Average Oxidation State Of Manganese Research Articles

Gd2CuZnMn4O12: A-site columnar-ordered perovskite with anisotropic thermal expansion and a gradual charge-order transition

A new A-site columnar-ordered quadruple perovskite, Gd2CuZnMn4O12, was synthesized by a high-pressure, high-temperature method at 6 GPa and 1500 K. Its structural properties between 100 K and 800 K were investigated by synchrotron powder X-ray diffraction. It has space group Pmmn (No. 59) between 100 K and about 450 K with a layered charge order (CO) of Mn3+ and Mn4+ cations at the perovskite B sites and space group P42/nmc (No. 137) above 475 K with one perovskite B site (having an average manganese oxidation state of +3.5). Anisotropic thermal expansion was observed as the c lattice parameter decreases with increasing temperature above 325 K. Temperature dependence of the unit-cell volume and the lattice parameters and differential scanning calorimetry measurements gave evidence that the CO transition (with TCO ≈ 475 K) is of the second order. Gd2CuZnMn4O12 shows a ferrimagnetic transition below TC = 105 K with an additional decrease of magnetic susceptibilities below about 45 K, probably caused by a gradual increase of the ordered moments on the Gd sublattices coupled antiferromagnetically with the existent ferrimagnetic structure.

Tuning electrocatalytic water oxidation by MnOx through the incorporation of abundant metal cations

Preparation and detailed characterization of metal cation doped layered manganese oxides (MyMnOx/CFP). Metal doping leads to tuning of the average manganese oxidation state and an OER performance boost under different pH conditions.

Facile design and synthesis of Co-free layered O3-type NaNi0.2Mn0.2Fe0.6O2 as promising cathode material for sodium-ion batteries

• Design the structure of the cathode material by capturing the cation potential. • The O3-type NaNi 0.2 Mn 0.2 Fe 0.6 O 2 were synthesized with co-precipitation method. • Valence Changes of Iron Elements by 57 Fe-M o ¨ ssbauer spectroscopy. O3-type layered oxides have received extensive attention as a class of battery layered materials. However, problems such as rate capability, stable period difference, and voltage deviation of these oxides hinder their practical application. The method of orthogonal experiment was adopted, and the reaction temperature, raw material concentration, reaction time and ammonia concentration were regarded as single factors. Finally, the optimal conditions for synthesizing cathode materials were obtained as follows: the reaction temperature was 50 °C, the raw material concentration was 0.2 mol·L -1 , the reaction time was 16 h, and the ammonia concentration was 1 mol·L -1 . A layered oxide cathode material Co-free NaNi 0.2 Mn 0.2 Fe 0.6 O 2 with high iron content was synthesized by co-precipitation method. When the charging voltage is greater than 4 V, the higher the utilization rate of sodium ions, the higher the energy density. But the higher the cut-off voltage, the worse the cycle stability. Due to the low manganese content and the high iron content, the average oxidation state of manganese is low. 57 Fe-M o ¨ ssbauer spectroscopy showed that the synthesized samples had high spin Fe 3+ . Under the control of the optimal synthesis conditions, the specific discharge capacity of the reaction increased from 105 mAhּ·g −1 to 112.5 mAhּ·g −1 at 0.1C.

Novel cryptomelane nanosheets for the superior catalytic combustion of oxygenated volatile organic compounds

This work offers a novel pathway to prepare cryptomelane manganese oxides nanosheets as an superior catalyst for the catalytic combustion of oxygenated volatile organic compounds. The tunnel cryptomelane manganese oxides nanosheets were prepared from layered birnessite via simultaneously tuning pH and molar ratio (ROK) of the redox-precipitation between oxalic acid and KMnO4. Thus, few-layered cryptomelane nanosheets possessing the most predominantly exposed (211) facet are generated at low pH (5.2–5.6), which intensifies the surface area of thin crystal cryptomelane nanosheets up to 177 m2g-1 and weakens Mn-O bonds. Moreover, high ROK results in low manganese average oxidation state (AOS), greater oxygen vacancies and better low-temperature reduction and oxygen mobility. Such features significantly maneuver the catalytic activity of the cryptomelane nanosheets catalysts for the complete oxidation of oxygenated volatile organic compound (e.g., 2-propanol, acetone) at low temperature (170–230 °C). Moreover, the catalysts show high stability for 48 h. The presented catalyst discloses an avenue to address current obstacles in the catalytic oxidation of volatile organic compounds.

Structural and Electrical Studies for Birnessite-Type Materials Synthesized by Solid-State Reactions.

The focus of this paper is centered on the thermal reduction of KMnO4 at controlled temperatures of 400 and 800 °C. The materials under study were characterized by atomic absorption spectroscopy, thermogravimetric analysis, average oxidation state of manganese, nitrogen adsorption–desorption, and impedance spectroscopy. The structural formulas, found as a result of these analyses, were and . The N2 adsorption–desorption isotherms show the microporous and mesoporous nature of the structure. Structural analysis showed that synthesis temperature affects the crystal size and symmetry, varying their electrical properties. Impedance spectroscopy (IS) was used to measure the electrical properties of these materials. The measurements attained, as a result of IS, show that these materials have both electronic and ionic conductivity. The conductivity values obtained at 10 Hz were 4.1250 × 10−6 and 1.6870 × 10−4 Ω−1cm−1 for Mn4 at 298 and 423 K respectively. For Mn8, the conductivity values at this frequency were 3.7074 × 10−7 (298) and 3.9866 × 10−5 Ω−1cm−1 (423 K). The electrical behavior was associated with electron hopping at high frequencies, and protonic conduction and ionic movement of the K+ species, in the interlayer region at low frequencies.

Creation of controllable cationic and anionic defects in tunnel manganese oxide nanowires for enhanced oxygen evolution reaction

The development of efficient oxygen evolution reaction (OER) catalysts is crucial to the environmental and economic feasibility of electrolysis for the production of hydrogen gas. In this work, two facile chemical treatments, acid leaching and transition metal doping, were shown to modify the chemical and structural properties of low cost and environmentally friendly α-MnO2 nanowire catalysts resulting in increased OER activity. Through a combination of XPS and XRD analyses, it was shown that a molten salt treatment of α-MnO2 nanowires with Co(NO3)2 introduced Co2+ ions into the structural tunnels. The introduction of minimal amounts of cobalt (<6 at. %) more than doubled ultimate current densities achieved in linear sweep voltammetry tests. This result was attributed to a reduction of the average oxidation state of manganese in Co-doped samples and a decrease in Tafel slope below 80 mV·dec−1. Acid leaching, on the other hand, is believed to modify nanowire topography through the creation of oxygen and manganese vacancies, exposing more active sites to participate in catalysis. A characterization approach combining atomic absorption spectroscopy and iodometric titration revealed a 5.1% increase in oxygen vacancies after 72h of interaction with nitric acid. When these controllable defect formation approaches were applied in tandem, the high activity of the cobalt-doped samples was combined with the increased number of exposed active sites achieved through acid leaching producing a highly efficient electrocatalyst with more than a 3-fold increase in OER activity over pristine α-MnO2 nanowires. Our results establish that scalable and easy-to-implement approaches, such as acid leaching and transition metal doping, can lead to more than a three-fold increase in OER activity of low-cost non-toxic manganese oxides. This methodology can be beneficial for other material systems used as OER electrocatalysts.

The enhanced catalytic activities of octahedral layer birnessite-type manganese oxide synthesized via precipitation method for the degradation of methylene blue

The catalytic activities of the octahedral layer birnessite-type manganese oxides for the degradation of methylene blue (MB) dye have been studied. The oxide catalysts were prepared via a facile precipitation method using the reaction between KMnO4 (oxidant) and citric acid or oxalic acid (reductant), where the acid is added dropwise into KMnO4 until certain volume achieved to generate a brownish black precipitate. The solid product was treated with either calcination or without calcination process, respectively. The samples without calcination display poor crystallinity with low intensity and broad peak and are identified as birnessite-type manganese oxides. The calcination, however, led to the formation of the highly crystalline birnessite materials. The birnessite catalysts synthesized by the different reductants displayed different crystallinity, morphology, average oxidation state (AOS) of manganese, zero-point charge pH (pHzpc) and particle size that affect their catalytic activities. The results indicated that the activities of birnessite-type manganese oxide catalysts for MB degradation correlated with the nature of catalysts. The citric acid-prepared manganese oxide catalyst (CAC) showed slight higher crystalline phase compared with the oxalic acid-prepared manganese oxide catalyst (OAC). The data from AOS of manganese suggested that the CAC catalysts have a lower value of AOS of manganese than OAC catalysts, indicating more defects due to the presence of more oxygen vacancy or more Mn3+ content. The smaller particle size and AOS of manganese for CAC catalysts are also considered to be responsible for their higher catalytic activities for MB degradation. The birnessite-type manganese oxide catalysts prepared without calcinations also showed much enhanced catalytic activities for MB degradation.

Copper modified manganese oxide with tunnel structure as efficient catalyst for low-temperature catalytic combustion of toluene

Copper-modified manganese oxide catalysts with tunnel, layer, and transitional structure were successfully prepared by a one-step hydrothermal method and investigated for the catalytic oxidation of toluene. The catalysts were characterized by XRD, SEM, TEM, BET, H2-TPR, XPS and ICP-AES techniques. The tunnel-structured catalyst exhibited the best activity among the three Cu-Mn oxide catalysts and excellent water resistance. Abundant surface oxygen species, good low temperature reducibility, low average oxidation state of manganese, and high surface area lead to remarkable catalytic performance of this catalyst. The toluene catalytic combustion over these catalysts was found to follow the Mars-van-Krevelen mechanism and the unique copper states found in the tunnel-structured catalyst play a key role in promoting the cyclic redox process.

A one-pot synthesis of Fe-doped cryptomelane type octahedral molecular sieve manganese oxide for degradation of methylene blue dye

The Fe-doped octahedral molecular sieve manganese oxides have been successfully synthesized by one-pot synthesis of the reaction between KMnO4 and glucose using sol-gel methods. The oxide products are then characterized by various techniques such as X-ray diffraction, scanning electron microscopy, atomic absorption spectroscopy and average oxidation state (AOS) of manganese in manganese oxides. The as-synthesized manganese oxide and Fe-doped manganese oxides are used as catalysts for the degradation of methylene blue dye using hydrogen peroxide as oxidants. The results indicated that the Fe-doped manganese oxide catalyst displayed much enhanced catalytic activities compared to undoped manganese oxide for methylene blue degradation. The differences in catalytic activities have been correlated with the difference in surface properties and crystallinity.

Chemical Regeneration of Manganese Oxide-Coated Sand for Oxidation of Organic Stormwater Contaminants.

Urban stormwater, municipal wastewater effluent, and agricultural runoff contain trace amounts of organic contaminants that can compromise water quality. To provide a passive, low-cost means of oxidizing substituted phenols, aromatic amines, and other electron-rich organic compounds during infiltration of contaminated waters, we coated sand with manganese oxide using a new approach involving the room-temperature oxidation of Mn2+ with permanganate. Manganese oxide-coated sand effectively oxidized bisphenol A under typical infiltration conditions and sustained reactivity longer than previously described geomedia. Because geomedia reactivity decreased after extended operation, chlorine was evaluated for use as an in situ geomedia regenerant. Geomedia regenerated by HOCl demonstrated similar reactivity and longevity to that of virgin geomedia. Chemical analyses indicated that the average manganese oxidation state of the coatings decreased as the geomedia passivated. X-ray absorption spectroscopy and X-ray diffraction showed that the reactive virgin and regenerated geomedia coatings had nanocrystalline manganese oxide structures, whereas the failed geomedia coating exhibited greater crystallinity and resembled cryptomelane. These results suggest that it is possible to regenerate the oxidative capacity of manganese oxide-coated sands without excavating stormwater infiltration systems. These results also suggest that manganese oxide geomedia may be a cost-effective means of treating urban stormwater and other contaminated waters.

Effect of heat treatment condition on magnetic, electrical transport and magnetoresistance properties of La0.67Sr0.33MnO3 manganite coatings

Effects of three different heat treatment conditions (air, vacuum and nitrogen) on the structure, magnetic, electrical transport and low-field magnetoresistance behaviors of La0.67Sr0.33MnO3 manganite coatings synthesized by sol-gel and screen printing methods have been systematically investigated. The detailed crystallographic analyses revealed that the diffraction peak positions of the samples subjected to annealing in vacuum and nitrogen ambients shifted to lower-angle regions, and lattice parameters and structure distortion increased as compared with those of the sample annealed in air, which were due to the introduction of oxygen deficiency under the annealing conditions of vacuum and nitrogen. The content of oxygen deficiency of the samples could be determined by the change of lattice parameters. Furthermore, the vacuum and nitrogen annealed samples exhibited a dramatic increase in the resistivity, accompanied by the reduction in ferromagnetism, the resistivity peak temperature and the ferromagnetism transition temperature. These phenomena were attributed to the decrease of average manganese oxidation state, as a result the mobile charge carriers were reduced, the ferromagnetic coupling and the double exchange effect between manganese ions decreased while the magnetic inhomogeneity increased. In the meanwhile, magnetoresistance ratios of the oxygen-deficient samples were enhanced around the percolation threshold.

Structural stability and electrochemical properties of gadolinium-substituted LiGdxMn2-xO4 spinel as cathode materials for Li-ion rechargeable batteries

Gadolinium-substituted spinel LiGdxMn2-xO4 (x=0.0 and 0.5) cathode crystals were synthesized by co-precipitation and dual calcination. Structural characterization using X-ray diffraction pattern revealed that gadolinium doping in LiMn2O4 resulted in the highly ordered cubic spinel structure with only slight increase in the average d-spacing and lattice parameters. X-ray photoelectron spectroscopic analysis indicated an increase in the average oxidation state of manganese in the Gd-doped material in comparison to its pristine spinel counterpart. Morphological characterization using field emission scanning electron microscopy and transmission electron microscopy revealed that gadolinium doping in LiMn2O4 resulted in a decrease of the average particle size. Electrochemical charge/discharge studies at various current rates showed that the LiGd0.5Mn1.5O4 spinel exhibited excellent and stable cycling stability in comparison to Gd-free LiMn2O4 spinel. Gd-substitution in LiMn2O4 brought structural stability via the expansion of LiO4 tetrahedra, contraction of MnO6 octahedra, and avoidance of the Jahn-Teller distortion effect, which translated in high-rate performance and less capacity fading. In addition Gd-doping was found via electrochemical AC impedance spectroscopy to lead to significant increase in electronic conductivity as evident by less charge transfer resistance than the pristine.

Post Synthesis Treatment of Tunnel Manganese Oxide Nanowires for Improved Electrocatalytic Activity in Oxygen Evolution Reactions (OER)

The oxygen evolution reaction (OER), the first step of the two-step electrolysis process used to split water into hydrogen and oxygen, is plagued by poor kinetics and requires catalysts to limit energy loss and improve efficiency. Precious metals and their alloys (Au, Pt, Ir) exhibit excellent catalytic behavior, but high material costs and concerns about the environmental impact caused by their production have led to intense investigation of period four transition metal oxides (manganese, iron, cobalt, and nickel oxides) as catalyst for OER.[1] In addition to their natural abundance and high electrochemical activity, manganese oxides represent a wide library of chemical compositions and morphologies that are stable in aqueous solutions allowing them to be utilized as OER catalysis. Tunnel manganese oxides are a family of manganese oxides composed of MnO6 octahedra that form tunnels around stabilizing ions and water molecules. α-MnO2 nanowires with square structural tunnels formed by two MnO6 octahedra on each side and potassium ions inside the tunnels were shown to have superior performance among various structurally diverse manganese oxide. [2]Additionally, high aspect ratio nanowire morphologies with diameters less than 100 nm and lengths on the order of microns, exhibiting high surface area, are attractive for catalytic applications. In this work, we investigated post synthesis treatment approaches for improving catalytic activity of α-MnO2 nanowires in OER. More specifically, acid leaching and cobalt doping were applied to modify α-MnO2 nanowires synthesized by hydrothermally treating a KMnO4 and NH3Cl solution at 150oC for 50 hours. During the acid leaching process, samples were dispersed in concentrated nitric acid for 72 hours under vigorous mixing. Such treatment is believed to produce vacancies in potassium, manganese, and oxygen crystallographic sites (Figure 1.a).[3,4] Cobalt ions were introduced into the crystal structure via hydrothermal treatment of 20 mg of α-MnO2 nanowires in 5 ml of Co(NO3)2 melt at elevated temperatures (T= 80-100oC, Figure 1.a). Electrochemical activity of Co-doped α-MnO2 nanowires (α-CoxMnO2 x=0.02;0.03;0.05), was evaluated in comparison with the electrochemical activity of pristine material (Figure 1.b). In addition, the effect of acid leaching and cobalt doping in conjunction was examined for α-MnO2 nanowires containing 0.02 mol % of cobalt (α-Co0.02MnO2, AL-α-Co0.02MnO2) and compared to that of α-MnO2 and AL-α-MnO2 nanowires (Figure 1.c). Synthesized materials were tested as OER catalysts using a Pine Research Instrumentation modulated speed rotator (MSP). Tests were performed at 1600 RPM in 0.1 M KOH electrolyte over a voltage window of 1 to 1.9 V vs RHE. Active electrodes were fabricated via ink casting of a 1:1 active material: carbon black ink prepared in ethanol with a Nafion® binding agent and mass loaded to have 0.05 mg of active material. Acid leaching for 72 hours resulted in the increase of electrocatalytic activity up to 26.6 mA/cm2 compared to that of pristine α-MnO2 nanowires (21.6 mA/cm2, Figure 1.c). For Co-doped samples, catalytic performance improved linearly with the increase of the cobalt content. The highest OER performance of 48.7 mA/cm2 was exhibited by α-Co0.05MnO2, which is a 125% increase as compared to the activity of pristine α-MnO2 (Figure 1.b). The defect rich sample (AL-α-Co0.02MnO2) that underwent both post synthesis treatments showed an increased activity when compared to materials individually modified by acid leaching or Co-doping (Figure 1.c). For example, cobalt doped 72-hour acid leached samples exhibited current densities of 35.0 mA/cm2, 20% higher than doped samples of the same chemical composition (29.4 mA/cm2). The increased catalytic activity of the acid-leached α-MnO2 was attributed to newly formed oxygen vacancies and a change in oxidation state of manganese. In manganese oxides, an average manganese oxidation state of +3.5 has been shown to result in the highest catalytic activity [5]. Acid leaching is believed to reduce the average oxidation state of manganese in α-MnO2. Co-doping potentially could further favorably affect the oxidation state of manganese while simultaneously creating additional active site for the oxygen evolution reaction leading to more efficient electrochemical water splitting. Acid leaching and cobalt doping both separately and in tandem were observed to have positive impacts on the OER catalytic activity of environmentally friendly and cost-efficient α-MnO­2 catalysts while also suggesting that they could be applied to other catalytically active tunnel manganese oxides. Yi Cheng et al. Progress in Natural Science: Materials International. 25, 6. 2015 545-553.Frey, Carolin E et al. Chemistry – A European Journal21 (42) 2015 14958-68.Yuan, YF, et al. NATURE COMMUNICATIONS, vol. 7, 2016, 13374Lee, Jin-Hyon, et al. Energy & Environmental Science, 5, 11, 2012, 9558-9565S.Y. Lee MS thesis. University of Freiburg, 2015. Figure 1

Jahn-Teller Distortion and Electrode Performance of P’2-Na2/3Mn0.9Me0.1O2 (Me = Mg, Ti, Co, Ni, Cu, and Zn)

Mn based P2-type layered oxides, Na2/3Me y Mn1-y O2 (Me=a variety of metals), have been widely studied as positive electrode materials for Na-ion batteries, especially consisting only of the abundant elements in the Earth’s crust, such as P2-Na2/3[Fe1/2Mn1/2]O2. [1] The end member, P2-type Na2/3MnO2 is known to have crystal polymorphism, P2- and P’2- Na2/3MnO2 with hexagonal and orthorhombic lattice, respectively. The orthorhombic system is stabilized by co-operative Jahn-Teller distortion (CJTD) derived from six-coordinated high spin MnIII (t2g 3-eg 1 ). Very recently, our group reported that the single-phase samples of these polymorphs, P2-Na0.59Mn0.9O2 and P’2-Na0.64MnO2, possess different stoichiometry and demonstrate the superior electrochemical performance of distorted P’2 sample [2], though the distortion in insertion materials is generally considered to be disadvantageous for battery application. Therefore, quantitative understanding how CJTD affects electrochemical properties such as reversibility, kinetics of sodium diffusion, and long-cycle stability is of great importance to realize high capacity performance of MnIII based oxide materials for high energy density NIBs. In this study, we demonstrate the systematic synthesis of differently distorted P’2-Na2/3MnO2samples by partial replacement of Mn with different metal elements to correlate their electrochemical properties and lattice distortion. Single phases of P’2-Na2/3Mn0.9Me0.1O2 (Me = Mg, Ti, Co, Ni, Cu, and Zn) are synthesized when we optimized facile solid-state reaction. The distortion calculated from lattice constant is highly correlated with average oxidation state of manganese varied by divalent metals (MgII, NiII, CuII, ZnII), trivalent metal (CoIII) and tetravalent metal (TiIV). Because of Jahn-Teller active CuII (t2g 6-eg 3 ), P’2-Na2/3Mn0.9Cu0.1O2 possesses anomalous distortion compared to other samples with divalent-metal dopants as shown in Figure 1a. All samples show high reversible capacity about 200 mAh g-1 and have smooth charge/discharge profiles. Fig. 1b compares charge/discharge curves of divalent-metal doped samples. The P’2-Na2/3Mn0.9Ni0.1O2 delivers the reversible capacity of 227 mAh g-1 and the Na2/3Mn0.9Cu0.1O2shows stable electrochemical cycling, 95% capacity retention after 25 cycles. In this presentation, we will further discuss the effect of metal-substitution and lattice distortion on electrochemical properties and demonstrate successful improvement of both cycle and rate performance by Cu doping.

Thermodynamics of manganese oxides: Sodium, potassium, and calcium birnessite and cryptomelane

Manganese oxides with layer and tunnel structures occur widely in nature and inspire technological applications. Having variable compositions, these structures often are found as small particles (nanophases). This study explores, using experimental thermochemistry, the role of composition, oxidation state, structure, and surface energy in the their thermodynamic stability. The measured surface energies of cryptomelane, sodium birnessite, potassium birnessite and calcium birnessite are all significantly lower than those of binary manganese oxides (Mn3O4, Mn2O3, and MnO2), consistent with added stabilization of the layer and tunnel structures at the nanoscale. Surface energies generally decrease with decreasing average manganese oxidation state. A stabilizing enthalpy contribution arises from increasing counter-cation content. The formation of cryptomelane from birnessite in contact with aqueous solution is favored by the removal of ions from the layered phase. At large surface area, surface-energy differences make cryptomelane formation thermodynamically less favorable than birnessite formation. In contrast, at small to moderate surface areas, bulk thermodynamics and the energetics of the aqueous phase drive cryptomelane formation from birnessite, perhaps aided by oxidation-state differences. Transformation among birnessite phases of increasing surface area favors compositions with lower surface energy. These quantitative thermodynamic findings explain and support qualitative observations of phase-transformation patterns gathered from natural and synthetic manganese oxides.

Tunnel Structured α-MnO2 with Different Tunnel Cations (H+, K+, Ag+) as Cathode Materials in Rechargeable Lithium Batteries: The Role of Tunnel Cation on Electrochemistry

α-MnO2 type manganese dioxide is an interesting prospective cathode material for reversible lithium insertion owing to its cation accessible tunnels (0.46 nm × 0.46 nm), high voltage, and low cost. The tunneled structure is synthetically formed by the assistance of cations acting as structure directing agents where the cations may remain in the tunnel. The electrochemistry of this family of materials is strongly dependent on the morphological and physicochemical (i.e. surface area, crystallite size, and average manganese oxidation state) properties as well as tunnel occupancy. In this work, we prepared a set of materials Mn8O16·0.81H2O, K0.81Mn8O16·0.78H2O and Ag1.33Mn8O16·0.95H2O with similar nanorod morphology, crystallite size, surface area, and tunnel water content. This set of samples allowed us to investigate the role of tunnel cations in the electrochemistry of α-MnO2 type manganese dioxide in a lithium based environment while minimizing the effects of the other parameters. The electrochemistry was evaluated using cyclic voltammetry, galvanostatic cycling, rate capability, and galvanostatic intermittent titration type testing. Mn8O16·0.81H2O showed higher loaded voltages, improved capacity retention, and higher specific energy relative to K0.81Mn8O16·0.78H2O and Ag1.33Mn8O16·0.95H2O. After 100 cycles, Mn8O16·0.81H2O delivered ∼200% more capacity than Ag1.33Mn8O16·0.95H2O (64 vs. 129 mAh/g) and ∼35% more capacity than K0.81Mn8O16·0.78H2O (85 vs. 129 mAh/g). Mn8O16·0.81H2O also showed higher effective lithium diffusion coefficients (DLi+) and higher rate capability compared to K0.81Mn8O16·0.78H2O and Ag1.33Mn8O16·0.95H2O suggesting faster Li+ ion diffusion in the absence of large metal tunnel cations.

Facile synthesis of birnessite-type manganese oxide nanoparticles as supercapacitor electrode materials

Manganese oxides are environmentally benign supercapacitor electrode materials and, in particular, birnessite-type structure shows very promising electrochemical performance. In this work, nanostructured birnessite was facilely prepared by adding dropwise NH2OH·HCl to KMnO4 solution under ambient temperature and pressure. In order to fully exploit the potential of birnessite-type manganese oxide electrode materials, the effects of specific surface area, pore size, content of K+, and manganese average oxidation state (Mn AOS) on their electrochemical performance were studied. The results showed that with the increase of NH2OH·HCl, the Mn AOS decreased and the corresponding pore sizes and specific surface area of birnessite increased. The synthesized nanostructured birnessite showed the highest specific capacitance of 245Fg−1 at a current density of 0.1Ag−1 within a potential range of 0–0.9V, and excellent cycle stability with a capacitance retention rate of 92% after 3000 cycles at a current density of 1.0Ag−1. The present work implies that specific capacitance is mainly affected by specific surface area and pore volume, and provides a new method for the facile preparation of birnessite-type manganese oxide with excellent capacitive performance.

Crystal Structure and Characterization of La1-xBixMnO3+δ

The La1-x Bi x MnO3+ δ (x=0.0-0.5) solid solutions were synthesized using three different methods: solid-state synthesis, citrate-nitrate combustion synthesis and mechanochemical activation. All obtained compositions crystallize in a rhombohedral perovskite structure, independent of the synthesis method. Two ranges with linear temperature dependence of the unit cell parameters have been observed during in-situ heating X-ray powder diffraction experiments. The transition between the two ranges occurs at 730°C and is explained by chemical expansion at high temperatures. The chemical compatibility between the obtained solid solutions and bismuth-containing electrolytes (Bi4V1.7Fe0.3O11, Bi13Mo5O34 and Bi3Nb0.8W0.2O7.1) was studied. Interaction between these materials has been observed at temperatures higher than 600°С. X-ray diffraction data collected at room temperature confirm that the La1-xBixMnO3+ δ solid solutions form in the whole range of the investigated bismuth concentrations independent of the synthesis method. All samples crystallize in a rhombohedral perovskite structure with a trigonal unit cell (S.G. R-3c). For a detailed microstructural characterization by transmission electron microscopy, the La0.8Bi0.2MnO3.15 composition obtained using the solid-state synthesis was chosen. All obtained diffraction patterns can be indexed in a trigonal unit cell with a=5.5300(1) Å, c=13.382(1) Å. An analysis of systematically absent reflections revealed the presence of two types of crystals. Extinction conditions for the fist type coincide with those obtained from XRD (hkl: -h+k+l=3n; h-hl: h+l=3n, l=2n; hhl: l=3n; 00l: l=6n), indicating two possible space groups: R-3c and R3c. A second type of crystals obeys the same extinction rules except for the 00l reflections for which l=3n and h-hl reflections for which l¹2n, pointing towards the following possible space groups: R3, R-3, R32, R3m, R-3m. The formation of two different types of crystallites with trigonal crystal symmetry has been observed in perovskite systems before and explained by the presence or absence of antiphase octahedral tilting about the [111] axis of the cubic perovskite sub-lattice. It is worth to mention that the X-ray powder diffraction pattern of this sample displays asymmetric line broadening of peaks, which can point towards the presence of an additional structurally related compound. A loss of oxygen was observed in the TG curves of La1-xBixMnO3+ δ (x=0.0-0.5; Δх=0.1) samples at temperatures >700°С, supporting the previously discussed results. TG/DSC curves of La0.9Bi0.1MnO3+ δ are shown in Figure 4. The loss of mass was found to be 0.6%. The excess of oxygen in La1-xBixMnO3+ δ (x=0.0-0.5; Δх=0.1) was determined using redox titration. The value of the oxygen nonstoichiometry (δ) was found to be positive and equal to 0.15 at all concentrations of dopant, which can be explained by the identical oxidation states of La3+ and Bi3+. To determine the concentration of metal atoms in the obtained solid solutions, surfaces and fractures of pellets have been studied using EDS and AAS. The elemental content of 15–20 crystallites of La1-xBixMnO3+ δ (x=0.1, 0.2) compositions was studied by EDS analysis inside the scanning electron microscope. It was found to be La0.9(1)Bi0.07(4)Mn0.9(7) and La0.9(8)Bi0.1(6)Mn0.9(6) for x = 0.1 and 0.2, respectively. The obtained results showed a slightly inhomogeneous cation distribution and a lower manganese concentration compared to the nominal value for both the bulk and the surface of the pellets. AAS measurements gave similar results for all synthesized solid solutions, showing the manganese content to be 0.9, and thus indicating that all obtained solid solutions lie in the homogeneity region of lanthanum manganite with a small manganese deficiency. Taking into account the obtained values of manganese deficiency and oxygen nonstoichiometry, the chemical formula of the solid solutions can be rewritten as La1-xBixMn1-yO[(3+(1-y) )/2] (where VMn is the average oxidation state of manganese). For example, for the La0.9Bi0.1MnO3+ δ compound with δ=0.15 and y=0.1, the average oxidation state of manganese (VMn) will be equal to 3.67. Thus, the ratio of manganese in different oxidation states (Mn4+/Mn3+) in the studied solid solutions is equal to 2, which means that 67% of the manganese cations in these compounds have oxidation state +4. This fact is probably the reason why we observe the formation of solid solutions with a rhombohedrally distorted perovskite structure already at room temperature. A study of the chemical compatibility between La1-x Bi x MnO3+ δ (x=0.0-0.5; Δх=0.1) solid solutions and bismuth-containing electrolytes shows that depending on the chemical nature of the electrolyte material, the formation of additional phases begins at different temperatures: at T³700°С for the studied niobate, at T³600°С for the studied vanadate and at T³500°С for the studied molybdate. The work was financially supported by Russian Fund of Fundamental Research, grant 14-03-92605.

Formation of todorokite from "c-disordered" H(+)-birnessites: the roles of average manganese oxidation state and interlayer cations.

BackgroundTodorokite, a 3 × 3 tectomanganate, is one of three main manganese oxide minerals in marine nodules and can be used as an active MnO6 octahedral molecular sieve. The formation of todorokite is closely associated with the poorly crystalline phyllomanganates in nature. However, the effect of the preparative parameters on the transformation of “c-disordered” H+-birnessites, analogue to natural phyllomanganates, into todorokite has not yet been explored.ResultsSynthesis of “c-disordered” H+-birnessites with different average manganese oxidation states (AOS) was performed by controlling the MnO4−/Mn2+ ratio in low-concentrated NaOH or KOH media. Further transformation to todorokite, using “c-disordered” H+-birnessites pre-exchanged with Na+ or K+ or not before exchange with Mg2+, was conducted under reflux conditions to investigate the effects of Mn AOS and interlayer cations. The results show that all of these “c-disordered” H+-birnessites exhibit hexagonal layer symmetry and can be transformed into todorokite to different extents. “c-disordered” H+-birnessite without pre-exchange treatment contains lower levels of Na/K and is preferably transformed into ramsdellite with a smaller 1 × 2 tunnel structure rather than todorokite. Na+ pre-exchange, i.e. to form Na-H-birnessite, greatly enhances transformation into todorokite, whereas K+ pre-exchange, i.e. to form K-H-birnessite, inhibits the transformation. This is because the interlayer K+ of birnessite cannot be completely exchanged with Mg2+, which restrains the formation of tunnel “walls” with 1 nm in length. When the Mn AOS values of Na-H-birnessite increase from 3.58 to 3.74, the rate and extent of the transformation sharply decrease, indicating that a key process is Mn(III) species migration from layer into interlayer to form the tunnel structure during todorokite formation.ConclusionsStructural Mn(III), together with the content and type of interlayer metal ions, plays a crucial role in the transformation of “c-disordered” H+-birnessites with hexagonal symmetry into todorokite. This provides further explanation for the common occurrence of todorokite in the hydrothermal ocean environment, where is usually enriched in large metal ions such as Mg, Ca, Ni, Co and etc. These results have significant implications for exploring the origin and formation process of todorokite in various geochemical settings and promoting the practical application of todorokite in many fields.Graphical abstractXRD patterns of Mg2+-exchanged and reflux treatment products for the synthetic “c-disordered” H+-birnessites.

Total Oxidation of Formaldehyde over MnOx-CeO2 Catalysts: The Effect of Acid Treatment

The effect of acid treatment in mixed MnOx–CeO2 samples has been investigated in the catalytic total oxidation of formaldehyde. The acid treatment has no effect on the textural and redox properties of the materials when Mn is stabilized in a MnOx–CeO2 solid solution (Mn content below 50%). However, these properties were found to be highly altered by acid treatment when the solubility limit of Mn in the ceria was exceeded (Mn content above 50%). This enabled access to the primary porosity and oxidized the manganese species to a higher oxidation state via a Mn dismutation reaction. As a result, the catalytic activity of pure manganese oxide, after chemical activation, in the oxidation of formaldehyde is greatly improved—at 100 °C, the conversion of formaldehyde is increased by a factor of 5 and the corresponding intrinsic reaction rate by 1.4. Combined in situ surface analysis unambiguously identified formate species as a result of formaldehyde oxidation at room temperature on the chemically activated pure MnOx. The evolution of various surface species was monitored by increasing the temperature and in situ FTIR, and XPS results provided direct evidence of the desorption of monodentate formate species into formaldehyde and the oxidation of bidentate-bridging formate species. Changes in the average oxidation state of surface manganese confirmed the participation of oxygen from MnOx in the formation of formate species at room temperature and their transformation into CO2 and H2O when increasing the temperature.