Disproportionation Of Mn Research Articles

Pressure-induced charge-transfer and structural transition in hexagonal multiferroic HoMnO3

The improper ferroelectricity and frustrated magnetism observed in hexagonal rare-earth manganites are tied to Mn trimerization in the basal plane. Multiferroic properties interplay via the lattice leading to complex structural behavior with temperature and pressure. Applying hydrostatic pressure to hexagonal HoMnO${}_{3}$ induces a phase transition to a rhombohedrally distorted superstructure, accompanied by disproportionation of Mn(III) and compressibility of the basal plane. These findings reveal new insights into multiferroic manganites under extreme conditions, highlighting the complicated interactions between their properties.

Specific countermeasures to intrinsic capacity decline issues and future direction of LiMn2O4 cathode

Spinel LiMn2O4 cathodes are particularly attractive in lithium-ion batteries (LIBs) owing to the non-toxic Mn sources, abundant reserves, and high specific power. However, poor cycling stability due to the significant capacity decay becomes the key limitation for its application. With the continuous exploration, some deep-rooted causes of capacity decay are being challenged. To date, three intrinsic mechanisms have been shown to cause capacity loss, including the Jahn-Taller (J-T) effect, Mn disproportionation, and oxygen vacancy formation. Specifically, the capacity loss especially below 3 V caused by J–T distortion hinders the achievement of theoretical capacity. Besides, the irreversible phase transitions arising from Mn(III) disproportionation exacerbates Mn dissolution. Even worse, the Mn ions migration due to oxygen loss during the electrochemical cycling also leads to severe phase transition. Although some reviews have involved various strategies to overcome the related drawbacks, a summary of more comprehensive and specific coping strategies along with recent advances and future development direction is still lacking. Herein, we first introduce the comprehensive intrinsic capacity fading mechanisms of LiMn2O4. Then, recent progress in suppressing the J-T distortion, Mn disproportionation, and Mn migration is systematically reviewed, with a special focus on the advances in the up-to-date strategies such as cation disorder and epitaxial coating. We also put forward future research directions and opportunities for the development of longer-life LiMn2O4 cathode. This review aims to offer some guidance for the rational designing of sufficiently durable LiMn2O4 cathodes and the maximizing of their inherent capacity for meeting the high demands in LIBs.

Understanding the activity decay of MnO2 for oxygen reduction in neutral media

MnO 2 has been studied as the catalyst for oxygen reduction reaction (ORR) in neutral due to its considerably good performance and low cost. Although many studies focus on activity enhancement, little attention has been paid to the activity degradation of MnO 2 . In this study, we first probe the mechanism of ORR activity decay of MnO 2 catalyst in neutral. Electrochemical measurements showed the apparent activity degradation of MnO 2 in both short-term and long-time stability tests. After chronoamperometry test, surface amorphization was observed on the catalyst, while no significant amorphization occurred for the absence of ORR catalysis. The activity decay and surface amorphization of MnO 2 catalyst are due to the disproportionation reaction of Mn 3+ ions in neutral. Mn 3+ ions involved in ORR catalysis disproportionate to generate soluble Mn 2+ ions, resulting in the surface amorphization together with the change of the highly ORR-active Mn 3+ /Mn 4+ redox to the low ORR-active Mn 2+ /Mn 3+ redox. This study clarifies the activity decay mechanism of MnO 2 for ORR in neutral media and provides theoretical support for improving and stabilizing the catalytic activity of MnO 2 in the future. The disproportionation reaction of Mn 3+ ions under neutral conditions destroys the redox conversion between Mn 3+ and Mn 4+ . • The ORR activity of MnO2 catalyst decays quickly in neutral. • The activity decay comes along with surface amorphization. • Surface amorphization is resulted by the disproportionation of Mn 3+ ions. • The loss of Mn 4+ converts highly active Mn 3+ /Mn 4+ to low active Mn 2+ /Mn 3+ .

Rechargeable zinc-ion batteries with manganese dioxide cathode: How critical is choice of manganese dioxide polymorphs in aqueous solutions?

In this work, different MnO2 polymorphs are applied as cathodes in zinc-ion batteries (ZIBs). All the polymorphs result in similar electrochemical behavior in weak acidic (1 М) ZnSO4 aqueous solutions at comparable specific capacity (200–225 mAh g−1), similar charge–discharge curves, and temporal stability owing to an irreversible modification of the pristine positive electrode during battery charge and discharge. This irreversibility stems from the dissolution and re-deposition of MnO2 and the formation of new manganese and zinc compounds (basic salt deposits such as ZnMn2O4, Mn2O3, and MnOOH). The additional (new) MnO2 phase is formed via two routes: disproportionation of Mn+3 ions formed during the discharge process and anodic oxidation of Mn2+ ions in a solution. According to X-ray diffraction and Raman spectroscopy analyses, the re-deposited MnO2 is in an amorphous state. The amorphous MnO2 covers the surface of the initial crystalline particles and affects the electrochemical behavior of the ZIBs. The strong pH dependence of the electrochemical response of the MnO2 electrodes is related to the H+ concentration effect on ionic equilibria in this system, such as the formation of basic zinc and manganese salts and hydroxides as well as the disproportionation of Mn+3 and anodic oxidation of Mn+2.

Tailoring manganese coordination environment for a highly reversible zinc-manganese flow battery

Zinc-manganese flow batteries have drawn considerable attentions owing to its advantages of low cost, high energy density and environmental friendliness. On the positive carbon electrode, however, unstable MnO 2 depositions can be formed during oxidation through disproportionation reaction of Mn 3+ , which result in poor reversibility of Mn 2+ /MnO 2 and bring instability to Zn–Mn flow battery limiting its performance and further development. To tackle this issue, in this study, we reported a highly reversible Zn–Mn flow battery by employing EDTA-Mn as the positive electrolyte. In aqueous solutions, EDTA exhibits a strong ligand field for Mn 2+ from ab initio calculations, which proves to form bonds with Mn 2+ through carboxyl/amino groups and replace bonded waters in the solvation structure of Mn 2+ . Benefiting from the coordination effect of EDTA, both electrochemical and material characterizations demonstrate a highly reversible Mn 2+ /Mn 3+ redox reaction in EDTA-Mn, which effectively inhibits the disproportionation reaction of Mn 3+ without forming any deposited MnO 2 on carbon electrode. The constructed Zn–Mn flow cell adopting EDTA-Mn not only demonstrates excellent rate performance with a high CE over 95 % operated at 10–50 mA cm −2 , but also realizes a superior cycling stability over 300 cycles at 20 mA cm −2 affording 98 % CE and 75 % EE. • A highly reversible Zn–Mn RFB is proposed based on EDTA-Mn catholyte. • EDTA proves to form bonds with Mn 2+ through carboxyl and amino groups. • A highly reversible Mn 2+ /Mn 3+ redox reaction is proved in EDTA-Mn. • Disproportionation of Mn 3+ is completely inhibited in EDTA-Mn without forming MnO 2 . • The Zn–Mn cell using EDTA-Mn shows superior rate performance and cycling stability.

Thermal plasma synthesis and electrochemical properties of high-voltage LiNi0.5Mn1.5O4 nanoparticles

The synthesis of LiNi0.5Mn1.5O4 has been reported to change the crystal structure with the oxygen partial pressure and affect the battery characteristics. LiNi0.5Mn1.5O4 involves the formation of impurities, such as LixNi1−xO, LixMn3−xO4, and Li2CO3, at a high temperature range exceeding 700 °C because oxygen loss occurs during synthesis. LiNi0.5Mn1.5O4 electrochemically contains Mn4+, however, Mn3+ is formed because of oxygen deficiency. The Li–Ni–Mn-oxide causes a disproportionation of Mn3+ in an oxygen-deficient state. The synthesized Li–Ni–Mn-oxide nanoparticles at 10,000 K by induction thermal plasma formed spinel-type LiNi0.5Mn1.5O4 (space group Fd3m) of Mn4+. The crystal structure of the cubic-spinel nanoparticles approached a LiNi0.5Mn1.5O4 single phase as the flow rate of O2 increased from 2.5 to 5 l min−1. The formation of LiNi0.5Mn1.5O4 was shown to be accelerated by increasing the O2 gas flow rate. The measured current–voltage characteristics of LiNi0.5Mn1.5O4 nanoparticles appeared at around 4.7–4.8 V as the reaction peak of Ni2+/Ni3+ and Ni3+/Ni4+. In contrast, the Mn of the Li–Ni–Mn-oxide nanoparticles synthesized in the oxygen-deficient state was less than trivalent, which caused disproportionation of Mn. The measured current-voltage characteristics showed peak of an oxygen desorption at near 4.6 V. This study investigated the factors affecting the crystal structure formation and electrochemical properties of high-voltage LiNi0.5Mn1.5O4 nanoparticles formed in thermal plasma.

Oxidative UO2 Dissolution Induced by Soluble Mn(III)

The stability of UO2 is critical to the success of reductive bioremediation of uranium. When reducing conditions are no longer maintained, Mn redox cycling may catalytically mediate the oxidation of UO2 and remobilization of uranium. Ligand-stabilized soluble Mn(III) was recently recognized as an important redox-active intermediate in Mn biogeochemical cycling. This study evaluated the kinetics of oxidative UO2 dissolution by soluble Mn(III) stabilized by pyrophosphate (PP) and desferrioxamine B (DFOB). The Mn(III)-PP complex was a potent oxidant that induced rapid UO2 dissolution at a rate higher than that by a comparable concentration of dissolved O2. However, the Mn(III)-DFOB complex was not able to induce oxidative dissolution of UO2. The ability of Mn(III) complexes to oxidize UO2 was probably determined by whether the coordination of Mn(III) with ligands allowed the attachment of the complexes to the UO2 surface to facilitate electron transfer. Systematic investigation into the kinetics of UO2 oxidative dissolution by the Mn(III)-PP complex suggested that Mn(III) could directly oxidize UO2 without involving particulate Mn species (e.g., MnO2). The expected 2:1 reaction stoichiometry between Mn(III) and UO2 was observed. The reactivity of soluble Mn(III) in oxidizing UO2 was higher at lower ratios of pyrophosphate to Mn(III) and lower pH, which is probably related to differences in the ligand-to-metal ratio and/or protonation states of the Mn(III)-pyrophosphate complexes. Disproportionation of Mn(III)-PP occurred at pH 9.0, and the oxidation of UO2 was then driven by both MnO2 and soluble Mn(III). Kinetic models were derived that provided excellent fits of the experimental results.

Hydrogen‐Atom Abstraction Reactions by Manganese(V)– and Manganese(IV)–Oxo Porphyrin Complexes in Aqueous Solution

High-valent manganese(IV or V)-oxo porphyrins are considered as reactive intermediates in the oxidation of organic substrates by manganese porphyrin catalysts. We have generated Mn(V)- and Mn(IV)-oxo porphyrins in basic aqueous solution and investigated their reactivities in C-H bond activation of hydrocarbons. We now report that Mn(V)- and Mn(IV)-oxo porphyrins are capable of activating C-H bonds of alkylaromatics, with the reactivity order of Mn(V)-oxo>Mn(IV)-oxo; the reactivity of a Mn(V)-oxo complex is 150 times greater than that of a Mn(IV)-oxo complex in the oxidation of xanthene. The C-H bond activation of alkylaromatics by the Mn(V)- and Mn(IV)-oxo porphyrins is proposed to occur through a hydrogen-atom abstraction, based on the observations of a good linear correlation between the reaction rates and the C-H bond dissociation energy (BDE) of substrates and high kinetic isotope effect (KIE) values in the oxidation of xanthene and dihydroanthracene (DHA). We have demonstrated that the disproportionation of Mn(IV)-oxo porphyrins to Mn(V)-oxo and Mn(III) porphyrins is not a feasible pathway in basic aqueous solution and that Mn(IV)-oxo porphyrins are able to abstract hydrogen atoms from alkylaromatics. The C-H bond activation of alkylaromatics by Mn(V)- and Mn(IV)-oxo species proceeds through a one-electron process, in which a Mn(IV)-oxo porphyrin is formed as a product in the C-H bond activation by a Mn(V)-oxo porphyrin, followed by a further reaction of the Mn(IV)-oxo porphyrin with substrates that results in the formation of a Mn(III) porphyrin complex. This result is in contrast to the oxidation of sulfides by the Mn(V)-oxo porphyrin, in which the oxidation of thioanisole by the Mn(V)-oxo complex produces the starting Mn(III) porphyrin and thioanisole oxide. This result indicates that the oxidation of sulfides by the Mn(V)-oxo species occurs by means of a two-electron oxidation process. In contrast, a Mn(IV)-oxo porphyrin complex is not capable of oxidizing sulfides due to a low oxidizing power in basic aqueous solution.

Synthesis and electrochemistry of Li 3MnO 4: Mn in the +5 oxidation state

Computational and experimental work directed at exploring the electrochemical properties of tetrahedrally coordinated Mn in the +5 oxidation state is presented. Specific capacities of nearly 700 mAh g −1 are predicted for the redox processes of Li x MnO 4 complexes based on two two-phase reactions. One is topotactic extraction of Li from Li 3MnO 4 to form LiMnO 4 and the second is topotactic insertion of Li into Li 3MnO 4 to form Li 5MnO 4. In the experiments, it is found that the redox behavior of Li 3MnO 4 is complicated by disproportionation of Mn 5+ in solution to form Mn 4+ and Mn 7+ and by other irreversible processes; although an initial capacity of about 275 mAh g −1 in lithium cells was achieved. Strategies based on structural considerations to improve the electrochemical properties of MnO 4 n− complexes are given.

Quantitative charge state analysis of manganese biominerals in aqueous suspension using scanning transmission X-ray microscopy (STXM)

We have applied Scanning Transmission Soft X-ray Microscopy (STXM) to investigate the charge state distribution of Mn in two kinds of Mn-biominerals, Mn nodules collected from Lake Michigan sediments and Mn precipitates formed by spores of a marine bacillus SG-1 under transport limited reaction conditions. A data analysis technique was developed, which allows for extraction of spatially resolved 2-d charge state maps of manganese on a submicron level. We find that the charge state of Mn dominates the spectral shape of L-edge spectra of environmentally important single oxidation state Mn minerals and that spectra of mixed oxidation state oxides can be modelled by a combination of appropriate single oxidation state reference spectra. Two-dimensional maps of charge state distributions clearly reveal domains of different oxidation states within single particles of Mn-micronodules. Spots of preferred accumulation of Mn(II) were found, which indicates biogenic precipitation of Mn(II)-species as a first step of nodule formation. The presence of Mn(III) in the studied sediment samples suggests the involvement of one-electron oxidation processes and reaction conditions which inhibit or slow down the disproportionation of Mn(III)-products. Under transport limited conditions, Mn oxidation products formed by spores of the marine bacillus SG-1 can vary from cell to cell. The presence of significant amounts of Mn(III) containing species points to the involvement of one-electron oxidation reactions as in the case of the micro-nodules. Our technique and the results obtained form a new basis for the mechanistic understanding of the formation of Mn biominerals in the environment.

Evaluation of Lead Anode Reactions in Acid Sulfate Electrolytes : II. Manganese Reactions

Lead alloys containing calcium-tin, silver, or similar combinations are the primary insoluble anodes used for copper and zinc electrowinning. One potential concern with these anodes is the detrimental effect on cathode purity from incorporation of lead corrosion products. Other factors which are important to process economics are the reaction potential and overpotential required for oxygen evolution. In addition to the inherent overpotential, impurities in the electrolyte can have a significant polarization impact, such as the films that may form when manganese is present. In order to gain a better understanding of the fundamental behavior of various concentrations of manganese ions at lead anodes, the polarization behavior of lead calcium-tin and lead-silver alloys in sulfuric acid was evaluated. A common depolarizing agent, Co(II), was also used in some cases to determine possible interactions with manganese. The data show that manganese ions are active on the lead anodes and may precipitate manganese oxide in addition to electrochemical deposition. One possible precipitation mechanism seems to occur by the disproportionation of Mn(III). Co(II) lowers the electrode potential at given current densities which decreases Pb corrosion but favors formation. © 2002 The Electrochemical Society. All rights reserved.

Effect of H 2SO 4 concentration on preparation and activity of activated manganese dioxide

In the preparation of activated manganese dioxide (AMD), H 2SO 4 solution is a better medium for dissolution of Mn 2O 3 and disproportionation of Mn 3+ ions. The preparation and activity of AMD is affected greatly by the H 2SO 4 concentration in the solution. Mn 2O 3 cannot dissolve completely if the H 2SO 4 concentration is too low. By contrast, less porous AMD is formed if the H 2SO 4 concentration is too high.

Thermopower analysis of substituted nickel manganite spinels

Information about the distribution of cations between tetrahedral (A) and octahedral (B) sites in the spinel structure of NiMn 2O 4 and some substituted spinel phases are derived from thermopower measurements in connection with some ion distribution models. The cubic spinel NiMn 2O 4 is characterized by a high degree of inversion coupled with disproportionation of Mn III with formation of Mn II on tetrahedral and Mn IV on octahedral sites. Spinels of the series Li zNiMn 2−zO 4 show increased concentrations of Mn IV on the B-sites and the Mn II content on the A-sites is reduced. FeNiMnO 4 is dominated by partial Fe octahedral site occupation connected with Mn disproportionation.

Kinetics of reductive dissolution of colloidal manganese dioxide

The temporal characteristics of electron transfer to and proton uptake by colloidal manganese oxides in acidic aqueous solutions have been examined in the time regime of milliseconds to tens of seconds. Time-resolved conductometric measurements coupled with electron-transfer-rate results indicate that electron deposition into the colloidal oxides and protonation occurs simultaneously; i.e., there is no polarization of the colloid during the reduction process. Reduction of the manganese oxides leads to their dissolution with quantitative amounts of Mn(II) produced in the bulk aqueous phase. A detailed examination of both thermal- and radiation-induced dissolution of the metal oxides indicates that Mn(III) centers present in the colloid are continually depleted to produce Mn(II). The driving force for this reaction is the chemical potential gradient between the colloid and the aqueous solution due to the homogeneous nature of mixed Mn(IV)/Mn(III) oxides. Equilibrium between the solution and the colloid is established in both acidic and alkaline solution by either conproportionation or disproportionation of Mn(III) depending only on pH or pMn.

KINETICS OF MANGANESE RELEASED FROM SELECTED MANGANESE OXIDE MINERALS AS INFLUENCED BY POTASSIUM CHLORIDE1

We studied the influence of KCl on the dynamics of Mn release from selected manganese oxide minerals. The release of Mn increased consistently with increasing concentration of KCl (0, 0.01, 0.1, and 1 M). As the concentration was increased from 0 to 1 M, the release in a 20-h reaction period at 297K increased 7.2, 2.9, and 1.6 times at pH 5.0 and 7.0, 2.0 and 1.4 times at pH 7.0 for pyrolusite, hausmannite, and cryptomelane, respectively. The release of Mn was attributed to the reduction and disproportionation of Mn(IV) and Mn(III) of the Mn oxides to Mn(II) in aqueous system, as confirmed by ESR spectroscopy, and to the subsequent formation of MnCl+ complex in the KCl system. The Mn release was rapid initially (0 to 0.25 h) and then decreased subsequently (1 to 20 h), following a multiple zero-order kinetics. The rate constants of the Mn release are in the same order as the specific surface of the minerals, i.e., cryptomelame > hausmannite > pyrolusite. The kinetic data indicate that more than one factor controlled the Mn release from the three Mn oxide minerals. The dependence of Mn release on KCl concentration, temperature, and time was evaluated and an empirical equation was derived relating these variables. The results indicate that the Mn release varies with the nature of Mn-bearing minerals studied.

Pulse radiolytic studies of aqueous manganese(II) perchlorate solutions

Using Mn(ClO/sub 4/)/sub 2/, the reactions of Mn/sup 2 +/ with CO/sub 2//sup -/, e/sup -//sub aq/, OH, and O/sub 2//sup -/ were studied at pH 6.7. The following reaction rate constants were measured: k(Mn/sup 2 +/ + CO/sub 2//sup -/) less than 2 x 10/sup 5/ M/sup -1/ s/sup -1/; k(Mn/sup 2 +/ + e/sup -//sub aq/) = (9 +- 2) x 10/sup 6/ M/sup -1/ s/sup -1/ (ionic strength 0.15 to 0.5 M); k(Mn/sup 2 +/ + OH) = (3.4 +- 0.8) x 10/sup 7/ M/sup -1/ s/sup -1/; k(Mn/sup 2 +/ + O/sub 2//sup -/) = (1.1 +- 0.2) x 10/sup 8/ M/sup -1/ s/sup -1/ (0.01 M formate). O/sub 2//sup -/ radical ions react with Mn/sup 2 +/ and produce Mn/sup 3 +/ or MnO/sub 2//sup +/ ions. The reaction was found to be reversible, and an equilibrium constant K = (1.5 +- 0.6) x 10/sup 4/ M/sup -1/ was measured at pH 6.7. Mn/sup 2 +/, however, does not catalyze the dismutation of O/sub 2//sup -/. The disproportionation of Mn(III) (as MnOH/sup 2 +/) and the subsequent nucleation of the product MnO/sub 2/ are reported.