Cryptomelane-type Manganese Oxide Research Articles

Synthesis of isomorphically substituted Ru manganese molecular sieves and their catalytic properties for selective alcohol oxidation

Ruthenium has been incorporated into the framework of the cryptomelane type manganese oxide K-OMS-2 ([Ru]-K-OMS2). Accordingly, Rietveld refinement of the X-ray diffractogram has allowed to estimate changes in the values of cell parameters.

Optimization of the preparation conditions of cordierite honeycomb monoliths washcoated with cryptomelane-type manganese oxide for VOC oxidation

ABSTRACT Ceramic honeycomb monoliths were washcoated with cryptomelane-type manganese oxides and their catalytic performance was evaluated in the oxidation of ethyl acetate. The effect of a mixture of ethyl acetate with toluene and of the presence of water vapour was also assessed. Different coating parameters, namely size of catalyst particles, number of immersions in the washcoating solution, presence of an initial coating with alumina, calcination temperature of this coating, as well as the amount of binding agent and ethanol in the washcoating solution were studied and optimized based on the catalytic activity of the structured catalyst. Small particles are required for a correct impregnation; however, since the smallest particles are less active, an intermediate size achieved the best catalytic results. Increasing the number of immersions over 3 did not significantly increase the catalytic activity of the structured catalyst. The presence of an initial coating with alumina and a binding agent (colloidal alumina) in the washcoating solution was found essential to increase the activity, whereas increasing the calcination temperature after the initial alumina coating above 500°C decreased the activity of the catalyst. The presence of ethanol in the washcoating solution did not significantly improve the activity of the structured catalyst. The optimized structured catalyst presented high catalytic activity in the removal of ethyl acetate (90% conversion into CO2 at 256°C) and high stability during 100 h of reaction. The addition of toluene or water vapour in the feed gas did not significantly affect the activity of the coated monolith.

The Tremendous Catalytic Activities of the Cryptomelane-Type Manganese Oxide Octahedral Molecular Sieve Prepared Without Calcination Process for Degradation of Methylene Blue

The cryptomelane-type manganese oxides octahedral molecular sieve with different crystalline phases have been successfully synthesized and characterized. The oxides were synthesized using precipitation method through the redox reaction between KMnO4 and C6H8O7 with a mole ratio of 5:1 by the variation of flow rates (1, 2, and 3 mL/min). The prepared catalysts were characterized by XRD to determine their phase structures, purity, and crystallinity. The XRD results indicated that the samples without heat treatment (calcination) displayed the poor tunnel structures of cryptomelane-type manganese oxides with low intensities and broad peaks. Upon the thermal treatment at 600°C for 4 hours, the much more crystalline phases of similar cryptomelane structures were obtained. The high flow rate results in the more crystalline cryptomelane for calcined samples, whereas the flow rate has no substantial influence on the crystallinity of uncalcined samples. The catalytic activities of both oxides were evaluated for the degradation of methylene blue (MB) with H2O2 as an oxidant. The catalytic degradation of MB greatly enhanced using the uncalcined cryptomelane compared to calcined cryptomelane. The high flow rate has a positive impact on the degradation of MB for calcined samples, but the effect of flow rate results in the higher degradation of MB until a certain flow rate. The 98.4% degradation of MB was achieved by uncalcined samples under the optimum condition.

PH-Control of Oxidative Degradation of Methylene Blue Over Octahedral Molecular Sieve Cryptomelane-Type Manganese Oxide (OMS-2) and Fe-OMS-2

The octahedral molecular sieve cryptomelane-type manganese oxide (OMS-2) and Fe-doped OMS-2 (1%, 5%, 10%) were successfully synthesized by the redox reaction between KMnO4 and citric acid using the sol-gel method. The sol products were heated at 120°C and followed by calcination at 600°C. The as-synthesized pristine manganese oxide and Fe-OMS-2 were then characterized by XRD to determine their crystallinity and phase structure. The XRD results indicated that both the pristine and Fe-doped manganese oxides produced tunnel cryptomelane-type manganese oxides. No other peaks due to other phases are detected, indicating that Fe ions are well distributed in the framework of cryptomelane. The oxidative activities, either the pristine and Fe-OMS-2, were evaluated for degradation of methylene blue (MB) at pH ranges 2-6 by the addition of HCl solution. The oxidative tests showed that the oxidative degradation of methylene blue greatly increase with a decrease in solution pH and decrease with an increase in concentration Fe species in Fe-OMS-2 compared to pristine cryptomelane.

The Tremendous Influence of Calcination Process on the Phase Structure and Catalytic Activity of Precipitation-Processed MnO2

The layer and tunnel manganese oxides are versatile materials and have been proposed for various applications. These materials are prepared by a wide range of methods such as sol-gel, solid-state, precipitation and etc. Here, both manganese oxides of birnessite (layer) and cryptomelane (tunnel) have been successfully synthesized using the precipitation method by the reaction between KMnO4 and glucose with a mole ratio of 3:1. XRD results indicated that the birnessite-type manganese oxide was obtained when the brownish-black precipitate was heated up to 120°C, whereas cryptomelane-type manganese oxide was generated when the as-synthesized sample was further calcined up to 600°C. The birnessite sample displays poor crystalline material with low intensity and broad peak. The heat-treated birnessite up to 600°C leads to the formation much more crystalline tunnel cryptomelane–type mangane oxide. The catalytic activities of the as-synthesized catalysts were tested for the degradation of methylene blue (MB) dye with H2O2 as an oxidant. The birnessite catalyst shows much higher catalytic activity for the degradation of methylene blue compared to the cryptomelane catalyst. The tremendous improved catalytic activity of birnessite catalyst are correlated with its poor crystallinity and higher surface area of as indicated by the BET surface area and the XRD results.

The Preparation of Cu-Modified Cryptomelane Type-Manganese Oxide Catalysts by one Step sol-gel Synthesis for Decomposition of Methylene Blue

The cryptomelane manganese oxide octahedral molecular sieve (OMS-2) and Cu-modified cryptomelane octahedral molecular sieve (Cu-OMS-2) with different Cu concentrations (1, 5, 10%) were successfully prepared by one-step sol-gel synthesis. The XRD patterns of OMS-2 and Cu-OMS-2 remain unchanged, indicating the Cu ions are well incorporated within OMS-2, except the peak due to (101) plane appears and grows significantly until 5% Cu incorporation. The SEM images revealed that the additions of Cu ions in the OMS-2 have a remarkable impact on the Cu-OMS-2 morphologies, especially for 10% Cu incorporation. The bulk analysis by AAS showed the steady decreases in K+ and Mn concentrations, and increase in Cu concentrations, suggesting that some Cu2+ ions are replacing K+ in the tunnel and Mn in the framework of OMS-2. The catalytic tests for the degradation methylene blue (MB) by heterogeneous Fenton processes revealed that the catalytic performance of Cu-OMS-2 increases remarkably with the increased concentrations of Cu doping. The increased catalytic activities for Cu-OMS-2 are associated with the smaller particles sizes are revealed by SEM results.

Preparation of ceramic and metallic monoliths coated with cryptomelane as catalysts for VOC abatement

Ceramic and metallic monoliths coated with cryptomelane-type manganese oxides were prepared and tested as catalysts for the oxidation of ethyl acetate. Several preparation conditions were optimized in order to enhance the activity of the monoliths. Monoliths with different cell sizes, prepared with the same number of immersions in the washcoating solution or with similar loadings were tested in order to evaluate the effect of this parameter on the adherence of the active phase and on the catalytic activity of the final monolith. Several characterization techniques were used to assess the properties of these monoliths and it was found that almost all properties of the active phase were maintained after the washcoating process.The thermal conductivity of the structured supports must be taken into account in order to interpret the catalytic results in highly endothermic or highly exothermic reactions. The location of the measurement thermocouple is decisive when analyzing the results since it can give rise to apparent differences in activity. In our specific case, the available data suggest that the specific activity of the catalyst is not significantly altered by the deposition process or by the properties of the supports used.

Synthesis of flexible cross-linked cryptomelane-type manganese oxide nanowire membranes and their application for U(VI) and Eu(III) elimination from solutions

The efficient elimination of radionuclides from aqueous solutions is crucial for environmental protection from radionuclides’ pollution. The construction of octahedral molecular sieve (OMS-2) nanoporous membranes is of great technical importance for different kinds of applications like purification filter, catalysts, oil separation, seawater desalination and pollutant elimination. The facile synthesis of freestanding materials demonstrated uniform and monodispersed nanostructures and a cross-sectional pattern of the membranes, which is easily recycled in the process of adsorption and separation of pollutants from aqueous solutions. The OMS-2 exhibited rapid adsorption kinetics and high adsorption capacities of U(VI) (348 mg/g) and Eu(III) (106 mg/g) at pH = 5.0 ± 0.1 and T = 298 K. The paper-like membranes still retained the original filiform morphologies and showed extraordinary stability after 6 adsorption cycles. The surface complexation and ion exchange were the main mechanisms for the uptake of radionuclides, and the change of water environment had little influence on radionuclide adsorption efficiency. Based on the high adsorption capacity and excellent recycle ability, the OMS-2 membranes are considered as superior adsorbents for the efficient elimination of metal ions in the decontamination of polluted water.

Effective reduction of metronidazole over the cryptomelane-type manganese oxide octahedral molecular sieve (K-OMS-2) catalyst: facile synthesis, experimental design and modeling, statistical analysis, and identification of by-products.

High concentrations of antibiotic compounds within pharmaceutical wastewater have hazardous impacts toward environment and human health. Therefore, there is an immediate requirement of efficient treatment method for removal of antibiotics from aquatic environment. In the present study, the cryptomelane catalyst-type manganese oxide octahedral molecular sieve (K-OMS-2) was synthesized in the presence of benzyl alcohol as a reducing agent and cetyltrimethylammonium bromide as a structure-directing agent and then utilized to reduce the metronidazole. The central composite design method was the experimental design adopted. The FESEM analysis revealed that the K-OMS-2 surface contained many uniformly cylindrical aggregates less than about 40nm in diameter and about 80-100nm in length. Besides, a high specific surface area of 129m2/g and average pore size of 45.47nm were recorded. According to the TGA/DTA analysis, the prepared catalyst revealed high thermal stability. The maximum metronidazole degradation (95.36%) was evident at conditions of pH = 3, catalyst mass = 0.97g/L, contact time = 200min, and metronidazole concentration = 20mg/L. Metronidazole did not form a complex with nitrate, fluoride, sulfate, or hardness. These ions exerted a negligible effect on metronidazole reduction using the K-OMS-2 catalyst, except for hardness, which reduced the removal efficiency of metronidazole by 17%. The FTIR and LC-MS revealed a complex mechanism involved in the metronidazole degradation by the K-OMS-2 involving the formation of an amino group, a hydroxyelated compound via N-denitration, and hydrogenation process on the K-OMS-2 catalyst surface.

Silver incorporated into cryptomelane-type Manganese oxide boosts the catalytic oxidation of benzene

Different amounts of silver were successfully incorporated into cryptomelane-type manganese oxide (K-OMS-2) via a one-step hydrothermal method for application in the catalytic combustion of benzene. Silver incorporation could promote the benzene oxidation performance of the catalysts. The silver doping effect was addressed in terms of the relationship between structure and activity. The prepared catalysts were characterized by ICP-OES, BET, XRD, Raman, FE-SEM, TEM, XPS, XAFS, H2-TPR and CO-TPD. The best precursor Mn/Ag mole ratio was 40. The resulting K/Ag-OMS-40 catalyst exhibited the highest activity in terms of benzene combustion and good tolerance to chlorine poisoning, all of which make it a promising candidate as an alternative to noble metal supported catalysts. All catalysts after silver incorporation maintained the structural integrity of the cryptomelane structure but with decreased crystalline size, which significantly increased the surface area and number of defects of the catalyst. The silver species, mostly in the form of Ag+, were well dispersed and partially replaced K+ in the tunnels of cryptomelane. K/Ag-OMS-40 had the largest surface area, the smallest nanorods and most abundant Mn octahedral defects. The large number of active oxygen species derived from the high Mn3+ content and Ag-O-Mn bridge bonds appeared to play critical roles in VOC decomposition.

Phase evolution and electronic properties of cryptomelane nanorods

Potassium functionalized cryptomelane-type manganese oxide (K-OMS-2) is gaining more and more attention due to its exceptional catalytic, electronic, magnetic and ion transport properties. It is therefore of great importance to have a general picture of the changes this material undergoes with the increase of the temperature in reducing and oxidizing atmospheres. Potassium cryptomelane was synthesized by the reflux method in the reaction of manganese(II) acetate and potassium permanganate. It was characterized by means powder X-ray diffraction and Raman spectroscopy at characteristic stages of high-temperature treatment, determined by means of thermogravimetric analysis. Moreover, the mobility of potassium and thus induced changes of electronic properties were determined by using temperature programmed potassium desorption experiments and by following work function changes at elevated temperatures. The presented results can be used as a guideline for potential application limits, where the operation temperature and atmosphere play a crucial role.

Cryptomelane-type manganese oxide (KMn8O16) nanorods cathode materials synthesized by a rheological phase for lithium ion batteries

Cryotolerance-type manganese oxide (KMn8O16) nanorods were prepared for the first time by a rheological phase reaction method. The KMn8O16 samples were characterized by X-ray diffraction, scanning electron microscopy, the effects of different annealed temperatures on the morphologies and electrochemical properties of the final products were systematically investigated. The result that the annealed samples exhibit the superior electrochemical performances compared to the unannealed sample. The KMn8O16 nanorods annealed at 400 °C show the highest reversible discharge capacity (147.9 mAh/g even after 80 cycles) at current density of 50 mA/g and the best cycling stability. These results indicate that the KMn8O16 nanorods could be a promising cathode material for lithium ion batteries.

Editors' Choice—Electrocatalyzed Oxygen Reduction at Manganese Oxide Nanoarchitectures: From Electroanalytical Characterization to Device-Relevant Performance in Composite Electrodes

We assess the effect of the pore–solid architecture of cryptomelane-type manganese oxide (MnOx) xerogels and aerogels on electrocatalysis of the oxygen-reduction reaction (ORR) using three different electrochemical test platforms. Rotating-disk electrode measurements at ink-cast films of carbon + MnOx show that both MnOx nanoarchitectures exhibit comparable intrinsic ORR activity for four-electron reduction with a low onset overpotential (∼310 mV). The MnOx xerogel and aerogel powders were also incorporated into practical powder–composite electrodes that include carbon and polymer binder. When evaluated in an air-breathing three-electrode electroanalytical cell, the aerogel-based composite electrode exhibits an overpotential lowered by ∼50 mV compared to the xerogel-based analog. The superior performance of the aerogel-based composite is also demonstrated in zinc–air button cells, with up to 100 mV improvement in discharge voltage at moderate-to-challenging current densities (5–125 mA cm–2). We ascribe the enhanced activity of the MnOx aerogel-based composite to a more uniform dispersion of the aerogel powder within the carbon/binder matrix, as verified by focused-ion beam−scanning electron microscopy and elemental mapping. The results reported herein highlight the importance of assessing the translation of electrocatalytic activity from fundamental measurements to technologically relevent electrode structures.

Polydopamine-Mediated Formation of MnSn(OH)6 on Cryptomelane-Type Manganese Oxide for Catalyzing Glucose Isomerization to Fructose

Efficient isomerization of glucose into fructose contributes to the utilization of biomass producing 5-hydroxymethylfurfural (HMF). In this work, we have explored a novel catalyst for isomerization of glucose into fructose. By the mediation of polydopmaine (PDA), MnSn(OH)6 was formed on cryptomelane-type manganese oxide (OMS-2). The composite MnSn(OH)6/PDA@OMS-2 can efficiently catalyze the glucose isomerization to fructose, with a yield of fructose 47 ± 1.6%. When used together with the Bronsted acid catalyst Amberlyst-15, the HMF yield from glucose was 53 ± 2.5%. Consecutive use of the composite demonstrated that after eight cycles the activity can be retained in terms of the yield of fructose. The mechanism of polydopamine-mediated formation of MnSn(OH)6 has been investigated. The desired yield of fructose has been obtained by MnSn(OH)6/PDA@OMS-2, and the preparation of the catalyst has also shown advantages, including low cost and easy preparation, short preparation time (36 h), and low temperature (≤...

Catalytic oxidation of carbon monoxide, toluene, and ethyl acetate over the xPd/OMS-2 catalysts: Effect of Pd loading

The Pd catalyst supported on cryptomelane-type manganese oxide octahedral molecular sieve (OMS-2) were prepared. The effect of Pd loading on the catalytic oxidation of carbon monoxide, toluene, and ethyl acetate over xPd/OMS-2 has been investigated. The results show that the Pd loading plays an important role on the physicochemical properties of the xPd/OMS-2 catalysts which outperform the Pd-free counterpart with the 0.5Pd/OMS-2 catalyst being the best. The temperature for 50% conversion was 25, 240 and 160 °C, and the temperature for 90% conversion was 55, 285 and 200 °C for oxidation of CO, toluene, and ethyl acetate, respectively. The lowtemperature reducibility and high oxygen mobility of xPd/OMS-2 are the factors contributable to the excellent catalytic performance of 0.5Pd/OMS-2.

Effect of preparation method on the surface characteristics and activity of the Pd/OMS-2 catalysts for the oxidation of carbon monoxide, toluene, and ethyl acetate

The cryptomelane-type manganese oxide octahedral molecular sieve (OMS-2)-supported Pd (0.5wt% Pd/OMS-2-DP, 0.5wt% Pd/OMS-2-PI, and 0.5wt% Pd/OMS-2-EX) catalysts were prepared by the deposition-precipitation, pre-incorporation, and ion-exchanging strategies, respectively. It is shown that the preparation method exerted an important effect on the physicochemical property of the sample. Among the OMS-2-supported Pd catalysts, 0.5wt% Pd/OMS-2-DP possessed the highest surface (Mn2++Mn3+)/Mn4+ atomic ratio and the highest surface Pd loading and acid sites. The 0.5wt% Pd/OMS-2 catalysts outperformed the Pd-free counterpart, among which 0.5wt% Pd/OMS-2-DP presented the best catalytic activity (T50% and T90% were 25 and 55°C for CO oxidation, 240 and 285°C for toluene oxidation, and 160 and 200°C for ethyl acetate oxidation, respectively). We believe that the high Pd surface loading, high surface atomic ratio of (Mn2++Mn3+)/Mn4+, and good low-temperature reducibility, good oxygen mobility, and high acidity were responsible for the excellent performance of the 0.5wt% Pd/OMS-2-DP catalyst.

Transition metal doped cryptomelane-type manganese oxide catalysts for ozone decomposition

Transition metal (Cerium, Cobalt, and Iron) doped cryptomelane-type manganese oxide (M-OMS-2) catalysts have been successfully synthesized and characterized. The different metal-ion-doped K-OMS-2 catalysts showed distinct differences in their ozone decomposition activity. Cerium-doped OMS-2 materials showed ozone conversion of 90% under RH=90% and space velocity of 600,000h−1. X-ray diffraction (XRD), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) data suggested that the as-synthesized M-OMS-2 materials were all crystalline with no segregated metal oxide impurities. ICP-OES and XAFS results confirmed that Co3+ and Fe3+ replace Mn3+ in the cryptomelane structure and Ce4+mainly replaces the K+ in the tunnel and partially replaces the Mn4+ in the framework of the cryptomelane structure. Because of the differences in the substitution sites and the ionic radius of dopants, the morphologies of the catalysts were different. The Mn3+content and number of surface defects play a key role during the decomposition of ozone. Ce-OMS-2 is a promising catalyst for purifying waste gases containing ozone under high-humidity conditions.

Tremendous effect of oxygen vacancy defects on the oxidation of arsenite to arsenate on cryptomelane-type manganese oxide

Manganese oxide as a benign and low-cost oxidant has been widely investigated for the oxidation of As(III) to As(V), but its As(III) oxidation rate is easily hindered due to the thermodynamically favorable adsorption of the As(V) species produced and coexisting ions. Herein, we develop a novel strategy of tremendously increasing the As(III) oxidation activity via tuning the concentration of oxygen vacancy defects (OVD) in cryptomelane-type manganese oxides (OMS-2). The result of the batch experiment of the As(III) oxidation on OMS-2 shows that increasing the concentration of oxygen vacancy defects in OMS-2 remarkably increases the specific As(III) oxidation rate from 0.12 to 0.22μmolm−2min−1, and dramatically reduces the unfavorable effect of the As(V) adsorption on the As(III) oxidation activity. The origin of the tremendous effect of OVD in OMS-2 on the As(III) oxidation activity is experimentally and theoretically investigated by the adsorption of the As(III) and As(V) species on OMS-2, the point of zero charge (pHPZC), the evolution of the concentration of Mn2+ ions dissolved in the solution, and density functional theory (DFT) calculations. The results reveal that the presence of OVD in OMS-2 not only makes the adsorption of As(III) on OMS-2 more thermodynamically favorable, but also accelerates the charge transfer from As(III) to Mn atom, thus significantly improving the As(III) oxidation activity.

CO oxidation over gold supported on Cs, Li and Ti-doped cryptomelane materials

Cryptomelane-type manganese oxides were synthesized by redox reaction under acid and reflux conditions. Different metals (cesium, lithium and titanium) were incorporated into the tunnel structure by the ion-exchange technique. Gold was loaded onto these materials (1wt%) by a double impregnation method. The obtained catalysts were characterized by high-resolution transmission electron microscopy, energy-dispersive X-ray spectrometry, scanning electron microscopy, X-ray diffraction and temperature-programmed reduction. The catalytic activity of these materials was evaluated in the oxidation of carbon monoxide. The incorporation of Cs, Li or Ti into cryptomelane was detrimental in terms of catalytic activity. Further addition of gold to cryptomelane doped materials significantly improved the catalytic performance, especially for Cs-K-OMS-2 and Li–K-OMS-2 (to a smaller extent). Addition of gold to the Ti containing material did not show a significant improvement. The observed trends are related to the effect of gold on samples reducibility and to the gold particle size. The lattice oxygen can also be considered accountable for the activity of the materials, since the most active cryptomelane catalysts are those with higher lattice oxygen donating ability for the oxidation of the CO molecule.

Oxidative coupling of anilines to azobenzenes using heterogeneous manganese oxide catalysts

Cryptomelane-type manganese oxide shows high efficiency for the NN formation reaction in the oxidative coupling of anilines to azobenzenes. The difference of the Hammett constants (Δσ) of two substituted groups determines the selectivity to unsymmetric azobenzenes in cross-coupling reactions, which are favored at Δσ< 0.32.