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.     

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.     

Studies on the formation of manganese molybdate

The reaction of MoO₃ with various oxides of manganese (MnO, Mn₂O₃, Mn₃O₄ and MnO₂) and with MnCO₃ has been studied in air and nitrogen atmospheres employing DTA, TG and X-ray diffraction methods, with a view to elucidating the conditions for the formation of MnMoO₄. Thermal decomposition of MnCO₃ has also been studied in air and nitrogen atmospheres to help understand the mechanism of the reaction between MnCO₃ and MoO₃. The studies reveal that, whereas MnO, Mn₂O₃ and MnO₂ react smoothly with MoO₃ to form MnMoO₄, Mn₃O₄ does not react with MoO₃ in the temperature range investigated (48O–6OO°C). An equimolar mixture of MnCO₃ and MoO₃ reacts in air to yield MnMoO₄, while only a mixture of Mn₃O₄ and MoO₃ remains as final product when the same reaction is carried out in nitrogen. Marker studies reveal that manganese ions are the main diffusing species in the reaction between MoO₃ and manganese oxides that result in MnMoO₄.     

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.     

Characterization and Activity of Iron-Manganese Catalysts for Carbon Monoxide Hydrogenation

Studies on the structural changes and catalytic behavior of iron-manganese catalysts for CO hydrogenation were conducted using Mossbauer spectroscopy, X-ray diffraction, temperature programmed reduction and kinetic measurements. It was observed that the reduction of the mixed oxide catalyst precursors proceeds via the formation of Fe_3-xMn_xO₄,Mn_3-xFe_xO₄ mixed spinel and Fe_1-zMn₂O mixed oxide to α-iron and MnO. After use for CO hydrogenation, catalysts are oxidized as well as carburized. The Mn_3-yFe_yO₄ mixed spinel and Fe_1-2Mn_zO mixed oxide are the most powerful phases for olefin production. The highest attainable 2–4 low carbon olefin selectivity is 41% with an 86% conversion level. Higher manganese content or lower reduction temperatures may change the carbide formed from χ-Fe₅C₂ to the more unstable ?′-Fe₂₂C. Carbide formation is greatly dependent on manganese content and activation procedure used.     

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.     

The structural characteristics of Zn-Mn ferrite synthesized by spray pyrolysis

The applicability of IR spectroscopy in studies of the structural characteristics of the ferrite spinel phase was shown for Zn_0.5Mn_0.5Fe₂O₄ samples prepared by the pyrolysis of aerosols of aqueous solutions of metal nitrates. The IR spectra of synthesized (ZnMn)Fe₂O₄ ferrites, Fe₂O₃, ZnO, MnO, and Mn₂O₃ pure oxides, and mixtures of these oxides in the region of characteristic M-O stretching vibration and M-O-H bending vibration frequencies were compared to determine the degree of concentration and structural uniformity of the ferrite spinel phase.     

Investigation of the defect manganese silicide MnnSi2n−m

Low-temperature magnetization studies upon melt-grown single crystals of the defect manganese silicide Mn_nSi_2n?m have shown this material to contain small quantities of plate-like MnSi precipitates. Metallographic and electron microprobe analyses have confirmed this result. The strongly magnetic MnSi precipitates dominate the diamagnetic Mn_nSi_2n?m matrix, and are responsible for the magnetic behavior reported in the literature. MnSi is metallic, and the plate-like metallic precipitates degrade the thermoelectric efficiency of the degenerate semiconductor Mn_nSi_2n?m.     

Submicronic particles of manganese carbonate prepared in reverse micelles: A new electrode material for lithium-ion batteries

A new preparation procedure based on the use of reverse micelles is used in the synthesis of manganese carbonate. A novel monodispersed form of MnCO₃ is obtained, in which particles with a regular shape and ca. 200 nm edges are observed by electron microscopy. The thermal decomposition at 400 °C of this solid under argon leads to the formation of MnO submicron particles. As-prepared MnCO₃ and the product of calcination, MnO, were tested in lithium cells. The electrochemical reaction with lithium of the new MnCO₃ material takes place by a different conversion reaction than the corresponding oxide. The low molecular-weight of MnCO₃ does not penalize the capacity while giving extra stability due to the formation of lithium carbonate as the main side product, which yields better capacity retention. In contrast to other anodes in recent commercial Li-ion product, the use of MnCO₃ submicron particles avoids the presence of the more toxic and expensive cobalt in the stoichiometry of the active electrode material.     

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.     

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.     

Activity—composition relations of solid solutions and stabilities of manganese and nickel titanates at 1250°C as derived from equilibria in the systems MnOCoOTiO2 and MnONiOTiO2

Equilibria between a metal phase (either cobalt or nickel), a gas phase of known oxygen pressures, and pairs of solid-solution phases in the systems MnOCoOTiO₂ and MnONiO₂ are used to calculate activity—composition relations in the solid-solution series Mn₂TiO₄Co₂TiO₄, MnTiO₃CoTiO₃, MnTi₂O₅CoTi₂O₅, Mn₂TiO₄Ni₂TiO₄ and MnTiO₃NiTiO₃, and free energies of formation of the manganese titanates Mn₂TiO₄, MnTiO₃ and MnTi₂O₅ and of nickel orthotitanate, Ni₂TiO₄.     

Electrochemical dissolution of Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> in acid solutions

The present study deals with the electrochemical reductive dissolution of Mn₃O₄, which was added to carbon-paste electroactive electrodes (CPEEs) in acid solutions. It was found that in the experimental conditions the thermodynamically stable form of manganese was . Kinetic features of the electrochemical reductive dissolution of Mn₃O₄, which was realized under potential cycling conditions (+1.0 V→−0.7 V→+1.0 V), were determined by the electrode polarization direction. It was shown that the cathodic reduction of Mn₃O₄ was accomplished in three stages. Manganese was dissolved in the supporting solution only at the third stage. The first two stages involved solid-phase reactions. The anodic cycling stage included an active dissolution of Mn₃O₄ and the lower manganese oxide (MnO) accumulated on the electrode surface during the cathodic reduction.     

Mössbauer studies on the thermolysis of manganese tris(malonato)ferrate(III)hexahydrate

Summary The thermal decomposition of manganese tris(malonato)ferrate(III) hexahydrate, Mn₃[Fe(CH₂C₂O₄)₃]₂ ^. 6H₂O has been investigated from ambient temperature to 600 °C in static air atmosphere using various physico-chemical techniques, i.e., simultaneous TG-DTG-DSC, XRD, M?ssbauer and IR spectroscopic techniques. Nano-particles of manganese ferrite, MnFe₂O₄, have been obtained as a result of solid-state reaction between a-Fe₂O₃ and MnO (intermediate species formed during thermolysis) at a temperature much lower than that for ceramic method. SEM analysis of final thermolysis product reveals the formation of monodisperse manganese ferrite nanoparticles with an average particle size of 35 nm. Magnetic studies show that these particles have a saturation magnetization of 1861G and Curie temperature of 300 °C. Lower magnitude of these parameters as compared to the bulk values is attributed to their smaller particle size.     

Chemical Approach to a New Crystal Structure: Phase Control of Manganese Oxide on a Carbon Sphere Template

The stabilization and growth of a non‐native structure, hexagonal wurtzite MnO (h‐MnO), is explored via kinetic control of manganese precursor on a carbon sphere template. MnO is most stable in the cubic rock‐salt structure (c‐MnO), and a number of studies have focused on the synthesis and properties of this rock‐salt phase. However, h‐MnO has not been fully characterized before our work. Prolonged heating at a relatively low temperature yields c‐MnO, whereas rapid heating of the reaction mixture at reflux produces h‐MnO in the presence of carbon spheres. The effect of benzyl amine concentration on the formation of two different oxidation states (c‐MnO and t‐Mn₃O₄) was examined as well. Moreover, the structural stability of the manganese oxides and phase transition of MnO in terms of the wurtzite to rock‐salt structural transformation have been investigated.     

Preparation of manganese oxide nanoparticles by thermal decomposition of nanostructured manganese carbonate

MnO and Mn₂O₃ nanoparticles were prepared in air and argon atmosphere by thermal decomposition of nanocrystalline manganese carbonate synthesized by reaction of manganese(II) nitrate with glycerol. Samples were characterized using transmission electron microscopy, simultaneous thermal analysis and X-ray diffraction analysis. Average sizes of prepared nanoparticles were calculated from XRD patterns using Scherrer equation. Also, the conditions for decomposition of manganese carbonate were optimized to obtain optimal nanoparticle sizes. Due to suitable sizes of prepared nanoparticles and the initial material, this method can be used in a wide range of industrial applications.     

Effect of manganese nanoparticles on the mechanical, thermal and fire properties of polypropylene multifilament yarn

Polypropylene filled with 10 wt% of inorganic nanoparticles has been prepared by melt blending. The fillers investigated were manganese oxides (MnO and Mn₂O₃) and manganese oxalate (MnC₂O₄). The morphology and thermal stability of these nanocomposites have been studied by transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). The experimental results reveal that the addition of 10 wt% manganese oxides improves the thermal stability in air of polypropylene by about 70-80 °C. In a second step, these nanocomposites have been processed by melt spinning in order to produce multifilament yarn. The mechanical properties of these filaments have then been characterized. It is shown that just the addition of Mn₂O₃ improves the mechanical properties of polypropylene filaments. The flammability of these nanocomposites used as knitted fabrics has finally been evaluated with a mass loss calorimeter at 35 kW/m². This kind of experiment has not revealed a real improvement of fire properties.     

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.     

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).