Iron Manganese Oxide Research Articles

Manganese Oxide Nanoparticle Synthesis by Thermal Decomposition of Manganese(II) Acetylacetonate.

For biomedical applications, metal oxide nanoparticles such as iron oxide and manganese oxide (MnO), have been used as biosensors and contrast agents in magnetic resonance imaging (MRI). While iron oxide nanoparticles provide constant negative contrast on MRI over typical experimental timeframes, MnO generates switchable positive contrast on MRI through dissolution of MnO to Mn2+ at low pH within cell endosomes to 'turn ON' MRI contrast. This protocol describes a one-pot synthesis of MnO nanoparticles formed by thermal decomposition of manganese(II) acetylacetonate in oleylamine and dibenzyl ether. Although running the synthesis of MnO nanoparticles is simple, the initial experimental setup can be difficult to reproduce if detailed instructions are not provided. Thus, the glassware and tubing assembly is first thoroughly described to allow other investigators to easily reproduce the setup. The synthesis method incorporates a temperature controller to achieve automated and precise manipulation of the desired temperature profile, which will impact resulting nanoparticle size and chemistry. The thermal decomposition protocol can be readily adapted to generate other metal oxide nanoparticles (e.g., iron oxide) and to include alternative organic solvents and stabilizers (e.g., oleic acid). In addition, the ratio of organic solvent to stabilizer can be changed to further impact nanoparticle properties, which is shown herein. Synthesized MnO nanoparticles are characterized for morphology, size, bulk composition, and surface composition through transmission electron microscopy, X-ray diffraction, and Fourier-transform infrared spectroscopy, respectively. The MnO nanoparticles synthesized by this method will be hydrophobic and must be further manipulated through ligand exchange, polymeric encapsulation, or lipid capping to incorporate hydrophilic groups for interaction with biological fluids and tissues.

ПЕТРОЛОГИЧЕСКИЕ АСПЕКТЫ СОВРЕМЕННОГО ОСТРОВОДУЖНОГО ВУЛКАНИЗМА НА ПРИМЕРЕ ЛАВ ВУЛКАНА БАТУР (О. БАЛИ, ИНДОНЕЗИЯ)

Актуальность изучения современного вулканизма заключается в понимании сложных процессов дифференциации, ассимиляции и смешения магм, зафиксированных в минералого-петрогеохимических и текстурно-структурных особенностях строения современных вулканитов. С другой стороны, с базитовым вулканизмом повышенной щелочности современных островодужных систем зачастую связано экономически значимое оруденение цветных и благородных металлов. Целью настоящей работы является изучение и интерпретация минералогических и петрохимических особенностей лав действующего вулкана Батур (о. Бали, Индонезия). Объектами исследования являются вулканиты, отобранные из лавовых потоков 1964 г. извержения действующего вулкана Батур. Породы современного островодужного магматизма и встречающиеся в них минералы изучены методами оптической и электронной микроскопии с применение энергодисперсионной спектроскопии. Химический состав основных петрогенных компонентов получен с помощью силикатного анализа, микроэлементный состав – с помощью масс-спектрометрии с индуктивно-связанной плазмой. В результате проведенных исследований было установлено, что все образцы изученных вулканитов имеют порфировую структуру и содержат вкрапленники плагиоклазов (17…23 об. %), оливинов (3 об. %) и пироксенов (2 об. %), расположенных в мелкозернистой массе мезостазиса, содержащего плагиоклазы, титаномагнетит, пироксены и вулканическое стекло. Для порфировых вкрапленников плагиоклаза характерна «ячеистая» структура, которая проявлена в виде ориентированных расплавных включений (~0,05 мм), содержащих первичный силикатный и сульфидный расплавы. По петрохимическим показателям породы относятся к высокоглиноземистым базальтам (Al2O3≥18,0). Сложный характер зональности порфировых вкрапленников плагиоклазов согласуется с быстрым охлаждением исходных расплавов, когда выделяющийся плагиоклаз не может прийти в равновесие с оставшейся еще жидкой магмой, по средневзвешенному составу он относится к лабрадор-битовниту. Оливин сочетает несколько вариантов перераспределения железа в своем составе. Отмечается прямая зональность с накоплением железа в краевых частях зерен, специфичные «структуры распада» с фазой «окси-оливина» и однородные кристаллы оливина с магнезиальностью Mg#(Fo) [=Mg/(Fe2++Mg)*100 %] ~ 66. Повышенная железистость оливина и примесь в нем MnO служат свидетельством того, что изучаемые породы являются производными базальтовых растворов с щелочным уклоном. По представлению авторов механизм формирования изучаемой серии андезибазальтов имеет следующий сценарий формирования. В период интенсивного вулканизма 1964 г. извержения вулкана Батур магматическая камера периодически обогащалась высокомагнезиальным (оливиновым) базальтом через определенные промежутки времени. Далее этот расплав поднимался в верхнюю часть камеры с последующим фракционированием оливина и клинопироксена при кристаллизации плагиоклаза и накоплением их в верхней части магматической камеры. В период поступления новой порции родоначального расплава существующая смесь не успевала прийти в равновесие с поступившей порцией расплава, изливаясь на поверхность и формируя лавы высокоглиноземистого базальта. Включения глобулей ковеллина и халькопирита в расплавных включениях порфировых вкрапленников плагиоклаза свидетельствуют о существовании высокофракционированного сульфидного расплава на более глубинном уровне.

Selective laser melting of thermal pre-treated metal oxide doped aluminum oxide granules

The influence of powder bed density on the final density and microstructure of aluminum oxide parts manufactured by direct selective laser melting has been studied. Iron oxide and manganese oxide nanoparticles were used to improve laser absorption by over eighty percent. To achieve such values, flowable doped alumina granules were prepared by spray drying. Thermal treatment of the granules at 1600 ​°C and consecutive mixing with coarse alumina allowed improvement of the tapped powder densities, reaching a maximum value of 56.4% of the theoretical density. This led to laser processed parts with densities up to 98.6% measured by tomographic microscopy. Measurements with an integrating sphere and an UV-VIS-NIR spectrophotometer employing Kubelka-Munk theory show the decrease of absorptance caused by thermal pre-treatment. 3D mapping by X-ray μ-beam fluorescence contrast tomography and high resolution synchrotron powder diffraction provide information about the variation of dopant distribution and composition within the granules.

Elucidating the Redox Mechanism of Battery Cathode Materials Made from Earth-Abundant Elements

Sodium ion batteries (NIBs) are a promising technology for grid storage due to the natural abundance, low cost, and suitable redox potential (-2.71 V vs. SHE) of Na. Exploration of NIB cathode materials that include abundant and inexpensive elements, has led to the investigation of sodium iron manganese oxides. However, these materials suffer from low electrochemical reversibility during cycling, and their reaction mechanisms are unresolved, presenting an obstacle for their further optimization. In literature, three different reaction mechanisms have been reported for sodium iron manganese cathodes including 1) Fe 2+/3+, 2) Fe 3+/4+, and 3) oxygen redox. Additionally, the reported redox measurements are ex situ, which leaves uncertainty if the oxidation state of these materials changes between electrochemical cycling and redox measurement. Herein, for the first time we combine operando x-ray absorption spectroscopy (XAS) and computation method to report the redox mechanisms of P2-Na2/3FexMn1-xO2 (x=1/4, 1/3, 1/2). The redox mechanism and capacity retention of these cathode materials is found to be dependent on the Fe content. When x=1/4, 1/3 the redox mechanism is Fe 2+/3+ and Mn 3+/4+, and the material exhibits high capacity retention. When x=1/2, the redox mechanism is Fe 3+/4+ and Mn 3+/ 4+, and the material shows poor capacity retention. These results indicate that improved reversibility of sodium iron manganese oxides can be obtained by limiting the amount of the Fe 3+/4+ redox through reducing the Fe content. These results will contribute to the rational design of long lifetime cathode materials made from earth-abundant elements. Figure 1

Iron manganese Oxide Modified Multi-walled Carbon Nanotube as Efficient Adsorbent for Removal of Organic Dyes: Performance, Kinetics and Mechanism Studies

In this study, iron manganese composite oxide (FeMnOx) were integrated with multi-walled carbon nanotube (MWCNTs) to synthesize an effective adsorbent for acid orange (AO) and basic violet (BV), two widely used organic dyestuffs belonging to anionic and cationic, respectively. Among all the composites evaluated, the FeMnOx-MWCNTs with Fe–Mn ratios of 1:1 and load ratio of 45% achieved superior adsorption capacity than others, and was selected as the best adsorbent. The Characterization data indicated that the physicochemical properties and structural characteristics of FeMnOx-MWCNTs have changed obviously after modification, with larger specific surface area and richer groups. The maximum adsorption capacity of AO (403.23 mg/g) and BV (165.29 mg/g) could be attained under optimal conditions: 300 min, 0.05 g adsorbent dosage, 100 mg/L dyes and pH 2.0 (for AO) or pH 10.0 (for BV). Moreover, the adsorption kinetics of the two dyes were both well fitted the pseudo-second-order kinetics model, and the adsorption isotherms agreed well with the Freundlich model for AO while Langmuir model for BV. Moreover, the FeMnOx-MWCNTs with optimal FeMnOx content showed relatively higher final adsorption amount and faster adsorption rate, thus better fitting with the kinetics. A mechanism was proposed that physical adsorption is the dominator during the process. The results indicated that the introduction of FeMnOx into the MWCNTs significantly enhanced the dye removal efficiency, providing a promising method for the improvement of carbon nanometer adsorbents. The selected graph can not only express the synthetic methods of FeMnOx-MWCNTs by chemical coprecipitation, but also presented the adsorption process and mechanism of AO and BV, which showed the purposes of this work simultaneously.

Reduction of iron–manganese oxide particles in a lab-scale packed-bed reactor for thermochemical energy storage

A packed-bed reactor heated with an IR furnace is designed and fabricated for high-temperature thermochemical energy storage (TCES) studies using 0.5–1.0 mm particles composed of iron–manganese oxide with a molar ratio of Fe/Mn 2:1 (Fe67) as the storage material. An axisymmetric, two-dimensional, transient model accounting for heat transfer (conduction, convection, and radiation), mass transfer, and thermochemical reaction kinetics is developed to describe the reduction reaction of Fe67 in the packed-bed reactor. The model and experiments are in good agreement, indicating that the model is useful for providing a fundamental understanding of the process.

Leaching behaviors and speciation of cadmium from river sediment dewatered using contrasting conditioning

Chemical conditioning is an effective strategy for improved river sediment dewatering affecting both the dewatering efficiency and subsequent resource utilization of the dewatered cake. Two types of conditioning agents, polyaluminium chloride (PAC)/cationic polyacrylamide (PAM) (coagulation precipitation conditioning agent, referred to as P–P conditioning) and ferrous activated sodium persulfate (advanced oxidation conditioning agent, referred to as F–S conditioning) were examined. With increasing leach liquid to solid (L/S) ratio the concentration of Cd for the real time leachates from the dewatered cakes decreased, but the leaching ratio of Cd in both P–P and F–S dewatered cakes increased. With the same L/S, the leaching ratio was reduced for both types of conditioning, as compared to no conditioning, with the leaching ratio being least with F–S conditioning. The leaching ratio of Cd in the dewatered cake with L/S of 100 L kg−1 was reduced from 21.3% of the total Cd present for the un-conditioned sediment to 12.5% upon P–P conditioning and 11.6% upon F–S conditioning. Furthermore, the different conditioning methods affected the Cd speciation in the dewatered cakes reducing the easy-to-leach speciation of exchangeable and carbonate-bound Cd species and increasing the potential-to-leach speciation of iron-manganese oxide and organically bound Cd species and also the difficult-to-leach species. Risk assessment indicates that the risk due to Cd leaching from the dewatered cakes at L/S of 100 L kg−1 was reduced from high risk to medium risk after P–P and F–S conditioning with reduced bioavailability.

A Moving Bed Reactor for Thermochemical Energy Storage Based on Metal Oxides

High-temperature thermal energy storage enables concentrated solar power plants to provide base load. Thermochemical energy storage is based on reversible gas–solid reactions and brings along the advantage of potential loss-free energy storage in the form of separated reaction products and possible high energy densities. The redox reaction of metal oxides is able to store thermal energy at elevated temperatures with air providing the gaseous reaction partner. However, due to the high temperature level, it is crucial to extract both the inherent sensible and thermochemical energies of the metal-oxide particles for enhanced system efficiency. So far, experimental research in the field of thermochemical energy storage focused mainly on solar receivers for continuously charging metal oxides. A continuously operated system of energy storage and solar tower decouples the storage capacity from generated power with metal-oxide particles applied as heat transfer medium and energy storage material. Hence, a heat exchanger based on a countercurrent moving bed concept was developed in a kW -scale. The reactor addresses the combined utilization of the reaction enthalpy of the oxidation and the extraction of thermal energy of a manganese–iron-oxide particle flow. A stationary temperature profile of the bulk was achieved with two distinct temperature sections. The oxidation induced a nearly isothermal section with an overall stable off-gas temperature. The oxidation and heat extraction from the manganese–iron oxide resulted in a total energy density of 569 kJ/kg with a thermochemical share of 21.1%.

Compositional study of slips and paintings in San José and Santa María pottery (Yocavil valley, Northwest Argentina): an approach by non-destructive and complementary techniques

From a multidisciplinary perspective, we propose a first comparative study on the pigments of archeological pottery San Jose and Santa Maria of Yocavil valley, Province of Catamarca, Northwest Argentina (ca. S XI–XVI AD). Both stylistic sets share a color palette in which the black, red and cream tones are structured. Considering that we are handling objects of cultural heritage, an analysis strategy was developed that combines non-destructive techniques (XRFp and XRD) and minimally destructive sampling methods with other techniques (EDS and micro-Raman spectroscopy) for the characterization of elements and crystalline phases in pigments. The results show not only some shared features—as hematite in red paints and cream slips resulting from a low-iron clay base preparation—but also some diversity in the composition of the pigments—as presence of rutile in some red paints and pyrolusite or series of solid solutions in the system manganese–iron oxides in black pigments.

Influences of stormwater concentration infiltration on the heavy metal contents of soil in rain gardens.

Many studies have been conducted on water volume reduction and pollutants purification of rain gardens. However, the pollutant variations in rain gardens are rarely explored. Seven soil sampling events were conducted from April 2017 to February 2019 to investigate the influences of stormwater concentration infiltration on soil heavy metals in two rain gardens. The results show that: (1) Cu, Zn, and Cd contents in rain garden soil are greater than those of the control soil. They vary with seasons and are trapped in the top layer of 0-30 cm; (2) Cu, Zn, and Pb exist as iron-manganese oxide combined form (S3), organic bound (S4) and residual forms (S5). However, Cd exists in exchangeable (S1) and carbonate bound (S2) forms, whereas Cr is in the S2, S3, and S4 forms. (3) According to the Soil Environmental Quality Standard in China, rain gardens, running for 8-9 years, are relatively clean and, within level II. However, compared with the background content of Shaanxi Province and the world, they are moderately or even heavily polluted by Cd and Zn and slightly polluted by Cu. It indicates that rain gardens have the risk of heavy metal pollution from stormwater concentration infiltration.

Deep-ocean polymetallic nodules as a resource for critical materials

Deep-ocean polymetallic nodules form on or just below the vast, sediment-covered, abyssal plains of the global ocean. Polymetallic nodules primarily consist of precipitated iron oxyhydroxides and manganese oxides, onto which metals such as nickel, cobalt, copper, titanium and rare earth elements sorb. The enormous tonnage of nodules on the seabed, and the immense quantities of critical metals that they contain, have made them a target for future mining operations. Mining of polymetallic nodules has been spurred by the need for critical metals to support growing populations, urbanization, high-technology applications and the development of a green-energy economy. Nevertheless, an improved understanding of the affected ecosystems and their connectivity, as well as the environmental impacts of deep-ocean mining, is required before operations begin. Opportunities exist, however, to ensure that this new industry applies adaptive management to continually refine operations with the goal of environmental protection and invests in the development of green technologies for extractive metallurgy and mining. In this Review, we explore the chemical processes that control the concentration of critical metals in deep-ocean polymetallic nodules, discuss the mining and metallurgical techniques required, and highlight the opportunities and potential risks that are presented by this new industry. Deep-ocean polymetallic nodules contain an enormous tonnage of critical metals, which are vital natural resources for green-energy technologies and vehicles. This Review highlights the formation processes of these nodules and outlines the advantages and disadvantages for this developing industry moving forward.

Numerical Investigations of a Counter-Current Moving Bed Reactor for Thermochemical Energy Storage at High Temperatures

High temperature storage is a key factor for compensating the fluctuating energy supply of solar thermal power plants, and thus enables renewable base load power. In thermochemical energy storage, the thermal energy is stored as the reaction enthalpy of a chemically reversible gas-solid reaction. Metal oxides are suitable candidates for thermochemical energy storage for solar thermal power plants, due to their high reaction temperatures and use of oxygen as a gaseous reaction partner. However, it is crucial to extract both sensible and thermochemical energy at these elevated temperatures to boost the overall system efficiency. Therefore, this study focuses on the combined extraction of thermochemical and sensible energy from a metal oxide and its effects on thermal power and energy density during discharging. A counter-current moving bed, based on manganese-iron-oxide, was investigated with a transient, one-dimensional model using the finite element method. A nearly isothermal temperature distribution along the bed height was formed, as long as the gas flow did not exceed a tipping point. A maximal energy density of 933 kJ/kg was achieved, when ( Mn , Fe ) 3 O 4 was oxidized and cooled from 1050 ° C to 300 ° C . However, reaction kinetics can limit the thermal power and energy density. To avoid this drawback, a moving bed reactor based on the investigated manganese-iron oxide should combine direct and indirect heat transfer to overcome kinetic limitations.

Cadmium Fractions in Two Calcareous Soils Affected by Incubation Time, Zinc and Moisture Regime

ABSTRACTThis study has been performed to determine the impact of zinc (Zn) on cadmium (Cd) fractions in two types of calcareous soils of the Fars province in Iran at the waterlogged condition. High-reduced conditions in soil reduce Cd solubility because of the development of Cd Sulfide. Cadmium and Zn have similar physical-chemical properties; they are maintained in soil’s exchangeable sites as a divalent ion. Since such few studies have been conducted in calcareous soils of Iran, we examined the effect of Zn on Cd fractions due to their similar characteristics. Treatments were three levels of Cd (0, 30 and 60 mg kg−1 of soil as CdSO4.8H2O), three levels of Zn (0, 5 and 10 mg kg−1 of soil as ZnSO4 .7 H2O), three levels of incubation times (2, 4 and 8 weeks), and two types of soils (clay and sandy clay loam). The randomized completed block design (RCBD) was used for this experiment. After 2, 4, and 8 weeks of treatments, the sequential extraction technique was used to determine the Cd concentration in (WsEx), (Fe-MnOx), (Car), (OM) and (Res.) fractions. The data indicated that based on the soil texture 64% to 70% of the Cd was extracted in the WsEx and Fe-MnOx fractions. The waterlogged condition decreased the Cd concentration in the WsEx fraction and increased the Fe-MnOx, Car and OM fractions. Cd concentration in Res. fraction was higher in the clay soil while WsEx, Car and OM fractions were higher in the sandy clay loam soil. Adding Zn as zinc sulfate reduced the concentration of Cd in Fe-MnOx, Car and OM fractions but had no significant effect on the WsEx and Res. fractions. It can be concluded that the change in the Cd concentration in its chemical fractions caused by Zn addition might be from the competitive transport and adsorption interaction between these two ions. The presence of Zn reduces the concentration of Cd in those forms that are easily released into the soil solution from where they can be absorbed by plants.Abbreviation: WsEx: Water Soluble and Exchangeable; Car: Carbonate; Fe-MnOx: Iron-Manganese Oxides; OM: Organic Matter; Res: Residual

Interaction mechanism of dissolved Cr(VI) and manganite in the presence of goethite coating

Hexavalent chromium has aroused a series of environmental concerns due to its high mobility and toxicity. Iron and manganese oxides usually coexist in the environments and influence the speciation and geochemical cycling of chromium. However, the interaction mechanism of iron-manganese oxides with dissolved Cr(VI) remains largely unknown. In this work, the interaction processes of dissolved Cr(VI) and manganite in the presence of goethite coating were investigated, and the effects of pH (2.0–9.0) and iron oxide content were also studied. Manganite-goethite composites were formed with uniform micromorphologies in the system of manganite and Fe(II). In the reaction system of single manganite and Cr(VI), manganite could only adsorb but not reduce Cr(VI), with the adsorption amount decreasing at higher pHs. In the reaction system of manganite-goethite composites and Cr(VI), adsorbed Cr(VI) was reduced to Cr(III) by Fe(II) on composites surface. The generated Cr(III) was then retained as Cr(OH)3 on the mineral surface. Goethite coating suppressed the re-oxidation of newly formed Cr(III) by manganite. The amounts of adsorbed Cr(VI) and generated Cr(III) increased with increasing iron oxide content, and increased first and then decreased with increasing pH. The Cr(III) formation and Cr(VI) adsorption amount reached the maximum at pH 5.0–6.0. The present work highlights the transformation and retention of Cr(VI) by iron-manganese oxides and provides potential implications for the use of such oxides in the remediation of Cr(VI) polluted waters and soils.

A particle-scale reduction model of copper iron manganese oxide with CO for chemical looping combustion

Chemical looping combustion is a promising power generation technology that produces sequestration-ready CO2 and heat/power from the combustion of fossil fuels with oxygen provided by an oxygen carrier, or metal oxide, rather than air. Successful implementation of chemical looping combustion depends highly on the choice of oxygen carrier and the development of reaction rate parameters for process design and scale-up of the multi-reactor system. In this work, the reaction profile of a promising trimetallic oxygen carrier, copper iron manganese oxide with CO, a component of coal-derived synthesis gas was characterized using differential scanning calorimetry/thermogravimetric analysis and in-situ X-Ray diffraction. A unique phase formation with reactivities different from that with single metal components and phase changes during the reaction with CO were identified. Three major reactions were identified from the phase changes to use for reaction modelling. A particle-scale reaction model was selected which best described the experimental thermogravimetric analysis data to determine the valuable intrinsic reaction values for reactor design and scale-up. A particle-scale reaction model based on nucleation and growth and 1-D phase boundary behavior exhibited the most accurate correlation with the experimental data and provided intrinsic rate constants which were validated with the conventional mass transport analysis.

Optimalisasi Penggunaan Gipsum dan Batu Kapur pada Pembuatan Semen Putih

White Portland cement is a fine, gray or white powder, which consists of a mixture of hydraulic cement materials consisting mainly of calcium silicate, aluminate, and aluminoferrite. The manufacture of white portland cement requires special raw material requirements and manufacturing processes, such as raw materials containing very low iron oxides and manganese oxides. Various tests were carried out to obtain white cement of good quality and meet Indonesian National Standards or Standar Nasional Indonesia (SNI). The tests included raw material testing (Lost on Ignition test, LOI) on various compositions and LOI test, % SO 3 test, and color test on white Portland cement. The results showed that the greater the % SO 3 , the strength of white Portland cement will decrease and the greater the LOI, the white Portland cement will be easily damaged. So from the results of the study obtained a good raw material composition for white Portland cement is 89% white slag, 4% limestone, and 7% gypsum. All test results meet Indonesian National Standards or SNI.

Novel iron manganese oxide modified mulberry biochar for the adsorption of phosphorus in aqueous solution

Novel iron manganese oxide modified mulberry biochar for the adsorption of phosphorus in aqueous solution

Efficient and stable degradation of chlorobenzene over a porous iron–manganese oxide supported ruthenium catalyst

Oxalate pyrolysis and subsequent wet impregnation are facile and effective for loading Ru nanoparticles in the mesopores of Fe–Mn oxides, which leads to a dramatic enhancement in their catalytic reactivity for chlorobenzene combustion.

The colors of San José pottery from Yocavil valley, Argentine Northwest. Strategy for the characterization of archaeological pigments using non-destructive techniques

Archaeological objects require the design of non-destructive strategies for their study since they are part of the cultural heritage. In this work, we present a characterization strategy that minimizes the risk of damage on the archaeological materials investigated, combining portable X-ray fluorescence spectroscopy to perform elemental analysis and X-ray diffraction to detect crystalline compounds. To confirm the obtained results, we used scanning electron microscopy coupled with an energy-dispersive X-ray spectrometer to perform elemental determination and micro-Raman spectroscopy to study crystalline and amorphous compounds. Using this strategy, we study the composition of paints and slips of different colors –cream, black and red– used by the old potters of the Yocavil valley (Catamarca Province, Argentine) to produce “San José” ceramics (ca. S XI-XIII AD). The results indicate that there are certain regularities –as hematite in red paints– but also some variability in the composition of the pigments of different fragments –as series of solid solution in the system manganese-iron oxides in black pigments–. In addition, post depositional compounds as gypsum and calcite were found in some cases.

Prussian blue analogue-derived Mn–Fe oxide nanocubes with controllable crystal structure and crystallinity as highly efficient OER electrocatalysts

A series of ternary manganese iron oxides with different crystal structures, oxidation states and crystallinities were successfully fabricated by modulating the calcination conditions of Mn–Fe Prussian blue analogue (PBA) precursor (Mn3 [Fe(CN)6]2·nH2O). The obtained Mn–Fe oxides retained the nanocubic morphology of the PBA precursor, and a mesoporous structure was acquired as a result of gas molecule release during the pyrolysis process. Electrochemical oxygen evolution reaction (OER) activity of the as-prepared catalysts was tested, and among the bimetallic oxides, the catalyst that had a crystal structure similar to cubic bixbyite Mn1.2Fe0.8O3 (space group: Ia-3) with low crystallinity exhibited the most advanced OER activity. An overpotential of only 245 mV was required to achieve a current density of 10 mA cm−2, and the Tafel slope value was only 38 mV dec−1. The excellent OER activity is likely due to the hollow porous morphology of the samples, the synergistic effect of Mn and Fe, the defect-rich low crystallinity of the catalyst, and the cubic Mn1.2Fe0.8O3 structure (space group: Ia-3), which has an intrinsic activity superior to that of spinel Mn1.8Fe1.2O4.