Average Oxidation State Of Manganese Research Articles

Synthesis, characterization, and catalytic activity of cryptomelane nanomaterials produced with industrial manganese sulfate

Industrial manganese sulfate from manganese mines has been utilized to synthesize cryptomelane in a simple and feasible route. K-birnessite precursor was prepared by air oxidation of the mixture of MnSO 4 and KOH solutions under alkaline conditions, and then transformed to cryptomelane under a heating process. The effects of OH − concentration, airflow rate, liquid reaction temperature, stirring rate, liquid reaction time, washing condition, calcination time, and temperature were investigated. X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) revealed that cryptomelane prepared under optimal conditions had a tetragonal symmetry and the particles were mostly in short lathy form with sizes of 20–30 nm. The average pore size and BET surface area of cryptomelane examined by N 2 adsorption methods were 24.15 nm and 32.21 m 2/g, respectively. X-ray photoelectron spectroscopy (XPS) studies demonstrated that the average oxidation state of manganese in cryptomelane was about 3.9 in comparison with prephase K-birnessite of 3.4. The synthesized cryptomelane sample showed improved catalytic activity for decomposition of hydrogen peroxide as compared with natural cryptomelane, but lower than those synthesized with hydrothermal and sol–gel methods. The results of this investigation will provide fundamental information for developing a large-scale production process for transforming manganese sulfate to cryptomelane.

Facile synthesis of Ag–OMS-2 nanorods and their catalytic applications in CO oxidation

Ag–OMS-2 nanorods were synthesized by a simple solid-state reaction of AgMnO 4 with manganese acetate. The nanorods had a diameter of about 8 nm and lengths of 50–150 nm with the co-existence of micropores and mesopores. Structural analysis revealed that the nanorods were amorphous but exhibited lattice fringes of Ag–hollandite structure. The silver species presented as Ag + and the average oxidation state of manganese was 3.7. Most importantly, the Ag–OMS-2 nanorods showed quite high catalytic performance for CO oxidation with 100% CO conversion at 90 °C, mainly because of the significantly enhanced oxygen activation and CO adsorption with the presence of Ag + in the tunnels.

Synthesis and Characterization of Ag−Hollandite Nanofibers and Its Catalytic Application in Ethanol Oxidation

Ag−hollandite nanofibers were synthesized through a simple hydrothermal process by oxidizing Mn(NO3)2 with AgMnO4 in aqueous solution. The temperature was found to play an essential role in determining both the crystalline structure and the morphology of the Ag−hollandite materials, whereas the AgMnO4/Mn(NO3)2 molar ratio affected only the morphology of the product. Ag−hollandite nanofibers with diameters of 20−40 nm and lengths of 0.5−4 μm were prepared at 160 °C and a 2/3 AgMnO4/Mn(NO3)2 molar ratio. X-ray photoelectron spectroscopy surface analysis revealed that the silver species presented as Ag+ and the average oxidation state of manganese was 3.9. More promisingly, the Ag−hollandite nanofibers showed quite high catalytic performance for ethanol oxidation, with ethanol conversion of 75% and acetaldehyde selectivity of 95% at 230 °C for 200 h time-on-stream. The high activity, selectivity, and stability were attributed to the stable presence of Ag+ species and the unique morphology of the Ag−hollandit...

Simultaneous synthesis of high crystalline manganese oxides with layered and tunnel structures

High crystalline manganese oxides with birnessite-type (H-BirMO) and large 2 × 4 tunnel-type (Na-2 × 4) were synthesized simultaneously using a low crystalline Na-form birnessite (NaBir) as a precursor in a 2 M NaOH solution at low temperature (150 °C) and autogenous pressure. The weight ratio of the obtained Na-2 × 4 to H-BirMO was about 50. Elemental analyses indicated that Na-2 × 4 had a formula in one unit cell of Na 8Mn 27O 53·9H 2O, and the manganese average oxidation state was 3.63. H-BirMO had a formula of Na 0.31Mn 2.04O 4·0.66H 2O and the manganese average oxidation state of 3.77. H-BirMO contained a set of basal reflections with d values that correspond to a minimum periodicity along c equal to 7.06 Å. H-BirMO belonged to a monoclinic system, and the lattice parameters were a = 5.18 Å, b = 2.85 Å, c = 7.35 Å, and β = 103.3°, respectively. Na-2 × 4 showed that the first peak appeared at a d spacing of 12.07 Å in the [0 0 2] direction, and the second peak had a similar d spacing of 7.16 Å in the [2 0 0] direction with Na-birnessite in the [1 0 0] direction. FE-SEM image of H-BirMO showed a very large plate-like morphology, while SEM photograph of Na-2 × 4 consisted of nanofibers with a thickness of about 80 nm and lengths ranging between 6 and 10 μm.

An investigation of intercalation-induced stresses generated during lithium transport through sol–gel derived Li xMn 2O 4 film electrode using a laser beam deflection method

The stress changes Δ σ generated during lithium transport through the sol–gel derived Li x Mn 2O 4 film electrodes annealed at 773 and 873 K were quantitatively determined as a function of the lithium stoichiometry x using a laser beam deflection method (LBDM). Δ σ generated during a real potential step between an initial electrode potential and a final applied potential was uniquely specified by the Δ σ versus x curve. The Li x Mn 2O 4 film annealed at 773 K for 24 h (low temperature (LT)-Li x Mn 2O 4) showed larger capacity than the Li x Mn 2O 4 film annealed at 873 K for 6 h (high temperature (HT)-Li x Mn 2O 4) and this result is ascribed to the fact that the smaller the grain size is, the more increases the electrochemically active area of the film electrode. From the analysis of the normalised Δ σ transient measured simultaneously along with the cyclic voltammogram in the potential range of 2.5–3.4 V Li / L i + , it is found that normalised Δ σ generated in the LT-Li x Mn 2O 4 was smaller than that in the HT-Li x Mn 2O 4 during the lithium intercalation/de-intercalation around 3.0 V Li / L i + region. This result gives an experimental evidence for the fact that the Jahn–Teller distortion is suppressed by the increase in the average oxidation state of manganese with decreasing in annealing temperature.

Lithium–manganese oxide electrodes with layered–spinel composite structures xLi 2MnO 3 · (1 − x)Li 1 + yMn 2 − yO 4 (0 < x < 1, 0 ⩽ y ⩽ 0.33) for lithium batteries

Lithium–manganese oxide electrodes containing structurally integrated domains of layered and spinel phases, represented in two-component notation as xLi 2MnO 3 · (1 − x)Li 1 + y Mn 2 − y O 4 (0 < x < 1, 0 ⩽ y ⩽ 0.33), were synthesized and evaluated in lithium cells. During an initial charge to 5 V, lithium and/or lithia (Li 2O) are extracted from the spinel and layered components at distinct voltages. When cycled between 5 and 2 V, a rechargeable capacity >250 mAh/g can be achieved from electrodes prepared at 400 °C during the early cycles. The discharge profiles confirm the layered–spinel character of the composite electrodes. The Li 2MnO 3 Mn(IV) content in the parent structure and the degree to which lithium and lithia are extracted from the electrode during the initial charge can be used to control the average manganese oxidation state at the end of discharge and electrochemical cycling stability.

A Magnetic Route to Measure the Average Oxidation State of Mixed-Valent Manganese in Manganese Oxide Octahedral Molecular Sieves (OMS)

A magnetic route has been applied for measurement of the average oxidation state (AOS) of mixed-valent manganese in manganese oxide octahedral molecular sieves (OMS). The method gives AOS measurement results in good agreement with titration methods. A maximum analysis deviation error of +/-7% is obtained from 10 sample measurements. The magnetic method is able to (1) confirm the presence of mixed-valent manganese and (2) evaluate AOS and the spin states of d electrons of both single oxidation state and mixed-valent state Mn in manganese oxides. In addition, the magnetic method may be extended to (1) determine AOS of Mn in manganese oxide OMS with dopant "diamagnetic" ions, such as reducible V5+ (3d0) ions, which is inappropriate for the titration method due to interference of redox reactions between these dopant ions and titration reagents, such as KMnO4, (2) evaluate the dopant "paramagnetic" ions that are present as clusters or in the OMS framework, and (3) determine AOS of other mixed-valent/single oxidation state ion systems, such as Mo3+(3d3)-Mo4+(3d2) systems and Fe3+ in FeCl3.

(La 0.4Ba 0.4Ca 0.2)(Mn 0.4Ti 0.6)O 3: A new titano-manganate with a high dielectric constant and antiferromagnetic interactions

A new perovskite-based titano-manganate, (La 0.4Ba 0.4Ca 0.2)(Mn 0.4Ti 0.6)O 3, has been prepared by the ceramic route at 1100°C. This oxide was found to possess the cubic perovskite structure with ‘a ’=3.9477(23) Å (space group Pm 3 ̄ m ). The refined composition as obtained by Rietveld analysis of powder X-ray data was found to be (La 0.44Ba 0.38Ca 0.18)(Mn 0.43Ti 0.57)O 2.91(3) ( R p=0.0704, w R p=0.0828). The composition was also ascertained by Energy dispersive X-ray analysis. Iodometric studies led to a slightly higher oxygen content (compared to Rietveld refinement) corresponding to an average manganese oxidation state of 3.05. The above oxide was found to exhibit high dielectric constant ( ε) of 6980 at 1 kHz decreasing to 590 at 100 kHz. At high temperatures (200°C) it shows an unusually high dielectric constant of 20,000 at 1 kHz. In addition to the dielectric properties, detailed magnetic studies show evidence of long-range antiferromagnetic interactions near 5 K. The presence of unusually high dielectric constant coupled with the long-range magnetic interactions may open up interesting applications.

Mössbauer Study of Lanthanum–Strontium Ferromanganite Oxides

The La0.88Sr0.2Mn(1−y)Fey0(3±δ, i (LSMF with y = 0, 0.2, 0.5, 0.8, 1) compounds are prospective cathode materials for advanced solid oxide fuel cells (SOFC) application operating at 700° C. Usual analysis methods like thermogravimetric analysis or redox titration enable to determine the average oxidation state of both manganese and iron cations. The comparative role of iron and manganese in B-site was evaluated by 57Fe Mossbauer spectroscopy. Spectra revealed that the complete substitution of iron for manganese induces the formation of Fe5+ for the compound with y = 1. However, no tetravalent iron cation was observed in air for the LSMF compounds with y = 0.2, 0.5 and 0.8. This means that only manganese cations are electronically active in the bulk with valence states Mn3+ and Mn4+. Then, when the iron content increases, the concentration in oxygen vacancies increases also facilitating anionic diffusion. The charge disproportionation Fe3+/Fe5+ can improve the electrical properties for the compound with y = 1.

Effects of Alkali Metal and Ammonium Cation Templates on Nanofibrous Cryptomelane-type Manganese Oxide Octahedral Molecular Sieves (OMS-2)

Four alkali cations (Li+, Na+, K+, and Rb+) and ammonium (NH4+), have been successfully used as templates for the synthesis of cryptomelane-type manganese oxide octahedral molecular sieves (A−OMS-2, A = the template cation). H−OMS-2 material has been prepared by the calcination of NH4−OMS-2. These A−OMS-2 materials have been characterized by X-ray diffraction, transmission electron microscopy, high-resolution scanning electron microscopy, thermogravimetric analysis, temperature-programmed desorption, inductively coupled plasma analysis, and BET surface area pore size distribution measurements. Catalytic evaluation of these A−OMS-2 materials has been done with the aerobic oxidation of cyclohexanol to cyclohexanone. All of these A−OMS-2 materials have cryptomelane tunnel structures with different degrees of crystallinity and nanofibrous morphology. The nature of the cation templates affects the formation and microstructures of the A−OMS-2 materials. The physical and chemical properties, such as the thermal stability, chemical composition, and average oxidation state of manganese, greatly depend on the nature of cations and amount of tunnel water. Catalytic activities of these A−OMS-2 materials are related to their surface areas.

Optimizing preparation conditions for 5 V electrode performance, and structural changes in Li 1− xNi 0.5Mn 1.5O 4 spinel

A new lithium-excess method is used for the synthesis of LiNi 0.5Mn 1.5O 4 electrode materials at temperatures in the 600–800 °C range. Higher average manganese oxidation state and lower impurity contents are detected from X-ray diffraction in the spinel samples, as compared with stoichiometric synthesis. These properties cause the virtual elimination of the 4 V capacity, thus allowing a higher performance of these 5 V materials. A two-phase model of lithium extraction–insertion is detected by X-ray diffraction of electrodes prepared at different extensions of charge.

6Li Magic Angle Spinning NMR Study of the Cathode Material LiNixMn2 − x O 4 : The Effect of Ni Doping on the Local Structure during Charging

magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy has been used to study the local structure of synthesized by solid-state reactions. When the extent of doping is small and 0.1), several resonances are observed, which are assigned to lithium local environments containing different numbers of and ions. As the doping level increases and the average manganese oxidation state rises, the intensity of the resonances assigned to lithium environments containing a greater number of higher-oxidation state manganese ions increases; this results in a gradual shift of the center of mass of the spectrum to higher frequency. By only ions are present, and only one major resonance, due to the lithium local environment is observed; this is consistent with Mn/Ni ordering on the octahedral sites of the spinel structure. The variable temperature NMR behavior of these samples is indicative of different magnetic behavior for the undoped and doped materials. A plot of shift) vs. temperature is consistent with the predominance of antiferromagnetic correlations for low doping level samples, while antiferromagnetic correlations clearly dominate the behavior of the sample, resulting in overall ferrimagnetic behavior. spectra were also collected following various levels of charging. In the case of is deintercalated from the different local sites sequentially: lithium ions in the local environment containing manganese ions with an average oxidation state of +3.5 are deintercalated first, followed by lithium ions in sites containing progressively more ions. In contrast, for where the deintercalation process involves oxidation of only, no changes in the lithium local environments were observed during the charging process: the shift position remained constant during the charging cycle and only a loss of the intensity of the lithium signal was observed. © 2001 The Electrochemical Society. All rights reserved.

Effects of Partial Acid Delithiation on the Electrochemical Lithium Insertion Properties of Nickel-Stabilized LiMn2O4 Spinel Oxides

Use of nickel-containing LiMn2O4 up to 20 at.% as an insertion electrode in lithium cells combined with preliminary acid treatment results in improved cycling performance. An increase in the average oxidation state of manganese, a decrease in the Li/Mn ratio, partial H+/Li+ exchange, and a decrease in the unit cell parameters of the solid products take place during treatment with aqueous HNO3. X-Ray diffraction line broadening analysis shows that for a sample with a Ni/Mn ratio of 0.25 and a temperature of preparation of 800°C, crystallinity increases slightly with acid treatment, as a result of the dissolution of the external layers of the solid in which defects concentrate. For samples prepared below 800°C an increase in line broadening associated with a larger extension of the cation-exchange process is observed. The use of acid-treated solids in lithium cells results in higher capacities and better capacity retention as compared with pristine spinels. Larger capacities are always observed for samples prepared at 550°C and a Ni/Mn ratio of 0.11. The role of nickel is of particular interest in stabilizing the spinel structure and allowing high average oxidation states of manganese in the solid.

Particle Size Control and Self-Assembly Processes in Novel Colloids of Nanocrystalline Manganese Oxide

The synthesis of semiconducting nanocrystals of manganese oxide of controlled sizes and their manipulation to form ordered arrays is described. Nanocrystalline mixed-valent manganese oxides have been prepared as colloidal solutions via reduction of tetraalkylammonium (methyl, ethyl, propyl, and butyl) permanganate salts in aqueous solutions with 2-butanol and ethanol. Reduction with the poorly water miscible 2-butanol produces aqueous colloids for the methyl, ethyl, and propyl systems, whereas 2-butanol colloids are produced for the butyl system. The colloids are reddish-brown, have an average manganese oxidation state of 3.70−3.79, and have been prepared in manganese concentrations up to 0.57 M. The sols will gel upon aging, and the gel time depends on the cation, the amount of alcohol, the temperature, and the concentration of manganese. Small angle neutron scattering (SANS) data indicate that the particles are disklike in shape with radii in the range 20−80 A and are largely unassociated in solution. T...

Relationship between Chemical Bonding Nature and Electrochemical Property of LiMn2O4 Spinel Oxides with Various Particle Sizes: “Electrochemical Grafting” Concept

Systematic Mn 2p XPS and Mn K-edge XAS analyses together with the electrochemical measurement have been carried out for the spinel LiMn2O4 prepared at various sintering temperatures in order to elucidate an origin of the dependence of electrochemical properties on synthetic conditions. From the comparative experiments, it becomes clear that a lowering of synthetic temperature gives rise to an increase of structural disorder and of the average oxidation state of manganese, which is more prominent on the surface than in the bulk. Such results suggest that the modification of surface property induced by a decrease of particle size is closely related to the electrochemical performance. The nanocrystalline LiMn2O4 prepared at 250 °C shows excellent cyclability at the 3 V region compared to that of microcrystalline LiMn2O4 prepared at 700 °C. For the purpose of examining the evolution of the chemical bonding nature of inserted lithium, 7Li MAS NMR studies have been performed for both the spinel compounds before...

Physical and electrochemical characterization of nanocrystalline LiMn2O4 prepared by a modified citrate route

Abstract Nanocrystalline LiMn 2 O 4 spinel oxides have been synthesized successfully by using a modified citrate route with ethanol dehydration of a Li–Mn–citrate complex solution. The effects of sintering temperature and atmosphere on the physicochemical properties of the LiMn 2 O 4 powders have been carefully examined by means of X-ray diffraction, electron microscopy, BET measurement, and X-ray absorption spectroscopy. It becomes clear that an oxidative atmosphere induces a partial agglomeration of particles at high temperature, but gives rise to an effective decomposition of the citrate precursors at low temperature. It is also found that a lowering of the sintering temperature enhances the average oxidation state of manganese with an increase in surface defects. The electrochemical property of the present LiMn 2 O 4 nanocrystals has also been measured to compare with that of lithium manganate prepared by a solid-state reaction, which confirms the effectiveness of the present modified citrate route. The evolution of geometrical and electronic structures during the charging process has also been investigated with X-ray absorption spectroscopy, and reveals shortening of the (Mn–O) bond distances induced by lithium de-intercalation.

Supertransferred Hyperfine Fields at7Li: Variable Temperature7Li NMR Studies of LiMn2O4-Based Spinels

The temperature dependence of the 7Li NMR shift was measured for LiMn2O4, LiMn2-yNiyO4 (y = 0.1, 0.25, 0.33), LiMn2-yCoyO4 (y = 0.25, 0.5, 1.0), Li[Mn2-yLiy]O4 (y = 0.1, 0.33), and λ-MnO2 spinel oxides. The 7Li NMR shift can be separated into temperature-independent and -dependent components. The temperature-dependent shift follows the Curie−Weiss behavior of the bulk magnetic susceptibility. The temperature-independent shift is attributed to contributions from van Vleck and diamagnetic susceptibilities. Pauli susceptibility may also contribute to the temperature-independent shift in the nickel- and cobalt-substituted spinels. Supertransferred hyperfine (STH) coupling constants were derived from the 7Li NMR shifts and bulk magnetic susceptibility data. The progressive increase in average nominal manganese oxidation state from +3.5 to +4 results in an increase in the supertransferred hyperfine field at the 7Li nucleus in the lithium-substituted samples. Replacement of manganese by either cobalt or nickel a...

Effect of crystallinity on the electrochemical behaviour of spinel Li 1.03Mn 2O 4 cathode materials

The spinel Li 1.03Mn 2O 4 powders with a different crystallinity were synthesized by a sol-gel method from an aqueous solution of metal acetates containing glycolic acid as a chelating agent. The relation between physicochemical and electrochemical properties of the Li 1.03Mn 2O 4 powders was studied. It was seen that as the calcination temperature increased, the lattice constant linearly increased whereas the average oxidation state of manganese linearly decreased, which resulted in an increase in crystallinity of the Li 1.03Mn 2O 4 host. The capacity in the 4 V region linearly increased with increasing the lattice constant of the cubic unit cell and decreasing the average oxidation state of manganese. The glycolic acid-assisted Li 1.0 3Mn 2O 4 powders calcined at 800°C have delivered a discharge capacity of 134 mAh/g and shown an excellent cyclability.

Structure, composition and magnetocaloric properties in polycrystalline La A MnO (A = Na, K)

The polycrystalline perovskitelike manganese oxides La1-xAxMnO\(_{3 + \delta }\) (A = Na, and K, 0.05 < x ≤ 0.20) have been fabricated by sol-gel technique. For all the compositions explored in this work, the average manganese oxidation state is practically constant, at 3.32 ± 0.02 for A = Na, and 3.40 ± 0.04 for A = K, respectively. A close relationship is confirmed to hold between the Curie temperature (Tc) and the bond distance of Mn-O. Results of magnetic measurements show that these materials can be utilized as suitable candidates for magnetic refrigerants with wide applied temperature span, for their significant entropy change and the easily tuned Curie temperature.

Thermochemistry of Framework and Layer Manganese Dioxide Related Phases

The energetics of a set of natural and synthetic manganese dioxides having a framework or a layer structure were determined by high-temperature solution calorimetry using sodium molybdate as a solvent. Enthalpies of formation of the metastable manganese dioxides were calculated from the measured values of the enthalpies of drop solution. The stability of the open structure depends mostly on the nature of and the amount of the tunnel (or interlayer) cation and on the degree of hydration. It depends less strongly on the topology of the structure itself and on the average oxidation state of manganese. The synthesis of these metastable microporous materials is not limited by the energetics of the structure.