Average Mn Oxidation State Research Articles

Effects of MnO2 of different structures on activation of peroxymonosulfate for bisphenol A degradation under acidic conditions

MnO2 with various structures, including three tunnel structures (α-, β-, γ-MnO2) and a layered structure (δ-MnO2), were synthesized and investigated for peroxymonosulfate (PMS) activation. The effects of different structured MnO2 on PMS activation in contaminant degradation, as quantified by the pseudo-first order rate constants of bisphenol A (BPA) oxidation, followed the order: α-MnO2 > γ-MnO2 > β-MnO2 > δ-MnO2. Results showed that under acidic conditions, BPA was degraded by both catalytic oxidation by PMS-MnO2 and direct oxidation by MnO2, and the relative importance of the two mechanisms differed for different MnO2. The direct oxidation accounted for 25.2, 7.4, 34.1, and 94.5% of the total reactivity of α-, β-, γ-, and δ-MnO2, respectively. Physicochemical properties of MnO2 including crystal structure, morphology, surface Mn oxidation states, surface area, oxygen species and conductivity were characterized and correlated with the catalytic reactivity. The results demonstrated that the crystallinity of MnO2 was the dominant factor in the catalytic reactivity, resulting in the lowest reactivity for the least crystalline δ-MnO2. For the crystalline MnO2, the catalytic reactivity linearly correlated with Mn average oxidation state, Mn(III) content, and conductivity. Electron spin resonance (ESR) and quenching experiments with ethanol and tert-butanol suggested that sulfate radicals (SO4−) were the dominant radicals in the systems, while hydroxyl radicals (OH) played a minor role. In addition, nonradical mechanisms such as singlet oxygen (1O2) also contributed to the BPA degradation, especially when δ-MnO2 was the catalyst. These findings offered new insights into the contaminant degradation mechanisms in PMS-MnO2 and provided guidance to develop cost-effective catalysts for water/wastewater treatment.

Study of mixed-valence Mn2BO4 using XRD, XPS and XAFS spectroscopies

The valence states and local structure around Mn atoms in mixed-valence Mn2BO4 have been studied by temperature dependent X-ray powder diffraction (XRPD), X-ray photoelectron (XPS) and Mn K-edge X-ray absorption (XAFS) spectroscopies measurements. X-ray absorption near-edge structure (XANES) and XPS have been used to measure the average oxidation state of Mn in bulk and near-surface of the material. The edge position, peak shapes and pre-edge features of Mn K-edge XANES spectra have been discussed. The pronounced temperature dependence of the Debye-Waller (DW) factor corresponding to the MnO coordination shell has been found from the extended x-ray absorption fine structure (EXAFS) analysis and has been associated with variations in the local distortions in MnO6 octahedra and emergence of short-range magnetic correlations at low temperatures. The XRPD measurements have been carried out at 298, 523 and 773 K. The monoclinic symmetry (P21/n) was found to persist up to highest temperature measured. The BVS calculations have revealed large valence difference between two manganese sites that strongly supports the presence of charge ordering up to high temperatures.

The effect of melt composition and oxygen fugacity on manganese partitioning between apatite and silicate melt

Oxygen fugacity and melt composition are both known to have a strong influence on the partitioning of trace elements between coexisting minerals and melt. Previous work has suggested that Mn partitioning between apatite and silicate melt may be strongly affected by oxygen fugacity and could, therefore, act as an oxybarometer. Here, we present a new study on the partitioning of Mn between apatite and melt at high temperature (1400–1250 °C) and 1 GPa pressure, for various melt compositions and oxygen fugacities (NNO +4.7 to NNO -10). We find that there is no demonstrable variation in the partition coefficient for Mn between apatite and silicate melt (DMnAp-m) across the range of fO2 conditions studied here. Instead, we find that DMnAp-m varies significantly with melt composition and that in particular, the proportion of non-bridging oxygens strongly influences partitioning of Mn between apatite and melt. We propose that variations in the Mn content of natural apatite, previously thought to reflect variations in fO2, are instead related to the degree of melt polymerisation. These findings are consistent with the results of Mn K-edge XANES spectroscopy, which demonstrate that Mn in coexisting apatite and silicate glass is present predominantly as Mn2+ regardless of fO2. Furthermore, XANES spectra from a series of silicate glasses synthesised at various oxygen fugacities demonstrate that Mn2+ is the predominant species, and that the average Mn oxidation state does not vary over a wide range of fO2-T conditions.

Influence of heavy metal sorption pathway on the structure of biogenic birnessite: Insight from the band structure and photostability

Birnessite, primarily formed by biooxidation, is a common semiconducting Mn oxide in nature and a major controller of heavy metals cycling processes. In turn, the heavy metal sorption pathway alters its structural chemistry and thus the electronic structure. We synthesize three kinds of birnessite, namely biogenic (bio-birnessite), Cu coprecipitated (co-birnessite) and Cu adsorbed birnessite (ad-birnessite), to investigate the influence of Cu(II) occupancy on the structure, semiconducting property and photostability of birnessite. XRD show co-birnessite holds the same hexagonal symmetry as bio-birnessite, whereas ad-birnessite transforms to triclinic symmetry as reflected by the splitting reflections at 2.44 and 1.42 Å. The adsorption process is accompanied by a more intense releasing of Mn(II) (156.88 μM/L) from a bio-birnessite analog δ-MnO2 in light than in dark (19.55 μM/L), suggesting the photoreductive releasing of Mn(II) promotes structure transformation. Conformably, both the Mn average oxidation state (AOS) (3.32) and mole ratio of Cu/Mn (0.08) in ad-birnessite is lower than those in co-birnessite (AOS: 3.47, Cu/Mn ratio: 0.17). EXAFS demonstrate different Cu complexing features in ad- and co-birnessite that the ratio of Cu incorporated (INC) into vacancies is 1.16 and 0.45 for ad- and co-birnessite, respectively. UV–vis diffuse reflection spectra (DRS) and photoelectron spectrometer (PS) exhibit the band gap (Eg) and valence band (VB) of bio-birnessite are 2.05 and −5.58 eV, while there is 0.1 and 0.05 eV decrease of Eg, and 0.08 and 0.16 eV lowering of VB in co- and ad-birnessite, respectively. The reduced Eg guarantee them to generate photoelectron-hole pairs under mild indoor visible light, and the lower energy level of VB in ad-birnessite makes its VB holes more reactive to accept electrons from electron donors. Further density functional theory (DFT) calculations focus on interpreting the fine structures brought by different Cu sorption pathways on the electronic structure of birnessite. A Mn vacancy in a 2 × 2 × 1 hexagonal birnessite cell significantly reduces Eg from 1.60 to 0.28 eV by hybridizing Mn 3d and O 2p states in VB. The doped Cu reduces Eg mainly through incorporating Cu 3d orbital in VB. The INC Cu in hexagonal birnessite contributes more to reduce Eg (1.60–0.34 eV) than TCS Cu (1.60–1.20 eV), while in triclinic birnessite cell, TCS Cu causes more obvious reduction of Eg (1.68–0.28 eV) than INC Cu (1.68–1.55 eV). Thus, we conclude the vacancy imposes more effect on the electronic structure of hexagonal co-birnessite than doped Cu in TCS or INC sites; and the band structure of co-birnessite is more sensitive to INC Cu, in contrast to TCS Cu affecting more in ad-birnessite. Overall, the crystal structure differs according to Cu fixation pathway, which imposes a comprehensive effect on band structure and photostability contributed by vacancies, Cu occupancy sites and structural symmetry. Cu coprecipitating with birnessite is suggested as a preferred method in practical clean-up of metals such as Cu, due to the higher adsorption capacity for Cu, better stability of TCS Cu and better photostability influenced by Cu.

Highly sensitive electrochemical analysis of tunnel structured MnO2 nanoparticle-based sensors on the oxidation of nitrite

Three MnO2 nanoparticles with different tunnel structures, α-, β-, and γ-MnO2, were synthesized and characterized. We demonstrated and compared their capabilities on the electrochemical oxidation of nitrite, providing a new perspective for MnO2 or MnO2 based materials on sensing application. α-MnO2 exhibited higher electrochemical reactivity than β- or γ-MnO2, which was ascribed to its higher conductivity, more exposure to MnO6 edges, longer average MnO bond length, and lower Mn average oxidation state(AOS). We hereby reported the first α-MnO2 nanoparticle-based electrochemical sensor for nitrite sensing. A highly controlled micro-plotter was used to deposit the MnO2 nanoparticles for the sensor fabrication, providing a micro-pattern of the sensing surface area of MnO2 and ensuring the reproducibility and sensitivity of this MnO2 based sensor. Using differential pulse voltammetry, a detection range of 10–800 μM of nitrite was accomplished along with a sensitivity of 17.1 μA μM−1 and a detection limit of 0.5 μM.

Cd2+ adsorption performance of tunnel-structured manganese oxides driven by electrochemically controlled redox

The heavy metal ion adsorption performance of birnessite (a layer-structured manganese oxide) can be enhanced by decreasing the Mn average oxidation state (Mn AOS) and dissolution−recrystallization during electrochemical redox reactions. However, the electrochemical adsorption processes of heavy metal ions by tunnel-structured manganese oxides are still enigmatic. Here, tunnel-structured manganese oxides including pyrolusite (2.3 Å × 2.3 Å tunnel), cryptomelane (4.6 Å × 4.6 Å tunnel) and todorokite (6.9 Å × 6.9 Å tunnel) were synthesized, and their electrochemical adsorptions for Cd2+ were performed through galvanostatic charge−discharge. The influence of both supporting ion species in the tunnel and tunnel size on the electrochemical adsorption performance was also studied. The adsorption capacity of tunnel-structured manganese oxides for Cd2+ was remarkably enhanced by electrochemical redox reactions. Relative to K+ in the tunnel of cryptomelane, the supporting ion H+ was more favorable to the electrochemical adsorption of Cd2+. With increasing initial pH and specific surface area, the electrochemical adsorption capacity of cryptomelane increased. The cryptomelane electrode could be regenerated by galvanostatic charge−discharge in Na2SO4 solution. Due to the differences in their tunnel size and supporting ion species, the tunnel-structured manganese oxides follow the order of cryptomelane (192.0 mg g−1) > todorokite (44.8 mg g−1) > pyrolusite (13.5 mg g−1) in their electrochemical adsorption capacities for Cd2+.

Effect of MnO2 Phase Structure on the Oxidative Reactivity toward Bisphenol A Degradation.

Manganese dioxides (MnO2) are among important environmental oxidants in contaminant removal; however, most existing work has only focused on naturally abundant MnO2. We herein report the effects of different phase structures of synthetic MnO2 on their oxidative activity with regard to contaminant degradation. Bisphenol A (BPA), a frequently detected contaminant in the environment, was used as a probe compound. A total of eight MnO2 with five different phase structures (α-, β-, γ-, δ-, and λ-MnO2) were successfully synthesized with different methods. The oxidative reactivity of MnO2, as quantified by pseudo-first-order rate constants of BPA oxidation, followed the order of δ-MnO2-1 > δ-MnO2-2 > α-MnO2-1 > α-MnO2-2 ≈ γ-MnO2 > λ-MnO2 > β-MnO2-2 > β-MnO2-1. Extensive characterization was then conducted for MnO2 crystal structure, morphology, surface area, reduction potential, conductivity, and surface Mn oxidation states and oxygen species. The results showed that the MnO2 oxidative reactivity correlated highly positively with surface Mn(III) content and negatively with surface Mn average oxidation state but correlated poorly with all other properties. This indicates that surface Mn(III) played an important role in MnO2 oxidative reactivity. For the same MnO2 phase structure synthesized by different methods, higher surface area, reduction potential, conductivity, or surface adsorbed oxygen led to higher reactivity, suggesting that these properties play a secondary role in the reactivity. These findings provide general guidance for designing active MnO2 for cost-effective water and wastewater treatment.

P2/O3 phase-integrated Na0.7MnO2 cathode materials for sodium-ion rechargeable batteries

We describe a phase-integrated P2-Na0.7MnO2/O3-NaMnO2 sodium manganese oxide (Na0.7MnO2) cathode material for sodium-ion rechargeable batteries. By changing the crystallization cooling rate after the calcination of P2/O3-Na0.7MnO2, the ratio between the two different (P2-Na0.7MnO2 and O3-NaMnO2) phases (and the Mn4+/Mn3+ ratio) in the Na0.7MnO2 material could be controlled. The P2/O3-Na0.7MnO2 sample prepared by fast cooling contains the optimum content of the two principal phases and showed excellent electrochemical performance because of the synergy between the fast kinetics of the P2-Na0.7MnO2 phase and high capacity of the O3-NaMnO2 phase. The stability of the layered crystal structure of Na0.7MnO2 was also improved by increasing the portion of the P2-Na0.7MnO2 phase, which has a higher average oxidation state of Mn compared to that in O3-NaMnO2; this suppressed the Jahn–Teller distortion during cycling.

CD-MUSIC-EDL Modeling of Pb2+ Adsorption on Birnessites: Role of Vacant and Edge Sites.

The surface complexation modeling of metal adsorption to birnessites is in its infancy compared to the charge-distribution multisite ion complexation (CD-MUSIC) models for iron/aluminum (hydr)oxides. Therefore, using X-ray diffraction with Rietveld refinement to obtain the reactive sites and their densities, a CD-MUSIC model combined with a Stern-Gouy-Chapman electrical double layer (EDL) model for the external surface and a Donnan model for the interlayer surface is developed for birnessites with different Mn average oxidation state (MnAOS). Proton affinity constants and the charge distributions of Pb surface complexes were calculated a priori. By fitting Pb adsorption data to the model the obtained equilibrium constants (log KPb) of Pb complexes were 6.9-10.9 for the double-corner-sharing and double-edge-sharing Pb2+ complexes on the edge sites and 2.2-6.5 for the triple-corner-sharing Pb2+ complex on the vacancies. The larger log KPb value was obtained for higher MnAOS. Speciation calculations showed that with increasing MnAOS from 3.67 to 3.92 the interlayer surface contribution to the total Pb2+ adsorption increased from 43.2% to 48.6%, and the vacancy contribution increased from 43.9% to 54.7%. The vacancy contribution from interlayer surface was predominant. The present CD-MUSIC-EDL model contributes to understand better the difference in metal adsorption mechanism between birnessite and iron/aluminum (hydr)oxides.

Tuning the K+ Concentration in the Tunnels of α-MnO2 To Increase the Content of Oxygen Vacancy for Ozone Elimination.

α-MnO2 is a promising material for ozone catalytic decomposition and the oxygen vacancy is often regarded as the active site for ozone adsorption and decomposition. Here, α-MnO2 nanowire with tunable K+ concentration was prepared through a hydrothermal process in KOH solution. High concentration K+ in the tunnel can expand crystal cell and break the charge balance, leading to a lower average oxidation state (AOS) of Mn, which means abundant oxygen vacancy. DFT calculation has also proven that the samples with higher K+ concentration exhibit lower formation energy for oxygen vacancy. Due to the enormous active oxygen vacancies existing in the α-MnO2 nanowire, the lifetime of the catalyst (corresponding to 100% ozone removal rate, 25 °C) is increased from 3 to 15 h. The FT-IR results confirmed that the accumulation of intermediate oxygen species on the catalyst surface is the main reason why it is deactivated after long time reaction. In this work, the performance of the catalyst has been improved because the abundant active oxygen vacancies are fabricated by the electrostatic interaction between oxygen atoms inside the tunnels and the introduced K+, which offers us a new perspective to design a high efficiency catalyst and may promote manganese oxide for practical ozone elimination.

Insight into pH‐Dependent Formation of Manganese Oxide Phases in Electrodeposited Catalytic Films Probed by Soft X‐Ray Absorption Spectroscopy

MnOx films electrodeposited under basic, neutral, and acidic conditions from an ionic liquid were investigated by means of X-ray absorption spectroscopy at the manganese L2,3 -edges and the oxygen K-edge. Such films can serve as catalysts for the water oxidation reaction. Previous studies showed that the catalytic activity could be controlled by varying the deposition parameters, which influence the formation of MnOx phases and the film composition. Herein the film compositions are investigated in detail, indicating different ratios of MnOx structural phases in the films. All films in the series predominately consist of varying proportions of three MnOx phases-Mn2 O3 , Mn3 O4 , and birnessite, while an increase of the average Mn oxidation state in the film is identified when going from basic to acidic conditions during electrodeposition. The contribution of these three phases shows a systematic dependency on the pH during electrodeposition. While no specific single MnOx phase was found to dominate the composition of samples that were previously found to show high catalytic activity, the X-ray spectroscopic results revealed the compositions of those samples prepared under close to neutral conditions to be most sensitive to changes in pH.

Effects of Mn average oxidation state on the oxidation behaviors of As(III) and Cr(III) by vernadite

Vernadite is a poorly crystalline phyllomanganate that widely distributed in natural environments, and plays a pivotal role in the geochemical transformations of heavy metal and other pollutants. Though many works have done about the reaction mechanisms between vernadite-like minerals and As(III)/Cr(III), the effects of some basic structure characteristics of the mineral, such as Mn average oxidation state (AOS), on the oxidation of As(III)/Cr(III) are not fully understood. In this study, vernadite samples with different Mn AOSs but almost the same particle sizes (Ver and Ver20) were synthesized, characterized, and their reactivities towards As(III)/Cr(III) oxidation and As(V)/Cr(VI) adsorption were compared. It is found that Ver and Ver20 have almost the same point of zero charge (PZC), but ver20 has increased contents of layer Mn(III) at particle edges and interlayer Mn(III) and Mn(II), thus greatly reduced Mn AOS. Decrease in Mn AOS greatly reduces the oxidation capacity and initial reaction rate (Kobs) of this mineral towards As(III) on mineral-water interfaces. The apparent As(III) oxidation amount and Kobs by Ver are 202 ± 8 mmol kg−1 and 0.3515 min−1 while that by Ver20 are 162 ± 7 mmol kg−1 and 0.0139 min−1. The oxidation capacity and Kobs for Cr(III) by these vernadie samples are also decreased but in a less extent compared to that in As(III) oxidation, e.g. Cr(III) oxidized by Ver and Ver20 are 2895 ± 47 and 1974 ± 109 mmol kg−1 and the Kobs are 0.0439 and 0.0169 min−1 respectively. Adsorption density of As(V) or Cr(VI) on Ver20 is ∼15–18% higher than that on Ver. These results suggest the more important role Mn AOS plays in As(III) oxidation than Cr(III) oxidation, and helps understand the geochemical behaviors of these toxic metals in natural environments mediated by Mn oxide minerals.

Mn Self-Doping of Orthorhombic RMnO3 Perovskites: (R0.667Mn0.333)MnO3 with R = Er-Lu.

Orthorhombic rare-earth trivalent manganites RMnO3 (R = Er-Lu) were self-doped with Mn to form (R0.667Mn0.333)MnO3 compositions, which were synthesized by a high-pressure, high-temperature method at 6 GPa and about 1670 K from R2O3 and Mn2O3. The average oxidation state of Mn is 3+ in (R0.667Mn0.333)MnO3. However, Mn enters the A site in the oxidation state of 2+, creating the average oxidation state of 3.333+ at the B site. The presence of Mn2+ was confirmed by hard X-ray photoelectron spectroscopy measurements. Crystal structures were studied by synchrotron powder X-ray diffraction. (R0.667Mn0.333)MnO3 crystallizes in space group Pnma with a = 5.50348(2) Å, b = 7.37564(1) Å, and c = 5.18686(1) Å for (Lu0.667Mn0.333)MnO3 at 293 K, and they are isostructural with the parent RMnO3 manganites. Compared with RMnO3, (R0.667Mn0.333)MnO3 exhibits enhanced Néel temperatures of about TN1 = 106-110 K and ferrimagnetic or canted antiferromagnetic properties. Compounds with R = Er and Tm show additional magnetic transitions at about TN2 = 9-16 K. (Tm0.667Mn0.333)MnO3 exhibits a magnetization reversal or negative magnetization effect with a compensation temperature of about 16 K.

Structure and properties of vanadium-doped α-MnO2 and enhanced Pb2+ adsorption phenol/photocatalytic degradation

Vanadium (V)-doped cryptomelane was synthesized and characterized by powder X-ray diffraction, electron microscopy, N2 adsorption, diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Vanadium doping does not change the basic crystal structure but slightly decreases the lattice parameters (a, b, c, β and cell volume) and increases the coherent scattering domain (CSD) sizes (16–19 nm). These V-doped cryptomelane materials are granular particles with increased specific surface areas (149–183 m2 g−1) and increased concentrations of hydroxyl groups on the surfaces with increasing V doping. Both V 2p3/2 spectra and V K-edge X-ray absorption near edge spectroscopy indicates that V exists in +5. Substitution of V5+ for Mn4+ in cryptomelane framework results in reduction in Mn average oxidation state and increase in bond lengths of Mn-O in [MnO6] unit, edge- and corner-sharing Mn-Mn(V) pairs. These V-doped materials have greatly increased adsorption capacities for Pb2+ (937–1146 mmol kg−1). Doping ∼1.4 wt.% V can significantly enhance cryptomelane optical absorption properties and thus photocatalytic degradation of phenol, i.e., with a TOC removal rate of ∼77%. These results can facilitate the modification and application of environmental friendly Mn oxide materials in pollution control and remediation.

Cosputtered Calcium Manganese Oxide Electrodes for Water Oxidation.

Calcium manganese oxide films were prepared by cosputter deposition from Mn and CaMnO3 targets and evaluated for their suitability as catalysts for the oxygen evolution reaction (OER). Scanning electron microscopy (SEM) revealed a compact morphology for the as-deposited films and the formation of nanorodlike features on the surfaces after annealing at 600 °C. X-ray-photoelectron-spectroscopy analysis showed that the surface oxidation state is close to +III (as in Mn2O3) for the as-deposited films and increases slightly to a mixture of III and IV after annealing occurs in dry air at 400-600 °C. Glancing-incidence X-ray diffraction (GIXRD) suggested that the CaMnxOy films are amorphous even when heated to 600 °C. However, transmission electron microscopy (TEM) showed that there is actually a polycrystalline component of the film, which best matches Mn3O4 (hausmannite with the average Mn oxidation state of ∼+2.7) but may have a slightly expanded unit cell because of the incorporation of Ca. Electrochemical analyses revealed that the as-deposited CaMnxOy films were OER-inactive. In contrast, annealing at 400 or 600 °C resulted in an increase of ∼15-fold in the current densities, which reached j ≅ 1.5 mA·cm-2 at OER overpotentials of η ≈ 550 mV in cyclic voltammetry (CV) sweeps. For the same η, annealed CaMnxOy electrodes also showed good electrochemical stabilities during 2 h of electrolysis, as rather constant steady-state current densities of j ≅ 0.4-0.5 mA·cm-2 were observed. The thicknesses and surface morphologies of the CaMnxOy films did not change during the electrochemical measurements, indicating that corrosion was negligible. In comparison with a previous study in which Ca-free thin layers of MnOx were evaluated, the results demonstrate that Ca2+ incorporation can enhance the OER activity of MnOx electrocatalysts prepared by sputter deposition. This work provides guidance for designing new electrodes for water oxidation on the basis of the abundant and nontoxic elements manganese and calcium.

Interaction of ammonium with birnessite: Evidence of a chemical and structural transformation in alkaline aqueous medium

The ammonium cation interaction with Na-birnessite in aqueous alkaline medium was studied. Solution and solid analysis give evidence that birnessite is not only acting as a cationic exchanger toward NH4+. The surface analysis performed by XPS showed that N1s spectra are characterized by the existence of two different environments: one assignable to an interlayer NH4+ and the second to a chemisorbed N-species. Structural and chemical transformations were observed on birnessite with nitrogen mass balance deficit. The monitoring of NH4+, Na+, Mn2+, NO3- and NO2- and solid changes (average oxidation state of Mn, cation exchange capacity, solid nitrogen content and symmetry evolution identified by XRD and FTIR) indicate unambiguously that NH4+ reacts chemically with the birnessite.

Substitution effects on the bulk and surface properties of (Li,Ni)Mn2O4

Manganese oxides of spinel structure, LiMn2O4, Li1-x Ni x Mn2O4 (0.25 ≤ x≤ 0.75), and NiMn2O4, were studied by EDS, XRD, SEM, magnetic (M-H, M-T), and XPS measurements. The samples were synthesized by an ultrasound-assisted sol-gel method. EDS analysis showed good agreement with the formulations of the oxides. XRD and Rietveld refinement of X-ray data indicate that all samples crystallize in the Fd3m space group characteristic of the cubic spinel structure. The a-cell parameter ranges from a = 8.2276 A (x = 0) to a = 8.3980 A (x = 1). SEM results showed particle agglomerates ranging in size from 2.3 μm (x = 0) down to 0.8 μm (x = 1). Hysteresis magnetization vs. applied field curves in the 5–300K range was recorded. ZFC-FC measurements indicate the presence of two magnetic paramagnetic-ferrimagnetic transitions. The experimental Curie constant was found to vary from 5 to 7.1 cm3 K mol−1 for the range of compositions studied (0 ≤ x ≤ 1). XPS studies of these oxides revealed the presence of Ni2+, Mn3+, and Mn4+. The experimental Ni/Mn atomic ratios obtained by XPS were in good agreement with the nominal values. A linear relationship of the average oxidation state of Mn with Ni content was observed. The oxide’s cation distributions as a function of Ni content from x = 0 Li+[Mn3+Mn4+]O4 to x = 1 $$ {\mathrm{Ni}}_{0.35}^{2+}{\mathrm{Mn}}_{0.65}^{3+}\left[{\mathrm{Ni}}_{0.65}^{2+}\right.\left.{\mathrm{Mn}}_{1.35}^{3+}\right]{\mathrm{O}}_4 $$ were proposed.

Local structure of Cu2 + in Cu-doped hexagonal turbostratic birnessite and Cu2 + stability under acid treatment

Geochemical behaviors of heavy metal contaminants, such as Cu2+, are strongly controlled by natural birnessite-like minerals in both marine and terrestrial environments. However, the mechanisms of the interaction of Cu2+ with birnessite are not fully understood yet. In the present study, Cu2+ was coprecipitated with Mn2+ to produce hexagonal turbostratic birnessite, which is analogous to natural birnessite. The obtained Cu-doped birnessite was characterized by powder X-ray diffraction, field-emission scanning electron microscopy, and X-ray absorption spectroscopy (XANES+EXAFS). The stability of Cu(II) in the birnessite structure was investigated by acid treatment. Increasing the dopant content reduces the mineral crystallinity in the [001] direction and the unit cell parameter b from the hexagonal layers. It also shortens the bond length of MnO in the [MnO6] unit and the edge-sharing MnMn distance in the layers, and increases the average oxidation state (AOS) of Mn and the specific surface area. Analysis of Cu K-edge XANES and EXAFS data indicates that, only a small part of Cu(II) is inserted into the birnessite layers, while most of it is adsorbed on the vacancies. When the Cu/Mn molar ratio is increased from 0.08 to 0.23, an increasing part of Cu(II) is present as polynuclear clusters on the birnessite edge sites in the pH range of ~3.3–5.3. Reaction with H2SO4 solution is found to easily dissolve the polynuclear Cu clusters and the highly distorted Cu octahedra in innersphere complexes on the birnessite-water interface, with ~53% of the Cu2+ released into the solution. On the other hand, the reaction with HCl solution leads to reductive dissolution of the mineral matrix, the release of Mn2+ into solutions, the decrease in the first MnO and edge-sharing MnMn distances and Mn AOS, in addition to the release of Cu2+. The release rate of Cu2+ is much faster than that of Ni2+ in Ni-doped birnessites, owing to the lower stability of distorted [CuO6] octahedron upon proton attack. These results indicate the formation of multinuclear Cu complexes on birnessite surfaces under the investigated conditions. The results also suggest the lower stability of Cu2+ in these minerals and thus higher potential toxicity in acidic conditions, in comparison with other metal pollutants, such as Ni2+. This study provides new insights into the interaction mechanisms between Cu2+ and birnessite-like minerals, and help to clarify the structural stability and geochemical behaviors of Cu2+ associated with birnessite-like minerals in natural environments.

Enhancing high-rate capability of MnO2 film electrodeposited on carbon fibers via hydrothermal treatment

MnO2 films electrodeposited on carbon fibers were subjected to hydrothermal treatment in deionized water and a K2SO4 solution. The effects of hydrothermal treatment on the structures and capacitive properties of the films were systematically evaluated. Compared to the as-deposited and K2SO4-treated films, the water-treated film exhibits a much lower capacitance at 2mVs−1 due to the phase transformation from ramsdellite to manganite during the hydrothermal treatment. The K2SO4-treated film maintains the ramsdellite structure although the average oxidation state of Mn in the film is decreased to some extent. At the charge-discharge current density of 1C, the K2SO4-treated and as-deposited films show specific capacitances of 281.9 and 274.7Fg−1, respectively. But the K2SO4-treated film displays a capacitance of 135Fg−1 at 15C and a good high-rate capability which is more than 3 times as high as that of the as-deposited one. These results and the related kinetic analysis suggest that the suitable ratio of Mn4+ to Mn3+ plays an important role in determination of the high-rate capability of the film.

Morphology-dependent enhancement of arsenite oxidation to arsenate on birnessite-type manganese oxide

Birnessite-type manganese oxide is a highly efficient oxidant that has been investigated widely for As(III) oxidation. Nevertheless As(III) oxidation rate is inevitably reduced due to favorable adsorption of coexisting ions and As(V) which passivate its surface. In this paper we explore a novel strategy to significantly improve As(III) oxidation performance by controlling birnessite morphology. The batch experiment results show that the nanoflower-like birnessite (Bir-NF) exhibits an incredible improvement in As(III) oxidation activity compared to nanowire-like (Bir-NW) and nanosheet-like (Bir-NS) birnessites. The morphology of birnessite varies from nanosheet to nanoflower not only promotes As(III) oxidation rate from 1.4 to 24.7μmolg−1min−1, but also reduces the adverse effect of adsorption of As(V) and coexisting ions on As(III) removal. The origin of morphology-dependent enhancement of As(III) removal was experimentally and theoretically studied by As(V) adsorption on birnessites, phosphate adsorption kinetics, detection of dissolved Mn2+ concentration, average Mn oxidation state, the point of zero charge, and density functional theory (DFT) calculations. The results reveal that significant enhancement of As(III) oxidation activity in Bir-NF as compared to Bir-NW and Bir-NS is attributed to its highly efficient contact between As(III) species and manganese oxide, as well as its fast charge transfer from As atom to Mn atom due to its highest oxygen vacancy defect concentration, thus significantly promoting As(III) oxidation activity.