Valence State Of Mn Research Articles

Three-Dimensional Nanostructures Formed from Morphology Controlled Synthesis of Pt Particles Based on Gas–Liquid Reaction for Electrocatalytic Application

In this paper, platinum (Pt) nanomaterials with controlled morphologies are grown on the surface of flowerlike manganese dioxide (MnO2) respectively based on gas–liquid reaction. Then flowerlike three-dimensional (3D) nanostructures are formed, with successful synthesis of corresponding Pt/MnO2 nanocomposites. The obtained nanocomposites are characterized by scanning electron microscopy, energy-dispersive X-ray spectrum, transmission electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. In addition, an interesting color-change phenomenon appeared with the Pt nucleation and growth progress which may be due to variation of the Mn valence state triggered by the reduction of Pt. This phenomenon can be used for naked-eye observation of materials’ growth states which is beneficial for investigation of synthetic mechanisms. At last, the Pt/MnO2 3D nanostructure exhibits perfect electrocatalytic properties toward oxidation of methanol. The four kinds of Pt/MnO2 composites are all us...

Tuning the Mn and Fe Valence States into New Ca0.7Mn2–xFexO4 (0 < x ≤ 0.60) Solid Solution during Reversible Redox Processes

New Mn4+-rich Ca0.7Mn2–xFexO4 complex oxides (0 < x ≤ 0.6) with Mn4+ amount varying from 33% to 43% have been prepared by aqueous self-combustion routes and characterized by powder X-ray Diffraction (XRD) and Mössbauer spectroscopy. These oxides crystallize with a monoclinic symmetry (CaMn3O6-type, space group P21/a) for low Fe content and with an orthorhombic symmetry (CaFe2O4-type, space group Pnma) for x > 0.14, thus stabilizing the highest Ca defects content (30%) for this series. A regular decrease of the Jahn–Teller Mn3+ site distortion is observed versus Fe content. For low iron content (x < 0.33), Fe3+ ions are preferentially but not exclusively located at the Mn4+-rich site which exhibits lower octahedral distortion, whereas for higher iron doping rates (x ≥ 0.33), Fe3+ ions are almost equally distributed in the two more or less distorted transition metal sites. The same distribution of Fe3+ in both of these atomic positions contributes to the stabilization within these phases of a high content of Mn4+ even at higher temperatures (T > 1000 °C), whereas the unsubstituted Ca0.66Mn2O4 phase is only stable up to 850 °C. Furthermore, these phases can be reduced at T = 550 °C under Ar/5% H2 into a Mn2+ and Fe2+/Fe3+ phase based on the rock salt structure and then reoxidized at T = 700 °C under air to get pure phases with the same orthorhombic unit cell and a Fe3+ distribution in the two atomic positions similar to the initial phase. Despite the additional stabilization of metal iron Fe0 during the reduction at T > 550 °C, a pure Mn4+/Mn3+/Fe3+-based oxide with CaFe2O4-type structure has been recovered after oxidation at T > 700 °C under air. Mössbauer spectroscopy of the reduced phase has revealed the presence of Fe2+ ions at two distinct octahedral sites corresponding to Fe3+ within the oxidized phase. The reversibility of this redox system could be explained by the occurrence of two rutile double chains in the CaFe2O4-type network, which is closely related to the rock salt structure with Ca2+ ions in the vicinity of Mn and Fe atoms. During the steps of oxygen release and storage, this network, derived from the rock salt framework, seems to preserve the Fe and Mn organization at the local scale.

Effect of Sintering Temperature on Interfacial Structure and Interfacial Resistance for All-Solid-State Batteries

Introduction Bulk-type all-solid-state rechargeable lithium batteries using oxide solid electrolytes (Ox-SSBs) require low-resistive electrode-solid electrolyte composite. Both electrode and solid electrolyte are fragile and hard ceramics and then they need a densification process to enlarge charge transfer reaction sites at the interface. Those composites have been developed by heating processes such as co-sintering [1], spark plasma sintering [2, 3], fusion bonding using low-melting solid electrolyte [4], etc. However, one of the crucial problems to apply such heating process is a formation of mutual diffusion layer at the interface. Formation of mutual diffusion layer can work as a driving force of solid-solid adhesion, but at the same time, that can retard charge transfer reaction at the interface, resulting in the reducing of the battery power density. Therefore, it is important to examine interfacial structure and interfacial resistance by a formation of mutual diffusion layer in detail. This work focuses on LiNi1/3Co1/3Mn1/3O2 (NMC: cathode) / LATP (solid electrolyte) interface. The NMC shows smaller volume change (less than 1%) during charge-discharge reactions among the various positive electrode materials [5]. Also, LATP is one of the highest Li+ conductive solid electrolytes (~10-4 S cm-1) [6]. Here, the NMC films were deposited by pulsed laser deposition (PLD) on mirror-polished LATP sheets (150 μm in thickness, Ohara Inc.) at different temperature as a model system of sintered LATP/NMC interface. Experimental NMC films were deposited on one side of LATP sheets by PLD at 700 °C or 900 °C, which will be denoted as NMC-700 and NMC-900, respectively. Bare side of the LATP sheet was immersed in a liquid electrolyte, where both reference and counter electrodes of lithium were immersed. Electrochemical properties of those NMC/LATP half-cells were measured by cyclic voltammetry (CV) and AC impedance spectroscopy. Structure analyses of NMC/LATP interface were carried out by XRD, TEM, EDX, and EELS. Results and discussion Thin films of NMC prepared at 700 °C or 900 °C on Pt/Ti-coated SiO2 substrates showed almost same redox reactions at around 3.6-3.8 V in liquid electrolyte, and the charge transfer resistance (R ct) at 4.0 V was 200-350 Ω cm2 in both cases. However, they showed quite different reactivity on the LATP sheets. Fig. 1(a) shows CVs of the NMC-700 and NMC-900 measured at 1 mV s-1. The NMC-700 appeared redox peaks at around 3.6-3.8 V as with the film in liquid electrolyte, while the NMC-900 did not show any redox peaks. The R ct of NMC-700 at 4.0 V was 315 Ω cm2, while that of NMC-900 was 114000 Ω cm2. These results suggest that the R ct of NMC/LATP strongly depend on the sintering temperature. Fig. 1(b) shows the EDX line profile of O, Ti, P, Ni, Co, and Mn around the interface of NMC-900. Also, integrated intensity ratio of L edge peaks, L3-edge/L2-edge, in Co (L3/L2-Co) and Mn (L3/L2-Mn) are shown below, which corresponds to the valence state of Co and Mn [7]. A mutual diffusion layer was observed around the interface, but its thickness of 30 nm was same with the case of NMC-700. XRD analysis detected impurity peaks in case of the NMC-900 while not NMC-700/LATP, indicating that mutual diffusion layer is crystallized. Also, both EDX and EELS analyses suggested that Co in the NMC films looks to diffuse toward the LATP as a reduced valence at higher sintering temperature. Based on these results, we will conclude that both structural disorder in NMC around the interface and crystallization of mutual diffusion layer will be key issues to increase the R ct of NMC/LATP interface at higher sintering temperature. Acknowledgement This work was supported by JST-ALCA.

An effective Mn-Co mixed oxide catalyst for the solvent-free selective oxidation of cyclohexane with molecular oxygen

Mn-Co mixed oxide (MnaCobOx) catalysts were prepared by the precipitation method, and the effect of Mn/Co (atom) on their structural and catalytic performance for the solvent-free selective oxidation of cyclohexane with oxygen was investigated. The results showed that the atomic ratio of Mn/Co would affect the phase constitution, surface area, surface Mn valence states and the reducibility of the MnaCobOx catalysts. When the atomic ratio of Mn/Co in the catalyst is 0.6, the Mn2Co3Ox catalyst behaved with excellent catalytic activity and stability, which should be attributed to its high specific surface area and the high atomic ratios of (Mn4++Mn3+)/Mn and Co3+/Co on its surface. The effects of the preparation conditions for the Mn2Co3Ox catalyst and the reaction conditions on the selective oxidation of cyclohexane over the Mn2Co3Ox catalyst was tested. Under the reaction conditions of 0.5MPa O2 and 140°C for 4h, 10.4% conversion of cyclohexane and 62.0% selectivity to a mixture of cyclohexanone and cyclohexanol (KA oil) were achieved. After this Mn2Co3Ox catalyst was repeatedly used 10 times, the cyclohexane conversion and the selectivity to KA oil had hardly changed.

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, Mn2 O3 , 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 eg states and highly covalent Mn-N bonding thus enhance the ORR activity of nitrogen-rich manganese oxynitrides.

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

AbstractThe 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 eg states and highly covalent Mn−N bonding thus enhance the ORR activity of nitrogen‐rich manganese oxynitrides.

Theoretical and experimental study of octahedral tilting of Ca1−xGdxMnO3 (x = 0.05, 0.1, 0.15, 0.2) nanometric powders

In order to estimate theoretical stability of the perovskite structure for synthesized Ca1−xGdxMnO3 (x = 0.05, 0.1, 0.15, 0.2) nanopowders, the Goldschmidt tolerance factor Gt and global instability index GII were calculated. Furthermore, we have performed structure prediction of Ca1−xGdxMnO3 perovskites and found several possible perovskite-related phases. The influence of gadolinium amount on Mn─O bond angles and distances, tilting of MnO6 octahedra around all three axes and deformation due to the presence of the Jahn–Teller distortion around Mn3+ cation, as well as the influence of the amount of Mn3+ cation on Ca1−xGdxMnO3 compound, was examined. Ion Mn valence states were determined by bond valence calculations (BVC). Infrared active phonon modes in Ca1−xGdxMnO3 were studied by infrared reflection spectroscopy and magnetic properties were studied by using EPR (electron paramagnetic resonance) measurements.

Overpotential-Induced Introduction of Oxygen Vacancy in La0.67Sr0.33MnO3 Surface and Its Impact on Oxygen Reduction Reaction Catalytic Activity in Alkaline Solution

Oxygen reduction reaction (ORR) catalytic activity of La0.67Sr0.33MnO3 epitaxial thin films was investigated in a KOH solution by using a rotating-disk electrode. We found that while the films exhibit ORR current, the current is not limited by oxygen transport resulting from the film electrode rotation and shows the large hysteresis against the potential sweep direction. This behavior is in stark contrast to the oxygen reduction reaction activity of an electrode ink made from LSMO bulk powder, whose ORR current is oxygen-transport limited. In situ synchrotron X-ray absorption spectroscopy also reveals that the valence state of Mn in the LSMO film surface is lowered under the reducing atmosphere caused by the overpotential. This indicates the overpotential-induced introduction of oxygen vacancies in the film surface. We also show that the ORR current of the LSMO films exposed to the reducing atmosphere is lowered than that of the original surface. These results indicate that the ORR catalytic activity of L...

Interface charge transfer process in ZnO:Mn/ZnS nanocomposites

ZnO:Mn/ZnS nanocomposites were prepared by seed-mediated growth of ZnS QDs onto the preformed ZnO:Mn nanoparticles. The formation of the nanocomposite structure has been evidenced by XRD, HRTEM, and XPS. The architecture of the nanocomposite with outer ZnS QDs around ZnO:Mn cores is sustained by the sulfur and oxygen depth profiles resulted from XPS. When the two components are brought together, the band gap of ZnS component decreases while that of ZnO:Mn increases. It is the result of interface charge transfer from ZnO:Mn to ZnS QDs. Here ZnO:Mn valence states are extended through the interface into unoccupied gap states of ZnS. The energy band setup is modified from a type II into a type I band alignment. The process is accompanied by enhancement of composite UV emission of PL spectra as compared to its counterparts. The charge transfer from valence band also determines the increase of the core-polarization effect of s shell electrons at Mn2+ nucleus, thus determining the increase of the hyperfine field through the reduction of the covalency degree of Zn(Mn)–O bonds. The quantum confinement in ZnS QDs promotes the ferromagnetic coupling of singly occupied states due to Zn vacancies determining a superparamagnetic behavior of the ensemble. When the nanocomposites are formed, due to interface charge transfer effects, an increased number of filled cation vacancies in ZnS QDs develop, thus disrupting the pre-existing ferromagnetic coupling between spins resulting in a significant reduction of the overall saturation magnetization. The possibility to modulate nanocomposite properties by controlling the interface interactions may be foreseen in these types of materials.

Tuning the Mn valence state in new Ca(0.66)Mn(2-x)Al(x)O4 (x≤ 0.4) oxides: impact on magnetic and redox properties.

New Ca(0.66)Mn(2-x)Al(x)O4 (x≤ 0.4) solid solutions crystallizing with the CaFe2O4-type structure (SG: Pnma) were synthesized for the first time by the glycine-nitrate process. The structures were determined on the basis of XRD Rietveld analysis and electron diffraction investigation. While the CaMn3O6 ('Ca(0.66)Mn2O4') oxide adopts a monoclinic unit cell, the Al substitution for Mn (x = 0.2, 0.4) leads to an orthorhombic cell with only two Mn atomic positions, with different valence states, and 33% of Ca sites empty. The Ca molar concentration decreases down to 0.6 in order to increase the Mn valence leading to a phase mixture, whereas a slight Ca content increase up to 0.7 leads to a decrease of Mn valence in the pure phase. The Al(3+) ions are located at a specific Mn site because their ionic radii are close to that of Mn(4+) and a more isotropic environment. The unit cell parameters and volume strongly decrease for a low Al content and tend to an asymptotic value of x = 0.33-0.4, around the limit of solubility. As the Al content increases, the Mn valence state in the same slightly distorted octahedral site increases up to 4+ whereas the other octahedral site is highly elongated and corresponds mainly to Jahn-Teller Mn(3+). At x = 0.33, these two Mn sites correspond to Mn(4+) and Mn(3+) respectively. Moreover, the aluminium content increase induces a weakening of the global antiferromagnetic long range interactions between the ferromagnetic chains. The Al substitution leads to the change of the Mn valence distribution as well as the unit cell symmetry of the CaMn3O6 phase. These 1D tunnel networks stabilizing the Mn(3+)/Mn(4+) valence states can be reduced under Ar/5%H2 between T = 300 °C and T = 600 °C (heating rate = 2 °C min(-1)) into pure Mn(2+) rocksalt solid solution despite the large difference in ionic radii. The re-oxidation leads to the same CaFe2O4-type structure and several redox cycles can be operated. The relationship between the two double chains of the edge-sharing octahedral sites and the rocksalt-type framework is clear and should appear as the driving force for the structural transformation during the reduction/oxidation processes. Finally, Al substitution allows an increasing of the Mn-O bond covalence and consequently the reduction in temperature.

Valence state of Mn in La&lt;sub&gt;1−&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Sr&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;MnO&lt;sub&gt;3&lt;/sub&gt; probed by resonant Mn 3&lt;i&gt;s&lt;/i&gt;→2&lt;i&gt;p&lt;/i&gt; x-ray emission spectroscopy

Valence state of Mn in La&lt;sub&gt;1−&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Sr&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;MnO&lt;sub&gt;3&lt;/sub&gt; probed by resonant Mn 3&lt;i&gt;s&lt;/i&gt;→2&lt;i&gt;p&lt;/i&gt; x-ray emission spectroscopy

Surface Valence States of Mn Ions and Magnetic Properties of La&amp;lt;sub&amp;gt;0.67&amp;lt;/sub&amp;gt;Sr&amp;lt;sub&amp;gt;0.33&amp;lt;/sub&amp;gt;MnO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; Films

La_0.67Sr_0.33MnO₃ (LSMO) films were prepared on SrTiO₃ single-crystal substrates by the pulsed laser deposition method. X-ray photoelectron spectra (XPS) were measured for the LSMO films as-prepared, annealed in oxygen and in vacuum, respectively. Multiple peaks fitting for Mn 2p_3/2 XPS spectra shows that Mn²⁺ appears and the ratio of Mn³⁺/Mn⁴⁺ increases on the surface of LSMO film annealed in vacuum comparing with the as-prepared film, though the saturation magnetization (<i>M_s</i>) changes little. Mn⁶⁺ and Mn⁷⁺ can be detected while Mn⁴⁺ decreases much on the surface of LSMO film annealed in oxygen. The change of Mn³⁺/Mn⁴⁺ results in the obviously reduce of <i>M_s</i>. O1s spectra show the gradual changes of Mn-O, Sr-O and La-O. Analysis indicates that Mn²⁺ forms at the surface defects of LSMO films. Annealing in vacuum hardly affects the magnetic property of LSMO films. But annealing in oxygen greatly changes the valence states of Mn, which weakens the magnetic property of LSMO films.

Structural Changes in a Li-Rich 0.5Li2MnO3*0.5LiMn0.4Ni0.4Co0.2O2 Cathode Material for Li-Ion Batteries: A Local Perspective

Local structural changes in a Li-rich 0.5Li2MnO3*0.5LiMn0.4Ni0.4Co0.2O2 cathode material are investigated using X-ray absorption spectroscopy (XAS). The element-selective nature of XAS revealed the composite structure of the material, where both Li2MnO3 and LiMn0.4Ni0.4Co0.2O2 components exist as separate domains and also exhibit a distinct electrochemical response. An irreversible oxygen release from Li2MnO3 domains contributes to a large irreversible capacity delivered by the material during activation and gives rise to the formation of a layered MnO2-type structure. Lithium reinsertion into this layered MnO2-type structure during discharge reforms the original Li2MnO3-type structure, which is lithium and oxygen deficient. The average valence state of Mn in Li2MnO3 domains remains unchanged at 4+ during charge and discharge, suggesting an unusual participation of oxygen anions of Li2MnO3 domains in redox processes. On the contrary, electrochemical processes in LiMn0.4Ni0.4Co0.2O2 domains involve conventional redox processes of transition-metal (TM) ions. In addition to Ni2+/Ni4+ and Co3+/Co4+ redox reactions, a small amount of Mn3+ detected in LiMn0.4Ni0.4Co0.2O2 domains also participates in electrochemical processes via a Mn3+/Mn4+ redox reaction. All structural modifications introduced into the material during activation are recovered upon discharge to 2.5 V, except those caused by the permanent removal of oxygen from Li2MnO3 domains.

Activated carbon doped with biogenic manganese oxides for the removal of indigo carmine

Indigo carmine (IC) is one of the oldest, most important, and highly toxic dyes which is released from the effluents of many industries and results in serious pollution in water. In this study, the biogenic Mn oxides were activated by NaOH and then heated for 3 h at 350 °C to produce activated carbon doped with Mn oxide (Bio-MnOx-C), which were produced by culturing Mn (II)-oxidizing bacterial strain MnI7-9 in liquid A medium at 28 °C with 10 mmol/L MnCl2. Bio-MnOx-C was characterized by SEM, TEM, IR, XPS, XRD, etc. It contained C, O, and Mn which comprised Mn (IV) and Mn (III) valence states at a ratio of 3.81:1. It had poorly crystalline ε-MnO2 with a specific surface area of 130.94 m2/g. A total of 0.1 g Bio-MnOx-C could remove 45.95 g IC from 500 mg/L IC solution after 0.5 h contact time. IC removal by Bio-MnOx-C included a rapid oxidation reaction and the removal reaction followed second-order kinetic equation. These results confirmed that Bio-MnOx-C could be a potential material for wastewater remediation.

Dielectric relaxation and magnetodielectric response in DyMn0.5Cr0.5O3

We investigate the structural, magnetic, and magnetodielectric properties of DyMn0.5Cr0.5O3. The sample can be indexed with an orthorhombic phase with B site disordered space group Pbnm. The valence state of both Mn and Cr ions are suggested to be +3 based on the results of x-ray photoelectron spectroscopy. Two thermally excited dielectric relaxation at temperatures TN2 &amp;lt; T&amp;lt; 300 K and large magnetodielectric effect (MDC = 20%–30%) due to the disordered arrangement of Mn3+/Cr3+ ions associated with electron hopping between them are observed. The absence of any noticeable magnetoresistance effect (MR &amp;lt; 0.5%) demonstrates that the observed magnetodielectric effect is an intrinsic behavior. These results suggest that DyMn0.5Cr0.5O3 is a magnetodielectric compound, whose dielectric properties are dependence of the applied magnetic field, which exhibits such effects near room temperature and holds great promise for future device applications.

Phase formation and crystallization of LiMnxFe1−xPO4-C olivine material with different Mn2+ contents fabricated at lower calcination temperatures

LiMnxFe1−xPO4-C cathode materials for lithium ion batteries were synthesized via solid-state method using Li2CO3, MnCO3, NH4H2PO4, FePO4 and sucrose as starting raw materials, followed by high-temperature reduction-annealing. A series of calcination experiments at different temperatures reveal that Mn2+-containing materials exhibit a lower temperature for olivine phase formation, for example LiMn0.5Fe0.5PO4 olivine phase forms at 275 °C, while manganese-free crystalline LiFePO4 generally forms at the required temperature of 350 °C. Increasing Mn2+ content is found to enhance crystallization degree of LiMnxFe1−xPO4 material prepared at lower calcination temperatures. X-ray photoelectron spectroscopy (XPS) results confirm that Mn valence state (+2) remains unchanged up to ~250 °C when calcined in ambient atmosphere. The above-mentioned beneficial effect of manganese on phase formation and crystallization of olivine can be well attributed to the stable nature of Mn2+ and its strong propensity to form olivine phases.

Linking Atomic Structure and Local Chemistry at Manganese‐Segregated Antiphase Boundaries in ZrO2–La2/3Sr1/3MnO3 Thin Films

This paper presents direct experimental evidence of Mn segregation at three types of antiphase boundaries in La2/3Sr1/3MnO3 thin films doped with ZrO2. The local atomic structure of these antiphase boundaries is investigated by high‐angle annular dark‐field and annular bright‐field imaging in a scanning transmission electron microscope. Chemical composition and cation valence are determined by electron energy‐loss spectroscopy. The evidence that strain relaxation and ZrMn substitution are driving forces for the formation of the antiphase boundaries is found in this study. Analysis of atomic structure, image contrast, Mn valence state, and Mn occupancy shows that the antiphase boundaries are charge‐neutral with minimized internal electric fields.

Synthesis, structure analysis and magnetic properties of Ca2TiMnO6 oxides

Ca2TiMnO6 compounds were synthesized by the conventional solid-state reaction in air. The crystal structure and magnetic properties were investigated. Characterization of X-ray diffractions and the refinement analysis confirmed the orthorhombic (space group Pbnm) structure of Ca2TiMnO6 at room temperature. Magnetic measurements reveal that the double perovskite Ca2TiMnO6 has antiferromagnetic interactions at low temperature and a paramagnetic order above 40K. According to the Curie–Weiss fitting at high temperature, the Neel constant θp and experimental efficient moment μeff was estimated to be about −161.98K and 4.91μB, respectively. The theoretical efficient moment was calculated to be 3.87μB. The XPS analysis indicates that the difference between experimental and theoretical efficient moment caused by the mixed valence state of Mn in Ca2TiMnO6. The temperature dependence of coercivity field shows a soft magnetic property in Ca2TiMnO6 oxides. The mechanism of the low temperature anomaly which was appeared in magnetization curve was discussed in detail.

Structural Evolution of High Capacity Li-Rich 0.5Li2MnO3*0.5LiMO2 (M=Mn, Ni and Co) Cathode Materials during Electrochemical Cycling

Li-rich cathode materials with a general chemical formula (x)Li2MnO3*(1-x)LiMO2 (M= Mn, Ni and Co) are in spotlights since last few years. They offer reversible capacities in excess of ~250 mAh/g when charged above 4.5 V, which is significantly higher than that delivered by the commercially available LiCoO2 (~140 mAh/g) cathode material, and exhibit high rate capabilities[1]. However despite their attractive properties, no clear consensus exists on how the two components, namely Li2MnO3 and LiMO2 of Li-rich materials coexist and how the structure evolves upon cycling. Herein, we report bulk and surface structural modifications in Li-rich cathode materials during electrochemical cycling using X-ray absorption spectroscopy (XAS). In-operando XAS measurements in the transmission mode at the Mn, Ni and Co K-edges of the material provided information about bulk structural changes, while the O K-edge Near-Edge X-ray absorption fine structure (NEXAFS) measurements in the total-electron yield (TEY) mode provided information about the near-surface structural modifications. It is observed that the type and amount of transition metal (TM) ions determine the structural integration between the two components of Li-rich materials. For instance, higher Co contents tend to reduce the formation of Li2MnO3-type domains and promote a layered Li(1+x)MO2-type structure, while higher Mn and Ni contents give rise to the formation of a large amount of well-ordered Li2MnO3 domains[2][3]. During activation, lithium extraction from Li2MnO3 domains occurs with a concurrent removal of oxygen, giving rise to the formation of a layered MnO2-type structure[4]. The observed preferential reduction in the amplitude of the first peak (Mn-O) in Fig.1, representing the first coordination shell of oxygen atoms around the central Mn atom, also confirms the oxygen removal from the material during activation. Lithium reinsertion into a layered MnO2-type structure formed during activation, gives rise to the formation of a structure which is similar to the original Li2MnO3, but Li and O deficient[4]. Consistent structural modifications were observed for the O K-edge NEXAFS spectra of the material during activation (Fig.2). It is observed that oxygen removal occurs only during the first cycle, and presumably, it is the necessary step in activating the Li2MnO3 component of Li-rich materials. During subsequent cycles, lithium extraction and reinsertion in Li2MnO3 domains may involve the unusual participation of oxygen anions into redox processes. The average valence state of Mn in Li2MnO3 domains remains 4+ at all times. Quite contrarily, electrochemical processes in LiMO2 domains involve conventional redox processes of TM ions. Lithium deintercalation from LiMO2domains results into the formation of O1-type stacking faults within the O3 structure, while lithium intercalation reverts these O1-type stacking faults back to the original O3-type. In order to clarify structural modifications occurring during cycling, we investigated charged and discharged samples in the 2nd, 10th and 25th cycles. It is observed that after the initial oxygen release during activation, no major changes occur in Li2MnO3 domains during subsequent cycling. On the other hand, structural changes in LiMO2 domains are also quite similar to those observed during the 1st cycle. Interestingly, there was no indication of a spinel-like phase being formed upon cycling, as speculated in the literature. As reported previously, both Li2MnO3 and LiMO2 experience structural degradation when used independently as cathode materials in cells and cycled above 4.5 V[4][5]. In case of Li-rich materials, however, none of these components show degradation after 25th cycle. These results highlight the role of both the components in Li-rich materials. Apparently, the randomly distributed Li2MnO3 domains in Li-rich materials act as a source of excess lithium for the redox-active LiMO2 domains and impart greater overall structural stability above 4.5 V. On the other hand, conventional redox processes of TM ions in the LiMO2 component help to overcome parasitic side reactions in the Li2MnO3 component above 4.5 V involving the electrolyte[4].

Identification of MnOx species and Mn valence states in MnOx/TiO2 catalysts for low temperature SCR

As the selective catalytic reduction (SCR) catalysts for NOx removal, various Mn-based catalysts have been widely investigated in the academic world. However, few studies focus on the identification of MnOx species and Mn valence states in Mn-based catalysts. In this study, Mn-based catalysts, which were prepared by using different precursors or pure MnOx species, were investigated for their activities for SCR of NO by NH3. Meanwhile, according to XRD, TG and H2-TPR measurement, a mass of crystalline and/or amorphous MnOx species were detected, while a very small amount of amorphous MnOx species could be observed with the TG–DSC-MS by detecting the oxygen coming from manganese oxides because of the decomposition. In addition, the contents and distributions of Mn2+, Mn3+ and Mn4+ as well as the average valences of manganese in catalysts were tested with the chemical titration method, while the XPS results could not identify the MnOx species, showing a semi-quantitative assessment of Mn2+, Mn3+ and Mn4+ contents. Furthermore, based on the SCR performance, Mn2O3 and Mn3O4 species were active for NOx reduction.