Average Oxidation State Of Manganese Research Articles

Decoration of the layered manganese oxide birnessite with Mn(II/III) gives a new water oxidation catalyst with fifty-fold turnover number enhancement.

The role of the manganese average oxidation state (AOS) in water oxidation catalysis by birnessite was investigated. Low AOS samples were most active, generating O2 immediately. Samples with a relatively high AOS showed an initial induction period and decreased turnover. Mn(ii- and iii)-enriched samples gave a 10-50 fold enhancement in turnover number.

QSAR analysis for nano-sized layered manganese–calcium oxide in water oxidation: An application of chemometric methods in artificial photosynthesis

Water oxidation is among the most important reactions in artificial photosynthesis, and nano-sized layered manganese–calcium oxides are efficient catalysts toward this reaction. Herein, a quantitative structure–activity relationship (QSAR) model was constructed to predict the catalytic activities of twenty manganese–calcium oxides toward water oxidation using multiple linear regression (MLR) and genetic algorithm (GA) for multivariate calibration and feature selection, respectively. Although there are eight controlled parameters during synthesizing of the desired catalysts including ripening time, temperature, manganese content, calcium content, potassium content, the ratio of calcium:manganese, the average manganese oxidation state and the surface of catalyst, by using GA only three of them (potassium content, the ratio of calcium:manganese and the average manganese oxidation state) were selected as the most effective parameters on catalytic activities of these compounds. The model’s accuracy criteria such as R2test and Q2test in order to predict catalytic rate for external test set experiments; were equal to 0.941 and 0.906, respectively. Therefore, model reveals acceptable capability to anticipate the catalytic activity.

Influence of vanadium doping on the supercapacitance performance of hexagonal birnessite

Nanostructured birnessite, promising candidate for supercapacitor electrode materials, shows greatly improved discharge capacity and cyclic stability with vanadium doping. The influence of vanadium doping on the physicochemical properties and supercapacitance performance of birnessite-type manganese oxides is investigated. Crystal structures, micromorphologies, bond lengths and chemical compositions of vanadium doped birnessites are characterized by XRD, SEM, XPS, TGA, and titration. The electrochemical properties are analyzed using galvanostatic charge–discharge test, cyclic voltammetry, and electrochemical impedance spectroscopy. The results exhibit that V/Mn molar ratio can reach 0.16:1 when Mn(IV) and K+ are partially substituted by V(V) in birnessite. The thickness of disk-shaped crystal, bond length of Mn–O1 and manganese average oxidation state decreases first and then increases with an increase in V/Mn molar ratio in synthesis system. Charge-transfer resistance decreases after doping vanadium, and increases with an increase in the content of vanadium in birnessite. Diffusion resistance increases first and then decreases due to the change of particle size and pore size distribution. The highest specific capacitance of 245 F g−1 is obtained with excellent cyclic stability for doped birnessite with V/Mn molar ratio of 0.14:1. Our study indicates that vanadium remarkably affects micromorphology, substructure, and electrochemical properties of manganese dioxides.

Study on the electrochemical behavior of nanostructure S-doping manganese oxide as a novel supercapacitor material.

A nanostructure S-doping manganese oxide (MnO(x)S(y)) amorphous powder was prepared by a homogeneous coprecipitation method for supercapacitor application. The structure and surface morphology of the as-prepared MnO(x)S(y) powder were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), elemental analysis and X-ray photoelectron spectroscopy (XPS). The as-prepared nanostructure S-doping manganese oxide powder was confirmed with a chemical composition of MnO1.26S0.23 by elemental analysis, in which the average oxidation state of manganese is about 3.1. The supercapacitive behavior of the as-prepared MnO1.26S0.23 was studied by means of cyclic voltammetry (CV) and constant current charge-discharge cycling (CD) experiments in 1 mol L(-1) Na2SO4 electrolyte. A maximum specific capacitance of 181 F g(-1) was obtained for MnO1.26S0.23. Furthermore, the specific capacitance remained 97.3% of the original value after 500 cycles. The superior capacitive behavior and better cycling stability indicate that the amorphous nanostructure MnO(x)S(y) maybe a promising electrode material for electrochemical capacitors.

Scalable synthesis and electrochemical investigations of fluorine-doped lithium manganese spinel oxide

A scalable synthesis route for the preparation of fluorine-doped lithium manganese spinel was developed. The effect of 2.5% fluorine doping on the properties of aluminum-doped lithium manganese spinel was investigated by examining the electrochemical performance and structural changes. The electrochemical study indicates that the initial discharge capacity increases by about 5% while other properties, like the capacity retention and rate performance, remain almost unchanged. The structural studies indicate that no impurity phase was formed, along with only little morphological changes and a slight increase in the lattice parameter. The Mn K-edge absorption spectra clearly show that the average manganese oxidation state decreases as a result of fluorine doping and Mn LII,III -edge spectra indicate that the electronic structure of the fluorine-doped spinel changes significantly, which may alleviate the intrinsic structural instability of this material.

Water oxidation by manganese oxides formed from tetranuclear precursor complexes: the influence of phosphate on structure and activity

Two types of manganese oxides have been prepared by hydrolysis of tetranuclear Mn(iii) complexes in the presence or absence of phosphate ions. The oxides have been characterized structurally using X-ray absorption spectroscopy and functionally by O2 evolution measurements. The structures of the oxides prepared in the absence of phosphate are dominated by di-μ-oxo bridged manganese ions that form layers with limited long-range order, consisting of edge-sharing MnO6 octahedra. The average manganese oxidation state is +3.5. The structure of these oxides is closely related to other manganese oxides reported as water oxidation catalysts. They show high oxygen evolution activity in a light-driven system containing [Ru(bpy)3](2+) and S2O8(2-) at pH 7. In contrast, the oxides formed by hydrolysis in the presence of phosphate ions contain almost no di-μ-oxo bridged manganese ions. Instead the phosphate groups are acting as bridges between the manganese ions. The average oxidation state of manganese ions is +3. This type of oxide has much lower water oxidation activity in the light-driven system. Correlations between different structural motifs and the function as a water oxidation catalyst are discussed and the lower activity in the phosphate containing oxide is linked to the absence of protonable di-μ-oxo bridges.

Preparation and capacitance behavior of manganese oxide hollow structures with different morphologies via template-engaged redox etching

A variety of amorphous manganese oxide electrode materials with uniform nonspherical hollow interiors are prepared via sacrificial template-engaged redox etching of the corresponding shape-controlled Cu2O nanocrystals in KMnO4 aqueous solution at room temperature. The obtained materials are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy and N2 adsorption–desorption. The amorphous manganese oxide hollow materials not only well inherit the size and shape of Cu2O nonspherical precursor, but also show a manganese average oxidation state of 3.7 and large specific surface area (about 198 m2 g−1). The electrochemical measurements show that the capacitance of the as-prepared amorphous manganese oxide electrode materials with uniform nonspherical hollow interiors is connects with their specific surface area. The amorphous manganese oxide star hollow architecture exhibits not only high specific capacitance of 366 F g−1 at a scan rate of 5 mV s−1, but also relatively good cycle stability (93.1% capacitance retention after 1000 cycles at a scan rate of 20 mV s−1), which make it have a potential application as a supercapacitor electrode material.

Catalytic oxidation of toluene by ozone over alumina supported manganese oxides: Effect of catalyst loading

This paper investigates effect of manganese loading on total oxidation of toluene by ozone using alumina-supported manganese oxide catalysts. Activities of four loadings of Mn (1, 5, 10 and 20%) were studied in the temperature range of 22–100°C. Catalysts were characterized by BET, X-ray diffraction (XRD), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). All catalysts became deactivated at room temperature and their activities were improved by increase of temperature. CHNS and Fourier transform infra red (FT-IR) spectroscopy were used to study catalyst deactivation at room temperature. Alcohols and carboxylic acids were identified as deposited species on the catalysts responsible for catalyst deactivation. A direct relationship between Mn loading and average oxidation state of manganese was found. Catalysts at lower loadings up to 10% were mostly composed of Mn2O3 while a mixture of MnO2 and Mn2O3 were present in catalysts with loadings higher than 10%. It was observed that lower Mn loadings have higher activity in oxidation of toluene. It was proposed that lower oxidation states of manganese are more favorable in decomposition of ozone resulting in higher rate of toluene oxidation.

Transition metal alloy-modulated lithium manganese oxide nanosystem for energy storage in lithium-ion battery cathodes

This paper explores the synergistic and catalytic properties of a newly developed lithium ion battery (LIB) composite cathode of LiMn2O4 modified with bimetallic (Au–Fe) nanoparticle. Spinel phase LiMn2O4 was doped with bimetallic nanoparticles, LiMxMn2−xO4 (M=FeAu), with concomitant oxidation of the Mn3+ ions (responsible for LIB capacity loss) to Mn4+. This nano-composite architecture accommodates the structural transformation that occurs during Li+ ion charge and discharge. Ultra-low scan rate (0.01mVs−1) cyclic voltammetry of the pure LiMn2O4 cathode material in 1M LiPF6/electrolyte solution, showed two sets of redox peaks with a third observed at lower potentials for LiMxMn2−xO4. The FeAu incorporation increased the reaction rate upon reduction of LiMn2O4 as indicated by the enhanced reduction peak seen by cyclic voltammetry. Nyquist plots of the electrochemical impedance spectroscopy (EIS) results showed LiFeAuxMn2−xO4 having increased conductivity with lower resistance of charge. X-ray diffraction studies showed the LiMxMn2−xO4 material retained well-developed octahedral structures bounded by (111) planes. The material crystallite size was ∼10nm with clear lattice fringes having a separation value of 0.48nm which concurrently improved the diffusion rate of Li+. Solid-state NMR results showed the progressive increase in average nominal manganese oxidation state from +3.5 to +4 resulted in an increase in the super-transferred hyperfine field at the 7Li nucleus of the FeAu doped cathode material. The LiFeAuxMn2−xO4 material also showed improved cycleability, especially at high C rate. This improvement was due to the enhanced physical stability of LiMn2O4 and its improved electrical conductivity ascribed to the incorporated FeAu nanoparticles.

Gold supported cryptomelane-type manganese dioxide OMS-2 nanomaterials deposited on AISI 304 stainless steels monoliths for CO oxidation

Gold supported on cryptomelane-type OMS-2 catalysts deposited on AISI 304 stainless steels monoliths have been prepared for the first time, characterised and tested in the CO oxidation reaction. An easy and non-conventional method of incorporation of gold to the cryptomelane solid is used. This method allows the preparation of the monolithic catalysts without altering the structural and textural characteristics of the parent OMS-2 material. Although these catalysts do not show an optimal performance for the oxidation of CO, the presence of small gold particles enhances the catalytic performances of the cryptomelane producing promissory CO oxidation catalysts. The non-conventional gold deposition favours a partial loss of K+ into the channels, resulting in an increment of the average oxidation state of manganese which favours the catalytic behaviour of these kinds of materials. This study can be taken as a starting point to obtain very active gold catalysts supported on OMS-2 materials through the optimisation of the gold–support interaction and the decrease in the gold particle size.

Co 2+-exchange mechanism of birnessite and its application for the removal of Pb 2+ and As(III)

Co-containing birnessites were obtained by ion exchange at different initial concentrations of Co 2+. Ion exchange of Co 2+ had little effect on birnessite crystal structure and micromorphology, but resulted in an increase in specific surface areas from 19.26 to 33.35 m 2 g −1, and a decrease in both crystallinity and manganese average oxidation state. It was due to that Mn(IV) in the layer structure was reduced to Mn(III) during the oxidation process of Co 2+ to Co(III). The hydroxyl groups on the surface of Co-containing birnessites gradually decreased with an increase of Co/Mn molar ratio owing to the occupance of Co(III) into vacancies and the location of large amounts of Co 2+/3+ and Mn 2+/3+ above/below the vacant sites. This greatly accounted for the monotonous reduction in Pb 2+ adsorption capacity, from 2538 mmol kg −1 for the unmodified birnessite to 1500 mmol kg −1 for the Co 2+ ion-exchanged birnessite with a Co/Mn molar ratio of 0.16. The amount of As(III) oxidized by birnessite was enhanced after ion exchange, but the apparent initial reaction rate was greatly decreased. The present work demonstrates that Co 2+ ion exchange has great influence on the adsorption and oxidation behavior of inorganic toxic metal ions by birnessite in water envrionments.

Laser microstructuring and annealing processes for lithium manganese oxide cathodes

It is expected that cathodes for lithium-ion batteries (LIB) composed out of nano-composite materials lead to an increase in power density of the LIB due to large electrochemically active surface areas but cathodes made of lithium manganese oxides (Li–Mn–O) suffer from structural instabilities due to their sensitivity to the average manganese oxidation state. Therefore, thin films in the Li–Mn–O system were synthesized by non-reactive radiofrequency magnetron sputtering of a spinel lithium manganese oxide target. For the enhancement of the power density and cycle stability, large area direct laser patterning using UV-laser radiation with a wavelength of 248nm was performed. Subsequent laser annealing processes were investigated in a second step in order to set up a spinel-like phase using 940nm laser radiation at a temperature of 680°C. The interaction processes between UV-laser radiation and the material was investigated using laser ablation inductively coupled plasma mass spectroscopy. The changes in phase, structure and grain shape of the thin films due to the annealing process were recorded using Raman spectroscopy, X-ray diffraction and scanning electron microscopy. The structured cathodes were cycled using standard electrolyte and a metallic lithium anode. Different surface structures were investigated and a significant increase in cycling stability was found. Surface chemistry of an as-deposited as well as an electrochemically cycled thin film was investigated via X-ray photoelectron spectroscopy.

Effects of excess manganese in lanthanum manganite perovskite on lowering oxidation light-off temperature for automotive exhaust gas pollutants

The effects of excess manganese on enhancement of oxidation activity of LaMn 1+ x O 3+ δ perovskite for CO and propane removal from a synthetic automotive exhaust gas are reported. Various LaMn 1+ x O 3+ δ ( x = 0.05–0.4) perovskite-type nanocatalysts were prepared by a microwave-assisted “gel combustion” method and calcined at 700 °C in air. Scanning electron microscopy (SEM) of the samples demonstrates highly porous and frothy particles. More than twice enhancement in the BET surface area of the catalyst samples is observed for the LaMn 1.2O 3+ δ catalyst, as compared to LaMnO 3. X-ray powder diffraction (XRD) analyses show only the perovskite structure for the catalysts. H 2-, CO-, and C 3H 8- temperature programmed reductions (TPR) indicate that the excess manganese oxides in LaMn 1+ x O 3+ δ significantly enhance the reducibility of the catalysts. The manganese oxides may be in the form of a separate phase of amorphous and/or small crystallites. Redox titration test results show that, as the amount of Mn in the structure increases, the average oxidation state of manganese reduces resulting in lowering cation vacancies. The light off temperature for CO and propane oxidation on LaMn 1.2O 3+ δ is decreased by 57 and 38 °C, respectively as compared to those of LaMnO 3 perovskite. LaMn 1.2O 3+ δ is stable under harsh conditions of the pollutants oxidation.

Characterization of Co-doped birnessites and application for removal of lead and arsenite

Nanostructured Co-doped birnessites were successfully synthesized, and their application for the removal of Pb 2+ and As(III) from aquatic systems was investigated. Powder X-ray diffraction, chemical analysis, nitrogen physical adsorption, field emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the crystal structure, chemical composition, micromorphologies and surface properties of the birnessites. Doping cobalt into the layer of birnessite had little effect on its crystal structure and micromorphology. Both chemical and XPS analyses showed that the manganese average oxidation state (Mn AOS) decreased after cobalt doping. The Co dopant existed mainly in the form of Co(III)OOH in the birnessite structure. Part of the doped Co 3+ substituted for Mn 4+, resulting in the gain of negative charge of the layer and an increase in the content of the hydroxyl group, which accounted for the improved Pb 2+ adsorption capacity. The maximum capacity of Pb 2+ adsorption on HB, CoB5, CoB10 and CoB20 was 2538 mmol kg −1, 2798 mmol kg −1, 2932 mmol kg −1 and 3146 mmol kg −1, respectively. The total As(III) removal from solution was 94.30% for CoB5 and 100% for both CoB10 and CoB20, compared to 92.03% for undoped HB, by oxidation, adsorption and fixation, simultaneously.

The role of lattice oxygen on the activity of manganese oxides towards the oxidation of volatile organic compounds

A series of manganese oxides differing in the structure, composition, average manganese oxidation state and specific surface area have been used in the total oxidation of volatile organic compounds (VOC). Ethanol, ethyl acetate and toluene were chosen as models of VOC.Among the manganese oxides tested, cryptomelane (KMn8O16) was found to be very active in the oxidation of VOC. The performance of cryptomelane was significantly affected by the presence of other phases, namely, Mn2O3 and Mn3O4. Temperature-programmed experiments combined with X-ray photoelectron spectroscopy (XPS) show that the mobility and reactivity of the oxygen species were significantly affected, explaining the catalytic performances of those samples. Mn3O4 improves the catalytic performance due to the increase of the reactivity and mobility of lattice oxygen, while Mn2O3 has the opposite effect. These results show that there is a correlation between the redox properties and the catalytic performance of the manganese oxides.Temperature-programmed surface reactions (TPSR) after adsorption of toluene or ethanol, in addition to reactions performed without oxygen in the feed, show that lattice oxygen is involved in the VOC oxidation mechanism. The conversion level was found to be influenced by the type of VOC, the reactivity into CO2 increasing in the following order: Toluene<Ethanol<Ethyl Acetate. The type of VOC is particularly important, as each VOC affects the reduction of the catalyst and, consequently, the incorporation rate of oxygen from the gas phase. Toluene decreases the oxygen mobility, so there is a slower incorporation rate of oxygen in the lattice, which explains the lower conversions observed.

Depth profile analysis of a cycled lithium ion manganese oxide battery electrode via the valence state of manganese, with soft X-ray emission spectroscopy

A 50-μm thick lithium manganese oxide (parent material LiMn 2O 4) battery electrode (positive electrode; cathode) was charged, slightly discharged and then sliced with a scotch tape test-type method. A selected number of slices was then subject to synchrotron soft X-ray emission spectroscopy near the Mn L α,β emission lines in order to determine changes in the oxidation state of the manganese as a function of sampling depth. The emission spectra showed a minute yet noticeable and systematic chemical shift of up to 0.25 eV between the layer near the current collector and the layer near the electrolyte separator. The average manganese oxidation state near the separator was smaller than the average oxidation state in the interior of the electrode, or near the current collector. Since the data provide an oxidation state depth profile of the cathode, a Li + depth profile can be inferred. This method provides information on the spatial chemical inhomogeneity of electrodes prior to and after electrochemical cycling, and thus can aid in degradation studies.

Significant enhancement of catalytic activities of manganese oxide octahedral molecular sieve by marginal amount of doping vanadium

Abstract Vanadium doped hollandite-type manganese oxide octahedral molecular sieve (OMS-2) was synthesized by a simple hydrothermal route. The results of catalytic tests showed that marginal amount of doping vanadium had greatly enhanced catalytic activities of OMS-2 not only in low-temperature complete oxidation of formaldehyde at 140 °C but also in high-temperature complete oxidation of methane at 450 °C. The results of XPS, HRTEM and H2-TPR measurements revealed that the enhancements of the catalytic activities primarily resulted from the increase of surface defect sites regardless of reaction temperatures and nature of reactants, rather than reducibility and average oxidation state of manganese.

Heavy-Fermion Behavior and Electrochemistry of Li1.27Mn1.73O4

The spinel of nominal composition Li4Mn5O12 has been prepared by the wet chemistry technique and characterized by structural analysis (X-ray diffractometry, Raman spectroscopy), magnetic properties including electron spin resonance (ESR), and thermal properties (specific heat). Both structural and magnetic analyses reveal that the physical and electrochemical properties are importantly influenced by the presence of Li2MnO3 impurity phase. The presence of this additional phase reduces the average oxidation state of manganese according to a disproportionation reaction so that the final composition of the spinel phase is Li3.8Mn5.2O12 (or Li1.27Mn1.73O4). Because of the important geometric frustration of the magnetic interactions and dilution of the antiferromagnetic interactions, no magnetic ordering is observed in the temperature range investigated. The anomalous magnetic properties, including the Dysonian profile of the ESR line, show that the material is metallic. The Sommerfeld constant is 308 mJ/(K2) p...

Tunnel structure effect of manganese oxides in complete oxidation of formaldehyde

Three manganese oxides with different square tunnel sizes, pyrolusite, cryptomelane and todorokite, were prepared, and characterized by X-ray powder diffraction, specific surface area measurement, temperature programmed reduction by H 2, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The results of catalytic tests in complete oxidation of formaldehyde revealed that cryptomelane had much higher activity than pyrolusite or todorokite. The characterization results suggested that the catalytic activity was primarily predominated by the tunnel structures of manganese oxides rather than specific surface area, degree of crystallinity, reducibility and average oxidation state of manganese. The effective tunnel diameter of cryptomelane similar to the dynamic diameter of formaldehyde molecule was proposed to be a major factor to give rise to the high catalytic activity.

Effects of thermal treatment on physico-chemical and catalytic properties of lanthanum manganite LaMnO3+y

Abstract Lanthanum manganite oxides were prepared by citric acid gel process. The precursor gel was thermally decomposed in four steps allowing high composition homogeneity and textural stabilisation. As the calcination temperature increased, the lattice constants increased whereas the average oxidation state of manganese decreased resulting in lower cation vacancies (lower y values in LaMnO 3+ y ). The specific surface area (SSA) was found to decrease from 13 to 1.5 m 2 g −1 when the sintering temperature was raised from 600 to 1000 °C. Well characterized (XRD, ATD-ATG, chemical analysis, surface area, SEM, XPS) perovskite phase LaMnO 3+ y synthetized after calcination at 700–1000 °C was tested for total methane oxidation in a fixed bed reactor with feed containing 1% methane, 4% oxygen and the balance helium. The catalytic activity was found to decrease with increasing calcination temperature and correlates not only with the bulk structure of the perovskite phase and consequently oxygen mobility but also with the content of manganese and its surface concentration. At low temperature ( 600 °C), the redox system is assumed to occur with the oxygen ions being transported through the lattice to the reduction site for reaction. This mechanism seems to be favoured by the increase of bulk cation vacancy amount.