MnO2 Phase Research Articles

Excess antimony induced 12R Ba4Sb0.85Mn3.15O12 phase and Mn3O4 exsolution from 10H Ba5Sb0.7Mn4.3O15: Crystal structure and magnetic properties

Hexagonal perovskite manganite has attracted much attention due to its rich structural chemistry and magnetic properties. The transformation between BaMxMn1−xO3 polymorphs can be caused by the change in substitution level or oxygen vacancies. In this study, we find a new polymorph change from 10H Ba5Sb0.7Mn4.3O15 (P63/mmc) to 12R Ba4Sb0.85Mn3.15O12 (R3‾m) via a ‘topochemical’ reaction with excess Sb2O3. The 10H Ba5Sb0.7Mn4.3O15 structure contains BaO3 layers in a (cchhh)2 sequence and face-shared MnO6 tetramers (Mn4 clusters), while the 12R Ba4Sb0.85Mn3.15O12 structure consists of BaO3 layers in a (cchh)3 stacking and face-shared MnO6 trimers (Mn3 clusters). The 10H → 12R phase conversion accompanies the exsolution of the Mn3O4 phase in trace amounts. For both 10H Ba5Sb0.71Mn4.29O15 and 12R Ba4Sb0.85Mn3.15O12, the Mn3+ cation on the corner-shared octahedra at M1 sites mediated competing ferromagnetic and antiferromagnetic exchanges between the magnetic Mn3 and Mn4 clusters, leading to spin-freezing transition at TF ≈ 11–15 K. The 10H → 12R phase change leads to an abrupt change of 2 K saturation moment from Ms ≈ 0.1 μB of 10H Ba5Sb0.71Mn4.29O15 to Ms ≈ 1.75 μB of 12R Ba4Sb0.85Mn3.15O12.

Designing reduced graphene oxide decorated Ni doped δ-MnO2 nanocomposites for supercapacitor applications

Reduced Graphene Oxide (rGO) decorated δ-MnO2 and Ni-doped δ-MnO2 samples have been synthesized by a microwave-assisted co-precipitation technique which is low cost, simple and fast reaction method. The structural and morphological properties of synthesized samples have been characterized using x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). XRD result indicates that all samples have monoclinic crystal structure with δ phase of MnO2. The morphologies of MnO2 transformed into perforated porous nanoflakes with Ni doping and rGO composition in MnO2. AFM images confirmed that surface roughness for 3 % Ni-doped MnO2 samples is higher due to the chemical adsorption of samples. Raman spectroscopy results show the presence of vibrational and Raman active modes of MnO2.X-ray photoelectronic spectroscopy revealed the presence of mixed valencies of Mn such as Mn2+, Mn3+, and Mn4+ in prepared nanocomposites while Ni shows 2+ valency. The specific capacitance (Cs) of pure δ -MnO2 samples were 68 Fg-1 and rGO decorated 3 % Ni-doped MnO2 composition 694 Fg-1 at 10 mVs−1 respectively. EIS data of the rGO decorated Ni-doped MnO2 composition samples confirm that the porous nanoflake-shaped rGO decorated 3 % Ni-doped MnO2 composite sample has a low resistance value among all samples. These results reveal that rGO-decorated 3 % Ni-doped MnO2 nanoflakes should be a promising material for designing and fabricating the electrode material for high-performance supercapacitors.

Base-free synthesis of renewable furan-2,5-dicarboxylic acid (FDCA) over an earth-abundant magnetic catalyst

Being a biobased alternative to petroleum-based terephthalic acid (TPA), furan-2,5-dicarboxylic acid (FDCA) requires a low-cost and efficient catalytic system for its commercialization. This study describes FDCA production using whey permeate powder (WPP)-derived 5-(hydroxymethyl)furfural (HMF) in a base-free system over a magnetically recyclable catalyst composed of earth-abundant metals (Mn and Fe). The impact of MnCl2·4 H2O loading on the iron core was studied at different levels and found optimal at the ratio of 1:1. The in-situ oxidation of HMF was carried out using ozone and t-BuOOH as the oxidants in a γ-valerolactone (GVL) -water solvent system. It was observed that the ozone pre-treatment (3 h at room temperature) of HMF increased the FDCA yield from 39.7 % (without ozonation) to 60 % at a reaction condition of 130 °C for 2 h. The concentration profiles for the intermediates formed during HMF to FDCA oxidation i.e., 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), furan-2,5-dicarbaldehyde (DFF), and 5-formyl-2-furancarboxylic acid (FFCA) were analyzed at different reaction temperatures to evaluate the reaction kinetics. The kinetic study revealed that the present solvent-catalytic system favored HMFCA formation over DFF due to the requirement of higher activation energy for the latter. The HMF to FDCA catalytic solvent was tested for WPP, which afforded an overall FDCA yield of 14.6 % at 130 °C in 120 min. In addition, the Mn1Fe1 consisted of mixed phases of Mn3O4, Mn2O3, MnO2, Fe2O3, and Fe3O4 and demonstrated variation in oxidation states for Mn (Mn3+↔Mn2+) and Fe (Fe2+↔Fe3+) species.

The green reductive leaching of manganiferous iron ore and Mn3O4 nanoparticles production: Kinetic modeling and comparison of various reductants

BackgroundSecuring critical metals is crucial for the transition from fossil fuels to renewable energies. In this regard, extracting metals from various sources and low-grade ores can lead to the sustainable production of metals. Manganese, as a strategic metal, can play a significant role in achieving this goal. MethodsIn this study, various reductants such as oxalic acid, citric acid, ascorbic acid, acetic acid, tannic acid, hydrogen peroxide, iron (II) sulfate, sodium thiosulfate, and DL-malic acid were used to evaluate the feasibility and comparison on the manganiferous iron ore leaching. Significant findingsDL-malic acid was chosen as the main reductant to investigate other factors because it is novel (as a reductant), eco-friendly, cost-effective, accessible, and easily storable. Therefore, more detailed studies on the dissolution of manganiferous iron ore in the presence of DL-malic acid as the reductant were carried out considering different levels of temperature, particle size, acid concentrations, and leaching time. Increasing the temperature from 298 K to 348 K notably boosted the leaching recovery from 27.00 % to 70.11 %. It was observed that decreasing the ore particle size from -212 μm to -38 μm resulted in an enhancement of leaching recovery from 57.74 % to 70.11 %. Also, adding only 250 % stoichiometry of DL-malic acid notably increased the leaching recovery to 70.11 %, compared to just 5.32 % in a reductant-free medium. It should be noted that the concentration of sulfuric acid had a direct impact on leaching recovery, increasing by 28.28 % with a concentration of 0.5 M and by 93.08 % with a concentration of 4 M. In this research work, the kinetic of the leaching process was modeled using the modified shrinking core model (MSCM). The calculated activation energy was about 33 kJ/mole, which confirmed that the mixed mechanism controlled the reaction. Mn3O4 nanoparticles were synthesized using a pregnant leach solution (PLS) and through a multi-stage co-precipitation method. In this method, hydrogen peroxide was used to modify the manganese hydroxide phase as a more eco-friendly method than other heat treatment methods. The SEM and EDX analyses revealed an average particle size of 80 nm with spherical shapes and no impurities. The XRD pattern of the synthesized nanoparticles confirmed the Mn3O4 phase composition.

Optical and electrochemical properties of manganese oxide (Mn3O4) nanoparticles: Investigating the influence of calcination temperature on supercapacitor performance

In this study, we investigate the influence of calcination temperature on the structural, optical, and electrochemical properties of Mn3O4 nanoparticles synthesized through a chemical co-precipitation route. Field-Emission Scanning Electron Microscopy (FESEM) images depict the formation of highly agglomerated, spherical-shaped particles. At calcination temperatures of 700 °C and 900 °C, the observation reveals the emergence of flat pellets and rod-like structures. The X-ray Diffraction (XRD) patterns confirm the presence of the Mn3O4 phase in M0 and M1 samples. However, the sample calcined at 500 °C (M3) showed a mixed phase of Mn3O4 and Mn2O3. Upon further calcination at 700 and 900 °C, this mixed phase undergoes complete transformation into the Mn2O3 phase. The change in absorption onset and photoluminescence intensity of samples is found with an increase in calcination temperature due to the crystal growth and phase modification. Furthermore, the synthesized samples are employed as electrode materials for supercapacitor applications. The sample calcined at 300 °C (M1) in three-electrode system demonstrated the specific capacitance (Csp) of 273.3 F/g at 0.5 A/g in an alkaline medium. The asymmetric cell assembled by using M1 and Hibiscus cannabinus stem-derived carbon material (HBC-900) delivered higher Csp of 205.7 F/g at 0.5 A/g. In addition, asymmetric supercapacitor device exhibited a high energy and power density of 43.1 Wh/kg and 476.2 W/kg with 82.07 % of initial Csp retention over 10,000 cycles at 10 A/g, which makes the sample M1 as the potential electrode material for supercapacitor application.

In English

Samples of MnOx-Na2WO4/SiO2 based on a mesoporous silica matrix were synthesized using manganese acetate, sodium tungstate, tetraethoxysilane (TEOS) as precursors and cetyltrimethylammonium bromide (CTAB), citric acid and triethanolamine as pore generating agent and characterized by scanning electron microscopy with energy dispersive elemental analysis (SEM/EDS), X-ray diffractometry (XRD), electron magnetic resonance (EMR), N2 adsorption-desorption measurements and tested as a catalyst for the reaction of oxidative condensation of methane (OCM). It is shown that the MnNaW/SiO2 catalyst consists of Mn2O3, MnO2, Na2WO4 and SiO2 phases. The EMR and SEM/EDS data indicate a noticeable effect of the reaction conditions on the surface structure and the distribution of catalytically active components in the catalyst structure. Significant changes in the values of the specific surface area and pore volume of the samples with increasing temperature and duration of the reaction were found (the values of the specific surface area and pore volume, respectively, 116.8 m2/g and 0.590 cm3/g - up to and 46.1 m2/g and 0.232 cm3/g after 15 hours of catalyst operation in the OCM reaction at 800 °C). It has been shown that under the conditions of the OCM reaction, the elemental composition of the surface of the MnOx-Na2WO4/SiO2 catalyst changes, and the degree of change depends on the duration of the OCM reaction. Under the influence of the reaction mixture CH4/O2, the content of Mn and W decreases in the first two hours, and the trend towards a decrease in their amount on the catalyst surface with an increase in the duration of the OCM reaction is generally preserved. A detailed X-ray phase analysis also indicates a change in the phase composition of this catalyst under the influence of the CH4/O2 reaction mixture. It is assumed that under the reaction conditions (700–800 °C) the MnOx-Na2WO4/SiO2 catalyst is silicon dioxide particles basely with the cristabolite structure, the surface of which is coated with molten sodium tungstate containing nanosized particles of MnOx structures activated by sodium ions and WOx.

Synthesis and Electrochemical Properties of One‐Dimensional γ‐MnOOH Nanorods

AbstractMnOOH has received much attention from researchers as a supercapacitor electrode material. In this article, one‐dimensional γ‐MnOOH nanorods were prepared by the hydrothermal method using the redox reaction between MnO4− and Mn2+, and the effects of temperatures on the crystalline phases and electrochemical properties of the samples were investigated. The materials were characterized using XRD, FT‐IR, SEM, TEM, Raman and XPS; electrochemical properties were tested using CV, GCD and EIS. The results show that the reaction temperature plays an important role in the generation of γ‐MnOOH. The samples were pure phases of γ‐MnOOH (M‐140) at the reaction temperature of 140 °C, showing interlaced nanorod morphology, and when the reaction temperature was increased to 180 °C, a mixed phase of MnOOH and Mn3O4 appeared. In contrast, the electrochemical performance of γ‐MnOOH nanorods (M‐140) was superior to the other samples, reaching a specific capacitance of 221 F/g at a current density of 0.2 A/g. The capacitance retention was 93.8 % after charging and discharging the M‐140 2000 times at a current density of 6 A/g. Supercapacitors assembled with γ‐MnOOHand activated carbon (AC) have excellent energy storage performance, and a high energy density of 40.6 Wh/kg is obtained at a power density of 196.7 W/kg.

Synergistic strategy of solute environment and phase control of Pb-based anodes to solve the activity-stability trade-off

The contradiction between the activity and stability of metal anodes exists extensively, especially in acid electrooxidation under industrial-level current density. Although the anode modification enhanced the initial activity of anodes, its long-term activity is limited by anode slime accumulation. Herein, a synergistic strategy, coupling the solute environment with the phase control of anodes, is proposed to achieve the trade-off between activity and stability of Pb-based anodes in concentrated sulfuric acid electrolysis. Non-exogenous Mn2+ motivated a series of positive behaviours of reactive-oxygen-species capture, anode reconstruction and corrosion-dependent activity alleviation. The synergistic effects, which are crystal phase-dependent, mainly benefit from the continuous self-healing ability of the specific crystal phase of MnO2 on the anodes by the coexisted Mn2+. Compared with Mn2+/α-MnO2, Mn2+/γ-MnO2 exhibited outperformed activity and stability in boosting oxygen evolution reaction (OER) and reducing hazardous pollutants, which resulted from the energy difference in the rate-determining step of OER and in the selectivity priority of Mn2+/MnO2 oxidation pathway. Interestingly, the pre-coated γ-MnO2 on the anode also presents excellent inheritance, guaranteeing the unchanged crystal phase of MnO2 and the high performance in ultra-low hazardous slime generation in subsequent Mn2+ oxidation. The sustainability of Mn2+/γ-MnO2 was proved in the operating hydrometallurgy conditions on Pb-based anodes. This strategy offers a promising approach for this common issue in electrooxidation-related areas.

Impressive efficiency of zinc oxide-manganese oxide/MAX composite in two-electrode system for photovoltaic-electrolyzer water splitting

The improvement and use of affordable and efficient electrocatalysts for complete water splitting are crucial for the future of energy supply. Here, a hybrid zinc oxide (ZnO)–manganese oxide (Mn3O4)/Ti3AlC2 (MAX) composite produced by a facile hydrothermal synthesis is utilized as a dual-function electrocatalyst for the hydrogen and oxygen evolution reactions (HER and OER, respectively). Its well-organized surfaces and bulky porous framework provide numerous catalytically active sites during electrochemical reactions, leading to significant improvement in the HER and OER activities. In particular, the ZnO and Mn3O4 phases increase the catalytic activity of the electrode, while the MAX phase enhances the conductivity and promotes low overpotential Ni sites through charge transfer effects. Consequently, the optimized ZnO-Mn3O4/MAX/Ni foam electrode exhibits relatively low overpotentials (η) of 244 and 340 mV at 20 mA cm−2 for the HER and OER at 1.0 M KOH, respectively. The electrodes also demonstrate exceptional long-term stability during cycling for both the HER and OER. A fully homemade water splitter, using these electrodes as anode and cathode, achieves a current density of 10 mA cm−2 at a cell voltage of only 1.62 V. When integrated with commercially available silicon-based solar cells, combined PV-EC water splitting devices enable increased hydrogen generation from stimulated sunlight. This study presents a typical demonstrated and proven strategy for real large-scale solar hydrogen generation using cost-effective PV-EC technology.

Catalytic Acetone Oxidation over MnOx Catalysts: Regulating Their Crystal Structures and Surface Properties

This study investigates the catalytic oxidation of acetone by different crystal phases of MnO2 prepared via different methods. Compared with β-MnO2 and γ-MnO2, α-MnO2 exhibited superior catalytic activity. Moreover, as replacements for traditional hydrothermal methods and air calcination, the use of microwave hydrothermal methods and N2 calcination significantly enhanced the catalytic performance of the MnO2 catalyst. The optimal catalyst, MnO2-WN (α-MnO2 synthesized via microwave hydrothermal method and N2 calcination), converted 100% of 100 ppm acetone below 150 °C, with the CO2 yields reaching 100%. Further, the stability of the catalyst and its potential for other volatile organic compounds (VOCs) were also determined. The experimental data demonstrated that its outstanding activity primarily stemmed from the improved preparation method, enhancing the specific surface area of the catalyst, optimizing the pore structure, improving the redox performance, and generating more acidic sites and active oxygen species, thereby creating a synergistic effect. Finally, the reaction pathway of acetone oxidation on the catalyst surface has been explored. This work provides a new perspective for developing economically efficient MnOx catalysts for removing VOCs.

Microstructure and electrochemical performance of plasma sprayed Fe45Mn35Co10Cr10 high−entropy alloy coating in different corrosive solutions

A Fe45Mn35Co10Cr10 high−entropy alloy coating was prepared on Ti6Al4V alloy by plasma spraying. The microstructure, chemical elements and phases of obtained coating were analyzed using a scanning electron microscope, energy dispersive spectrometer and X−ray diffraction, respectively. The electrochemical performances in 0.1 M H2SO4, 0.1 M NaOH and 3.5 % NaCl solutions were investigated using an electrochemical workstation, and the corrosion model was also established to reveal the corrosion mechanism. The results show that the HEA coating is composed of face−centered cubic, FeCr2O4, MnCr2O4 and MnO phases, which forms the mechanical bonding with the substrate. The HEA coating has the lowest corrosion current density icorr, highest polarization resistance Rpo and largest charge transfer resistance Rct in 0.1 M H2SO4 solution, exhibiting the highest electrochemical corrosion resistance. The corrosion mechanism of HEA coating is pitting corrosion, which is attributed to the formed stable corrosion products.

Structure and electrical properties of novel Mn3O4–LaMnO3 composite ceramics with NTC effect

Novel Mn3O4 - LaMnO3 composite ceramics have been successfully designed by the Pechini sol-gel routine, ceramics were calcined at 1450 °C for 12 h in air, and the electrical properties of its negative temperature coefficient at room temperature (300K) were investigated. X-ray diffraction and SEM results indicate that all the samples consist of two phases, LaMnO3 and Mn3O4, and illustrate highly crystalline quality. The electrical transport measurements show that the temperature coefficient of resistance (TCR) vs. temperature of the composite presents a ‘U’ shape at wide-temperature-range(188–350K), which is beneficial for the performance of practical devices. Low resistivity ρ and TCR revealed for pure LaMnO3, While TCR and room temperature range were increased with the addition of Mn3O4 phase into LaMnO3. Enhanced electrical properties with moderate resistivity ρ make it a prominent and potential candidate to be utilized in portable, uncooled bolometer.

Base free HMF oxidation over Ru-MnO2 catalysts revisited: Evidence of Mn leaching to Mn-FDCA complexation and its implications on catalyst performance

2,5-furandicarboxylic acid (FDCA) is a high value bio-based chemical useful for the production of next generation renewable polymers, epoxy-resins and surfactants. Herein, we show that the performance and stability of MnO2 based oxidation catalysts used in FDCA production are considerably influence by process conditions (especially initial HMF conc. and presence of base additives). Although, base free aerobic oxidation of HMF to FDCA is known to be highly effective over Ru modified MnO2 catalysts, our experiments showed considerable leaching of MnO2 phase under reaction conditions with the formation of insoluble Mn-FDCA complexes. The formation of Mn-FDCA complex and Mn leaching was also supported by control experiments between FDCA and various Mn oxides under hydrothermal conditions and its structure confirmed by single crystal XRD. Formation of Mn-FDCA and Mn leaching was also identified as the underlying reason for carbon balance deficit, catalyst mass loss and deactivation observed upon reuse.

Magnetic effect and chemical distribution study of LCMNO3 perovskite by photoelectron spectroscopy

The crystal structure, x-ray photoemission spectroscopy, magnetic characteristics, and magnetocaloric performance of perovskite La0.55Ca0.45Mn0.8Nb0.2O3 (LCMNO) ceramic powder were analyzed. The orthorhombic structure with a Pnma space group was found as the main phase in the as-received compound after the XRD pattern refinement. The mixed-valence states of manganese were detected by x-ray photoemission spectroscopy (XPS). The analysis of the La3d line revealed the complex nature of the lanthanum states. The evaluation of Zero-Field-Cooled and Field-Cooled magnetization performance provided conclusive evidence for the existence of the antiferromagnetic Mn3O4 phase at a temperature of TN = 42 K and the coexistence of CO- AFM and FM domains in LCMNO. Despite the observed significant (δTFWHM) value due to relatively low magnetic entropy change (ΔSM), the relative cooling power (RCP) is relatively moderate. By adding Nb4+, the changes in Mn valences and their effect on the magnetic cooling effect are lessened.

Mn-promoted MoS2 catalysts for CO2 hydrogenation: enhanced methanol selectivity due to MoS2/MnOx interfaces.

Considering the alarming scenario of climate change, CO2 hydrogenation to methanol is considered a key process for phasing out fossil fuels by means of CO2 utilization. In this context, MoS2 catalysts have recently shown to be promising catalysts for this reaction, especially in the presence of abundant basal-plane sulfur vacancies and due to synergistic mechanisms with other phases. In this work, Mn-promoted MoS2 prepared by a hydrothermal method presents considerable selectivity for CO2 hydrogenation to methanol in comparison with pure MoS2 and other promoters such as K and Co. Interestingly, if CO is used as a carbon source for the reaction, methanol production is remarkably lower, which suggests the absence of a CO intermediate during CO2 hydrogenation to methanol. After optimization of synthesis parameters, a methanol selectivity of 64% is achieved at a CO2 conversion of 2.8% under 180 °C. According to material characterization by X-ray Diffraction and X-ray Absorption, the Mn promoter is present mainly in the form of MnO and MnCO3 phases, with the latter undergoing convertion to MnO upon H2 pretreatment. However, following exposure to reaction conditions, X-ray photoelectron spectroscopy suggests that higher oxidation states of Mn may be present at the surface, suggesting that the improved catalytic activity for CO2 hydrogenation to methanol arises from a synergy between MoS2 and MnOx at the catalyst surface.

First-principles predictions of room-temperature ferromagnetism in orthorhombic MnX2 (X = O, S) monolayers.

Two-dimensional ferromagnets with high spin-polarization at ambient temperature are of considerable interest because they might be useful for making nanoscale spintronic devices. We report that even though bulk phases of MnO2 are generally antiferromagnetic with low ordering temperatures, the corresponding MnO2 and MnS2 monolayers are ferromagnetic, and MnS2 is a high temperature half metallic ferromagnet. Based on first-principles calculations, we find that the MnO2 monolayer is an intrinsic ferromagnetic semiconductor with a Curie temperature TC of ∼300 K, while the half-metallic MnS2 monolayer has a remarkably high TC of ∼1150 K. Both compounds have substantial magnetocrystalline anisotropy, out of plane in the case of MnO2 monolayers, and in plane along the b-axis of orthorhombic MnS2 monolayer. Interestingly, a metal-insulator phase transition occurs in the MnS2 monolayer when the applied biaxial strain is beyond -2%. Tuning near this metal-insulator transition offers additional possibilities for devices. The present work shows that MnX2 (X = O, S) monolayers have the properties required for ultrathin nano-spintronic devices.

A new magnetic heterogeneous catalyst for fast degradation of rhodamine B by peroxymonosulfate oxidation: Monodisperse-porous and Fe3O4 incorporated manganese oxide microspheres

A new synthetic method, “staged shape templating decomposition” suitable for transition or rare-earth metal oxide microspheres was developed for obtaining magnetic, uniform and porous manganese oxide (MagMnOx) microspheres. The microspheres 4.7 μm in size were obtained with a specific surface area of 45.5 m2 g−1. Fe3O4, Mn5O8, MnO2 and Mn2O3 phases were shown by X-ray diffraction spectroscopy. Mn2+, Mn3+ and Mn4+ states were observed on the surface by X-ray photoelectron spectroscopy. Catalase-like and oxidase-like activities of MagMnOx microspheres were explained by coexistence of Mn and Fe crystallites in a mesoporous structure with a sufficiently large surface area and a particle interior with large mesopores facilitating substrate diffusion. The maximum substate consumption rates for catalase-like and peroxidase-like activities were determined as 15.6 mM/min and 20.3 μM/min, respectively. By considering their ROS generation ability, MagMnOx microspheres were evaluated as a heterogeneous catalyst for rhodamine B (Rh B) removal via peroxymonosulfate (PMS) activation. Rh B was removed in the periods shorter than 15 min via chemical degradation with a limited physical adsorption. Superoxide anion (O2−●) and singlet oxygen (1O2●) radicals generated by MagMnOx-PMS system were responsible for degradation of Rh B. The highest apparent first order rate constant for Rh B degradation was calculated as 0.5771 min−1. The apparent first order rate constant increased with increasing MagMnOx and PMS concentrations and decreasing initial Rh B concentration. Rh B was almost completely removed in each of five recycling runs and only lower than 1.5 % w/w of Fe and Mn contents of MagMnOx microspheres were leached over five recycling runs. The catalyst with large particle size, resistant to agglomeration exhibited high removal rate, low dye adsorption, high magnetization, and high reusability for Rh B removal via peroxymonosulfate activation.

Tracking Mn and Zn in Rechargeable Aqueous Zn-MnO2 Batteries By Operando X-Ray Absorption

Zn-MnO2 batteries with mildly acidic electrolytes are a promising chemistry for large scale storage thanks to their remarkable energy density, low cost, and high safety. This is mainly obtained thanks to the high capacity of the Zn metal anode, and the nonflammable character of the aqueous electrolyte. MnO2 is one of the most common cathode of choice, not only for being Earth-abundant, but also because it can undergo a two-electron mechanism, which is however complex and still not fully understood. There is currently agreement in considering for discharge a MnO2 dissolution, leading to soluble Mn2+ and simultaneous precipitation of Zinc Hydroxide Sulfate (ZHS, ZnSO4[Zn(OH2)]3·xH2O). When charging the process is not simply reverted. In fact, a distinct electrochemical profile is observed, with at least two distinct plateaus and a third, apparently pseudocapacitive stage (Figure 1a). A similar multistage profile is observed during the second discharge. Although such profile is characteristic and observed with different MnO2 phases and architectures, the underlying mechanism remains elusive, as it seems to involve mainly poorly crystallized phases.We studied the mechanism by operando X-ray absorption (XAS) at the Mn and Zn K-edges to follow speciation simultaneously and quantitatively in the cathode and in the electrolyte via principal component analysis. Beam intensity needed appropriate regulation to avoid interference with the experiment.Simultaneous X-ray diffraction allowed precise correlation with the MnO2 dissolution and ZHS formation. We found evidence of Mn(III) intermediate occurring during local bond reorganization, which is inferred by the significant evolution of the absorption fine structure region (EXAFS) of the Mn K-edge (Figure 1b). In contrast, minor Zn spectral changes reflect primarily processes of precipitation and dissolution, suggesting that no Zn-Mn mixed phases form during cycling. Figure 1

Effect of Mn on the ZnBiMnO varistor ceramics sintered at a low-temperature of 875 °C

The effect of Mn on ZnO–Bi2O3 (ZnBiO) based varistor ceramics sintered below 900 °C has not been fully understood due to the conventional ZnBiO-based varistor ceramics usually being sintered at temperatures higher than 900 °C. Therefore, (99.5-x) ZnO+0.5Bi2O3+xMnCO3 (x = 0, 1, 2, and 3; all in mole ratio) ceramics were prepared by solid-state sintering at 875 °C for 3 h to study the effect of Mn on the microstructure, electrical nonlinear properties and their stability within the range of 25–100 °C. The results show that doping MnCO3 results in the formation of Mn3O4 and Bi12MnO20 secondary phases within the sintered samples. Mn3O4 mainly exists at grain boundary as particle phase, restricting the growth of ZnO grains during sintering. The presence of Bi12MnO20 indicates MnCO3 may enhance the sintering of ZnBiMnO ceramics by increasing the amount of Bi-rich liquid phase. MnCO3 effectively improves the electrical nonlinear properties of ZnBiMnO varistor ceramic and their stability against the variations in ambient temperature. The 2 mol% MnCO3 doped ZnBiMnO varistor ceramic shows the best electrical nonlinearity, with a nonlinear coefficient, a breakdown voltage and a leakage current density of 53.61, 503.11 V/mm, and 2.85 μA/cm2 respectively. This study may improve our understanding of the effect of Mn on the low-temperature sintered ZnBiO based varistor ceramics, and provide a promising candidate material for manufacturing low-cost ZnO based varistor.

Plasma-assisted deposition of Mn and Fe phases on CeO2 biomorphic fibers for soot combustion and CO oxidation

In the present work, the gliding arc (glidarc) plasma technique is for the first time explored as a novel precipitation-deposition method to generate fibrous catalysts. Precisely, Mn and Fe oxides were plasma-precipitated on a fibrous ceria support and evaluated in two reactions of environmental interest, (1) soot combustion and (2) CO oxidation. Pure CeO2 fibers (Fib Ce) were obtained via a biotemplate route with the idea of starting from a support already active in oxidation reactions, and then further improving its activity by supporting on its surface Fe and Mn oxides. The precipitates were identified by XRD, FTIR and LRS, and their loadings and distributions assessed by ICP and EDX respectively. α-Fe2O3 and Mn3O4 phases were detected. XPS confirmed the presence of Mn and Fe on the surface of Fib Ce and revealed that the deposition of Fe logically gave a superficial Fe/Ce ratio much higher than the bulk one and provoked an increase in Ce3+/Ce4+ and Oads/Ce ratios. On the contrary, Mn-supported samples did not affect considerably the Ce3+/Ce4+ and Oads/Ce ratios and presented a Mn/Ce ratio closer to that of the bulk. The latter suggests that Mn has integrated the ceria structure. For reaction (1), the deposition of Fe by plasma drastically degraded the performance of the bare support, whereas Mn deposition greatly increased the activity of Fib Ce. The same activity order was observed for reaction (2): Mn-Fib Ce > Fib Ce > Fe-Fib Ce.