Amorphous Manganese Dioxide Research Articles

Mild and Selective Heterogeneous Catalytic Hydration of Nitriles to Amides by Flowing through Manganese Dioxide

A sustainable flow chemistry process for the hydration of nitriles, whereby an aqueous solution of the nitrile is passed through a column containing commercially available amorphous manganese dioxide, has been developed. The product is obtained simply by concentration of the output stream without any other workup steps. The protocol described is rapid, robust, reliable, and scalable, and it has been applied to a broad range of substrates, showing a high level of chemical tolerance.

The capacitive properties of amorphous manganese dioxide electrodeposited on different thermally-treated carbon papers

The surface structure of the commercial carbon paper (CP) has been modified through thermal treatment, and then MnO2 was electrodeposited on the thermally-treated carbon paper (TTCP) by cyclic voltammetry (CV) method in the phosphate buffer solution (PBS, pH 9.18) containing KMnO4. The obtained samples were characterized by X-ray diffraction (XRD), X-ray photoelectron energy spectroscopy (XPS) and scanning electron microscopy (SEM). The results showed that the phase of the obtained MnO2 was amorphous, and that the morphology of the MnO2 deposited on the substrates was influenced strongly by the substrates. The capacitive properties of MnO2 deposited on the carbon substrates were investigated by CV, galvanostatic charge/discharge, electrochemical impedance spectroscopy technique. It was found that the specific capacitance of MnO2 deposited on the optimum TTCP can reach to 749Fg−1 based on the charge/discharge measurement, which was far larger than that on the CP (345Fg−1).

Ultrathin amorphous manganese dioxide nanosheets synthesized with controllable width

We present a one-step synthesis process for ultrathin non-crystalline MnO2 nanosheets. Their thickness is about 2 nm and the width varies from 5 to 20 nm. They induce a higher capacitance than zero-dimensional spheres or one-dimensional rods. When heat treated at elevated temperature MnO2 nanosheets partly grow into crystalline Mn2O3 rods.

Manganese hexacyanoferrate/MnO2 composite nanostructures as a cathode material for supercapacitors

A composite of manganese hexacyanoferrate (MnHCF) coated by an amorphous manganese dioxide layer was synthesized by a facile co-precipitation method and a further step called “deep electro-oxidation”. The structure and components of the resulting MnHCF/MnO2 composites were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Electrochemical testing showed a capacitance of 225.6 F g−1 at a sweep rate of 5 mV s−1 within a voltage range of 1.3 V and a high energy density of 74.5 W h kg−1 at a current density of 0.5 A g−1 during galvanostatic charge/discharge cycles, which is superior to most cathode materials, including some reported graphene/MnO2 nanocomposites. It is confirmed that the two components, manganese hexacyanoferrate and manganese dioxide, lead to an integrated electrochemical behavior and a capacitor with enhanced performance. The electrochemical testing and corresponding XPS analysis also demonstrated that the manganese coordinated by cyanide groups via nitrogen atoms in MnHCF did not get involved in the charge storage process during potential cycles.

ChemInform Abstract: Manganese Oxide‐Catalyzed Additive‐ and Solvent‐Free Aerobic Oxidative Synthesis of Primary Amides from Primary Amines.

AbstractPrimary benzylic amines undergo aerobic oxidation to the corresponding amides (II) in the presence of amorphous manganese dioxide as reusable catalyst.

ChemInform Abstract: Efficient Hydration of Nitriles Promoted by Simple Amorphous Manganese Oxide Using Reduced Amounts of Water.

AbstractHydration of various (het)aromatic and aliphatic nitriles can efficiently be promoted by amorphous manganese dioxide using two equivalents of water at the most.

Effect of Ag-Doping on the Capacitive Behavior of Amorphous Manganese Dioxide Electrodes

Amorphous MnO2 and Ag-doped MnO2 thin films were galvanostatically deposited on a polished stainless steel substrate from 20 mM KMnO4 aqueous solutions without and with AgNO3 additions at cathodic current density of 1 mA/cm 2 for the application in electrodes of electrochemical supercapacitors. The supercapacitive properties of the deposited thin films have been studied in 0.5 M Na2SO4 electrolyte by cyclic voltammetry, impedance spectroscopy, and charge-discharge measurement techniques. The Ag-free MnO2 film showed better capacitive behavior and lower charge transfer resistance compared to the Ag-doped MnO2 films. The specific capacitance and charge transfer resistance values for Ag-free MnO2 films were 160 F/g at a scan rate 10 mV/s and 3.87 Ω, respectively. Cyclic stability using charge-discharge measurement technique indicates the excellent stability of Ag-free MnO2 films and its possible use as a low cost electrode for superca- pacitor applications.

Capacitive Behavior of Manganese Dioxide/Stainless Steel Electrodes at Different Deposition Currents

Amorphous manganese dioxide thin films were prepared by galvanostatic cathodic deposition at current densities of 0.5-1 mA/cm 2 on etched stainless-steel substrate from 20 mM KMnO4 solution. The structure of the deposited oxides was investigated using X-ray diffraction analysis. The capacitive behavior of the manganese dioxide electrodes was characterized by cyclic voltammetry and electrochemical impedance spectroscopy in 0.5 M Na2SO4 electrolyte. The capacitive performance was found to increase with the increase in the deposition current density. The electrode deposited at current density of 1 mA/cm 2 showed specific capacitance of 174 F/g at a scan rate of 10 mV/s, equivalent series resistance of 3.53 Ω, and charge transfer resistance of 1.39 Ω. The improvement in the capacitive behavior of the electrode with the increase in the deposition current density was attributed to the increase in the electronic properties of the deposited oxides.

Oxidation of alkyl aromatics to ketones by tert-butyl hydroperoxide on manganese dioxide catalyst

The catalytic activity of the manganese oxide was investigated for the oxidative functionalization of alkylaromatics to benzylic ketones using tert-butyl hydroperoxide (TBHP) as an oxidant. Manganese oxides of different types were tested for this reaction. Of all the oxides, the nano amorphous manganese dioxide exhibited significant catalytic activity and selectivity for the reaction. The nano amorphous MnO2/TBHP catalytic system could also be reused for six consecutive cycles with no considerable loss in catalytic activity.

High energy density capacitor using coal tar pitch derived nanoporous carbon/MnO 2 electrodes in aqueous electrolytes

Asymmetric aqueous electrochemical capacitors with energy densities as high as 22 Wh kg −1, power densities of 11 kW kg −1 and a cell voltage of 2 V were fabricated using cost effective, high surface carbon derived from coal tar pitch and manganese dioxide. The narrow pore size distribution of the activated carbon (mean pore size ∼0.8 nm) resulted in strong electroadsorption of protons making them suitable for use as negative electrodes. Amorphous manganese dioxide anodes were synthesized by chemical precipitation method with high specific capacitance (300 F g −1) in aqueous electrolytes containing bivalent cations. The fabricated capacitors demonstrated excellent cyclability with no signs of capacitance fading even after 1000 cycles.

Nanostructured amorphous MnO 2 prepared by reaction of KMnO 4 with triethanolamine

Amorphous manganese dioxide is prepared by reaction of potassium permanganate with an organic reductant triethanolamine. The effect of heat-treatment temperature is studied on the characteristics of the materials. Power X-ray diffraction (XRD), scanning electron microscope (SEM) and N 2 adsorption and desorption measurements are employed to investigate crystalline structure, surface morphology, the specific surface area and the pore size distribution. It is found that when the annealing temperature reaches up to 400 °C, the crystalline convert to α-MnO 2 from amorphous MnO 2. The electrochemical characteristics of the prepared MnO 2 powder are characterized by means of cyclic voltammetry (CV), experiments in 1.0 mol L −1 Na 2SO 4 electrolyte. The specific capacitance (SC) value is 251 F g −1 that is obtained from the product annealing at 350 °C at a CV scan rate of 2 mV s −1. And charging–discharging measurement reveals the good stability of the prepared material.

High-voltage asymmetric supercapacitors operating in aqueous electrolyte

The performance of supercapacitors based on different materials with pseudocapacitive properties such as several conducting polymers (ECPs), amorphous manganese dioxide (a-MnO2), and activated carbon is reported. Composite electrodes of high resiliency and good electronic conductivity were obtained by mixing the active materials with carbon nanotubes. The various limitations of all the above-mentioned materials, when used as negative and positive electrodes in traditional symmetric systems, are shown. It is demonstrated that a successful application of ECPs and a-MnO2 in supercapacitor technologies is possible only in an asymmetric configuration, i.e. with electrodes of different nature for positive and negative polarizations. Several types of asymmetric capacitors were developed by combining ECPs, a-MnO2, and activated carbon and characterized in aqueous electrolyte by galvanostatic charge–discharge, cyclic voltammetry, and impedance spectroscopy. The best device considering the specific energy and power is the asymmetric supercapacitor using a-MnO2 and poly(3,4-ethylenedioxythiophene) (PEDOT) for the positive and negative electrodes, respectively. It has an operating voltage of 1.8 V, which is attributed to different operating potentials of both electrodes, and good electrochemical stability in neutral aqueous electrolyte. According to the voltage value, the energy density of the asymmetric capacitor at a current density of 250 mA/g is found to be 13.5 W h/kg, which is about ten times more than for a symmetric capacitor based on PEDOT in an aqueous medium. The asymmetric capacitor provides two times higher power than a symmetric capacitor based on activated carbon in organic electrolyte, and is thus extremely promising for the development of environmentally friendly systems.

Amorphous Manganese Dioxide as a Material for an Electrochemical Pseudocapacitor

The charge storage process of amorphous hydrated manganese dioxide (MnO2) as a pseudo-capacitor involves a fast redox reaction at the electrode surface. In order to understand the charge storage mechanism of MnO2 as a pseudo-capacitor in an aqueous KCl solution, we monitored the change of the capacitance by varying the pH of the solution, the cation of the electrolyte, the concentration of the KCl electrolyte, and the solvent. The charge storage mechanism of a metal oxide electrode such as MnO2 is concluded to involve a fast redox reaction through both the potassium ion exchange, MnO2 + δK+ + δe- ⇔ MnO2-δ(OK)δ and the proton exchange, MnO2 + δH+ + δe- ⇔ MnO2-δ(OH)δ dependent upon the availability of the cations in the electrolyte.

Performance of Manganese Oxide/CNTs Composites as Electrode Materials for Electrochemical Capacitors

Nanocrystalline metal oxides can be prepared with large surface area, electrochemical stability, and pseudocapacitive behavior, being able to be used as supercapacitor electrodes. Among the various metal oxides studied, amorphous and hydrated manganese oxide is the most promising for supercapacitor electrodes due to the low cost of the raw material. In the present work, amorphous manganese dioxide is prepared by chemical co-precipitation of Mn(VII) and Mn(II) in water medium, giving small particles of relatively high surface area. Carbon nanotubes (CNTs) are proposed as an alternative additive of carbon black for improving the electrical conductivity of the manganese oxide electrodes used to build capacitors. The results demonstrate that CNTs are effective for increasing the capacitance and improving the electrochemical properties of the electrodes which show a better capacitive behavior than with carbon black. This enhancement is due to the high entanglement of CNTs which form a network of open mesopores, allowing the bulk of to be easily reached by the ions. The performance optimization requires a careful control of the electrolyte pH in order to avoid the irreversible reactions Mn(IV) to Mn(II) at the negative electrode and Mn(IV) to Mn(VII) at the positive one. © 2004 The Electrochemical Society. All rights reserved.

Ramsbeckite from the Hirao mine at Minoo, Osaka, Japan

Ramsbeckite was found as a vein forming mineral in altered shale at the Hirao mine, Minoo, Osaka, Japan. It occurred as aggregates of euhedral granular crystals up to 0.5 mm across, in association with sphalerite, chalcopyrite, smithsonite, aurichalcite, amorphous manganese dioxide, schulenbergite, brochantite, serpierite, limonite and an unidentified mineral. It was emerald green to blue-green in color with a vitreous luster in hand specimen. An EPMA and CHNS/O analyzer gave the empirical formula (Cu9.380Zn5.542Ni0.034Co0.027Fe0.020Mn0.013)Σ15.016[(SO4)3.871(CO3)0.166]Σ4.037(OH)21.958·6.06H2O on the basis of O = 44. The unit cell parameters were a = 16.106 (3), b = 15.568 (2), c = 7.109 (1) A, β = 90.23 (1)°, and Z = 2. The mineral was optically biaxial negative with refractive indices α = 1.676, β = 1.704 and γ = 1.707, and 2VX(calc) = 37.8°. The Vickers microhardness was 162 (144-182) kg/mm2 (10 g load), and the Mohs hardness was 3.5. The measured density was 3.36 g/cm3. It is likely that ramsbeckite at the Hirao mine crystallized from Cu and Zn-bearing fluids.

Ramsbeckite from the Hirao mine at Minoo, Osaka, Japan

Ramsbeckite was found as a vein forming mineral in altered shale at the Hirao mine, Minoo, Osaka, Japan. It occurred as aggregates of euhedral granular crystals up to 0.5 mm across, in association with sphalerite, chalcopyrite, smithsonite, aurichalcite, amorphous manganese dioxide, schulenbergite, brochantite, serpierite, limonite and an unidentified mineral. It was emerald green to blue-green in color with a vitreous luster in hand specimen. An EPMA and CHNS/O analyzer gave the empirical formula (Cu9.380Zn5.542Ni0.034Co0.027Fe0.020Mn0.013)Σ15.016[(SO4)3.871(CO3)0.166]Σ4.037(OH)21.958·6.06H2O on the basis of O = 44. The unit cell parameters were a = 16.106 (3), b = 15.568 (2), c = 7.109 (1) A, β = 90.23 (1)°, and Z = 2. The mineral was optically biaxial negative with refractive indices α = 1.676, β = 1.704 and γ = 1.707, and 2VX(calc) = 37.8°. The Vickers microhardness was 162 (144-182) kg/mm2 (10 g load), and the Mohs hardness was 3.5. The measured density was 3.36 g/cm3. It is likely that ramsbeckite at the Hirao mine crystallized from Cu and Zn-bearing fluids.

Amorphous Manganese Oxides as Lithium Intercalation Hosts Prepared by Oxidation of Mn(II) Precursors

Amorphous manganese oxides were prepared by oxidation of Mn(II) precursors in aqueous solutions at room temperature. The oxidation procedure offers a number of advantages including a good control of chemical composition and morphology. X-ray and electron diffraction of as-synthesized materials reveal a complete lack of long-range order. Synthesized amorphous manganese dioxide with a low sodium content (0.09 Na/Mn) exhibits a specific capacity of 350, 290, and 250 mAh/g at discharge/charge rates of C/100, C/50, and C/5, respectively. The performance indicates not only extremely high lithium intercalation capacities, but also excellent rate capabilities. Electrochemical behavior during discharge/charge cycling suggests occurrence of local structural relaxations in the amorphous material; however, no trend toward a spinel-like behavior is observed. It is found that the presence of a large amount of sodium in the amorphous material (0.27 Na/Mn) lowers the lithium intercalation capacity but greatly improves capacity retention upon cycling. The sodium ions, which reside stably in the amorphous host as an electrochemically inactive species, may help stabilize the local structure thus leading to excellent cycling performance.

Effect of Copper Doping on Intercalation Properties of Amorphous Manganese Oxides Prepared by Oxidation of Mn(II) Precursors

Copper-doped amorphous manganese oxides were synthesized via a room-temperature aqueous solution route involving oxidation of Mn(II) precursors. Doping of copper ions did not significantly affect the low rate specific capacity or the intrinsic lithium intercalation capacity of the amorphous materials. However, even a small amount of copper doping dramatically increased the specific capacity at high rates. Larger amounts of copper doping also significantly enhanced capacity retention upon cycling. For an amorphous manganese dioxide doped with 0.18 Cu per Mn, a specific capacity of 275 mAh/g was attained between 4.0 and 1.5 V vs. Li + /Li at a high discharge/charge rate of C/5 (0.78 mA/cm 2 ), with capacity fading of less than 0.5% per cycle upon cycling. It is believed that copper doping has the dual effect of both improving the lithium intercalation kinetics and stabilizing the local structure of the amorphous materials during lithium intercalation and cycling.

Anaerobic mineralization of toluene by enriched sediments with quinones and humus as terminal electron acceptors.

The anaerobic microbial oxidation of toluene to CO(2) coupled to humus respiration was demonstrated by use of enriched anaerobic sediments from the Amsterdam petroleum harbor (APH) and the Rhine River. Both highly purified soil humic acids (HPSHA) and the humic quinone moiety model compound anthraquinone-2,6-disulfonate (AQDS) were utilized as terminal electron acceptors. After 2 weeks of incubation, 50 and 85% of added uniformly labeled [(13)C]toluene were recovered as (13)CO(2) in HPSHA- and AQDS-supplemented APH sediment enrichment cultures, respectively; negligible recovery occurred in unsupplemented cultures. The conversion of [(13)C]toluene agreed with the high level of recovery of electrons as reduced humus or as anthrahydroquinone-2,6-disulfonate. APH sediment was also able to use nitrate and amorphous manganese dioxide as terminal electron acceptors to support the anaerobic biodegradation of toluene. The addition of substoichiometric amounts of humic acids to bioassay reaction mixtures containing amorphous ferric oxyhydroxide as a terminal electron acceptor led to more than 65% conversion of toluene (1 mM) after 11 weeks of incubation, a result which paralleled the partial recovery of electron equivalents as acid-extractable Fe(II). Negligible conversion of toluene and reduction of Fe(III) occurred in these bioassay reaction mixtures when humic acids were omitted. The present study provides clear quantitative evidence for the mineralization of an aromatic hydrocarbon by humus-respiring microorganisms. The results indicate that humic substances may significantly contribute to the intrinsic bioremediation of anaerobic sites contaminated with priority pollutants by serving as terminal electron acceptors.

Abiotic hydrolysis of the detergent builder tripolyphosphate by hydrous manganese dioxide

The hydrolysis of tripolyphosphate to orthophosphate is facilitated by suspensions of amorphous manganese dioxide. Pyrophosphate is observed as an intermediate. The rate enhancement decreases with increasing pH. The weak sorption of orthophosphate on the oxide also decreases with increasing pH, indicating that the orthophosphate product would be bioavailable. The presence of calcium ions in natural waters increases both the rate of hydrolysis of tripolyphosphate and the extent of sorption of orthophosphate. Under some circumstances this abiotic mechanism could play a significant role in the hydrolysis of tripolyphosphate in the aquatic environment.