Todorokite-type Manganese Oxide Research Articles

Near 100% selectivity for ammonia synthesis at a high current density by promoting nitrate protonation on the copper dispersed todorokite-type manganese oxide

The Cu-dispersed OMS-1 promotes NO3− protonation to produce ammonia with a Faraday efficiency of ∼100% and production rate of 0.51 mmol h−1 cm−2.

Synthesis and Aerobic Oxidation Catalysis of Mesoporous Todorokite-Type Manganese Oxide Nanoparticles by Crystallization of Precursors

The pursuit of a high surface area while maintaining high catalytic performance remains a challenge due to a trade-off relationship between these two features in some cases. In this study, mesoporous todorokite-type manganese oxide (OMS-1) nanoparticles with high specific surface areas were synthesized in one step by a new synthesis approach involving crystallization (i.e., solid-state transformation) of a precursor produced by a redox reaction between MnO4- and Mn2+ reagents. The use of a low-crystallinity precursor with small particles is essential to achieve this solid-state transformation into OMS-1 nanoparticles. The specific surface area reached up to ca. 250 m2 g-1, which is much larger than those (13-185 m2 g-1) for Mg-OMS-1 synthesized by previously reported methods including multistep synthesis or dissolution/precipitation processes. Despite ultrasmall nanoparticles, a linear correlation between the catalytic reaction rates of OMS-1 and the surface areas was observed without a trade-off relationship between particle size and catalytic performance. These OMS-1 nanoparticles exhibited the highest catalytic activity among the Mn-based catalysts tested for the oxidation of benzyl alcohol and thioanisole with molecular oxygen (O2) as the sole oxidant, including highly active β-MnO2 nanoparticles. The present OMS-1 nanomaterial could also act as a recyclable heterogeneous catalyst for the aerobic oxidation of various aromatic alcohols and sulfides under mild reaction conditions. The mechanistic studies showed that alcohol oxidation proceeds with oxygen species caused by the solid, and the high surface area of OMS-1 significantly contributes to an enhancement of the catalytic activity for aerobic oxidation.

Atomic-level tunnel engineering of todorokite MnO2 for precise evaluation of lithium storage mechanisms by in situ transmission electron microscopy

Todorokite-type manganese oxide (τ-MnO2) with p × 3 tunneled structure is especially captivating as charge storage material for rechargeable batteries, since the enlarged tunnel dimensions enable rapid electrode kinetics and superior rate performance. However, its congenitally rich polytypism associated with tunnel heterogeneity impedes our precise understanding of structure-property relationship in this polytypic material. In this regard, this work has taken substantial effort to preliminarily achieve uniform 4 × 3 tunnel-structured τ-MnO2 nanorods, as corroborated via atomically resolved imaging. Afterwards, the (de)lithiation mechanisms of the tunnel-specific phase are investigated via in situ transmission electron microscopy including electron diffraction, high-resolution imaging, and electron energy loss spectroscopy, coupled with density functional theory calculations and phase field simulations. Upon initial lithiation, the intercalation reaction region β (less than 1.43 Li insertion) is observed as result of rapid lithium-ion diffusion through the tunnels with slightly increased lattice constants. By tracing the full lithiation procedure, the evolution of intermediate Mn2O3 phase and the development of final Mn and Li2O phases are identified in the conversion reaction region α (more than 1.43 Li insertion). These results indicate that τ-MnO2 can be applied to a cathode by intercalation reaction and to an anode by conversion reaction in corresponding to voltage ranges in a lithium-based battery. Upon delithiation, we observe an unusual reciprocating-motion reaction front (different from one-way lithiation reaction front), for which the driven dynamics are delineated based on a phase field model. Impressively, a reversible and symmetric conversion reaction between Mn2O3 phase and Mn + Li2O phases is established upon subsequent (de)lithiation cycles. This work can be regarded as a stepping-stone arousing the appetite of a comprehensive understanding of the highly polytypic material with other tunnel-specific phases.

Electrode Properties of Todorokite-type Tunnel-structured Manganese Oxide for Calcium Secondary Batteries

The electrode properties of todorokite-type tunnel-structured manganese oxides (TodMO) were examined for their potential use as cathode materials in calcium batteries. TodMO with a chemical composition of Mg0.19Na0.07MnO2·0.37H2O was prepared through hydrothermal treatment of layer-structured manganese oxide using magnesium ions as interlayer guest ions. The TodMO exhibited a dis-charge and charge capacity of 100 and 80 mAh g−1, respectively, at a relatively large current density of 100 mA g−1. The reaction mecha-nism was studied in detail using X-ray absorption spectroscopy measurements. Consequently, it became clear that the newly formed Mn3+-compound converted from TodMO is responsible for the reversible capacity.

Todorokite-type manganese oxide nanowires as an intercalation cathode for Li-ion and Na-ion batteries

Extended hydrothermal treatment at an elevated temperature of 220 °C allowed high yield synthesis of manganese oxide nanowires with a todorokite crystal structure suitable for reversible intercalation of Li+ and Na+ ions.

Cesium Adsorption by Synthetic Todorokite-Type Manganese Oxides

Abstract Synthetic todorokite-type manganese oxides containing magnesium (todorokite(Mg)), calcium (todorokite(Ca)), and iron-doped todorokite containing magnesium (Fe-todorokite(Mg)) were treated with acid solutions to obtain protonated todorokite(Mg-H), todorokite(Ca-H), and Fe-todorokite(Mg-H), respectively, and studied for cesium adsorption from the aqueous solutions. The cesium adsorption rate was slow to attain equilibrium in 24 h at pH 4.4–5.5 (uncontrolled); the adsorptive capacities were 0.50 mmol g−1 for todorokite(Mg-H), 0.49 mmol g−1 for todorokite(Ca-H), and 0.27 mmol g−1 for Fe-todorokite(Mg-H). The cesium adsorptive capacity was significantly dependent on pH, increasing with increase in pH. A partial substitution of Mn by Fe (7.2 wt %) in the framework of todorokite greatly decreased the cesium adsorptive capacity compared to metal-undoped todorokite. The distribution coefficient Kd values of the protonated materials for Na+, K+, Cs+, Mg2+, and Ca2+ at 1.0 mmol dm−3 in acidic pH showed that the materials preferred Cs+ over other cations. Both todorokite(Mg-H) and todorokite(Ca-H) were effective for cesium adsorption completely (ca. 100%) from drinking water intentionally contaminated with 0.1 mmol dm−3 CsCl at solution-to-solid ratio of 0.1–1.0 dm3 g−1.

Cr-hollandite: Breaking tradition with todorokite-type manganese oxides

The synthesis of a tunneled hollandite-type manganese oxide with interstitial and framework Cr3+ is described. This unique material is prepared from a layered buserite precursor under conditions previously believed to only yield todorokite-type manganese oxides with larger tunnels. The influence of Cr3+ in promoting the hollandite structure has been investigated by selectively placing the cation either in interstitial or framework sites. The use of framework Cr3+ in combination with other interstitial cations generates related hollandite and todorokite derivatives. Catalytic oxidation reactions with benzyl alcohol and carbon monoxide have also been examined.

Supercapacitive behavior and high cycle stability of todorokite-type manganese oxide with large tunnels

A todorokite-type manganese oxide (T-MnO x ) with a 3 × 3 large tunnel structure was successfully synthesized by a hydrothermal method and employed as an active electrode material for a supercapacitor. The todorokite structure of the as-prepared manganese oxide was confirmed by X-ray diffraction (XRD), and the fragmented platelets were observed by scanning electron microscopy (SEM). The electrochemical performances of the as-prepared T-MnO x were evaluated by cyclic voltammetry (CV), galvanostatic charge–discharge (CD) experiments and electrochemical impedance spectroscopy (EIS) in 1 M Na 2SO 4 electrolyte. The specific capacitance of the T-MnO x electrode sharply increased during the initial 1000 cycles and then continued improving gradually. It reached a maximum of 220 F g −1 at approximately the 4000th cycle at a scan rate of 2 mV s −1, and further studies showed that the morphology and structure of the T-MnO x were maintained after 4000 cycles, indicating that an electrochemical activation process occurred during the initial thousands of cycles. The electrochemical activation mechanism is discussed in detail. The specific capacitance remained an almost constant value and decreased slightly upon prolonged cycling. After 23,000 cycles, it decreased by 6% of the maximum specific capacitance. The experimental results demonstrate that T-MnO x is a promising candidate as an electrode material for supercapacitors.

Large-scale preparation of hierarchical manganese oxide octahedral molecular sieves (OMS-1) composed of nanoplate microspheres via a facile one-pot reflux method

A facile one-pot reflux method was explored for large-scale preparation of todorokite-type manganese oxide octahedral molecular sieves (OMS-1) composed of hierarchical nanoplate microspheres. The diameters of the microspheres are usually 1.5–2.5 μm, and the thickness of the nanoplate is approximately 10 nm.

Synthesis of a Nanofibrous Manganese Oxide Octahedral Molecular Sieve with Co(NH3)63+ Complex Ions as a Template via a Reflux Method

Todorokite-type manganese oxide octahedral molecular sieves (OMS-1) have been extensively studied due to their potential applications as materials. In this study, a nanofibrous todorokite-type, tun...

Single-step synthesis of manganese oxide octahedral molecular sieves with large pore sizes

A facile single-step method was developed for synthesizing todorokite-type manganese oxide octahedral molecular seieves (OMS-1) and 2 x 4 tunnel structured manganese oxide (OMS-5) materials. Selection of starting materials and initial pH conditions in the syntheses are crucial.

Synthesis of todorokite-type manganese oxide from Cu-buserite by controlling the pH at atmospheric pressure

A todorokite-type manganese oxide molecular sieve material was successfully synthesized at atmospheric pressure by refluxing treatment of Cu 2+ exchanged Na-buserite (named as Cu-OMS-1). TEM (transmission electron microscope) and HRTEM (high resolution transmission electron microscope) images revealed that this material has a needle-phase crystal morphology with thickness 0.1–1 μm, and the lattice fringes spacing 1.0 nm corresponding to the [1 0 0] plane of the todorokite structure. Such morphological and intergrowth characteristics were similar to those of hydrothermally synthesized todorokites. The Cu-OMS-1 with a chemical composition of Cu 0.34MnO 2.19 · 1.11H 2O, was stable below 400 °C. The BET surface area was found to be 62.5 m 2/g, and a major micropore size distribution peak centered at 0.70 nm for Cu-OMS-1 by the Horvath–Kawazoe (HK) method. As determined by the t-plot method, 31% of the surface area was contributed by micropores. The pH of reaction solution plays an important role in the sub-structure modification of formed Cu-buserites and the formation of todorokite at atmospheric pressure. A mechanism for the influence of pH on the transformation is discussed.

Amperometric biosensors based on carbon paste electrodes modified with nanostructured mixed-valence manganese oxides and glucose oxidase

Nanostructured, multivalent, manganese-oxide octahedral molecular sieves (OMS), including cryptomelane-type manganese oxides and todorokite-type manganese oxides, were synthesized and evaluated for chemical sensing and biosensing at low operating potential. Both cryptomelane-type manganese oxides and todorokite-type manganese oxides are nanofibrous crystals with subnanometer open tunnels that provide a unique property for sensing applications. The electrochemical and electrocatalytic performance of OMS for the oxidation of H 2O 2 have been compared. Both cryptomelane-type manganese oxides and todorokite-type manganese oxides can be used to fabricate sensitive H 2O 2 sensors. With glucose oxidase (GOx) as an enzyme model, amperometric glucose biosensors are constructed by bulk modification of carbon paste electrodes with GOx as a biocomponent and nanostructured OMS as a mediator. A Nafion thin film was applied as an immobilization/encapsulation and protective layer. The biosensors were evaluated as an amperometric glucose detector at phosphate buffer solution with a pH 7.4 at an operating potential of 0.3 V (vs Ag/AgCl). The biosensor is characterized by a well-reproducible amperometric response, linear signal-to-glucose concentration range up to 3.5 mmol/L and 1.75 mmol/L, and detection limits (S/N = 3) of 0.1 mmol/L and 0.05 mmol/L for todorokite-type manganese oxide and cryptomelane-type manganese oxide–modified electrodes, respectively. The biosensors based on OMS exhibit considerable good reproducibility and stability, and the construction and renewal are simple and inexpensive.

Studies on the formation of todorokite-type manganese oxide with different crystalline birnessites by Mg 2+-templating reaction

The formation of todorokite-type manganese oxide (TodMO) by hydrothermal soft chemical reaction (Mg 2+ exchange followed by hydrothermal treatment) was studied using three kinds of Na-type birnessite (Na-BirMO) with different crystallinities. Buserite (BusMO) formation by Mg 2+ exchange and TodMO formation by hydrothermal treatment progressed in a similar manner regardless of the crystallinity of the initial Na-BirMO, but the crystallinity of the synthesized TodMO depended on that of the initial Na-BirMO. Particle morphology of the synthesized TodMO was related to the crystallinity of the initial Na-BirMO. The amounts of fibrous morphology were gradually increasing, accompanied by increased crystallinity of the initial Na-BirMO. TodMO with a neat fibrous morphology was obtained using a high crystalline Na-BirMO as precursor. Most of the interlayer metal ions (Na + and Mg 2+) were extracted from Na-BirMO and BusMO by treatment with a 0.1 M HCl solution, while only 37% of Mg 2+ was extracted from TodMO. Both the acid treatment and the particle morphology may be the most convenient method to distinguish TodMO from BusMO.

Effects of Microwave Processing on Chemical, Physical, and Catalytic Properties of Todorokite-Type Manganese Oxide

A microwave-assisted hydrothermal method of synthesis was used to prepare todorokite. Synthetic todorokite, OMS-1, is a multivalent manganese oxide with a 3×3×∞ tunnel structure. OMS-1 was synthesized from its layered precursor much faster by the microwave method than by conventional heating (8 vs 48 h). In addition, the microwave-synthesized materials reveal some properties superior to those of the conventionally synthesized ones such as better stability, crystallinity, and catalytic activity in the production of phthalic anhydride. Microwave-prepared todorokite also shows novel cubic morphology, which was not found for conventionally made OMS-1 and is catalytically active in the oxidation of benzyl alcohol. Materials synthesized by microwave as well as conventional methods were characterized with XRD, TGA, SEM, TEM, TPD, and BET. Physical, chemical, and catalytic properties of todorokite prepared by the two different methods were compared based on the experimental results.

Sorption Behavior of Radionuclides on Crystalline Synthetic Tunnel Manganese Oxides

Cryptomelane-type (potassium) and todorokite-type (magnesium and calcium) manganese oxides were prepared and studied for the removal of radionuclides from aqueous solutions containing inactive metal ions of interest in radionuclide-bearing nuclear waste effluents. Batch distribution coefficients were measured as a function of pH and sodium, potassium, magnesium, and calcium ion concentrations. Trace strontium (89Sr) and cesium (137Cs) ions were taken up by ion-exchange mechanisms. Selectivity coefficients were estimated as KCs/K = 0.6 and KSr/K = 1.0 for cryptomelane and KCs/Mg = 7550, KSr/Mg = 50, and KSr/Ca = 10 for todorokite. The cryptomelane analogue was particularly effective for the separation of trace silver ions (110Ag) in low-pH solutions. Todorokite analogues were effective for a wider range of radionuclides, including 57Co across a wide pH range (1−10) and 137Cs in acidic solution. The affinity sequence for magnesium and calcium ion-extracted (acid-treated) todorokites in 0.1 HNO3 was 137Cs > 59Fe > 51Cr ≈ 57Co ≈ 241Am > 54Mn > 63Ni > 65Zn > 236Pu > 89Sr.

A new synthetic route to todorokite-type manganese oxides

Microporous todorokite-type manganese oxides have been synthesized by a new route in which the key Na-birnessite precursor is prepared by oxidation of Mn(OH) 2 with K 2S 2O 8 in aqueous NaOH. The reaction is promoted by foreign metal cations such as Mg 2+, Co 2+, Ni 2+, and Cu 2+, which are incorporated into the manganese oxide layer framework. These same divalent cations are used in a subsequent ion-exchange reaction that converts the Na-birnessite into a related layered material, buserite. Hydrothermal treatment of the buserite ultimately yields Mg-, Co-, Ni-, or Cu-todorokite. The todorokites have been characterized by powder X-ray diffraction, elemental analysis, Mn oxidation state determination, scanning electron microscopy, and cyclic voltammetry. The composition of Mg-doped Na-birnessite is Na 0.26Mg 0.13MnO 2.04(H 2O) 1.26, with the average Mn oxidation state being 3.55. Mg-todorokite has a composition of Mg 0.33MnO 2.14(H 2O) 0.97, with a Mn oxidation state of 3.62. A mixed Co/Ni-todorokite has been synthesized to assess the distribution of foreign cation in framework and tunnel sites. For Co-todorokite, 42% of the Co is in the manganese oxide framework and 58% is in the interlayer galleries, which gives the formula Co 0.21(Co 0.16Mn)O 2.21(H 2O) 0.97. If the Co percentages are applied to Mg-todorokite, a formula of Mg 0.19(Mg 0.14Mn)O 2.14(H 2O) 0.97 is obtained. Thermal stability experiments reveal that Mg-todorokite is more robust compared to the other todorokites and remains intact up to 400°C. The Co, Ni, and Cu-todorokites have similar thermal stabilities and their structures collapse at about 300°C. Na-birnessite prepared by the Mn(OH) 2/K 2S 2O 8 route can further be used to generate other birnessite derivatives such as H-birnessite and alkylammonium-birnessites. These derivatives can be synthesized both with and without Mg 2+, Co 2+, Ni 2+, and Cu 2+ as isomorphous framework dopants.

Hydrothermal Soft Chemical Process for Synthesis of Manganese Oxides with Tunnel Structures

A soft chemical process is used for the synthesis of manganese oxides with tunnel structures. This process comprises two steps: the first step is the preparation of a precursor with layered structure and insertion of templates (structure-directing agents) into its interlayer space by a soft chemical reaction [1], and the second step is the transformation of the template-inserted precursor into a tunnel structure by hydrothermal treatment. The usefulness of this process was demonstrated by synthesizing six kinds of tunnel manganese oxides from a birnessite-type manganese oxide with layered structure. The transformation reactions from layered structure to tunnel structures were investigated by X-ray, DTA-TG, and chemical analyses, and IR spectroscopy. Spinel-, hollandite-, romanechite-, and todorokite-type manganese oxides, which have (1×3), (2×2), (2×3), and (3×3) tunnel structures, can be obtained by using Li+, K+, Ba2+, and Mg2+ ions as the templates, respectively. Pyrolusite- and ramsdellite-type manganese oxides with (1×1) and (1×2) tunnel structures can be obtained in acidic solutions. The size of the resulting tunnels correspond to the size of the templates.

Metal ion extraction/insertion reactions with todorokite-type manganese oxide in the aqueous phase

Metal ion extraction/insertion reactions with todorokite-type manganese oxide in the aqueous phase