Carbon-supported manganese oxide nanoparticles as electrocatalysts for oxygen reduction reaction (orr) in neutral solution

Manganese oxides (MnO _x ) catalysts were chemically deposited onto various high specific surface area carbons. The MnO _x /C electrocatalysts were characterised using a rotating disk electrode and found to be promising as alternative, non-platinised, catalysts for the oxygen reduction reaction (ORR) in neutral pH solution. As such they were considered suitable as cathode materials for microbial fuel cells (MFCs). Metal [Ni, Mg] ion doped MnO _x /C, exhibited greater activity towards the ORR than the un-doped MnO _x /C. Divalent metals favour oxygen bond splitting and thus orientate the ORR mechanism towards the 4-electron reduction, yielding less peroxide as an intermediate.     

Mixed oxides as catalysts for the selective reduction of nitrobenzene

The catalytic behaviour of the PbO-Mn₃O₄ and the Bi₂O₃-MoO₃ systems was investigated in the selective reduction of nitrobenzene to nitrosobenzene. Lead compounds appeared to be good catalysts, and co-catalysts when used with Mn₃O₄. Different from oxidations by di-oxygen, Bi₃O₃ alone is a good catalyst and formation of mixed Bi-Mo-O compounds does not enhance the catalytic activity. It is suggested that the difference between these catalysts in the mentioned reaction is related to the way in which the oxygen vacancy is represented by the oxygen donor.     

Electrocatalytic activity of manganese oxides prepared by thermal decomposition for oxygen reduction

The oxygen reduction reaction (ORR) was studied in KOH electrolyte on manganese oxides supported on Vulcan carbon (Mn_yO_x/C). The oxides were prepared by thermal decomposition of manganese nitrate at different conditions. The oxides were characterized by X-ray diffraction (XRD) and in situ X-ray absorption near edge structure (XANES). The electrochemical studies were conducted using cyclic voltammetry (CV) and steady state polarization measurements carried out with a thin layer rotating ring/disk electrode. XRD results showed that the manganese oxide prepared at 200 °C in air is formed by a major phase of β-MnO₂ and the polarization curves indicated the highest activity for this material. In situ XANES evidenced the occurrence of a redox process involving Mn(II)/Mn(III) and Mn(III)/Mn(IV) in the range of potentials of the CV measurements. The electrocalytic activity was related to the occurrence of a mediation process involving the reduction of Mn(IV) to Mn(III), followed by the electron transfer of Mn(III) to oxygen and by a disproportionation reaction of the HO₂⁻ species in the Mn_yO_x sites. In situ XANES results showed that the Mn(IV) species is MnO₂ and the Mn(III) is most likely MnOOH.     

A novel alkaline air electrode based on a combined use of cobalt hexadecafluoro-phthalocyanine and manganese oxide

Electrocatalytic reduction of O₂ with dual catalysts of cobalt 1, 2, 3, 4, 8, 9, 10, 11, 15, 16, 17, 18, 22, 23, 24, 25-hexadecafluoro-29 H, 31H-phthalocyanine (CoPcF₁₆) and MnOOH was studied in alkaline media with cyclic and rotating ring-disk electrode (RRDE) voltammetry. Cyclic voltammetric results show that CoPcF₁₆ possesses a good catalytic activity for redox-catalyzing an apparent two-electron reduction of O₂ with superoxide (O₂⁻) as an intermediate. The combined use of CoPcF₁₆ with MnOOH which shows a bifunctional catalytic activity toward the sequential disproportionations of the reduction intermediate and product, i.e. O₂⁻ and peroxide (HO₂⁻), eventually enables an apparent four-electron reduction of O₂ to be achieved at a positively-shifted potential in alkaline media. The possibility of utilizing the dual catalysts for the development of practical alkaline air electrodes was further explored by confining the catalysts in active carbon (AC) and carbon black (CB) matrices that are generally used as the substrate for constructing air electrodes. The RRDE voltammetric results suggest that an apparent four-electron reduction of O₂ reduction can be obtained at the as-prepared carbon-based air electrode at a potential close to that at the Pt-based air electrode, and that the as-prepared electrode shows a high tolerance against methanol and glucose crossover.     

Catalytic reduction of NO by propene in the presence of oxygen over mechanically mixed metal oxides and Ce-ZSM-5

The mechanical mixing of Mn₂O₃ or CeO₂ to Ce-ZSM-5 considerably enhanced the rate of the reduction of NO by propene in the low to medium temperature region, although Mn₂O₃ or CeO₂ itself was much less active for this reaction. In contrast, Mn₂O₃ was highly active and CeO₂ was moderately active for the oxidation of NO to NO₂. On the basis of the comparison of the rates of the C₃H₆ + O₂, NO + C₃H₆ + O₂ and NO₂ + C₃H₆ + O₂ reactions over these catalysts, a bifunctional mechanism is proposed, in which Mn₂O₃ and CeO₂ accelerate the oxidation of NO and the subsequent reaction steps between NO₂ and propene proceed on Ce-ZSM-5.     

A comparative study of oxygen storage capacity over Ce0.6Zr0.4O2 mixed oxides investigated by temperature-programmed reduction and dynamic OSC measurements

The effects of redox-ageing on the temperature-programmed reduction and dynamic oxygen storage were investigated on two samples of Ce_0.6Zr_0.4O₂ prepared under different synthesis conditions. It was observed that a high-temperature reduction/mild oxidation redox cycle can generate temperature-programmed reduction (TPR) profiles featuring a reduction peak at a temperature as low as 537 K. However, despite such favourable reduction behaviour, a strong deactivation of the oxygen storage is observed under dynamic conditions, indicating the limitations of the TPR method for investigation of oxygen storage.     

Encapsulation of manganese oxides nanocrystals in electrospun carbon nanofibers as free-standing electrode for supercapacitors

Flexible composites with manganese oxides (MnO_x) nanocrystals encapsulated in electropun carbon nanofibers were successfully fabricated via a simple and practical combination of electrospinning and carbonization process. The as-formed MnO_x/carbon nanofibers composites have a rough surface with MnO_x nanoparticles well embedded in the carbon nanofibers backbones. When used as electrodes for supercapacitor, the resulting MnO_x/carbon nanofiber composites exhibit good electrochemical performance with a specific capacitance of 174.8 F g⁻¹ at 2 mV s⁻¹ in 0.5 M Na₂SO₄ electrolyte, a good rate capability at high current density and long-term cycling stability. It is expected that such freestanding composites could be promising electrodes for high-performance supercapacitors.     

Liquid-phase oxidation of toluene by molecular oxygen over copper manganese oxides

Copper manganese oxides (Cu–Mn oxides) were prepared by coprecipitation method and characterized by several techniques, such as X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and Temperature-programmed reduction (TPR). Catalytic activities of the Cu–Mn oxides were tested by the oxidation of toluene with molecular oxygen in liquid phase and solvent-free conditions. The molar ratio of Cu:Mn in catalyst was optimized to be 1:1 and thus the corresponding crystalline material was designated as Cu_1.5Mn_1.5O₄.     

Multivalent manganese oxides with high electrocatalytic activity for oxygen reduction reaction

A noble-metal-free catalyst based on both Mn₃O₄ and MnO was prepared by using the dielectric barrier discharge technique at moderate temperature. The prepared catalyst shows a higher electrocatalytic activity towards the oxygen reduction reaction than the catalyst prepared by using the traditional calcination process. The enhanced activity could be due to the coexistence of manganese ions with different valences, the higher oxygen adsorption capacity, and the suppressed aggregation of the catalyst nanoparticles at moderate temperature. The present work would open a new way to prepare low-cost and noble-metal-free catalysts at moderate temperature for more efficient electrocatalysis.

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Liquid-phase oxidation of toluene by molecular oxygen over copper manganese oxides

Copper manganese oxides (Cu–Mn oxides) were prepared by coprecipitation method and characterized by several techniques, such as X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and Temperature-programmed reduction (TPR). Catalytic activities of the Cu–Mn oxides were tested by the oxidation of toluene with molecular oxygen in liquid phase and solvent-free conditions. The molar ratio of Cu:Mn in catalyst was optimized to be 1:1 and thus the corresponding crystalline material was designated as Cu_1.5Mn_1.5O₄.     

Nanostructured manganese dioxides: Synthesis and properties as supercapacitor electrode materials

Low-cost layered manganese oxides with the rancieite structural type were prepared by reduction of KMnO₄ or NaMnO₄ in acidic aqueous medium, followed or not by successive proton- and alkali-ion-exchange reactions. Samples were characterized by X-ray diffraction, energy dispersive X-ray analysis, BET surface area measurements, thermal analyses and X-ray photoelectron spectroscopy. As a result of successive exchange steps, compounds with high surface area (reaching 200 m² g⁻¹) can be obtained, and in the case of syntheses made with KMnO_4, the α-MnO₂ phase is formed. Capacitive properties of the synthesized materials were studied using potentiodynamic cycling in K₂SO₄. Correlations between the electrochemical and the physicochemical properties of the samples were investigated. The interesting conclusion is that the morphology and the size of the particles influence directly the capacitance, and that among the samples presenting the best morphology, the compounds derived from K-containing rancieite-type compounds (and containing α-MnO₂) present a better cycleability.     

Evaluation of oxygen storage profile on CeO2–ZrO2 mixed oxides by periodic injections of O2 pulse and their reduction behavior

Oxygen storage profile on CeO₂–ZrO₂ mixed oxide (CZ) has been observed by periodic injections of O₂ pulse. The reduction behavior of oxygen after the O₂ pulse injection was also investigated using Temperature Programmed Reduction (TPR) method. The oxygen storage profiles of the CZ catalyst with κ-CeZrO₄ phase indicate that the solid solution phase facilitates to diffuse oxygen into the bulk, and TPR profiles suggest that oxygen is preferentially stored by the reaction with Ce³⁺ species derived from CeO₂ phase compared with those from the κ-CeZrO₄ phase, especially at low temperatures.     

Synthesis of macroporous ZrO2–Al2O3 mixed oxides with mesoporous walls, using polystyrene spheres as template

Macroporous ZrO₂–Al₂O₃ mixed oxides with mesoporous walls were synthesized. The three-dimensional interconnected macroporous structures, of inorganic zirconia–alumina mixed oxides containing different alumina compositions (25, 50, 100 wt%), were prepared by sol–gel method from inorganic precursors and using polystyrene microspheres with diameters of 685 and 1520 nm as templates. The final porous arrays with controllable pore size in the submicrometer range could be obtained by calcination of the organic template. The structural characteristics are discussed. The physicochemical characterization of the samples was carried out by N₂ physisorption (S_BET), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The shrinkage of pore diameter was approximately 35%, and the wall thickness of inorganic framework varied between 135 and 154 nm. The specific surface areas, of the samples, were between 123 and 287 m²/g, obtaining the largest surface area with the highest alumina contents and the smallest templates.     

A study on capacity fading of lithium-ion battery with manganese spinel positive electrode during cycling

The capacity fading mechanism of lithium-ion cell was studied by disassembling the charge-discharged cells and analyzing their electrodes using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), etc. Cu ion dissolved from current collector of anode and Mn ion dissolved from LiMn₂O₄ spinel (cathode) were all existing in solid electrolyte interface (SEI) layer on carbon anode as Cu₂O and MnO or MnO₂, respectively. These depositions of Cu and Mn oxides did not uniformly deposited on the anode side, and most of them were detected on the carbon surface nearby to the separator side. The SEI layer is hard and about 0.3 μm in thickness. Furthermore, the cycling performance of the cells can be improved by adding 1,2,3-benzotrazole (a corrosion inhibitor of Cu) before assembling the cell, it then coordinates strongly with Cu ions into the electrolyte. From the results, it is obvious that the existing of Cu oxide as well as Mn oxide in the SEI layer, which blocks the normal intercalation of the lithium ions, is one of the factors for the capacity fading of the cells.     

MnOx/C composites as electrode materials II. Reduction of oxygen on bifunctional catalysts based on manganese oxides

MnO_x/C composites prepared by the reduction of KMnO₄ by active carbon black and doped with Ca(II), Mg(II), Ni(II), Bi(III), and Cr(III) ions were tested as catalysts of oxygen reduction in alkaline electrolyte. The polarisation curves were analysed by logarithmic wave analysis. MnO_x doped with the transition metal salts are slightly more active than those with Ca(II) and Mg(II).     

Additive treatment effect of TiO2 as supports for Pt-based electrocatalysts on oxygen reduction reaction activity

In this study, we investigated the additive treatment effect of TiO₂ as alternative support materials to common carbon black for Pt-based electrocatalysts on electrocatalytic activity for oxygen reduction reaction (ORR). The shape of TiO₂ was varied by hydrothermal treatment with various additives, such as urea, thiourea, and hydrofluoric acid. From the results of transmission electron microscopy (TEM) images and ultraviolet-visible spectroscopy (UV-vis) spectra, it was identified that the morphology of hydrofluoric acid (HF)-treated TiO₂ was changed into a round shape having lower aspect ratio than other samples, and its band gap was decreased. Notably, the electronic state of HF-treated TiO₂ support was changed into highly reduced (electron rich) state which led to the increase of ORR activity, compared to other samples treated with different additives or before treatment. The electrocatalytic characteristics changes after treatment with various additives were investigated by using X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), cyclic voltammograms (CV), and rotating disk electrode (RDE) techniques.     

Spinels as cathodes for the electrochemical reduction of O2 and NO

Spinels were synthesised and investigated as electro-catalyst for the electrochemical reduction of oxygen and nitric oxide using cyclic voltammetry and cone shaped electrodes. The following four spinels were investigated; CoFe₂O₄, NiFe₂O₄, CuFe₂O₄ and Co₃O₄. The composition CuFe₂O₄ revealed the largest difference in activity between reduction of oxygen and the reduction of nitric oxide, the activity being highest for the reduction of nitric oxide. The material is probably not stable when polarised cathodically. However it seems that the electrode material can be regenerated upon oxidation. NiFe₂O₄ is also more active for the reduction of nitric oxide than for the reduction of oxygen, whereas the cobalt containing spinels have a higher activity for the reduction of oxygen than for the reduction of nitric oxide.     

Effects of poly-1,5-diaminoanthraquinone morphology on oxygen reduction in acidic solution

The poly-1,5-diaminoanthraquinone (P15DAAQ) modified Pt electrodes show electrocatalytic activity for oxygen reduction reaction (ORR) with oxygen reduction peak at about 0.39 V in 0.1 M H₂SO₄. The P15DAAQ with different thickness has different morphology. The effects of morphologies on the electrocatalytic behaviors of P15DAAQ for oxygen reduction reaction are investigated using cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) measurements. We propose two different O₂-transport processes on electrodes modified with thin P15DAAQ and thick P15DAAQ. Together with the quantitative analysis with O₂-transport dynamics, electron-transfer resistance, and catalytic reaction rate during ORR, thin P15DAAQ electrode performs better electrocatalysis for ORR, although thick P15DAAQ provides higher real surface area and more reactive sites which is beneficial for ORR within a short time.     

The influence of oxygen additions to hydrogen in their electrode reactions at Pt/Nafion interface

The influence of oxygen gas added to hydrogen in their electrode reactions at the Pt/Nafion interface was investigated using ac impedance method. The electrochemical cell was arranged in either electrolytic (hydrogen enrichment) or galvanic (fuel cell) mode. The impedance spectra of the electrode reaction of a H₂/O₂ gas mixture were taken in each mode as a function of the gas composition, electrode surface roughness and the cell potential. The spectrum taken for the anodic reaction of electrolytic arrangement confirmed the anodic oxygen reduction reaction (AOR, the local consumption of hydrogen by the added oxygen) by showing an independent arc distinguishable from that for hydrogen oxidation. But the independent arc was not revealed in the spectrum taken on a smooth (low surface area) electrode or on a Pt/C anode of the galvanic cell. At any cell current density, the electrolytic mode showed its anodic overpotential much higher (nearly three times higher at the current density of 100 mA cm⁻²) than the potential registered in galvanic mode implying that the oxygen gas in the mixture engages more active and independent AOR at the anode of the electrolytic cell.     

Study of IrxRu1−xO2 oxides as anodic electrocatalysts for solid polymer electrolyte water electrolysis

Electrocatalysts of the general formula Ir_xRu_1−xO₂ were prepared using Adams’ fusion method. The crystallite characterization was examined via XRD, and the electrochemical properties were examined via cyclic voltammetry (CV) in, linear sweep voltammetry (LSV) and chronopotentiometry measurements in 0.5 M H₂SO₄. The electrocatalysts were applied to a membrane electrode assembly (MEA) and studied in situ in an electrolysis cell through electrochemical impedance spectroscopy (EIS) and stationary current density–potential relations were investigated. The Ir_xRu_1−xO₂ (x = 0.2, 0.4, 0.6) compounds were found to be more active than pure IrO₂ and more stable than pure RuO₂. The most active electrocatalyst obtained had a composition of Ir_0.2Ru_0.8O₂. With an Ir_0.2Ru_0.8O₂ anode, a 28.4% Pt/C cathode and the total noble metal loading of 1.7 mg cm⁻², the potential of water electrolysis was 1.622 V at 1 A cm⁻² and 80 °C.