Average Oxidation State Of Manganese Research Articles

Catalytic combustion of methane over LaMnO 3 perovskite supported on La 2O 3 stabilized alumina. A comparative study with Mn 3O 4, Mn 3O 4-Al 2O 3 spinel oxides

Ten and 20 wt % LaMnO3 perovskites supported on La2O3-stabilized γ-Al2O3 were studied for catalytic combustion of methane. A comparison with Mn3O4 and Mn3O4-Al2O3 spinel oxides was also drawn. The catalysts were characterized by microanalysis, X-ray diffraction (XRD), temperature-programmed reduction (TPR), and O2 temperature-programmed desorption (TPD) techniques. Catalytic activity tests were carried out in a fixed-bed reactor at T=300–800°C, space velocity = 40000 h−1, CH4 concentration = 0.4% v/v, O2 concentration = 10% v/v. Both XRD and microanalysis indicated a uniform dispersion of the perovskite phase. The structure of γ-alumina was retained after the treatment at 800°C, the treatment at 1100°C led to the transition to the θ and β phases. TPR measurements suggested the presence of a fraction of Mn4+ in supported perovskites. The possible interaction of manganese with alumina, which stabilizes Mn2+, led to the reduction of the initial average oxidation state of manganese with the perovskite content and the temperature of treatment. O2 desorption in TPD measurements was significant from spinel oxides, whereas negligible from supported perovskites. Supported perovskites gave complete CH4 conversion within 650°C with 100% selectivity to CO2. The activation energy value, evaluated from a methane first-order rate equation, suggested the occurrence of the same reaction mechanism of unsupported LaMnO3. The preexponential factors of the catalysts treated at 800 °C were proportional to the perovskite content, in agreement with a monolayer model. Samples treated at 1100°C showed the same activity not depending on the perovskite content, suggesting that only a fraction of manganese in the 20 wt % LaMnO3 is available for the reaction. This was related to the stabilization of a fraction of Mn2+, probably not involved in the reaction. Spinel oxides catalyze the reaction at lower temperature, giving complete conversion within 600°C with 100% selectivity to CO2. The activation energy was lower than that of supported perovskites. A correlation with the ability to desorb O2 was hypothesized.

Manganese Oxide Mesoporous Structures: Mixed-Valent Semiconducting Catalysts

Hexagonal and cubic phases of manganese oxide mesoporous structures (MOMS) have been prepared by means of the oxidation of Mn(OH) 2 . The hexagonal MOMS materials form a hexagonal array of pores with an open porous structure, thick walls (1.7 nanometers), and exceptional thermal stability (1000°C). The walls of the mesopores are composed of microcrystallites of dense phases of Mn 2 O 3 and Mn 3 O 4 , with MnO 6 octahedra as the primary building blocks. The calcined hexagonal MOMS have an electrical conductivity of 8.13 × 10 −6 per ohm·centimeter, an average manganese oxidation state of 3.55, and a band gap of 2.46 electron volts. Catalytic oxidations of cyclohexane and n -hexane in aqueous solutions in a batch reactor show conversions of ∼10 and ∼8 percent, respectively. Characterization and catalytic data suggest that MOMS systems show significant enhancement in thermal stability with respect to octahedral molecular sieve materials.

Cu Containing Octahedral Molecular Sieves and Octahedral Layered Materials

Manganese oxide octahedral molecular sieves (OMS) with Cu2+in the tunnel (Cu-OMS-1) and octahedral layer (OL) manganese oxides with Cu2+in the interlayer of buserite-like materials (Cu-OL-1) were prepared and characterized by X-ray diffraction, scanning electron microscopy/energy dispersive X-ray analysis, temperature programmed reduction, thiosulfate titrations, and volumetric adsorption of carbon monoxide (CO). The materials were tested as CO oxidation catalysts. Results show that autoclave treatment of Cu-OL-1 suspensions induces complete removal of chlorine as well as a change of the Cu/Mn ratio and average manganese oxidation state. Such changes depend on the initial MnO−4/Mn2+ratio. At a ratio of 0.34–0.40, autoclave treatment increases the manganese oxidation state and the Cu/Mn ratio. At a ratio of 0.44, however, treatment decreases manganese oxidation state and the Cu/Mn ratio. A possible mechanism is proposed for the hydrothermal transformation of Cu-OL-1 to Cu-OMS-1. CO oxidation data show that Cu-OMS-1 has a CO conversion of 89–99% at 60–100°C, while Cu-OL-1 has almost no activity in the same temperature region. This dramatic difference is due to the fact that Cu-OMS-1 has more surface oxygen and a larger CO adsorption capacity, as compared to Cu-OL-1. Different structural properties may also contribute to the difference in catalytic activity, since the todorokite tunnel structure of OMS-1 is correlated with CO conversion. Blockage of the tunnels by water and tunnel collapse cause a significant decrease in CO oxidation. Kinetic studies of CO oxidation show that water vapor in the feed stream induces a small and reversible inhibition and that a positive reaction order is found with respect to CO partial pressure under these experimental conditions.

Chemical and Magnetic Characterization of Spinel Materials in the LiMn2O4–Li2Mn4O9–Li4Mn5O12System

Lithium manganospinels belonging to the LiMn2O4–Li2Mn4O9–Li4Mn5O12system were prepared by solid-state reaction of various lithium salts and manganese sources at temperatures ranging from 400 to 900°C. Their exact stoichiometry, which is highly dependent on the preparation temperature, was determined by X-ray diffraction, atomic absorption spectroscopy, and redox titration. Nearly stoichiometric LiMn2O4shows a reversible phase transition at 30°C upon heating and at 10°C upon cooling, which is associated with a tetragonal-cubic transformation. An increase in the average oxidation state of manganese from 3.5+ to 4+ in the two systems Li1−δMn2−2δ(0 ≤ δ≤ 0.11) and Li1+δMn2−δO4(0 ≤ δ ≤ 0.33) results in (i) the suppression of the Jahn–Teller distortion observed around room temperature for stoichiometric LiMn2O4and (ii) a progressive passage from antiferromagnetic (LiMn2O4, θ = − 266 K,nMn+ = 3.5) to ferromagnetic behavior (Li4Mn5O12, θ = + 40 K,nMn+ = 4.0), where the lattice parameter is not too small.

The 2D Rancieite-type manganic acid and its alkali-exchanged derivatives: Part I — Chemical characterization and thermal behavior

The 2D Rancieite type manganic acid was prepared by reduction of KMnO 4 in acidic medium. Its ion exchange behavior allows to prepare alkali derivatives. All compounds were characterized with use of a combination of X-ray diffraction, chemical analyses, TGA, magnetic measurements and spectroscopic techniques. The evolution of their chemical composition versus temperature was studied between 180 and 400 °C. It shows that the dehydration process is partly reversible in these compounds whereas the weak reduction is irreversible. The 2D Rancieite-type manganic acid is readily different from a Birnessite-type phyllomanganate, as shown by several features: the interlayer distance, the ion exchange capacity, the thermal behavior, the interlayer cation content, the manganese average oxidation state, the magnetic behavior and the IR spectrum.

Variation of (Mn-O) covalency in ALaMgMnO6-x(A = Ca, Sr, Ba) prepared under high pressure of 60kbar

Abstract The ALaMgMnO6-x (A = Ca, Sr, Ba) oxides with cubic perovskite structure have been prepared under high oxygen pressure. From the chemical and thermogravimetric analyses, the extent of oxygen deficiency and the average oxidation state of manganese were estimated. The content of Mn(V) increases with a cation going from Ca to Sr, and to Ba. The magnetic measurements indicated that CaLaMgMnO6-x follows the curie-Weiss law with μeff = 3.80 B.M., whereas SrLaMgMnO6-x and BaLaMgMnO6-x are ferromagnetic. Through EPR study, Mn(VI) in a tetrahedral site and Mn(IV) in an octahedral site have been detected in CaLaMgMnO6-x while weak ferromagnetic couplings were observed in SrLaMgMnO6-x and BaLaMgMnO6-x due to the superexchange interactions between the t 2g electrons of Mn(IV) and Mn(V) ions. The evolution of all the physical properties could be explained by the variation of covalent bonding character between Mn—O bond in three compounds; the π(Mn—O) bond competing with σ[(A, La)—O] bond becomes stronger with ...

Synthesis and Lithium Intercalation Properties of a Layered Lithiated Manganese Oxide with Rancieite Type

AbstractThe 2D lithiated manganese oxide with Rancieite-type structure (lithium phyllomanganate), synthesized via a soft chemistry route (T≤60ºC) is hydrated at room temperature and can be dehydrated progressively by a thermal treatment. This compound and a series of samples obtained after annealing at different temperatures were characterized and studied for their electrochemical lithium intercalation properties. They are poorly crystallized, with however the advantage of a manganese oxidation state very close to 4, a mustto get high specific capacity. Their chemical characterization was achieved after gathering results obtained from complementary techniques such as AAS, redox titration, TGA and XPS measurements. Special care was taken for the chemical characterization of electrode compositions effectively used in the electrochemical cells.Electrochemical lithium intercalation was systematically studied for the series of samples, when starting in charge or in discharge after assembly of the Li battery. The electrochemical behavior is discussed in relation with the manganese average oxidation state and the interlayer water content. Materials of the series showing the larger specific capacities were further examined on the application point of view, for their reversibility, cyclability and high rate capability upon Li intercalation.This work shows that the anhydrous material which is obtained at 300ºC, with the composition Li0.42MnIII0.20MnIV0.8002.11, is a promising rechargeable layered manganese dioxide material.

ChemInform Abstract: A Novel Tetranuclear Manganese Complex That Displays Multiple High‐Potential Redox Processes. Synthesis, Structure, and Properties of ((Mn2(TPHPN)(O2CCH3)(H2O))2O)(ClO4)4·2 CH3OH.

AbstractThe novel tetranuclear complex (III) with a septadentate ligand and with an average manganese oxidation state of +2.5 is prepared and crystallographically characterized (space group P21/n, Z=2).

Determination of manganese oxidation states in solids by electron energy-loss spectroscopy

Electron energy-loss spectra of manganese-bearing phases vary systematically with changes in the average oxidation state of manganese. The energy region recorded, 0 to 1000 eV, includes electronic transitions from manganese and oxygen core orbitals, as well as from states that have contributions from both elements. We used a spectrometer attached to a transmission electron microscope (TEM) to obtain reference spectra from manganese metal and from manganese oxides that contain Mn 2+, Mn 3+, and Mn 4+. Such reference spectra can be used to estimate the manganese oxidation state for uncharacterized manganese-bearing solids. Areas with diameters as small as 0.1 μm can be analyzed, thus avoiding the uncertainties associated with bulk analyses of fine-grained, heterogeneous mixtures. Similar determinations of reference energy-loss spectra for other polyvalent elements will broaden the applicability of this technique for the study of small volumes of solids.