Amorphous MnO2 Research Articles

Amorphous MnO2 Coated 3D Ni Nanomesh as a Thin-film Hybrid Cathode Material under O2 Atmosphere

In this study, we apply the economically feasible liquid ALD process to deposit a 2 nm amorphous MnO2 thin-film on our high surface-area 3D Ni nanomesh substrate, which shows a high surface to footprint area ratio (30 cm2:1 cm2 for 1 μm thick Ni nanomesh), and demonstrate its preliminary electrochemical activity as a cathode under N2 and O2. The excellent footprint charge density and faradaic efficiency can be attributed to the catalytic oxygen reduction reaction, followed by Li-intercalation thus forming a hybrid combination of Li-ion and Li-O2 activity, alleviating the detrimental deactivation process of pristine MnO2.

Effects of K and Mn promoters over Fe2O3 on Fischer–Tropsch synthesis

Abstract Structural and compositional design of core-shell structure is an effective strategy towards enhanced catalysis. Herein, amorphous MnO2 nanosheets and K+-intercalated layered MnO2 nanosheets are controllably assembled over Fe2O3 spindles, in which the MnO2 nanosheets are perpendicularly anchored to the surface of Fe2O3. Such a core shell structure contributes to a high specific surface area and abundant pore channels on the surface of catalysts. In addition, the existence of K+ provides large numbers of basic sites and restrains the formation of unpleasant (Fe1−xMnx)3O4. Benefiting from the merits in structure and composition, CO adsorption is enhanced and remaining time of intermediates is prolonged on the surfaces of catalysts during the Fischer–Tropsch synthesis (FTS), facilitating to the formation of active iron carbides and C–C coupling reactions. Resultantly, the Fe2O3@ K+-MnO2 shows both a high CO conversion of 82.3% and a high C5+ selectivity of 73.1%. The present study provides structural and compositional rationales on design high-performance catalysts towards FTS.

High-temperature dielectric properties and pyrolysis reduction characteristics of different biomass-pyrolusite mixtures in microwave field

Exploring the dielectric properties of mineral-biomass mixtures is fundamental to the coupled application with biomass pyrolysis and microwave technology to mineral reduction. In this work, the microwave dielectric properties of five pyrolusite-biomass mixtures were measured by resonant cavity perturbation technique and the pyrolysis reduction characteristics were systematically investigated, including poplar, pine, ageratina adenophora, rapeseed shell and walnut shell. Results indicated that the dielectric properties commonalities of five mixtures with temperature represented by increasing firstly, dropping intensely and finally rising slightly, with excellent responsiveness to microwaves; which the change trend was mainly attributed to the crystal transformation of amorphous MnO2 and pyrolusite reduction reactions by biomass pyrolysis. Meanwhile, the heating characteristics successfully matched the dielectric properties of the mixtures, and the pyrolusite reduction process by biomass can be divided into two stages: biomass pyrolysis and pyrolusite reduction. The work highlights the universal feasibility of the novel coupled method for mineral reduction.

Synergistic Control of Structural Disorder and Surface Bonding Nature to Optimize the Functionality of Manganese Oxide as an Electrocatalyst and a Cathode for Li-O2 Batteries.

An efficient way to improve the electrocatalyst and Li-O2 battery performances of metal oxide is developed by an exquisite synergistic control over structural disorder and surface bonding nature. The effects of amorphous nature and surface chemical environment on the functionalities of metal oxide are systematically investigated with well-crystalline and amorphous MnO2 nanocrystals with/without surface anchoring of highly oxidized iodate clusters. The amorphous MnO2 nanocrystal containing anchored iodate clusters shows much better performance as an oxygen evolution electrocatalyst and cathode catalyst for Li-O2 batteries than both iodate-free amorphous and well-crystalline homologues, underscoring the remarkable advantage of simultaneous enhancement of structural disorder and surface electron density. In situ X-ray absorption spectroscopic analysis demonstrates the promoted formation of double (MnO) bond, a critical step of oxygen evolution reaction, upon amorphization caused by the poor orbital overlap inside highly disordered crystallites. The beneficial effects of iodate anchoring and amorphization on electrocatalyst functionality are attributable to the alteration of surface bonding character, stabilization of Jahn-Teller active Mn3+ species, and enhanced charge transfer of interfaces. The present study underscores that fine-tuning of structural disorder and surface bonding nature provides an effective methodology to explore efficient metal oxide-based electrocatalysts.

Amorphous MnO₂ on Carbon Substrate with Ordered Submicron Pore Array Structure for High Performance Supercapacitor Electrode.

MnO₂/C composite with ordered submicron pore array structure was obtained by using Atrophaneura horishana butterfly wings as template and carbon source for high performance supercapacitor electrode. Micro/nanostructured carbon substrate was firstly constructed by calcining the wing specimen under nitrogen atmosphere. Then, amorphous MnO₂ was formed on the structure via surface solution reaction. The as-prepared composite shows a high specific capacitance of 1342 F g-1 at 1 A g-1 with good rate capability and stability. The excellent charge-discharge performance should be attributed to the coupling effect of pseudocapacitive amorphous MnO₂ and robust porous carbon. The pore array structured substrate not only provides high specific surface area for sufficient active sites, but also exhibits good connectivity for efficient charge and mass transport.

Activity of manganese oxides supported on halloysite towards the thermal catalytic oxidation of formaldehyde: Constraint from the manganese precursor

Mn-based catalysts have been widely studied for the oxidation of volatile organic compounds (VOCs) and appear to be the most active catalysts among the transition metal oxides. In this study, manganese oxides are supported on halloysite, a nanoscale and porous clay mineral, by an impregnation method using manganese nitrate, manganese acetate, and potassium permanganate as precursors. The nitrate results in scattered 35–249 nm agglomerates of pyrolusite (β-MnO2) on the external surface of the halloysite nanotubes; the acetate produces hausmannite (Mn3O4) particles with sizes of 15–40 nm dispersed on the external surface, while the permanganate creates nanoparticles and agglomerates of amorphous K-containing MnO2 on the external surface as well as in the lumen. These differences in structure and distribution are related to the oxidizability of the precursor, surface charge and the cylindrical lumen of halloysite, and the interaction between manganese ions (Mn2+ or MnO4−) and the surface groups of halloysite (SiOSi or AlOH). The obtained samples were tested for the thermal catalytic oxidation of formaldehyde. The halloysite-supported manganese oxide with the permanganate precursor exhibits the best activity and achieves 90% of CO2 generation at 149 °C. The activity of formaldehyde oxidation is positively correlated to the reducibility, which depends on the phase, oxidation state, and dispersion of manganese oxides. This study illuminates the constraint of the manganese precursor on halloysite-supported manganese oxides for the thermal catalytic oxidation of formaldehyde and will be beneficial for the development of transition metal oxide-based catalysts in the abatement of VOCs.

Amorphous Manganese Dioxide Coated Polydopamine Nanoparticles for Acid-Sensitive Magnetic Resonance Imaging-Guided Tumor Photothermal Therapy.

Photothermal therapy (PTT) is a powerful technique for tumor ablation. However, there is a problem that PTT cannot accurately locate the tumor site, so it is easy to cause heat damage to the surrounding normal tissues. In this study, PEGylated amorphous manganese dioxide (MnO₂) coated polydopamine (PDA) core-shell nanoparticles (PDA@MnO₂-PEG) with regular morphology and uniform dimensions were prepared for acid-sensitive magnetic resonance imaging-guided tumor photothermal therapy. The results showed that amorphous MnO₂ shell provided markedly acid-sensitive T1-weighted magnetic resonance imaging (MRI). The relaxation rate (r1 value) of PDA@MnO₂-PEG in pH=7.4 was measured to be 0.310 mM-1·S-1, while that in pH=6 became 4.364 mM-1·S-1. In mice tumor models, MRI of tumors exhibited dramatically whitening effects compared with the signal before injection. Besides, the in vivo experiments revealed that the tumors in PTT group with PDA@MnO₂-PEG injection and NIR laser irradiation were almost eliminated within 14 days, indicating the effective photothermal therapy of tumor generated by the PDA core. In conclusion, we successfully synthesized amorphous MnO₂ coated PDA core-shell nanoparticles with the function of acid-sensitive magnetic resonance imaging guided photothermal therapy, which provide a new approach for improving the effect of photothermal therapy of tumors.

The enhanced performance of Ti doped MnOx for the removal of NO with NH3

• Ti doped MnO x show an excellent SCR activity below 200 °C and above 300 °C. • Ti doping yields amorphous oxides, developed pore structure and more Mn 4+ . • Ti doped to MnO x brings more acid sites with proper acid strength. • Mn 1 Ti 1 O x forms more nitrites and shows a better low-temperature SCR activity. • Nitrates and strong NH 3 adsorption over Mn 1 Ti 2 O x delay the deactivation. Ti doped MnO x catalysts for the removal of NO with NH 3 (NH 3 ‒SCR) were prepared by coprecipitation method. The results showed that pure MnO x is composed of dominant Mn 3 O 4 with (101) plane and small amorphous MnO 2 , while TiO x only forms anatase TiO 2 . When Ti is doped into MnO x , as-prepared samples can form amorphous phase, and Mn 1 Ti 2 O x samples achieve 446 m 2 /g of specific surface area and 0.65 cm 3 /g of total pore volume. NH 3 can be adsorbed on MnO x and TiO 2 surface, but the former is weak and the latter is strong. Ti doping changes the acid site distribution and surface adsorbed oxygen of MnO x . Mn 1 Ti 1 O x has more available oxygen vacancies to produce NO 2 and achieves a good deNO x activity below 200 °C. With the increase of reaction temperature, more O 2 can be dissociated into O ‒ , causing that Mn‒containing catalysts show a complete NO removal at 200‒300 °C. When the temperature is higher than 300 °C, adsorption and activation of NH 3 become the rate‒determining step with the strong supply of NO 2 . TiO x can achieve 30% NO removal efficiency while MnO x is 100% at 200‒300 °C. Mn 1 Ti 1 O x samples have 95% of NO removal efficiency at 100 °C and maintain 100% in the range of 150‒320 °C. Nearly 100% N 2 selectivity is attained for all samples in the range of 80–160 °C, but the N 2 selectivity gradually decreases when the temperature is higher than 200 °C.

Further insights into the combination of permanganate and peroxymonosulfate as an advanced oxidation process for destruction of aqueous organic contaminants

Recent studies have reported a novel advanced oxidation process (AOP) by combining permanganate (KMnO4) and peroxymonosulfate (PMS) for destruction of organic contaminants (i.e., acid orange 7, trichloroethylene, and benzene), where hydroxyl (•OH) and sulfate radicals (SO4•−) are proposed to be generated from PMS activation by amorphous manganese dioxide (MnO2) formed in situ from KMnO4 reduction. In this work, appreciable degradation of p-chlorobenzoic acid (p-CBA) was confirmed in KMnO4/PMS system, while KMnO4 or PMS alone showed inert reactivity toward p-CBA. Moreover, it was found that pre-synthesized amorphous MnO2 showed invalid PMS activation for p-CBA degradation, and pre-addition of inorganic or organic reducing agents to promote the formation of amorphous MnO2 showed negligible influence on p-CBA degradation as well. In these regards, a tentative mechanism for PMS activation by KMnO4 rather than its product MnO2 was proposed, involving the substitution of oxo atoms of KMnO4 by peroxo groups, subsequent reductive generation of peroxomanganese (VI) complexes, and intramolecular disproportionation of these complexes to generate radicals. Efficient degradation of p-CBA was achieved at acid or basic conditions with a maximum rate occurring at pH 3. The coexisting chloride anions showed suppressive effect on p-CBA degradation for scavenging SO4•− and •OH, while metal ions accelerated the degradation of p-CBA, possibly due to the cation bridging function between negatively-charged MnO4− and HSO5−. Hydroxylated intermediates of p-CBA were identified in KMnO4/PMS system. This work improved the fundamental understanding of a new class of AOPs by combining KMnO4 and PMS for environmental decontamination.

Synthesis of MnO2-CuO-Fe2O3/CNTs catalysts: low-temperature SCR activity and formation mechanism.

MnO2–CuO–Fe2O3/CNTs catalysts, as a low-dimensional material, were fabricated by a mild redox strategy and used in denitration reactions. A formation mechanism of the catalysts was proposed. NO conversions of 4% MnO2–CuO–Fe2O3/CNTs catalyst of 43.1–87.9% at 80–180 °C were achieved, which was ascribed to the generation of amorphous MnO2, CuO and Fe2O3, and a high surface-oxygen (Os) content.

An Amorphous MnO2/Lithiated NiO Nanosheet Array as an Efficient Bifunctional Electrocatalyst for Iron Molten Air Batteries

Iron molten air battery is considered one of the promising batteries in next‐generation energy storage due to high theoretical specific energy density, cost efficiency, and environment‐friendly merits. Herein, an amorphous MnO2/lithiated NiO nanosheet array is grown directly on a highly conductive nickel foam substrate to use as a bifunctional electrocatalyst in iron molten air batteries. The advantages of the nanosheet array structure are having more highly exposed active sites for electrochemical reactions, as well as a conductive stable 3D framework for allowing barrier‐free diffusion and transport of ions and oxygen gas. As a result, the amorphous MnO2/lithiated NiO nanosheet array exhibits a promising catalytic activity for oxygen evolution reaction and oxygen reduction reaction with a voltage gap of only 0.29 V. The as‐assembled iron molten air battery shows an impressive rechargeability with the highest coulombic efficiency of nearly 100% through 200 cycles with an average discharge potential of ≈1.14 V at 500 °C. Moreover, the cell has a high rate response up to 6.7 C with a discharge voltage of above 0.9 V. The study achieves critical advances for iron molten air batteries without the use of precious metals in contrast with previous results using the Pd‐based catalyst.

Surface deformation studies of MnO2 film by double exposure digital holographic interferometry technique

In the present paper, the MnO2 films have been deposited on stainless steel support by potentiostatic mode of electrodeposition at different times. The x-ray diffraction, Fourier transform infrared spectroscopy along with Energy-dispersive x-ray spectroscopy analysis confirmed formation of amorphous MnO2 coating. The surface analysis by field emission scanning electron microscope indicated development of fractures with increasing film thickness. The in situ measurement of stress to the substrate and MnO2 film thickness during deposition is performed by non-destructive double exposure digital holographic interferometry (DEDHI) technique. It was observed that the stress to the substrate decreases with increasing MnO2 film thickness. This study opens application of DEDHI for non-destructive measurement using simple mathematical interpretation.

Amorphous MnO2 surviving calcination: an efficient catalyst for ozone decomposition

Calcination at 300 °C of amorphous MnO2 maintains the structure and results in superior stability owing to the enhanced water-resistant ability.

Low-Temperature Assembly of Ultrathin Amorphous MnO2 Nanosheets over Fe2O3 Spindles for Enhanced Lithium Storage

Carbon coating is an effective method to enhance the lithium storage of metal oxides, which, however, suffers from harsh conditions in high-temperature hydrolysis of organic mass at inert atmosphere and compromised capacity due to the presence of low-capacity carbon. We herein report a direct assembly of ultrathin amorphous MnO2 nanosheets with thickness less than 3 nm over Fe2O3 nanospindle backbones at 95 °C as a mild-condition, short-process, and upscalable alternative to the classic carbon-coating method. The assembly of the amorphous MnO2 nanosheets significantly increases the electrical conductivity of Fe2O3 nanospindles. When evaluated as an anode for lithium-ion batteries, the Fe2O3@amorphous MnO2 electrode shows enhanced capacity retention compared to that of the Fe2O3 nanospindle electrode. In situ transmission electron microscopy and in situ X-ray diffraction observations of the electrochemically driven lithiation/delithiation of the Fe2O3@amorphous MnO2 electrode indicate that the assembled amorphous MnO2 nanosheets are in situ transformed into a Fe-Mn-O protection layer for better electrical conductivity, uncompromised Li+ penetration, and enhanced structural integration. The Fe2O3@amorphous MnO2 electrode therefore has a reversible capacity of 555 mAh g-1 after 100 galvanostatic charge/discharge cycles at 1000 mA g-1, comparable with that of the Fe3O4@C electrode derived via the classic carbon-coating route.

Oxidation of bisphenol A by nonradical activation of peroxymonosulfate in the presence of amorphous manganese dioxide

This work demonstrated that bisphenol A (BPA) was rapidly degraded by peroxymonosulfate (PMS) in the presence of amorphous manganese dioxide (MnO2). Chemical quenching experiments and electron paramagnetic resonance spectroscopy (EPR) suggested that hydroxyl radical (OH), sulfate radical (SO4−), and singlet oxygen (1O2) were unlikely responsible for BPA oxidation. As such, a nonradical mechanism involving the formation of reactive complexes between amorphous MnO2 and PMS was tentatively proposed based on the PMS decomposition and Raman spectra. The presence of phosphate ions (H2PO4−) remarkably suppressed the degradation of BPA, while the addition of divalent metal ions (Ca2+, Mg2+, and Zn2+) appreciably enhanced BPA degradation. The discrepancy was likely resulted from their contrasting influences on the formation of reactive PMS-MnO2 complexes. Based on identified oxidation products (i.e., dimers, 4-hydroxycumyl alcohol, mono-hydroxylated BPA and its quinone derivative) by liquid chromatography tandem mass spectrometry, the transformation pathways of BPA in amorphous MnO2/PMS system involving one-electron oxidation, radical coupling, bond cleavage, and hydroxylation were proposed. In addition to BPA, thirteen other selected phenolic compounds were also efficiently degraded by amorphous MnO2/PMS system, and good correlations between apparent pseudo-first-order reaction rate constants (kobs) and descriptor variables (i.e., Hammett constants σ+ and half-wave potentials E1/2) were obtained.

The promotional role of Nd on Mn/TiO2 catalyst for the low-temperature NH3‑SCR of NOx

A series of rare-earth metal doped 30% Mn-3% RE/TiO2 (RE = Ce, Sm, Nd, Er, Y) catalysts were prepared by adopting two step incipient wetness impregnation method and investigated for the low-temperature SCR of NOx with NH3. It was found that 3% of rare earth elements (Nd, Er, Y) was added to 30% Mn/TiO2 catalyst even could increase the catalytic performance at the low temperature, in addition to usual used Ce and Sm. And the promoting effect of Nd was among the most significant. Then the 30%Mn-3%Nd/TiO2 catalyst was synthesized by another two methods, Coprecipitation and Sol-gel, for a comparative study. Various techniques were applied to characterize the catalysts. The catalysts prepared by Coprecipitation and Sol-gel method exhibited higher surface area and smaller average particle size in comparison with catalyst prepared by impregnation with TiO2 support. The addition of Nd made surface area and dispersion of MnOx species increase. It also led both the content of amorphous MnO2 and the content of crystalline Mn2O3 to decrease, but the proportion of MnOx species with low oxidation state increased. H2-TPR results showed that the low temperature reduction peak of MnOx was shifted to much lower temperatures with the doping of Nd, which improved the redox property of the catalysts. The all characteristics shown above could well explain the promotional role of Nd doping in the 30%Mn-3%Nd/TiO2 catalyst, that improved the low temperature SCR catalytic activity.

Abiotic hydroxylamine nitrification involving manganese- and iron-bearing minerals

Hydroxylamine (NH2OH) undergoes biotic and abiotic transformation processes in soil, producing nitrous oxide gas (N2O(g)). Little is known about the magnitude of the abiotic chemical processes in the global N cycle, and the role of abiotic nitrification is still neglected in most current nitrogen trace gas studies. The abiotic fate of NH2OH in soil systems is often focused on transition metals including manganese (Mn) and iron (Fe), and empirical correlations of nitrogen residual species including nitrite (NO2−), nitrate (NO3−), and N2O(g). In this study, abiotic NH2OH nitrification by well-characterized manganese (Mn)- and iron (Fe)-bearing minerals (pyrolusite, amorphous MnO2(s), goethite, amorphous FeOOH(s)) was investigated. A nitrogen mass balance analysis involving NH2OH, and the abiotic nitrification residuals, N2O(g), N2O(aq), NO2−, NO3−, was used, and specific reactions and mechanisms were investigated. Rapid and complete NH2OH nitrification occurred (4–5 h) in the presence of pyrolusite and amorphous MnO2(s), achieving a 95–96% mass balance of N byproducts. Conversely, NH2OH nitrification was considerably slower by amorphous FeOOH(s) (14.5%) and goethite (1.1%). Direct reactions between the Mn- and Fe-bearing mineral species and NO2− and NO3− were not detected. Brunauer–Emmett–Teller surface area and energy dispersive X-ray measurements for elemental composition were used to determine the specific concentrations of Mn and Fe. Despite similar specific concentrations of Mn and Fe in crystalline and amorphous minerals, the rate of NH2OH nitrification was much greater in the Mn-bearing minerals. Results underscore the intrinsically faster NH2OH nitrification by Mn minerals than Fe minerals.

Manganese translocation and concentration on Quercus cerris decomposing leaf and wood litter by an ascomycetous fungus: an active process with ecosystem consequences?

Decomposing fungi translocate manganese (Mn) as demonstrated by the fact that Mn has been found to accumulate on decomposing leaves associated with individual fungal hyphae forming insoluble Mn(III,IV) oxides that remain concentrated in diffuse patches. Here, we studied Mn translocation and precipitation by the saprophytic fungus Alternaria sp. strain FBL507 both on naturally decomposing oak leaves and in vitro experiments. Manganese was translocated and precipitated in beads and encrustations along the fungal hyphae. The combination of X-ray diffraction and scanning electron microscopy-energy dispersive X-ray spectroscopy chemical data showed that the precipitates found on leaves were rhodochrosite (MnCO3), birnessite ([Na, Ca, K]Mn2O4× 1.5H2O) and possibly Mn oxalate. The precipitates on wood were an amorphous Mn-O compound, probably MnO. Thus, Mn oxidation state in the precipitates spanned from +2 to +4, with +3 and +4 only in the birnessite on the leaves. In vitro experiments showed that Mn precipitates formed in living hyphae, suggesting the possibility that Mn precipitation is actively produced by the fungus. Such a possibility raises interesting questions regarding the role of readily available Mn in the activity of saprophytic fungi and other soil microorganisms, such as would result in a large involvement of Mn in the cycles of the major nutrient elements.

Unconventional Preparation of Polymer/Amorphous Manganese Oxide-Based Biodegradable Nanohybrids for Low Premature Release and Acid/Glutathione-Activated Magnetic Resonance Imaging

Smart poly(methacrylic acid-co-N,N-bis(acryloyl)cystamine)/DOX/MnO2-2/polyethylene glycol theranostic nanohybrids (PMAABACy/DOX/MnO2-2/PEG TNs) were rationally fabricated using in situ generation of amorphous MnO2 by taking advantage of the spatial confinement effect of PMAABACy nanohydrogels, and its PEGylation was accomplished through Mn–N coordinate bonding. The amorphous MnO2 of PMAABACy/DOX/MnO2-2/PEG TNs was synthesized through the chelation between Mn2+ ions and carboxyl groups of PMAA chains, which served as a gatekeeper to prevent the premature leakage of DOX during blood circulation. In the presence of intracellular acidic glutathione (GSH), the release of Mn2+ ions from amorphous MnO2 as a dual T1/T2 contrast agent endowed the nanohybrids with enhanced acid/GSH-activated magnetic resonance imaging (MRI). In addition, the site-specific release of a chemotherapeutic drug (DOX) was also realized because of the disintegration of both amorphous MnO2 and PMAABACy in response to biological endogenous ...

Sea-Buckthorn-Like MnO2 Decorated Titanate Nanotubes with Oxidation Property and Photocatalytic Activity for Enhanced Degradation of 17β-Estradiol under Solar Light

A new class of sea-buckthorn-like composite material, MnO2/TNTs (TNT = titanate nanotube), was developed and used for 17β-estradiol (E2) degradation in this study. The new material was characterized as amorphous MnO2 nanoparticles decorated on titanate nanotubes, with chemical formula 0.4MnO2·Na1.1H0.9Ti3O7. The MnO2 fraction can preactivate E2 through one-electron oxidation, while the TNT skeleton is the primary photocatalysis center. Unlike the traditional weak oxidation and photocatalytic degradation, the synergetic effect of these two processes leads to efficient E2 removal by MnO2/TNTs under simulated solar light. The apparent first-order rate constant (k1) was determined to be 0.198 min–1, which was ∼28 times that for MnO2/TNTs direct oxidation without light and ∼15 times of that for calcined TNTs photocatalysis with light. Moreover, the higher TOC elimination rate (82.6% at 1 h) was also obtained compared to that in the pure MnO2 system. Dual-enhanced mechanisms are proposed to interpret the high E...