Über die Oxydation von Manganoxidhydroxid

The oxidation of γ-MnOOH (manganite) in oxygen and its disproportionation in HNO₃ lead topotactically to β-MnO₂. The oxidation of synthetic α-MnOOH (groutite) in oxygen depends on its cristallite size; finely divided crystals oxidise rapidly to Mn₅O₈ which usually is stable but yields β-MnO₂ by further oxidation. Larger crystals of disperse synthetic α-MnOOH are topotactically transformed to γ-MnO₂. In HNO₃ α-MnOOH disproportionates into γ-MnO₂ and Mn²⁺. Though strictly topotactical, the reaction α-MnOOH → γ-MnO₂ is not single-phase as might be expected. The discontinuity in the function: JAHN -TELLER distortion vs. reaction rate, may simply be interpreted as the crosspoint of two different functions attributed to the crystal species α-MnOOH and γ-MnO₂, respectively. This distortion confirms the presence of Mn³⁺ in manganite and nsutite. The wide variety of possible X ray powder patterns of the phase γ-MnO₂ is explained by the superposition of, (i) cristallite size broadening, (ii) intergrowth structure effects on the profile, and (iii) BRAGG angle shifts due to substitution of part of Mn⁴⁺ by Mn³⁺.     

Thermal decomposition of managanese oxyhydroxide

Temperature-programmed thermal decomposition of γ- and α-manganese oxyhydroxide has been studied between 20 and 670°C under vacuum and under a low pressure (10 Torr) of oxygen. Solid products at various temperatures have been analyzed by X-ray diffractometry. Under vacuum γ-MnOOH decomposed below 400°C to a mixture of Mn₅O₈, α-Mn₃O₄, and water according to the reaction scheme: 8MnOOH → Mn₅O₈ + Mn₃O₄ + 4H₂O. Above this temperature Mn₅O₈ was converted to α-Mn₃O₄ as a result of oxygen removal. The vacuum dehydration at 250°C of oxyhydroxide rich in α-MnOOH led to the formation of a new modification of Mn₂O₃ isostructural with corundum (α-Al₂O₃). In oxygen both oxyhydroxides decomposed to β-MnO₂. γ-MnOOH transformed directly to β-MnO₂ while α-MnOOH appeared to transform via corundum-phase Mn₂O₃ as an intermediate.     

Catalytic reduction of NOx and oxidation of dichloroethane over α-MnO2 catalysts: Properties-Reactivity relationship

The reduction ability of NO to N₂ and the oxidation performance of 1,2-dichloroethane (DCE) over α-MnO₂ catalysts were investigated. The results show that α-MnO₂-3 exhibited the highest catalytic activity in 63.5 % conversion of NO_x reduction by C₃H₈ at 250 °C, and 80 % conversion of DCE combustion by O₂ at 338 °C. It is revealed the active phase of α-MnO₂-3 is tetragonal α-MnO₂ with the selectively exposed plane of (2 1 1). It was proposed the high DCE decomposition of α-MnO₂-3 was ascribed to the redox properties. The overall characterization results revealed that α-MnO₂-3 catalyst preserves more active sites of low valence Mn and higher surface adsorbed oxygen (O_ads) /lattice oxygen (O_latt) at the outermost layers, and lower reduction temperature in H₂-TPR profiles than that of other catalysts. Meanwhile, NH₃-TPD profile of α-MnO₂-3 also shows a large number of acid sites promote NO_x reduction.     

Effect of alkaline and alkaline–earth cations on the supercapacitor performance of MnO2 with various crystallographic structures

The electrochemical performances of the α-, γ-, and δ-MnO₂ with different crystallographic structures were systematically investigated in 0.5 mol/L Li₂SO₄, 0.5 mol/L Na₂SO₄, 1 mol/L Ca(NO₃)₂, and 1 mol/L?Mg(NO₃)₂ electrolytes. The results showed that the electrochemical performances of the manganese dioxides depended strongly on the crystallographic structures of MnO₂ as well as the cation in the electrolytes. Because the δ-MnO₂ consists with layers of structure and the interlayer separation is 7 Å, which is suitable for insertion/extraction of some alkaline and alkaline–earth cations, the δ-MnO₂ electrode showed the higher specific capacitance than that of α-MnO₂ and γ-MnO₂. We also found that the α-, γ-, and δ-MnO₂ electrodes in the Mg(NO₃)₂ electrolyte showed a higher specific capacitance, while all the α-, γ-, and δ-MnO₂ electrodes in the Li₂SO₄ electrolyte exhibited a better cycle life. The reason for the different behavior of Li⁺ and Mg²⁺ during the charge/discharge process can be ascribed to the charge effect of the cations in the electrolytes. The ex situ X-ray diffraction (XRD) and long-time cyclic voltammogram measurements were used to systematically study the energy storage mechanism of MnO₂-based electrodes. A progressive crystallinity loss of the materials is also observed upon potential cycling at the oxidized states. A reasonable charge/discharge mechanism is proposed in this work.     

Hydrothermal synthesis,structure and photocatalytic property of nano-TiO<Subscript>2</Subscript>-MnO<Subscript>2</Subscript>

TiCl₄ and MnSO₄· H₂O as raw materials are hydrolyzed stiochiometrically, following the intermediate of oxide hydrating reacts at 150°C, 0.5 MPa in high-pressure reactor, after filtering, washing and drying, nanometric TiO₂-MnO₂ (Ti_1-X Mn _X O₂) is prepared. The effects of the reaction temperature and time on nanometric TiO₂-MnO₂ are also discussed. XRD shows that the product is TiO₂-MnO₂ with amorphous phase. After being sintered at above 780 °C, it transfers into Ti_1-X Mn _X O₂ with a rutile structure. TEM shows that TiO₂-MnO₂ is the spherical particle. And the average diameter of the particles is 20 nm. The optical absorbance was determined by UV-265 spectrophotometer after dispersing the sample in the mixture of water and glycerol with the ratio of 1 : 1 equably. It is found that the nano-material possesses the advantages of both nano-TiO₂ and nano-MnO₂, and it has strong absorption in the UV and visible region. Photodegradation of dyes in an aqueous solution is investigated using nanometricTiO₂-MnO₂ as a photocatalyst. The results show that after 60 min illumination, the decolorization rate of the acidic red B and acidic black 234 dye can be as high as 100%.     

Reactivity of η-, γ-, and α-Al2O3 for ZnAl2O4 Formation

The rate of ZnAl₂O₄ formation was measured for η-, γ-; and α- Al₂O₃ in order to distinguish the reactivity of them. The reactivity decreased as follows: η- > γ- > α-Al₂O₃. The reaction rate fitted to Jander's equation and the activation energies calculated were 33, 47 and 113 Kcal/mol for η-, γ- and α-Al₂O₃ systems, respectively. These differences are explained by an assumption that η- and γ-Al₂O₃ resulted in a ZnAl₂O₄ with imperfect spinel structure, but α-Al₂O₃ gave the perfect spinel structure. This assumption is based on the theoretical consideration of the activation energy needed for the diffusion-controlled reaction and date of lattice constant of each ZnAl₂O₄ obtained from three aluminas. The fact that η-Al₂O₃ shows very high reactivity compared with that of γ-Al₂O₃ was found to be explained on the basis of Jander's equation, a comparison of specific surface area and the defect structures of the aluminas.     

Hydrothermal Microwave Synthesis of MnO<Subscript>2</Subscript> in the Presence of Melamine: The Role of Temperature and pH

Nanocrystalline manganese dioxide have been prepared by hydrothermal microwave treatment of mixed solutions of potassium permanganate and 2,4,6-triamino-1,3,5-triazine (melamine) in pH range 0.5–3. Phase and chemical composition and morphology of the samples was studied by XRD, Raman spectroscopy, and SEM. Conditions (solution pH and temperature) for the formation of single phase MnO₂ powders (α-MnO₂, γ-MnO₂, δ-MnO₂, and δ*-MnO₂) under hydrothermal microwave treatment were determined.     

Selective hydrothermal microwave synthesis of various manganese dioxide polymorphs

Hydrothermal microwave treatment of mixed solutions of potassium permanganate and hexamethylenetetramine within the pH range 0.5–6.9, resulted in various polymorphs of nanocrystalline manganese dioxide: α-MnO₂ (cryptomelane), γ-MnO₂ (nsutite), β-MnO₂ (pyrolusite), and δ-MnO₂ (birnessite). The pH values of the medium at which single-phase samples form were determined.     

The γ-MnO2 electrode: the electrochemical de-insertion of the (H+, e–) couple from the partly reduced grain

A considerable understanding of the re-oxidation properties of partly reduced γ-MnO₂ is gained through simple phenomenological analysis of the experimental results and exploitation of the advances in the science of carrier injection in semiconductors. It is essentially demonstrated that the de-insertion reaction is still possible if the free or untrapped charges (H⁺, e^–) in the bulk of the host material remain the majority carriers. In contrast, in the existence zone (r _d>0.80) of δ-MnOOH the trapped particles become the majority (the free ones become minority) carriers and the flux may be stopped or gives rise to further types of reaction (complex formation, oxygen regeneration, etc.). Besides being of interest in themselves, these results are useful in studying the properties of electrode materials or semiconductors in which they occur. Electronic Publication     

The particle dimension controlling synthesis of α-MnO2 nanowires with enhanced catalytic activity on the thermal decomposition of ammonium perchlorate

Hydrothermal method synthesis of α-MnO₂ nanowires has been achieved at different temperatures in this work. X-ray diffraction and transmission electron microscopy confirmed the pure phase of the α-MnO₂ nanowires. All of the samples crystallized in a single-phase nanowires shape. The α-MnO₂ nanowires diameter increased from 11 nm to 21 nm with the increase in hydrothermal temperature from 120 °C to 200 °C. The α-MnO₂ catalytic activity on the decomposition of ammonium perchlorate (AP) was characterized through thermogravimetric analysis. The decomposition rate of AP with the addition of α-MnO₂ was size relative. The 11 nm MnO₂ nanowires exhibited the best catalytic activity, which lowered the high-temperature peak of AP by 130 °C.     

Über Oxidhydroxide des vierwertigen Mangans mit Schichtengitter 2. Mitteilung: Mangan (III)-manganat (IV)

The preparation and probable structure of managanese (III)-manganate (IV) Mn₇O₁₃, 5H₂O (a₀ = 2,84, c₀ = 7,27 Å) and manganous (II)-manganate (IV) Mn₇O₁₂, 6H₂O are described. Both consist of platelets. Digesting in diluted HNO₃ leads to γ-MnO₂ (nsutite). Manganese (III)-manganate (IV) is much less stable than the sodiummanganese (II, III)-manganate-(IV) described earlier and looses water easily when heated or in vacuo. Water loss results in breaking down of the double layer lattice, and the product is only two-dimensionally ordered, producing only prism reflections on the X ray diagramm. Heating results in a continuous transition to the two-dimensionally ordered phase, then to a finely divided and very disordered γ-MnO₂, and eventually to a finely divided and disordered β-MnO₂ (pyrolusite). The transition is topotactical so far, but further heating produces δ-Mn₂O₃ without obvious topotactical relations to the earlier products. The so-called ‘δ-MnO₂’ (birnessite) appears to be a family of finely divided and very disordered varieties of such manganates (IV) with part of the Mn³⁺ substituted by Mn⁴⁺. Since such products usually contain remarkable amounts of alkali ions, they are rather varieties of the earlier described sodiummanganese (II, III)-manganate (IV). A provisional explanation of the streaking in the electron diffractions of these manganate (IV). A provisional explanation of the streaking in the electron diffractions of these manganates (IV) is given. With respect to these results the so-called ‘δ-MnO₂’ can no longer be attributed to the true manganese dioxides.     

Nanorods of manganese oxides: Synthesis, characterization and catalytic application

Single-crystalline nanorods of β-MnO₂, α-Mn₂O₃ and Mn₃O₄ were successfully synthesized via the heat-treatment of γ-MnOOH nanorods, which were prepared through a hydrothermal method in advance. The calcination process of γ-MnOOH nanorods was studied with the help of Thermogravimetric analysis and X-ray powder diffraction. When the calcinations were conducted in air from 250 to 1050 °C, the precursor γ-MnOOH was first changed to β-MnO₂, then to α-Mn₂O₃ and finally to Mn₃O₄. When calcined in N₂ atmosphere, γ-MnOOH was directly converted into Mn₃O₄ at as low as 500 °C. Transmission electron microscopy (TEM) and high-resolution TEM were also used to characterize the products. The obtained manganese oxides maintain the one-dimensional morphology similar to the precursor γ-MnOOH nanorods. Further experiments show that the as-prepared manganese oxide nanorods have catalytic effect on the oxidation and decomposition of the methylene blue (MB) dye with H₂O₂.     

Conversion efficiency of γ/β-MnO2 in composites with natural graphite and carbon nanotubes in a prototype lithium battery

Chemically synthesized manganese dioxide γ/β-MnO₂ was studied in composites with multi-walled carbon nanotubes and natural graphite of EUZ-M brand was studied in the redox reaction with lithium. It was shown that nanometer carbon electron-conducting filler is advantageous over the micrometer filler (EUZ-M) in the efficiency of influence exerted on power characteristics and cycling capacity of a prototype lithium battery with a cathode based on γ/β-MnO₂. The effective chemical diffusion coefficient of lithium ions in MnO₂ composites with multi-walled carbon nanotubes and EUZ-M was estimated and hodographs of electrodes based on γ/β-MnO₂ without a carbon additive and with EUZ-M, brought in contact with an electrolyte, were analyzed.     

Synthesis, characterization and study of arsenate adsorption from aqueous solution by α- and δ-phase manganese dioxide nanoadsorbents

Single-phase α-MnO₂ nanorods and δ-MnO₂ nano-fiber clumps were synthesized using manganese pentahydrate in an aqueous solution. These nanomaterials were characterized using the Transmission Electron Microscope (TEM), Field Emission Scanning Electron Microscope (FE-SEM), Powder X-ray diffraction (XRD) and the Brunauer-Elmet-Teller nitrogen adsorption technique (BET-N₂ adsorption). The structural analysis shows that α-MnO₂ (2×2 tunnel structure) has the form of needle-shaped nanorods and δ-MnO₂ (2D-layered structure) consists of fine needle-like fibers arranged in ball-like aggregates. Batch adsorption experiments were carried out to determine the effect of pH on adsorption kinetics and adsorption capacity for the removal of As(V) from aqueous solution onto these two types of nanoadsorbents. The adsorption capacity of As(V) was found to be highly pH dependent. The adsorption of As(V) onto α-MnO₂ reached equilibrium more rapidly with higher adsorption capacity compared to δ-MnO₂.     

Synthese makrocyclischer, α,β-ungesättigter γ-Oxolactone durch Ringerweiterungsreaktionen; ein neuer Weg zum makrocyclischen Lacton-Antibiotikum A 26771 B

Synthesis of Macrocyclic α,β-Unsaturated γ-Oxolactones by Ring-Enlargement Reactions; a New Path to the Macrocyclic Lactone Antibiotic A 26771 B A new synthetic route to the α,β-unsaturated γ-oxolactones 2a and 2b , involving two ring-enlarement reactions, is described. Ring opening of bicyclic α-nitroketones of the type 3 gave ring-enlarged compounds of the type 4 which were converted to monoprotected diketones of the type 10 by using a variation of the Nef reaction as a key step. Macrocyclic lactones of the type 11 were obtained by Baeyer-Villiger oxidation and converted into compounds of the type 2 . The conversion of 2b to the macrocyclic lactone antibiotic A 26771 B ( 1 ) is already described in the literature.     

Kinetics of Mn2O3 digestion in H2SO4 solutions

The kinetics of Mn₂O₃ digestion in various H₂SO₄ solutions (0.5-2.0 M) and at various temperatures (ambient to 80 °C) to form solid γ-MnO₂ and soluble Mn(II) have been examined using X-ray diffraction. Using a modified first-order Avrami expression to describe digestion kinetics, rate constants in the range 0.02-0.98 h⁻¹ were found for Mn₂O₃ disappearance, and 0.03-0.42 h⁻¹ for γ-MnO₂ formation, with higher H₂SO₄ concentrations and temperatures leading to faster conversion rates. Also, for a particular set of experimental conditions, the rate of γ-MnO₂ formation was always slower than Mn₂O₃ disappearance. This was interpreted in terms of the solubility and stability of the soluble Mn(III) intermediated formed during the digestion. Activation energies for Mn₂O₃ dissolution and γ-MnO₂ formation were also determined.     

锰负载量对活性炭载锰氧化物的结构及催化分解臭氧性能的影响简

将高锰酸钾与活性炭（AC）原位氧化还原制备的活性炭载锰氧化物（MnO_x/AC）用作臭氧分解的催化剂. 采用扫描电镜、X射线光电子能谱、X射线衍射、电子自旋共振波谱、拉曼光谱以及程序升温还原研究了设计Mn负载量对负载锰氧化物性质（形貌、氧化态和晶体结构）的影响. 结果表明,Mn负载量由0.44%增至11%,负载锰氧化物在活性炭表面由疏松的地衣状变为堆叠的纳米球状体,负载层的厚度由~180 nm增加至~710 nm,结构由氧化态+2.9到+3.1的低结晶β-MnOOH生长为由氧化态+3.7到+3.8的δ-MnO₂结晶. MnO_x/AC室温催化分解低浓度臭氧的活性与负载锰氧化物的形貌及含量密切相关. Mn负载量为1.1%的MnO_x/AC具有疏松的地衣状形貌,催化分解臭氧的性能最高,Mn负载量为11%的MnO_x/AC具有紧密的堆积结构,因而表现出最低的催化臭氧分解活性.     

A Novel Method for the Synthesis of α,β‐Unsaturated Amides Mediated by Samarium Diiodide

Promoted by Samarium diiodide (SmI₂), α,β‐unsaturated amides were formed from nitrogen anions (formed in situ by the reduction of nitro compounds) and α,β‐unsaturated esters. This reaction contrasts with the conjugate addition between amines and α,β‐unsaturated esters promoted by samarium triiodide (SmI₃) and provides an alternative attractive way to obtain α,β‐unsaturated amides using SmI₂.     

Reductive dissolution of microparticulate manganese oxides

Electrochemical dissolution of immobilised microparticulate Mn(III,IV) oxides in slightly acidic solution (pH 4.4) was found to be a very general reaction, which is responsible for well-defined voltammetric peaks. Dissolution of six Mn(III,IV) oxides is initiated by the reduction of Mn(IV) to Mn(III) in the solid phase, which is followed by a massive dissolution via further reduction of Mn(III) to Mn(II), which finally yields soluble Mn²⁺. The reactivity of manganese oxides depends on their structure: the most reactive are amorphous (δ-MnO₂) and layered structures (birnessite); more resistant toward reductive dissolution are α- and λ-MnO₂ and electrochemical manganese dioxide; and least reactive is β-MnO₂. Reductive dissolution of LiMn₂O₄ resembles that of λ-MnO₂, whereas CaMnO₃ dissolves via a different reaction mechanism.     

Enantiomeric separation of α-phenylethylamine and its analogs by mixed chiral and achiral stationary phase

A mixed stationary phase of modified β-cyclodextrin, 2,6-di-O-butyl-3-O-butyryl-β-cyclodextrin (phase A), and SE-54 (phase B) was used for enantiomeric separation of α-phenylethylami-ne, o,m,p-methyl and o,m,p-methoxy-substituted analogs. The composition of mixed phase was selected by comparison of each calculated amin(= (γm,i+1)/(γm,i))> the relative retention values of the most adjacent peaks, and γm,last, the relative retention values of the last eluting peak at each preselected ratio. Values of γm,i,α calculated by derived equations were in good agreement with the experimental results obtained with two specified mixed phases. All solutes investigated were almost baseline separated at a predicted composition of phase A and phase B in a single run within 18 minutes.