Transformations of manganese oxides under different thermal conditions

Thermal decomposition of various synthetic manganese oxides (MnO, Mn₃O₄, Mn₂O₃, MnOOH) and a natural manganese dioxide (MnO₂) from Gabon was studied with the help of termogravimetry in inert, oxidizing and reducing atmospheres. The compounds were characterized by XRD and electrochemical activity was tested by cyclic voltammetry using a carbon paste electrode. The natural manganese dioxide showed the best oxidizing and reducing capacity, confirmed by the lower temperatures of the transitions, the extent of the reactions and electrochemical performance in cyclic voltammograms.     

Effect of the oxidation state of manganese in the manganese oxides used in the synthesis of Mn-substituted cordierite on the properties of the product

Catalysts based on Mn-substituted cordierite 2MnO · 2Al₂O₃ · 5SiO₂ have been synthesized using different manganese oxides (MnO, Mn₂O₃, and MnO₂) at a calcination temperature of 1100°C. The catalysts differ in their physicochemical properties, namely, phase composition (cordierite content and crystallinity), manganese oxide distribution and dispersion, texture, and activity in high-temperature ammonia oxidation. The synthesis involving MnO yields Mn-substituted cordierite with a defective structure, because greater part of the manganese cations is not incorporated in this structure and is encapsulated and the surface contains a small amount of manganese oxides. This catalyst shows the lowest ammonia oxidation activity. The catalysts prepared using Mn₂O₃ or MnO₂ are well-crystallized Mn-substituted cordierite whose surface contains different amounts of manganese oxides differing in their particle size. They ensure a high nitrogen oxides yield in a wide temperature range. The product yield increases with an increasing surface concentration of Mn³⁺ cations. The highest NO_x yield (about 76% at 800–850°C) is observed for the MnO₂-based catalyst, whose surface contains the largest amount of manganese oxides.     

Electrode reactions of manganese oxides for secondary lithium batteries

Nanorods of MnO₂, Mn₃O₄, Mn₂O₃ and MnO are synthesized by hydrothermal reactions and subsequent annealing. It is shown that though different oxides experience distinct phase transition processes in the initial discharge, metallic Mn and Li₂O are the end products of discharge, while MnO is the end product of recharge for all these oxides between 0.0 and 3.0 V vs. Li⁺/Li. Of these 4 manganese oxides, MnO is believed the most promising anode material for lithium ion batteries while MnO₂ is the most promising cathode material for secondary lithium batteries.     

Correlation between structure and catalytic activity of manganese oxides in liquid-phase oxidation of 1-octene

We have studied the correlation between the crystal structure and the catalytic activity of manganese oxides MnO, MnO₂, Mn₃O₄, and Mn₂O₃ in liquid-phase oxidation of 1-octene by molecular oxygen. The catalytic activity decreases in the series of oxides with octahedral coordination environment for the manganese atoms MnO−Mn₂O₃−MnO₂. The oxide Mn₃O₄ (with mixed tetrahedral and octahedral environment for the Mn atoms) catalyzes the process according to a different mechanism. L'vov Polytechnic State University, 12 S. Bandery ul., L'vov-13 290646, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 34, No. 5, pp. 324–327, September–October, 1998.     

Application of Differential Scanning Calorimetry to the Reduction of Several Manganese Oxides

The aim of this work was to study the thermal transitions of several manganese oxides (MnO, MnOOH, Mn₂O₃, Mn₃O₄ and MnO₂) under reducing conditions. Differential scanning calorimetry (DSC) was used to analyse the transitions of some oxides into others. A comparison of the behavior of the synthetic samples with that of a natural manganese dioxide demonstrated that DSC is a quick tool for the distinction of natural manganese dioxide from synthetic γ-MnO₂ from other manganese oxides. This revised version was published online in July 2006 with corrections to the Cover Date.     

New baths for the deposition of manganese and cobalt oxides

New methods have been adopted for the anodic deposition of the different manganese and cobalt oxides. The deposition of the diferent oxides is usually carried out from their metal salt solutions in presence of a reducing agent. The oxides deposited are as follows: Mn₂O₃ from manganous sulphate in presence of boric, acid and formaldehyde at pH=5.5, Mn₃O₄ from manganous sulphate in presence of formic acid at pH=5.0 MnO from manganous sulphate-ammonium chloride solution in presence of telluric acid, Co₂O₃ from cobalt chloride in presence of telluric acid and sodium fluoride, Co₃O₄ from cobaltite in presence of formaldehyde and potassium chloride and finally CoO from cobalt chloride in presence of alcohol. The results of chemical analysis revealed that the purity of the oxides is 99.99% and their molecular formulae are MnO_1.5, MnO_1.33, MnO, CoO_1.5, CoO_1.33 and CoO respectively.     

Thermal Decompositions of Pure and Mixed Manganese Carbonate and Ammonium Molybdate Tetrahydrate

The thermal decompositions of pure and mixed manganese carbonate and ammonium molybdate tetrahydrate in molar ratios of 3:1, 1:1 and1:3 were studied by DTA and TG techniques. The prepared mixed solid samples were calcined in air at 500, 750 or 1000°C and then investigated by means of an XRD technique. The results revealed that manganese carbonate decomposed in the range 300–1000°C, within termediate formation of MnO₂, Mn₂O₃ andMn₃O₄. Ammonium molybdate tetrahydrate first lost its water of crystallization on heating, and then decomposed, yielding water and ammonia. At 340°C,MoO₃ was the final product, which melts at 790°C. The thermal treatment of the mixed solids at 500, 750 or 1000°C led to solid-solid interactions between the produced oxides, with the formation of manganese molybdate. At 1000°C, Mn₂O₃ and MoO₃ were detected, due to the mutual stabilization effect of these oxides at this temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Manganese oxide with hollow rambutan-like morphology as highly efficient electrocatalyst for oxygen evolution reaction

The research about oxygen evolution reaction (OER) has attracted extensive attention. In this work, different manganese oxides with different shell thickness were firstly grown on the surface of carbon ball template, and then the carbon ball was removed by high-temperature calcination in air to obtain hollow rambutan-like Mn₂O₃ and MnO₂–Mn₂O₃ with long nanowires. The concentration of inorganic manganese salt and the reaction time has a determining influence on the morphologies of manganese oxide. The as-prepared MnO₂–Mn₂O₃ exhibits a lower overpotential than the Mn₂O₃ to achieve a current density of 10 mA cm^?2. The Faradic efficiency of MnO₂–Mn₂O₃ reaches to 94.1% during the bulk electrolysis, and the morphology of MnO₂–Mn₂O₃ remains virtually unchanged after electrolysis, indicating the outstanding stability of the as-obtained MnO₂–Mn₂O₃.     

Characterisation of workplace aerosols in the manganese alloy production industry by electron microscopy

Workplace aerosols in a combined FeMn and SiMn alloy smelter were studied by scanning and transmission electron microscopy. Special emphasis was placed on the characterisation of individual particles with diameters below 500 nm and on identification of the different manganese phases present in the workroom air. In high-carbon FeMn production, the submicron size fraction is dominated by MnO particles forming chain-like or compact agglomerates. Minor amounts of MnO₂, Mn₃O₄, Mn₂O₃ and Fe₃O₄ are also observed. During production of SiMn, the submicron size fraction consists predominantly of MnSi particles, but small amounts of Mn₃Si, Mn₆Si and Mn₅Si₂ are also found. Workplace aerosols from the manganese oxide refinement (MOR) process consist mostly of Mn oxides. Minor amounts of carbonaceous particles occurring as sheets, ribbons and as hollow carbon structures are observed along the whole production line. Carbonaceous particles are either amorphous or consist of poorly crystallised graphite. Particles with fibre morphology were encountered at all sampling locations but most prominently during tapping of FeMn with fibre concentrations between 0.1 and 0.7 per cm³. The pronounced differences in particle composition along the production line clearly show that workers are exposed to a variety of Mn-containing species. MnO particles have a higher solubility than MnSi particles and are thus more bioaccessible, suggesting a higher risk of adverse health effects in the FeMn production than in the SiMn production.     

Physicochemical properties of manganese oxides obtained via the sol–gel method: The reduction of potassium permanganate by polyvinyl alcohol

Experimental data on the sol–gel synthesis of manganese oxides formed during the reduction of potassium permanganate by polyvinyl alcohol in an aqueous medium are presented. The physicochemical properties of the obtained manganese oxide systems that depend on the conditions of the synthesis are studied by means of DTA, XRD, SEM, and the low temperature adsorption–desorption of nitrogen. It is found that the obtained samples have a mesoporous structure and predominantly consist of double potassium–manganese oxide K₂Mn₄O₈ with a tunnel structure and impurities of oxides such as α-MnO₂, MnO, α-Mn₂O₃, and Mn₅O₈. It is shown that the proposed method of synthesis allows us to regulate the size and volume of mesopores and, to a lesser extent, the texture of the obtained oxides, which can be considered promising sorbents for the selective extraction of strontium and cesium ions from multicomponent aqueous solutions.     

Nitrogen‐Rich Manganese Oxynitrides with Enhanced Catalytic Activity in the Oxygen Reduction Reaction

The catalytic activity of manganese oxynitrides in the oxygen reduction reaction (ORR) was investigated in alkaline solutions to clarify the effect of the incorporated nitrogen atoms on the ORR activity. These oxynitrides, with rock‐salt‐like structures with different nitrogen contents, were synthesized by reacting MnO, Mn₂O₃, or MnO₂ with molten NaNH₂ at 240–280 °C. The anion contents and the Mn valence states were determined by combustion analysis, powder X‐ray diffraction, and X‐ray absorption near‐edge structure analysis. An increase in the nitrogen content of rock‐salt‐based manganese oxynitrides increases the valence of the manganese ions and reinforces the catalytic activity for the ORR in 1 m KOH solution. Nearly single‐electron occupancy of the antibonding e_g states and highly covalent Mn?N bonding thus enhance the ORR activity of nitrogen‐rich manganese oxynitrides.     

Complete oxidation of ethanol over MnOx

MnO_x was synthesized through the oxidation of MnCO₃ and its catalytic activity in the complete oxidation of ethanol was determined. Analyses of the catalyst through in situ X-Ray Diffraction and in situ DRIFTS spectroscopy showed the presence of Mn₂O₃ (bixbyite) and γMnO_2. The catalytic activity of MnO_x depends on the mixtures of oxides and the treatment before reaction. This revised version was published online in June 2006 with corrections to the Cover Date.     

Phase equilibrium in the system Ln-Mn-O VI: Ln=Ho and Tb at 1100°C

Phase equilibria were established in Ho-Mn-O and Tb-Mn-O systems at 1100°C by varying the oxygen partial pressure from −log(P_O2/atm)=0-13.00, and phase diagrams for the corresponding Ln₂O₃-MnO-MnO₂ systems at 1100°C were presented. Stable Ln₂O₃, MnO, Mn₃O₄, LnMnO₃, and LnMn₂O₅ phases were found at 1100°C, whereas Ln₂Mn₂O₇, Ln₂MnO₄, Mn₂O₃, and MnO₂ were not found to be stable. Small nonstoichiometric ranges were found in the LnMnO₃ phase, with the composition of LnMnO₃ represented as functions of log(P_O2/atm), and . Activities of the components in the solid solution were calculated from these equations. The composition of LnMnO₃ may range from Ln₂O₃ rich to Ln₂O₃ poor, while MnO is slightly nonstoichiometric, being oxygen rich and LnMn₂O₅ seems to be nonstoichiometric. Lattice constants of LnMnO₃ quenched at different oxygen partial pressures and of LnMn₂O₅ quenched in air were determined. The standard Gibbs energy changes of the reactions appearing in the phase diagrams were also calculated. The relationship between the tolerance factor of LnMnO₃ and ΔG⁰of reaction, (1/2)Ln₂O₃+MnO+(1/4)O₂=LnMnO₃, is shown graphically.     

DTA studies on preparation of MnSO4 by reacting pyrite and pyrolusite at high temperatures

The preparation of MnSO₄ by reacting pyrolusite at high temperatures with SO₂ generated from pyrite was followed by DTA, and the process conditions were optimized to fix the minimum time and temperature of reaction required to obtain the maximum yield of pure MnSO₄ from stoichiometric amounts of reactants in a natural draught of air. The presence of MnO and Fe₃O₄ in the reaction products, detected by DTA, indicates that the SO₂ is initially oxidized to SO₃ by reducing MnO₂, Mn₂O₃ and Fe₂O₃ to MnO and Fe₃O₄. SO₃ finally attacks MnO to form MnSO₄. When an intimate stoichiometric blend of pyrite and pyrolusite is heated at temperatures ranging from 873 K to 973 K for 3 hrs, about 93% of the Mn is converted to ironfree MnSO₄.     

Thermal Decomposition of Manganese Carbonate Supported on Active Alumina

The effect of aluminium oxide support on the thermal behaviour of manganese carbonate was investigated using TG, DTA, dDTA and XRD techniques. The concentrations of MnCO₃ were 0.025, 0.05 and 0.125 mol/mol Al₂O₃. The results obtained showed that the employed support material retards the thermal decomposition of manganese carbonate due to the formation of a manganese/aluminium adduct which decomposes readily at 350°C instead of 250°C in the absence of Al₂O₃, to give γ-MnO₂. This compound decomposed at 500°C into Mn₂O₃ (partridgeite). The produced Mn₂O₃ decomposed at 940°C yielding Mn₃O₄ which interacted with atmospheric oxygen during the cooling processes to give Mn₂O₃. However, Mn₃O₄ formed in the case of unloaded Mn₂O₃ did not interact easily with O₂ and remained stable. The results might indicate the role of Al₂O₃ in increasing the degree of dispersion of the produced manganese oxides thus increasing their reactivity towards reoxidation by O₂. The produced manganese oxide (Mn₂O₃) enhanced markedly the crystallization of aluminium oxide at 800°C into ?-Al₂O₃. Solid-solid interaction between Mn₂O₃ and Al₂O₃ occurred at 800°C giving MnAl₂O₄ which decomposed at 1000°C yielding Mn₂O₃ and α-Al₂O₃ (corundum).     

The reduction of alkali substituted LaMnO3

In order to determine the stability of some potential NO_x reduction catalysts (La_0.8M_0.2MnO₃, M  Na, K, Rb) the accelerated reduction of these catalysts in H₂, N₂ atmospheres was studied. La_0.8K_0.2MnO₃ goes through a reversible oxygen loss at about 350°C corresponding to the reduction of the available Mn⁴⁺ to Mn³⁺ in H₂, N₂ atmospheres. By reduction at higher temperatures a previously unreported phase La₂MnO₄ is formed. The most reducing conditions (10% H₂ in N₂, >940°C) formed only La₂O₃ and MnO. Between 700 and 880°C in 10% H₂ in N₂ potassium was eliminated from the sample by reduction to the metal and evaporation. Analogous results were found for Na and Rb substituted LaMnO₃ except that the intermediate phase La₂MnO₄ was not observed in the reduction of La_0.8Rb_0.2MnO₃.     

Effects of Metal Ion Sources on Synthesis and Electrochemical Performance of Spinel LiMn2O4 Using Tartaric Acid Gel Process

The effect of lithium and manganese ions on the synthesis, phase purity, and electrochemical properties of tartaric acid gel processed lithium manganese oxide spinel were investigated. The poor bonding between both lithium and manganese ions with tartaric acid was shown by the FT-IR analysis when lithium nitrate and/or manganese nitrate were used as sources. Li₂MnO₃ and Mn₂O₃ impurities formed in addition to lithium manganese oxides when nitrate salts were used as the sources. When acetate salts were used as sources for the lithium and manganese ions, single-phase LiMn₂O₄ was obtained. These results indicate that homogeneous bonding between acetate salt and tartaric acid was formed. The capacity of single-phase LiMn₂O₄ calcined at 500°C was 117 mAh/g which was much higher than those containing Mn₂O₃ and Li₂MnO₃ impurity compounds. Thus, sources of lithium and manganese ions play an important role in the synthesis and electrochemical behaviors of lithium manganese oxide spinel.     

The thermodynamic method for selecting oxygen carriers used for chemical looping air separation

Chemical looping air separation (CLAS) has been suggested as a new and energy saving method for producing oxygen from air. The selection of suitable oxygen carriers is the key issue for CLAS system. This paper shows a comprehensive thermodynamic method for selecting oxygen carriers used for CLAS through studying the properties of 34 different oxygen releasing reactions referring to 18 elements at different temperatures. The research mainly includes analysis of oxygen releasing capacity by calculating the Gibbs free energy change (ΔG) and the equilibrium partial pressure of oxygen of the reduction or oxidation reaction at different temperatures. Oxygen content and transport capacity were calculated. The spontaneous reaction temperatures for oxygen releasing reactions were presented to determine the operating temperatures. Also, the minimum demand of the steam for the reduction reaction was discussed. On the basis of the comprehensive thermodynamic study, the oxide systems of CrO₂/Cr₂O₃, PbO₂/Pb₃O₄, PbO₂/PbO, Pb₃O₄/PbO, MnO₂/Mn₂O₃, and Ag₂O/Ag have been found suitable for the CLAS process in low temperatures (500–800 K). The systems of PdO₂/PdO, PdO₂/Pd, PdO/Pd, MnO₂/MnO, and MnO₂/Mn₃O₄ were suitable for medium temperatures (800–1100 K) CLAS process. And Co₃O₄/CoO, CuO/Cu₂O, Mn₂O₃/Mn₃O₄, and OsO₂/Os systems only worked successfully in high temperatures (1100–1400 K). In addition, the CaO₂/CaO system was not suitable for CLAS because of the reaction with steam. The various binders such as SiO₂, TiO₂, Al₂O₃, Y₂O₃, ZrO₂, and YSZ which have been used for CLC could also be the supports for CLAS oxygen carriers.     

Solid-solid interactions in pure and Li2O-doped manganese and magnesium mixed oxides system

The solid-solid interactions between manganese and magnesium oxides in absence and in presence of small amounts of Li₂O have been investigated. The molar ratios between manganese and magnesium oxides in the form of Mn₂O₃ and MgO were varied between 0.05:1 to 0.5:1. The mixed solids were calcined in air at 400-1000°C. The techniques employed were DTA, XRD and H₂O₂ decomposition at 20-40°C.The results obtained revealed that solid-solid interactions took place between the reacting solids at 600-1000°C yielding magnesium manganates (Mg₂MnO₄, Mg₆MnO₈, MgMnO₄ besides unreacted portions of MgO, Mn₂O₃ and Mn₃O₄). Li₂O-doping (0.75-6 mol%) of the investigated system followed by calcination at 600 and 800°C decreased progressively the intensity of the diffraction lines of Mn₂O₃ (Bixbyite) with subsequent increase in the lattice parameter 'a' of MgO to an extent proportional to the amount of Li₂O added. This finding might suggest that the doping process enhanced the dissolution of Mn₂O₃ in MgO forming solid solution. This treatment led also to the formation of Li₂MnO₃. Furthermore, the doping with 3 and 6 mol% Li₂O conducted at 800°C resulted in the conversion of Mn₂O₃ into Mn₃O₄, a process that took place at 1000°C in absence of Li₂O. The produced Li₂MnO₃ phase remained stable by heating at up to 1000°C. Furthermore, Li₂O doping of the investigated system at 400-1000°C resulted in a progressive measurable increase in the particle size of MgO.The catalytic activity measurements showed that the increase in the molar ratio of Mn₂O₃ in the samples precalcined at 400-800°C was accompanied by a significant increase in the catalytic activity of the treated solids. The maximum increase in the catalytic activity expressed as reaction rate constant measured at 20°C (k _20°C) attained 3.14, 2.67 and 3.25-fold for the solids precalcined at 400, 600 and 800°C, respectively. Li₂O-doping of the samples having the formula 0.1 Mn₂O₃/MgO conducted at 400-600°C brought a progressive significant increase in its catalytic activity. The maximum increase in the value of k _20°C due to Li₂O attained 1.93 and 2.75-fold for the samples preheated at 400 and 600°C, respectively and opposite effect was found for the doped samples preheated at 800°C.This revised version was published online in November 2005 with corrections to the Cover Date.     

A fast route to obtain manganese spinel nanoparticles by reduction of K-birnessite

The K-birnessite (K_xMnO₂·yH₂O) reduction reaction has been tested in order to obtain manganese spinel nanoparticles. The addition of 0.25 weight percent of hydrazine hydrate, the reducing agent, during 24 hours is efficient to transform the birnessite powder in a hausmanite Mn₃O₄ powder. Well crystallised square shape nanoparticles are obtained. Different birnessite precursors have been tested and the reaction kinetics is strongly correlated to the crystallinity and granulometry of the precursor. The effects of aging time and hydrazine hydrate amount have been studied. Well crystallised Mn₃O₄ is obtained in one hour. The presence of feitknechtite (MnO(OH)) and amorphous nanorods has been detected as an intermediate phase during birnessite conversion into hausmanite. The conversion mechanism is discussed.