide (CRGO) and thermally reduced graphene oxide (TRGO) decorated with Au nanoparticles (Au wt% = 1 %, 50 % & 90 %) for the selective sensing of reducing gases.

Enhanced electrocatalytic water splitting by Sm and Gd-doped ceria electrocatalysts on Ni foam substrate

CeO2 is attractive for water splitting due to its various valence oxidation states, however, its catalytic performance in alkaline medium is challenging. In this study, we synthesized novel ceria-based electrocatalysts: Gd-doped CeO2 nanocrystals (GDC) and Sm-doped CeO2 nanospheres (SDC) on Ni foam using sol–gel and one-step hydrothermal methods, respectively. These ceria-based electrocatalysts, GDC and SDC, depict excellent activity in the OER and HER, respectively. The GDC/NF showed promising OER performance, achieving an exceptional overpotentials of 300 and 420 mV to keep current densities of 10 and 100 mA cm−2, respectively. In addition, SDC/NF exhibited excellent HER activity with overpotentials of 117 and 325 mV to reach current densities of 10 and 100 mA cm-2, respectively. Furthermore, both GDC and SDC were ultra-stable at a fixed current density of 50 mA cm−2 for 25 h. Moreover, the complete anion-exchange membrane (GDC/NF||SDC/NF) demonstrated 1.6 Vcell to accomplish 10 mA cm-2 current density, with ultra-durability for 40 h at 50 mA cm-2. The versatile electrochemical performance of ceria-based catalysts is attributed to diverse-valence Ce3+/Ce4+ redox states and positive entropy contribution of the f0–f1 transition. Notably, doping with Gd and Sm led to an easy and strong reduction owing to the oxygen voids created and hydroxyls. Moreover, dopants stabilized ceria and assisted the generation of metal–H groups, which improved the kinetic activity of ceria for electrochemical water splitting.

Segregation Phenomena in Size-Selected Bimetallic CuNi Nanoparticle Catalysts.

Surface segregation, restructuring, and sintering phenomena in size-selected copper-nickel nanoparticles (NPs) supported on silicon dioxide substrates were systematically investigated as a function of temperature, chemical state, and reactive gas environment. Using near-ambient pressure (NAP-XPS) and ultrahigh vacuum X-ray photoelectron spectroscopy (XPS), we showed that nickel tends to segregate to the surface of the NPs at elevated temperatures in oxygen- or hydrogen-containing atmospheres. It was found that the NP pretreatment, gaseous environment, and oxide formation free energy are the main driving forces of the restructuring and segregation trends observed, overshadowing the role of the surface free energy. The depth profile of the elemental composition of the particles was determined under operando CO2 hydrogenation conditions by varying the energy of the X-ray beam. The temperature dependence of the chemical state of the two metals was systematically studied, revealing the high stability of nickel oxides on the NPs and the important role of high valence oxidation states in the segregation behavior. Atomic force microscopy (AFM) studies revealed a remarkable stability of the NPs against sintering at temperatures as high as 700 °C. The results provide new insights into the complex interplay of the various factors which affect alloy formation and segregation phenomena in bimetallic NP systems, often in ways different from those previously known for their bulk counterparts. This leads to new routes for tuning the surface composition of nanocatalysts, for example, through plasma and annealing pretreatments.

(Invited) Resolving Sites for Catalysis in Amorphous Metal Oxide and Molecular Water-Oxidation Catalysts Using in-Situ X-Ray Techniques

A key challenge for the development of materials and technologies for solar-to-fuels energy conversion lies in resolving structure, structural dynamics, and mechanisms underlying solar fuels catalysis, and measuring these structural parameters during functional conditions. This presentation will discuss the resolution of sites for solar-driven catalysis in amorphous cobalt oxide and molecular water-oxidation catalysts using a combination of in-situ high energy X-ray (60 keV) scattering with atomic pair distribution function (PDF) analysis, cobalt L3 edge X-ray absorption spectroscopy, and anomalous wide angle X-ray scattering (AWAXS), with a goal of resolving structure, "one electron at a time", following successive, single-electron, photo-initiated electron transfers. We have extended the in-situ PDF technique by developing 3-D porous electrode architectures that enable structural characterization of interfacial thin films during photo-electrochemistry with 0.2 Å spatial resolution. PDF-electrochemistry measurements for the cobalt borate OEC film, CoBi, poised across the potentials from 0.5 V and 1.4 V vs NHE show lattice contraction of the domains as a result of bond shortening by the accumulation of metal centers in high valence oxidation states. The structural change is associated with increased metal-oxo covalency and charge delocalization. Comparable in-situ PDF measurements for the cobalt phosphate, CoPi, OEC show that charge accumulation leads to similar domain lattice contraction. In addition, for CoPi the CoIICoIII to CoIIICoIII transition is found to be coupled to Co-O peak broadening and amplitude decreases for pairs involving edge O atoms, identifying these as the sites for redox activity. Corresponding changes are seen in the P-O peak. These results provide experimental evidence for a Pi edge-associated model with bond length disorder. The PDF data shows that Pi remains edge-bound at the onset of water oxidation, suggesting the possible positioning of Pi for function in proton-coupled electron transfer. The cobalt OEC are also distinguished by X-ray absorption at the L3 edge, with CoPi showing both octahedral and tetrahedral Co(II) assigned to lattice and edge or extra-domain connecting sites, respectively. Finally, we demonstrate new AWAXS measurements for structure resolution in molecular and mixed metal OEC.

Synthesis and Modification of Zn-doped TiO2 Nanoparticles for the Photocatalytic Degradation of Tetracycline.

The synthesis of Zn-doped TiO2 nanoparticles by solgel method was investigated in this study, as well as its modification by H2 O2 . The catalyst was characterized by transmission electron microscopy, X-ray diffraction, Brunauer-Emmett-Teller, UV-visible reflectance spectra and X-ray photoelectron spectroscopy (XPS). The results indicated that doping Zn into TiO2 nanoparticles could inhibit the transformation from anatase phase to rutile phase. Zn existed as the second valence oxidation state in the Zn-doped TiO2 . Zn-doped TiO2 that was synthesized by 5% Zn doping at 450°C exhibited the best photocatalytic activity. Then, the H2 O2 modification further enhanced the photocatalytic activity. Zn doping and H2 O2 modifying narrowed the band gap and efficiently increased the optical absorption in visible region. The optimal degradation rate of tetracycline by Zn-doped TiO2 and H2 O2 modified Zn-doped TiO2 was 85.27% and 88.14%. Peroxide groups were detected in XPS analysis of H2 O2 modified Zn-doped TiO2 , favoring the adsorption of visible light. Furthermore, Zn-doped TiO2 modified by H2 O2 had relatively good reusability, exhibiting a potential practical application for tetracycline's photocatalytic degradation.

Reverse water–gas shift reaction over ceria nanocube synthesized by hydrothermal method

A well-defined ceria nanocube with six (100) planes was successfully prepared by a facile hydrothermal method. Hydrogenation tests on the carbon dioxide, and several advanced analysis techniques, were used to investigate the catalytic performance of ceria nanocube for reverse water–gas shift (RWGS) and understand the governing reaction mechanism. The results demonstrated that the obtained ceria was a typical mesoporous material with a fluorite structure, and mainly had cerium with +4 valence oxidation state. As-obtained ceria nanocube showed good performance for RWGS reaction, while nickel on ceria evidently promoted the hydrogenation of CO2. An oxygen-transformation and metal-dissociation mechanism for RWGS reaction was proposed. The dissociation of carbon dioxide over ceria by directly oxidized oxygen vacancy was considered as a main reaction pathway of RWGS. Meanwhile, dissociated adsorption of CO2 and hydrogen over nickel surface directly formed CO and supplied spillover hydrogen to nearby oxygen vacancies, respectively. The neighboring oxygen vacancies at the interface of nickel and ceria were considered as efficient active sites for CO2 hydrogenation.

Hydrogen-transfer dehydration between alcohols over V2O3 and MoO2 catalysts for the formation of corresponding alkanes and aldehydes

Conversion of alcohols in a gas phase under N2 flow at 573K was carried out using V2O3 and MoO2 oxides with low valence oxidation states. It was found in the reaction of ethanol that equimolar amounts of ethane and acetaldehyde were catalytically formed as the main products over the oxides. Bi-products were small amounts of ethene and C4 compounds. Reactions of other alcohols (methanol, 1-propanol and 2-propanol) over the V2O3 and MoO2 catalysts also led to the equimolar formation of corresponding alkanes and aldehydes or ketone. It was confirmed by XRD and XPS that the low valence states of V2O3 and MoO2 were unchanged during the reactions and the oxides stably worked as the catalyst. Based on catalytic reaction results obtained under various reaction conditions (reaction temperature, contact time, introduction of H2 and C2H4 into reaction stream) and on experiments of kinetic isotope effects on the ethanol reaction, a reaction scheme is proposed, in which hydrogen transfer reaction between two alcohol molecules adsorbed on metal–O2−–metal sites on the surface of V2O3 and MoO2 catalysts takes place via 6-membered transition state, followed by dehydration.

Fabrication of predominantly Mn4+ -doped TiO2 nanoparticles under equilibrium conditions and their application as visible-light photocatalyts.

The chemical state of a transition-metal dopant in TiO(2) can intrinsically determine the performance of the doped material in applications such as photocatalysis and photovoltaics. In this study, manganese-doped TiO2 is fabricated by a near-equilibrium process, in which the TiO(2) precursor powder precipitates from a hydrothermally obtained transparent mother solution. The doping level and subsequent thermal treatment influence the morphology and crystallization of the TiO(2) samples. FTIR spectroscopy and X-ray photoelectron spectroscopy analyses indicate that the manganese dopant is substitutionally incorporated by replacing Ti(4+) cations. The absorption band edge can be gradually shifted to 1.8 eV by increasing the nominal manganese content to 10 at %. Manganese atoms doped into the titanium lattice are associated with the dominant 4+ valence oxidation state, which introduces two curved, intermediate bands within the band gap and results in a significant enhancement in photoabsorption and the quantity of photogenerated hydroxyl radicals. Additionally, the high photocatalytic performance of manganese-doped TiO(2) is also attributed to the low oxygen content, owing to the equilibrium fabrication conditions. This work provides an important strategy to control the chemical and defect states of dopants by using an equilibrium fabrication process.

Logic Control of Enzyme-Like Gold Nanoparticles for Selective Detection of Lead and Mercury Ions

Functional logic gates based on lead ions (Pb(2+)) and mercury ions (Hg(2+)) that induce peroxidase-like activities in gold nanoparticles (Au NPs) in the presence of platinum (Pt(4+)) and bismuth ions (Bi(3+)) are presented. The "AND" logic gate is constructed using Pt(4+)/Pb(2+) as the input and the peroxidase-like activity of the Au NPs as the output; this logic gate is denoted as "Pt(4+)/Pb(2+)(AND)-Au NPPOX". When Pt(4+) and Pb(2+) coexist, strong metallophilic interactions (between Pt and Pb atoms/ions) and aurophilic interactions (between Au and Pb/Pt atoms/ions) result in significant increases in the deposition of Pt and Pb atoms/ions onto the Au NPs, leading to enhanced peroxidase-like activity. The "INHIBIT" logic gate is fabricated by using Bi(3+) and Hg(2+) as the input and the peroxidase-like activity of the Au NPs as the output; this logic gate is denoted as "Bi(3+)/Hg(2+)(INHIBIT)-Au NPPOX". High peroxidase-like activity of Au NPs in the presence of Bi(3+) is a result of the various valence (oxidation) states of Bi(3+) and Au (Au(+)/Au(0)) atoms on the nanoparticle's surface. When Bi(3+) and Hg(2+) coexist, strong Hg-Au amalgamation results in a large decrease in the peroxidase-like activity of the Au NPs. These two probes (Pt(4+)/Pb(2+)(AND)-Au NPPOX and Bi(3+)/Hg(2+)(INHIBIT)-Au NPPOX) allow selective detection of Pb(2+) and Hg(2+) down to nanomolar quantities. The practicality of these two probes has been validated by analysis of Pb(2+) and Hg(2+) in environmental water samples (tap water, river water, and lake water). In addition, an integrated logic circuit based on the color change (formation of reddish resorufin product) and generation of O2 bubbles from these two probes has been constructed, allowing visual detection of Pb(2+) and Hg(2+) in aqueous solution.

Structural Formability of ABO<sub>3</sub>-Type Perovskite Compounds: Bond Valence Analysis

On the basis of a total of 382 ABO3-type compounds, the structural formability of ABO3-type perovskite compounds is investigated by using the bond valence model method. A new two-dimensional structural map approach for predicting the formability of ABO3-type perovskite compounds that relies on the ideal bond distances with the combination of bond valence parameter, coordination number and oxidation state is proposed. The sample points representing compounds of forming perovskite and non-perovskite are distributed in distinctively different regions. Some misclassified compounds are analyzed and some new compounds are tested within the new structure map. The developed approach can be used to search for new perovskite and perovskite-related compounds by screening all possible elemental combinations.

Chirality Driven Metallic versus Semiconducting Behavior in a Complete Series of Radical Cation Salts Based on Dimethyl-Ethylenedithio-Tetrathiafulvalene (DM-EDT-TTF)

Enantiopure (S,S) and (R,R) dimethyl-ethylenedithio-tetrathiafulvalene (DM-EDT-TTF) 1 donors are synthesized by cross coupling followed by decarboxylation reactions. In the solid state the methyl groups are arranged in axial positions within sofa-type conformation for the six-membered rings. Crystalline radical cation salts formulated as [(S,S)-1]2PF6, [(R,R)-1]2PF6, and [(rac)-1]2PF6 are obtained by electrocrystallization. When the experiment is conducted with enantioenriched mixtures both enantiopure and racemic phases are obtained. The monoclinic enantiopure salts, containing four independent donors in the unit cell, show semiconducting behavior supported by band structure calculations of extended Hückel type. The racemic salt contains only one independent donor in the mixed valence oxidation state +0.5. Under ambient pressure the racemic material is metallic down to 120 K, while an applied pressure of 11.5 kbar completely suppresses the metal-insulator transition. Band structure calculations yield an open Fermi surface, typical for a pseudo-one-dimensional metal, with unperfected nesting, thus ruling out the possibility of charge or spin density modulations to be at the origin of the transition. Raman spectroscopy measurements, in agreement with structural analysis at 100 K, show no indication of low-temperature charge ordering in the racemic material at ambient pressure, thus suggesting Mott-type charge localization for the observed metal-insulator transition.

Electronic structure effects in the vectorial bond-valence model

The vectorial bond-valence model (VBVM) describes the spatial distribution of bonds to each atom in a system in terms of the vector sum of the incident bond valences. It has been applied in the past to cations not subject to electronic structure effects (e.g., lone-pair or Jahn-Teller effects) in which case the expectation is that the vector sum will be approximately zero. Here we analyze 178 simpleoxide crystal structures and show that the vectorial bond-valence sum is a predictable function of the atomic valence (oxidation state) of each atom and the valence of the strongest bond to atoms for which second-order Jahn-Teller and lone-pair effects play a role in determining molecular geometry. Outliers are uniformly metastable or unstable under ambient conditions, suggesting that deviation from ideal vectorial bond-valence sums might be used as a proxy for some aspect of structural potential energy. These results are all strictly in harmony with the VSEPR model of molecular geometry, but may allow for more quantitative prediction.

Combined method to prepare core–shell structured catalyst for proton exchange membrane fuel cells

This paper reports a modified core–shell structured CuPd@Pt/C catalyst, which was synthesized by combining a two-step reduction method and chemical dealloying step using carbon black Vulcan XC-72R as the supporting material. The physical measurements confirmed that the final catalyst has a complete core–shell structure. The interaction between the core and the shell as well as the particle size, particle size distribution, and morphology of the catalyst particles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results of inductively coupled plasma atomic emission spectroscopy (ICP-AES) and X-ray photoelectron spectroscopy (XPS) indicated that the Pt and Pd on the surface of the catalyst nanoparticles are mainly in the zero valence oxidation state. Cyclic voltammetry (CV) and rotating disk electrode (RDE) tests were used to measure the electrochemical performance, and the results showed that the modified CuPd@Pt/C catalyst has higher electrochemical catalytic activity in catalyzing the oxygen reduction reaction (ORR) than Pt/C, making it possible to reduce the usage of platinum and leading to a promising low-Pt catalyst for proton exchange membrane fuel cells (PEMFCs).

Interfacial effects in La2/3Sr1/3MnO3 thin films with different complex oxide capping layers

Interfacial effects in sputtered La2/3Sr1/3MnO3 thin films with different capping layers (MgO, LaAlO3, SrTiO3, NdGaO3, and Au) have been locally investigated by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Mn L3,2-edge. Data were acquired by using the total electron yield detection mode thus guaranteeing maximum sensitivity to the interface. The data show that LaAlO3 capping almost does not modify the bulklike Mn valence at the interface. In case of SrTiO3 and Au, the presence of divalent Mn is detected, whereas MgO and NdGaO3 capping lead to an increase of the Mn valence oxidation state. The modification of the nominal Mn valence state leads to depressed surface magnetization.

Crystal Structures and Characterizations of Mixed Valence 12 l-Perovskites Ba4EuM3O12 (M = Ru and Ir)

The crystal structures and characterizations of Ba(4)EuM(3)O(12) (M = Ru and Ir) are reported. They crystallize in a monoclinic 12-l-perovskite-type structure with space group C2/m. The M(3)O(12) trimers and EuO(6) octahedra are alternately linked by corner-sharing and form the perovskite-type structure with 12 layers. The (151)Eu Mossbauer and X-ray photoelectron spectra reveal that the Eu and M ions are in the trivalent and mixed valence (+4.33) oxidation states, respectively. Their electrical conductivities show semiconducting behavior with an activation energy of approximately 0.2 eV above room temperature and demonstrate two-dimensional Mott variable range hopping behavior at low temperatures. The magnetic susceptibility and specific heat measurements indicate that an antiferromagnetic ordering occurs at 4 K, which is due to the magnetic interaction between Ru(3)O(12) trimers. On the other hand, the Ir(3)O(12) trimer is found to be nonmagnetic.

Oxygen-vacancy-induced green emission and room-temperature ferromagnetism in Ni-doped ZnO nanorods

Single-crystalline Ni-doped ZnO nanorods have been synthesized through a chemical method. The average length and diameter of these nanorods are in the ranges of 400–700 nm and 25–40 nm, respectively. Structural analyses reveal that the Ni-doped ZnO nanorods are of pure wurtzite hexagonal phase and grow along the preferred c-axis direction. X-ray photoelectron spectroscopy (XPS) gives evidence that the Ni dopant is in the +2 valence oxidation state and is uniformly distributed in the nanorods. Full multiple-scattering ab initio calculations of Ni K-edge x-ray absorption near edge structure (XANES) analysis reveal that Ni impurity atoms are substitutionally incorporated into ZnO host without formation of secondary phases (Ni metal and Ni2O3). The comparison of experimental and simulated XANES spectra on Ni K edge shows the presence of the oxygen vacancy (native defect) in the prepared nanorods. Photoluminescence spectrum shows two emission peaks, which are ascribed to near band edge (NBE) transitions and broadened intensive green emission associated with oxygen-vacancy defects. Furthermore, the magnetic measurements reveal that the nanorods exhibit intrinsic room-temperature ferromagnetism. Ferromagnetic ordering is interpreted by the overlapping of polarons mediated through oxygen vacancy based on the bound magnetic polaron (BMP) model.

Spectroelectrochemical properties of homo- and heteroleptic ruthenium and osmium binuclear complexes: intercomponent communication as a function of energy differences between HOMO levels of bridge and metal centres

A series of binuclear ruthenium and osmium complexes [(bipy)(2)Ru(qpy)Ru(bipy)(2)](4+) (1), [(bipy)(2)Os(qpy)Os(bipy)(2)](4+) (2), [(bipy)(2)Ru(pytr-bipy)Ru(bipy)(2)](3+) (3), [(bipy)(2)Ru(pytr-bipy)Os(bipy)(2)](3+) (4), [(bipy)(2)Os(pytr-bipy)Ru(bipy)(2)](3+)(5) and [(bipy)(2)Os(bpbt)Os(bipy)(2)](2+) (6) {bipy = 2,2'-bipyridyl; qpy = 2,2':5',5'':2'',2'''-quaterpyridyl; pytr-bipy = 3-(2,2'-bipyrid-6-yl)-5-(pyrid-2-yl)-1,2,4-triazolato, and bpbt = 5,5'-bis-(pyrid-2''-yl)-3,3'-bis-1,2,4-triazolato} are reported. Analysis of the electrochemical data focuses on structural factors and on determining the extent of electronic communication between the metal centres in the mixed valence oxidation state. Intervalence charge transfer (IT) bands could be identified in the spectra of the complexes 4 and 6 only. Analysis of their spectroelectrochemical data leads to the conclusion that the IT is superexchange mediated through the HOMO of the bridging ligand.

A study on mechanism of charging/discharging at amorphous manganese oxide electrode in 0.1 M Na 2SO 4 solution

In the present work, the mechanism of charging/discharging at the amorphous manganese oxide electrode was investigated in 0.1 M Na 2SO 4 solution with respect to amount of hydrates and valence (oxidation) states of manganese using a.c.-impedance spectroscopy, anodic current transient technique and cyclic voltammetry. For this purpose, first the amorphous manganese oxide film was potentiostatically electrodeposited, followed by heat-treatment at 25–400 °C to prepare the electrode specimen with different amounts of hydrates and oxidation states of manganese. For as-electrodeposited electrode with the most hydrates, the anodic current transient clearly exhibited a linear relationship between the logarithm of current density and the logarithm of time, with a slope of −0.5, indicating that the charging/discharging is purely limited by Na +/H + ion diffusion. From the analyses of the impedance spectra combined with anodic current transients measured on the hydrated electrode heat-treated at 25–150 °C, it was found that as the amount of hydrates decreases, the depth of cation diffusion in the electrode becomes shallower and the ratio of charge-transfer resistance to diffusion resistance also increases. This suggests that a transition occurs of pure diffusion control to a mixed diffusion and charge-transfer reaction control. For the dehydrated electrode heat-treated at 200–400 °C, the charging/discharging purely proceeds by the charge-transfer reaction. The reversibility of the redox reaction increases with increasing amount of hydrates and oxidation states of manganese, which provides us the higher power density. On the other hand, the pseudocapacitance decreases in value with increasing heat-treatment temperature, thus causing the lower energy density.

Oxidative catalysis by Mn 4O 46+ cubane complexes

The oxidation of a variety of substrates (thioethers, hydrocarbons, alkenes, benyzl alcohol and benzaldehyde) by t BuOOH catalyzed by Mn 4O 4(O 2PPh 2) 6 ( 1) and Mn 4O 4(O 2P( p-MePh) 2) 6 ( 2) is reported. These reactions illustrate the first examples of oxidative catalysis using a manganese-oxo complex with a Mn 4O 4 cubane core. These uncharged complexes contain Mn ions in a mixed valence oxidation state, formally Mn 4(2III, 2IV), and are bridged by bulky diphenylphosphinate chelates across each of the six faces of the cube. Using this system, methyl phenyl sulfide is selectively mono-oxygenated to methyl phenyl sulfoxide with high catalytic efficiency, and no evidence for further oxidation to the thermodynamically preferred sulfone. Toluene is oxidized to a mixture of benzyl alcohol, benzaldehyde, and benzoic acid with high catalytic efficiencies. Lower catalytic efficiencies are observed in the oxidation of styrene to a mixture of styrene oxide and benzaldehyde, of cyclohexene to a mixture of cyclohexene oxide, 2-cyclohexen-1-ol, and 2-cyclohexen-1-one, and of cyclohexane to a mixture of cyclohexanol and cyclohexanone. The observed product distribution from the oxidation of hydrocarbons has the characteristics of a free radical-based oxidation mechanism. However, the sulfoxidation and epoxidation activity of the 1 / t BuOOH system, as well as the observed steric preferences for less congested substrates, suggest that a metal-oxo centered oxidation mechanism is active in the reactions studied here. An intermediate species, characterized by a UV–VIS band centered at 610 nm is observed in all reaction mixtures, and forms upon reaction of 1 or 2 with t BuOOH . Preliminary evidence suggests this reactive intermediate may correspond to a Mn(V)O species. Kinetic studies suggest two pathways for oxidation: one involving an oxygen atom transfer (two-electron branch), and the other involving a hydrogen atom abstraction (one-electron branch).