Manganese Oxide Materials Research Articles

Li<sup>+</sup>-Doping-Induced Changes of Phase Composition in Electrodeposited Manganese(IV) Oxide Materials

The impact of Li+dopant-ions in fluorine-containing electrolytes on electrodeposited manganese (IV) oxide material was under investigation in this paper. The dependence of phase composition of this material at Li+-concentration range in the electrolyte below the stoichiometric content of lithium in hollandite A2Mn8O16(Mn:Li ≈ 4:1) was established. The hollandite phase stabilization as a template effect caused by Li+-ions is gradually reduced with the Li+concentration growth from 0.025 to 0.15mol∙L-1LiOH concentration range. The hollandite content sharply drops at close to the stoichiometric Mn:Li ratio for the hollandite phase. In contrary, the concentration of cation-deficient ε-MnO2becomes significant. Thus, the template effect of Li+cations at electrolytic doping from fluorine-containing electrolytes consists of stabilization of the hollandite tunnels at longer distance with the size of coherent scattering regions of this phase more than of about 20—50 Å comparing with undoped materials. It is supposed that Li+-ions presence makes tunnel space unavailable unlike water molecules or ammonium cations. Therefore, to realise molecular sieves based on manganese (IV) oxides the availability of tunnels should be taken into account.

Porous MnO/Mn3O4 nanocomposites for electrochemical energy storage

Controlling the morphology of nanostructured manganese oxide materials can be an effective way to improve capacitance for supercapacitor applications. Herein we demonstrated for the first time the synthesis of MnO/Mn3O4 nanocomposite tetrahedrons with a porous structure, and a new method to synthesize porous urchin-shaped MnO/Mn3O4 nanocomposite. Compared with the non-porous MnO nanocrystalline octahedrons and the mixture of non-porous MnO and Mn3O4, the porous MnO/Mn3O4 nanocomposite ‘urchins’ exhibited superior capacitance in supercapacitor application, while the porous MnO/Mn3O4 nanocomposite tetrahedrons displayed superior stability. The excellent capacitance and stability of these nanocomposites could be explained in terms of the much higher surface area associated with their porous structure. These porous nanostructures offered a good model to investigate the effects of morphology and surface area on the capacitance of nanocomposites.

The structure–property relationship of manganese oxides: highly efficient removal of methyl orange from aqueous solution

A-MnO2 has the best adsorption capacity for methyl orange among other manganese oxide materials.

Emission of highly excited electronic states of potassium from cryptomelane nanorods.

Cryptomelane (KMn8O16) nanorods were synthesized, characterized (XRD, Raman spectroscopy, TEM/SAED) and investigated by species resolved thermal desorption of potassium from the material in the range of 20-620 °C. The desorbing fluxes of ions, atoms and highly excited electronic states (field ionizable Rydberg states) were measured using an ion collector, surface ionization and field ionization detectors, respectively, in a vacuum apparatus. The non-equilibrium emission of potassium Rydberg species (principal quantum number > 30) strongly depends on the surface positive voltage bias with a broad maximum at 1-8 V. The stimulation of Rydberg species emission is discussed in terms of spatial and energetic overlapping between the electron cloud above the cryptomelane surface and the desorbing potassium ion.

Compare study cellulose/Mn3O4 composites using four types of alkalis by sonochemistry method

The purpose of this article was to explore the influences of alkalis types on the cellulose/Mn3O4 composites via a sonochemistry method. In this study, cellulose/Mn3O4 composites were successfully fabricated using four types of alkalis (urea (CO(NH2)2), hexamethylenetetramine ((CH2)6N4, HMT), NaOH, and KOH) by an environmentally-friendly sonochemistry method. The phase, shape, thermal stability, and the formation mechanism of the cellulose composites were researched in detail. Experimental results demonstrated that the types of alkalis played an important role in the phase, shape, dispersion, and thermal stability of cellulose/Mn3O4 composites. By thermal treatment of cellulose/Mn3O4 composites at 600°C for 3h in air, the Mn3O4 crystals were obtained. This novel method reported here maybe has a guiding significance for the synthesis of manganese oxide materials and other metal oxides using cellulose as template.

Facile synthesis route of porous MnCo2O4and CoMn2O4nanowires and their excellent electrochemical properties in supercapacitors

Two types of porous cobalt manganese oxide nanowires (MnCo2O4 and CoMn2O4) with different structures have been successfully synthesized by thermal decomposition of organometallic compounds for the first time. Nitrilotriacetic acid (NA) was used as a chelating agent to coordinate Co(II) and Mn(II) ions in various molar ratios, in a hydrothermal condition. The microstructure of as-synthesized cobalt manganese oxides, composed of numerous nanoparticles, completely retains the 1D network structure of the Co–Mn–NA coordination precursors without structure collapse. Electrochemical properties of the cobalt manganese oxide materials have been tested for supercapacitors at room temperature. Both the MnCo2O4 and CoMn2O4 electrodes display the outstanding capacitive behaviors and superior electrochemical properties. The CoMn2O4 nanowire shows excellent capacitance and desirable rate performance (2108 F g−1 at 1 A g−1 and 1191 F g−1 at 20 A g−1) compared to that of the MnCo2O4 nanowire (1342 F g−1 at 1 A g−1 and 988 F g−1 at 20 A g−1). Electrochemical impedance spectra (EIS) results also reconfirm that the CoMn2O4 nanowires display more facile electrolyte diffusion and higher capacitor response frequency than MnCo2O4 nanowires. This can be ascribed to the facile electrolyte/OH− ion penetration and better Faradaic utilization of the electroactive surface sites that generated by the smaller particle size and higher surface area.

A high-capacity, low-cost layered sodium manganese oxide material as cathode for sodium-ion batteries.

A layered sodium manganese oxide material (NaMn3 O5 ) is introduced as a novel cathode materials for sodium-ion batteries. Structural characterizations reveal a typical Birnessite structure with lamellar stacking of the synthetic nanosheets. Electrochemical tests reveal a particularly large discharge capacity of 219 mAh g(-1) in the voltage rang of 1.5-4.7 V vs. Na/Na(+) . With an average potential of 2.75 V versus sodium metal, layered NaMn3 O5 exhibits a high energy density of 602 Wh kg(-1) , and also presents good rate capability. Furthermore, the diffusion coefficient of sodium ions in the layered NaMn3 O5 electrode is investigated by using the galvanostatic intermittent titration technique. The results greatly contribute to the development of room-temperature sodium-ion batteries based on earth-abundant elements.

An investigation on the structure and catalytic activity of cryptomelane-type manganese oxide materials prepared by different synthesis routes

Cryptomelane is an octahedral molecular sieve with a 2 × 2 tunnel structure (OMS-2) and a stoichiometric formula of approximately KMn8O16. Cryptomelane OMS-2 is gaining increasing attention in a range of applications, including catalysis. However, the impacts of synthesis methods on catalytic performance are still not well understood. Cryptomelane OMS-2 was prepared in this study by three methods: (1) a sol–gel method, (2) a reflux method, and (3) a milling method. The physical and chemical characteristics of these materials were investigated, and the catalytic performances of these materials were compared for the gas phase oxidation of ethanol. A novel aspect of this study is that cryptomelane was prepared by the milling method using post-consumer alkaline battery waste, and this novel cryptomelane material was compared with others prepared from commercially-available precursors and published laboratory methods.

Novel Li₂MnO₃ nanowire anode with internal Li-enrichment for use in a Li-ion battery.

Anode materials which undergo a conversion reaction can achieve larger specific capacities than conventional carbon-based materials. They can even achieve higher energy densities when used at low voltages. However, the large amounts of Li₂O generated in the interior of these structures when Li ions are inserted can cause volume expansion and mechanical fracturing from the inside out. This leads to a poor cycling performance and limits their commercial application. To overcome this limitation, we introduced Li ions into the interior of the cells of manganese oxide materials and successfully synthesized a novel Li-rich anode material (Li₂MnO₃). The reversible capacity reached 1279 mA h g(-1) after 500 cycles, much higher than that of pure MnO₂ or other commercial anodes. This optimization of the internal Li-enrichment and its application in Li₂MnO₃ nanowires used as low voltage anodes in Li-ion batteries have rarely been reported. Further investigations by X-ray diffraction and photoelectron spectroscopy suggested that the strategy of optimizing the internal Li-enrichment of this novel Li₂MnO₃ anode is a promising development for Li-ion batteries.

Selective oxidation of p-chlorotoluene to p-chlorobenzaldehyde with molecular oxygen over zirconium-doped manganese oxide materials

Nano-fibrous zirconium-doped manganese oxide octahedral molecular sieves (denoted as Zr-OMS-2) were prepared and characterized by XRD, SEM, EDXS, FTIR, NH3-TPD, and N2 adsorption-desorption techniques. The materials were used as catalysts for liquid-phase aerobic oxidation of p-chlorotoluene to p-chlorobenzaldehyde. The Zr-OMS-2 catalysts show much higher catalytic activities than OMS-2, and the Zr-OMS-2 with 6wt% Zr content shows the highest activity, giving approximately 93% p-chlorotoluene conversion and 86% p-chlorobenzaldehyde selectivity in 10h at 100°C. The promotion effect of Zr is deduced to be related with the suitable acidity and the enhancement of specific surface area. In addition, effects of reaction parameters such as catalyst amount, oxygen flow, temperature and time were studied.

Solvothermal synthesis of shape-controlled manganese oxide materials and their electrochemical capacitive performances

Abstract

Synthesis of P-doped mesoporous manganese oxide materials with three-dimensional structures for catalytic oxidation of VOCs

P-doped cryptomelane-type manganese oxide K-OMS-2 materials with three-dimensional structures were easily prepared using KMnO4 and MnPO4 as precursors via hydrothermal route. The scanning electron microscopy (SEM) and Energy dispersive X-ray spectra (EDS) results revealed that the morphology of P-doped K-OMS-2 transformed from hierarchical microspheres assembled from nanosheets and nanoneedles to nest-like structure with the increase of P/Mn molar ratio. Nitrogen sorption analysis showed that the isotherms of P-doped K-OMS-2 materials were typically type-IV with hysteresis loops, which meant that the samples had a mesoporous structure. The surface area and average pore diameter of the as-prepared K-OMS-2 materials were 138–153m2g−1 and 18–56nm respectively. In addition, the obtained materials showed good catalytic ability for the catalytic oxidation of a typical volatile organic compound (VOCs), o-xylene.

Energetic basis of catalytic activity of layered nanophase calcium manganese oxides for water oxidation

Previous measurements show that calcium manganese oxide nanoparticles are better water oxidation catalysts than binary manganese oxides (Mn3O4, Mn2O3, and MnO2). The probable reasons for such enhancement involve a combination of factors: The calcium manganese oxide materials have a layered structure with considerable thermodynamic stability and a high surface area, their low surface energy suggests relatively loose binding of H2O on the internal and external surfaces, and they possess mixed-valent manganese with internal oxidation enthalpy independent of the Mn(3+)/Mn(4+) ratio and much smaller in magnitude than the Mn2O3-MnO2 couple. These factors enhance catalytic ability by providing easy access for solutes and water to active sites and facile electron transfer between manganese in different oxidation states.

Comparative Performances of Birnessite and Cryptomelane MnO2 as Electrode Material in Neutral Aqueous Lithium Salt for Supercapacitor Application

Na-doped Birnessite-type manganese oxide (δ-MnO2) has been synthesized using the chemical method and characterized through X-ray diffraction and SEM, showing the lamellar structure and high crystal structure. A comparative study of the electrochemical performances of this material with those of the commercial Cryptomelane-type MnO2 has then been undertaken in ten neutral aqueous electrolytes for supercapacitor applications. Aqueous electrolytes, containing a lithium salt, LiX (where X = SO42–, NO3–, CH3CO2–, CH3SO3–, ClO4–, C7H15CO2–, TFSI–, Beti–, BOB–, or Lact–), have been first prepared under neutral pH conditions to reach the salt concentration, providing the maximum in conductivity. Their transport properties are then investigated through conductivities, viscosities, and self-diffusion coefficient measurements. Second, the thermal behaviors of these electrolytic aqueous solutions are then evaluated by using a differential scanning calorimeter from (213.15 to 473.15) K in order to access their liquid range temperatures. Cyclic voltammograms (CV) in three electrode configurations are thereafter investigated using Na Birnessite and Cryptomelane as working electrode material from (−0.05 to 1.5) V versus Ag/AgCl at various sweep rates from (2 to 100) mV·s–1. According to anion nature/structure and manganese oxide material type, different CV responses are observed, presenting a pure capacitive profile for Beti– or C6H13CO2– and an additional pseudocapacitive signal for the smallest anions, such as ClO4– and NO3–. The capacitances, energies, and efficiencies are finally calculated. These results indicate clearly that electrolytes based on a mineral lithium salt under neutral pH condition and high salt concentration (up to 5 mol·L–1) have better electrochemical performances than organic ones, up to 1.4 V with good material stability and capacity retention. The relationship between transport properties, electrostatic and steric hindrance considerations of hydrated ions, and their electrochemical performances is discussed in order to understand further the lithium intercalation–deintercalation processes in the lamellar or tunnel structure of investigated MnO2.

Nanostructuring of β-MnO2: The Important Role of Surface to Bulk Ion Migration

Manganese oxide materials are attracting considerable interest for clean energy storage applications such as rechargeable Li ion and Li–air batteries and electrochemical capacitors. The electrochemical behavior of nanostructured mesoporous β-MnO2 is in sharp constrast to the bulk crystalline system, which can intercalate little or no lithium; this is not fully understood on the atomic scale. Here, the electrochemical properties of β-MnO2 are investigated using density functional theory with Hubbard U corrections (DFT+U). We find good agreement between the measured experimental voltage, 3.0 V, and our calculated value of 3.2 V. We consider the pathways for lithium migration and find a small barrier of 0.17 eV for bulk β-MnO2, which is likely to contribute to its good performance as a lithium intercalation cathode in the mesoporous form. However, by explicit calculation of surface to bulk ion migration, we find a higher barrier of >0.6 eV for lithium insertion at the (101) surface that dominates the equilibrium morphology. This is likely to limit the practical use of bulk samples, and demonstrates the quantitative importance of surface to bulk ion migration in Li ion cathodes and supercapacitors. On the basis of the calculation of the electrostatic potential near the surface, we propose an efficient method to screen systems for the importance of surface migration effects. Such insight is valuable for the future optimization of manganese oxide nanomaterials for energy storage devices.

Supercritical N,N-dimethylformamide for the exfoliation and phase transition of layered manganese oxide materials to obtain trimanganese tetroxide nanosheets

A novel method using supercritical N,N-dimethylformamide (SC-DMF) exfoliation is proposed for the preparation of trimanganese tetroxide (Mn3O4) nanosheets. The obtained Mn3O4 nanosheets have nanoscale thickness and lateral dimensions of hundreds of nanometers. The change in morphology from a bulk to a two-dimensional structure when exfoliated with SC-DMF is characterised. The exfoliated products can be uniformly dispersed in N,N-dimethylformamide (DMF), the dispersion has a molar extinction coefficient of approximately 5.52 × 102 mol−1 dm3 cm−1 at a peak wavelength of 266 nm. The fourier transform infrared (FTIR) spectra show that intercalation mainly occurs between 300 °C and 400 °C. Raman spectroscopy indicates that the phase transition from MnO2 to Mn3O4 takes place between 200 °C and 400 °C. X-ray diffraction (XRD) confirms that the phase transition between the manganese oxides begins at around 350 °C. The exfoliated products possess strong coercivity, approximately 7 kOe, at low temperatures. It is demonstrated that the exfoliated Mn3O4 products result from exfoliation and phase transition in SC-DMF. The exfoliated nanosheet materials can be easily cracked in the course of exfoliation and aggregate when collected, these phenomena are attributed to the fragility and magnetic properties of the material, respectively.

Biodiesel production using calcium manganese oxide as catalyst and different raw materials

The use of heterogeneous catalysts for biodiesel production aims to simplify the production process as well as to reduce purification costs and related environmental impacts. Calcium manganese oxide was recently identified by the authors as an interesting heterogeneous catalyst for biodiesel production from animal fat; however, the difference between this and other catalysts, the catalyst activation/deactivation mechanisms, its behaviour in the synthesis using different raw materials as well as the impacts of its use on product quality remained unclear. Therefore, the present work: (i) compared biodiesel production using calcium manganese oxide and other catalysts (CaO and NaOH); (ii) studied the reasons leading to activation/deactivation of the heterogeneous catalyst; (iii) analysed biodiesel heterogeneous synthesis using calcium manganese oxide and different raw materials (lard, waste frying oil and a mixture); and (iv) evaluated raw material and catalyst impact on the product quality. Considering the use of different catalysts, the results showed that, after 8h of reaction, product purity was similar using the different catalysts, being 92.5wt.% using both NaOH and calcium manganese oxide and 93.8wt.% using CaO. The active species of the heterogeneous catalysts were CaO, in the case of calcinated calcium carbonate, and Ca0.9Mn0.1O, in the case of calcinated calcium manganese oxide. Because the deactivating species were different for both catalysts, the calcium manganese oxide required lower activation temperature, which should be an advantage. When using different raw materials, the reaction was slower when lard was used for biodiesel synthesis. Leaching results showed that catalytic behaviour was heterogeneous but further purification of biodiesel might be needed. Using heterogeneous catalysis, after 8h of reaction, independently of the raw materials used, similar results were obtained regarding the reaction progression; the quality was generally maintained but the water content of the product was improved compared to the homogeneous process.

Electrical behavior of an octahedral layered OL-1-type manganese oxide material

OL-1-type material (birnessite) is synthesized by an oxidoreduction process. Different physicochemical techniques were used to characterize the obtained material. AC impedance spectroscopy results show processes associated to the electrical conduction in bulk and grain boundary at high frequency, and an ionic conduction at low frequency. Here σ′(ω) shows a universal Jonscher’s law behavior associated to the electron hopping and charge polarization, and the value of 8.39 × 10−6 Ω−1 cm−1 found in the high frequency region at room temperature suggests its semiconductor nature. The combined results of AOS, TGA, and AA suggest the following chemical formula \( {\text{N}}{{\text{a}}^{ + }}_{{0.28}}\left( {{\text{M}}{{\text{g}}^{{2 + }}}_{{0.16}}{\text{M}}{{\text{n}}^{{4 + }}}_{{0.46}}{\text{M}}{{\text{n}}^{{3 + }}}_{{0.54}}} \right){{\text{O}}_{{2.03}}} \cdot 0.6{{\text{H}}_2}{\text{O}} \). Finally, the XRD pattern is characteristic of OL-1-type materials, the BET area was 56,25 m2 g−1, and the behavior of N2 isotherms suggests the presence of microporous and mesoporous structures. With the purpose of obtaining a better understanding of the ionic conductivity in these types of materials, magnesium exchange material was prepared and electrical properties at room temperature were analyzed. These results indicate that there is interplay among the structural, morphological, and textural properties with the electrical performance of these materials.

Synthesis and Electrochemistry of Nanocrystalline Iron and Manganese Oxide Materials

Identification of viable energy storage materials based on inexpensive, earth abundant metal centers is one of the most pressing issues for development of future battery technologies. In addition, low temperature, scalable methodologies are needed for active material synthesis. Preparation, characterization, and electrochemical evaluation of three promising battery materials are described herein. This work will contribute to materials advances in the development of environmentally sustainable materials for new battery technologies.

Cu-modified cryptomelane oxide as active catalyst for CO oxidation reactions

Manganese oxide octahedral molecular sieves (cryptomelane structure) were synthesized by a solvent-free method and tested in the total oxidation of CO (TOX), and preferential oxidation of CO in presence of hydrogen (PROX). The influence of Cu in the cryptomelane structure was evaluated by several characterization techniques such as: X-ray fluorescence (XRF), thermogravimetric analysis (TGA), hydrogen temperature programmed reduction (TPR-H2) and X-ray photoelectron spectroscopy (XPS). The Cu-modified manganese oxide material (OMS-Cu) showed very high catalytic activity for CO oxidation in comparison to the bare manganese oxide octahedral molecular sieve (OMS). The improved catalytic activity observed in OMS-Cu catalyst was associated to a high lattice oxygen mobility and availability due to the formation of CuMnO bridges. In addition, under PROX reaction conditions the catalytic activity considerably decreases in the presence of 10% (v/v) CO2 in the feed while the same amount of water provokes an improvement in the CO conversion and O2 selectivity.