Catalytic Activity Of Manganese Oxides Research Articles

Tin-doped manganese octahedral molecular sieve catalysts with efficient water resistance for CO oxidation

It is challenging to improve the low-temperature catalytic activity and water resistance of manganese oxide for CO oxidation. Herein, Sn is successfully doped into manganese octahedral molecular sieve (OMS-2) by hydrothermal synthesis. The doping of Sn promotes the low-temperature activity and water resistance of OMS-2 catalysts, and the OMS-5Sn catalyst exhibits the best performance for CO oxidation, which T90 under dry and humid condition are 100 °C and 179 °C, respectively. According to the results of CO-TPR, O2-TPD, XPS, FT-IR, and ex situ/in situ Raman, the appropriate Sn doping increases the oxygen vacancies and enhances the surface hydrophilicity. Furthermore, the H218O isotope experiment reveals that water can participates in CO oxidation with a surface hydroxyl-mediated Mars-Van Krevelen reaction pathway. Significantly, the study improves the catalytic activity of manganese oxide for CO oxidation and could provide a new proposal to design the water-resistant catalysts incatalytic oxidation reaction.

Enhancing Oxygen Reduction Reaction Activity of α-MnO2 Nanowires Through Ag Doping

Enhancing the catalytic activity of manganese oxide in oxygen reduction reaction (ORR) is a key issue for its large-scale application in metal-air fuel cells. Ag-doped [Formula: see text]-MnO2 nanowires without Ag or Ag2O have been successfully synthesized via a facile hydrothermal method, and the changes in both the structure and electrochemical catalytic performances after Ag doping are investigated. Compared with the pristine [Formula: see text]-MnO2, the as-prepared Ag-doped MnO2 exhibits a significantly enhanced catalytic activity in both ORR and Mg-air fuel cell application. With Ag/Mn ratio of 1:25, Ag-doped MnO2 exhibits a typical 4e-reaction pathway and presents a 163 mV higher half-wave potential than that of the pristine [Formula: see text]-MnO2. Furthermore, it demonstrates a power density of 75.1[Formula: see text]mW[Formula: see text]cm[Formula: see text] at current density of 134.5[Formula: see text]mA[Formula: see text]cm[Formula: see text] in the Mg-air fuel cells. The enhanced ORR performances are considered to be contributed from the activation of surface lattice oxygen, the improvement in conductivity and the increase in oxygen vacancies of [Formula: see text]-MnO2. These findings provide new understanding for developing high-performance manganese oxide catalysts.

Single-ion copper doping greatly enhances catalytic activity of manganese oxides via electronic interactions.

Single-ion copper doping significantly improves the catalytic activity of α-MnO2 in CO oxidation, reducing the apparent activation energy to ∼0.3 eV via strong electronic interactions between the frontier orbitals of copper ions and manganese ions.

Construction of manganese oxide nanowire-like cluster arrays on a DNA template: Application to detection of hydrogen peroxide

Improved electron transfer properties and catalytic activity of manganese oxide (MnOx) was demonstrated following its electrochemical deposition on a deoxyribonucleic acid (DNA) modified glassy carbon electrode. The MnOx showed different morphologies, electrocatalytic properties and electrochemical kinetics. Scanning electron microscopy showed that electrodeposition of MnOx on a bare glassy carbon electrode led to the formation of irregular-shapes while a nanowire cluster (NWC) was formed on a GCE/DNA due to the DNA serving as a template. Electrochemical impedance spectroscopy (EIS) revealed lower charge transfer resistance of the MnOxNWC compared with MnOx. A new mechanism is presented for the electrodeposition of MnOx on the surface of a GC/DNA electrode. An electrochemical biosensor was fabricated based on depositing MnOx onto a glassy carbon /DNA electrode (GCE/DNA/MnOxNWC) and was used to detect hydrogen peroxide (H2O2). The MnOx nanowire cluster and DNA exhibited significant electrocatalytic activity for simultaneous electrocatalytic oxidation at two oxidation potentials (0.6 V and 0.98 V vs Ag/AgCl) and one reduction potential (−0.5 V vs Ag/AgCl) for H2O2 at pH 6.0. A new mechanism for the detection of H2O2 is presented. Excellent electrocatalytic activity, stability and facility for simultaneous detection of H2O2 at different of applied potentials are proposed advantages of the proposed electrochemical biosensor.

Catalytic Carbon Monoxide Oxidation over Potassium-Doped Manganese Dioxide Nanoparticles Synthesized by Spray Drying

Manganese oxides are promising catalysts for the oxidation of CO as well as the removal of volatile organic compounds from exhaust gases because of their structural versatility and their ability to reversibly change between various oxidation states. MnO2 nanoparticles doped with Na+ or K+ were synthesized by a semi-continuous precipitation method based on spray drying. Specific surface area, crystallite size, and morphology of these particles were predominantly determined by the spray-drying parameters controlling the quenching of the crystallite growth, whereas thermal stability, reducibility, and phase composition were strongly influenced by the alkali ion doping. Pure α-MnO2 was obtained by K+ doping under alkaline reaction conditions followed by calcination at 450 °C, which revealed a superior catalytic activity in comparison to X-ray amorphous or Mn2O3-containing samples. Thus, the phase composition is identified as a key factor for the catalytic activity of manganese oxides, and it was possible to achieve a similar activation of a K+-doped X-ray amorphous catalyst under reaction conditions resulting in the formation of crystalline α-MnO2. The beneficial effect of K+ doping on the catalytic activity of MnO2 is mainly associated with the stabilizing effect of K+ on the α-MnO2 tunnel structure.

Generating more Mn4+ ions on surface of nonstoichiometric MnO2 nanorods via microwave heating for improved oxygen electroreduction

Catalytic activities of transition metal oxides are usually influenced by their surface states. Herein, microwave heating is exploited as a fast and economic alternative approach to conventional postsynthesis calcination to effectively tune the surface states of nonstoichiometric MnO2 nanorods. After heating with microwave for 30 min, although the crystalline phase and morphology of nonstoichiometric MnO2 nanorods are largely reserved, the content of high-valent Mn cations, i.e. Mn4+, on the surface of nonstoichiometric MnO2 is maximized, which leads to a remarkably reduced charge transfer resistance for ORR, and the corresponding catalytic activity of oxygen reduction reaction (ORR) is markedly enhanced and becomes close to that of commercial Pt/C catalyst, within the context of onset potential, diffusion-limiting current density and average electron transfer number. The present work not only enriches the way to tune the ORR activity of catalysts, but also provides a deeper insight into the influence of surface state on electrochemically catalytic activity of manganese oxides.

Charge-transfer modified embedded-atom method for manganese oxides: Nanostructuring effects on MnO2 nanorods

In spite of the superior performance of nanostructured manganese oxides as materials for electro-catalysis, there has been low progress in their theoretical understanding because of the large scale of the required simulation cells. In this work, we have developed a large-scale simulation method for manganese oxides based on the Charge-Transfer Modified Embedded-Atom Method (CT-MEAM), which is not only capable of describing complex atomic bonding in transition metal oxides but also dynamic oxidation states, as those induced by the widely exposed surface area of nanostructures. The reliability of the method is confirmed by comparison with the corresponding ab initio reference results, and afterwards used to investigate the nanostructuring effects on manganese oxide nanorods. We found that nanostructuring introduces surface and edge structural distortions, driven by emerged unsaturated surface ions. These ions are highly mobile, and easily migrate to form new bonds with neighboring ions. Nanostructuring also weakens charge transfer between Mn and O ions located at the outer layers, thus bringing in more reductive Mn ions and enabling oxygen vacancy formation, which ultimately enhances the catalytic activity of manganese oxides. Our results also show that large scale surface defects, such as voids, could produce a similar effect than size reduction in improving the catalytic performance of the nanorod, thus avoiding undesirable amorphization instabilities, which are inevitable at reduced sizes. These findings will help to the optimization of the design and synthesis of nanostructured manganese oxides with enhanced catalytic activity.

Understanding the role of gold nanoparticles in enhancing the catalytic activity of manganese oxides in water oxidation reactions.

The Earth-abundant and inexpensive manganese oxides (MnOx) have emerged as an intriguing type of catalysts for the water oxidation reaction. However, the overall turnover frequencies of MnOx catalysts are still much lower than that of nanostructured IrO2 and RuO2 catalysts. Herein, we demonstrate that doping MnOx polymorphs with gold nanoparticles (AuNPs) can result in a strong enhancement of catalytic activity for the water oxidation reaction. It is observed that, for the first time, the catalytic activity of MnOx/AuNPs catalysts correlates strongly with the initial valence of the Mn centers. By promoting the formation of Mn(3+) species, a small amount of AuNPs (<5%) in α-MnO2/AuNP catalysts significantly improved the catalytic activity up to 8.2 times in the photochemical and 6 times in the electrochemical system, compared with the activity of pure α-MnO2.

Understanding the Role of Gold Nanoparticles in Enhancing the Catalytic Activity of Manganese Oxides in Water Oxidation Reactions

AbstractThe Earth‐abundant and inexpensive manganese oxides (MnOx) have emerged as an intriguing type of catalysts for the water oxidation reaction. However, the overall turnover frequencies of MnOx catalysts are still much lower than that of nanostructured IrO2 and RuO2 catalysts. Herein, we demonstrate that doping MnOx polymorphs with gold nanoparticles (AuNPs) can result in a strong enhancement of catalytic activity for the water oxidation reaction. It is observed that, for the first time, the catalytic activity of MnOx/AuNPs catalysts correlates strongly with the initial valence of the Mn centers. By promoting the formation of Mn3+ species, a small amount of AuNPs (&lt;5 %) in α‐MnO2/AuNP catalysts significantly improved the catalytic activity up to 8.2 times in the photochemical and 6 times in the electrochemical system, compared with the activity of pure α‐MnO2.

Catalytic Activity of Manganese Oxide Anchored on to Alumina and Zirconia for the Synthesis of Dihydropyrimidinones: Role of Acidity, Crystal Phase and Morphology

Manganese oxide supported on alumina and zirconia has been prepared by precipitation-impregnation method. The materials have been characterized by FT-IR, P-XRD, BET, TPD-NH3, TGA, SEM and TEM techniques for their textural properties. The catalytic activity of these materials has been explored in the synthesis of dihydropyrimidinones via.,Biginelli reaction. A rod like morphology is observed for zirconia containing 5% manganese oxide. These materials exhibited good catalytic activity in terms of isolated yield (>97%) and selectivity (100%) for the synthesis of dihydropyrimidinones. Acidity of the catalysts and the tetragonal phase of zirconia were found to play an important role in catalyzing this reaction. The catalyst with 5%Mn/ZrO2 exhibited distinctly different catalytic activity. The catalyst is reusable without losing its activity upto 5 cycles.

Catalytic activity of manganese oxide supported on alumina in the synthesis of quinoxalines

Two catalysts, alumina and manganese oxide supported on alumina, have been prepared by calcination and precipitation-impregnation methods, respectively. The catalysts are characterised by the following techniques: Brunner-Emmett-Teller-N2 adsorption-desorption for surface area, temperature programmed desorption of NH3 and n-butyl amine back titration methods for surface acidity, powder X-ray diffraction for textural properties, and Fourier transform infrared spectroscopy for the anionic radicals. The catalytic activity has been determined under heterogeneous conditions in the condensation reaction between o-phenylenediamine and benzil. The product purity is checked by thin-layer chromatography and melting point. The products are also analysed by LC-MS and 1H-NMR techniques. The yields of the products have been found to be good and catalysts exhibited excellent recyclability. The effect of changing the reaction parameters such as temperature, reaction time, amount of the catalyst, nature of solvent and molar ratio of reactants on the yield of the product has been studied. The surface acidity of the catalysts plays an important role in activating the reaction.

H2O and SO2 deactivation mechanism of MnOx/MWCNTs for low-temperature SCR of NOx with NH3

The effect of H2O and SO2 on the catalytic activity of manganese oxides supported on multi-walled carbon nanotubes (MnOx/MWCNTs) for low-temperature selective catalytic reduction (SCR) of NOx with NH3 was studied. Also, N2 adsorption, transient response experiments, X-ray powder diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FT-IR) spectroscopy and in situ FT-IR spectroscopy were performed to investigate the deactivation mechanism of MnOx/MWCNTs catalyst in the presence of H2O or SO2. Experimental results showed that H2O had a reversible negative effect on the catalytic activity of the catalyst. When the temperature was higher than 270°C, the effect of H2O could be negligible. The competitive adsorption of H2O and NH3 on the Lewis acid sites contributed to the deactivation of the catalyst. The integrity increase of MWCNTs in the presence of H2O might be another reason for the deactivation of the catalyst. However, SO2 led to the irreversible deactivation of the catalyst. The higher the reaction temperature, the more dramatically the catalystic activity decreased. The sulfation of the active center atoms was the main poisoning route. Also, formation of ammonium sulfates on the catalyst surface and the competitive adsorption between SO2 and NO were responsible for the partial deactivation of the catalyst to some extent.

Manganese oxide–graphene composite as an efficient catalyst for 4-electron reduction of oxygen in alkaline media

The catalytic activity of manganese oxide (MnOx)–graphene composite prepared by a physical method towards oxygen (O2) reduction reaction in 0.1M KOH solution has been investigated. For a better performance, MnOx materials with different crystallographic structures and morphologies have been synthesized, and their catalytic properties and stability for O2 reduction are evaluated with linear sweep voltammetry and current–time chronoamperometry. The results show that the catalytic activity decreases in the sequence: α-MnO2 nanowire>amorphous MnOx nanoparticle>β-MnO2 microprism, and α-MnO2 nanowire is more stable than amorphous MnOx nanoparticle. Thus, α-MnO2 nanowire is selected to combine with graphene. The modification of graphene on GC electrode leads to ca. 130mV positive shift of the half-wave potential for the reduction of O2 to peroxide (HO2−), and further introduction of α-MnO2 nanowire catalyzes the disproportionation of HO2− produced effectively, which enables 4-electron reduction of O2 at low overpotential. In addition, the composite possesses good durability.

Synthesis and Characterization of Manganese Oxide and Investigation of its Catalytic Activities for Oxidation of Benzyl Alcohol in Liquid Phase

Manganese oxide was prepared by mechano-chemical process in solid state and was characterized by chemical and physical techniques. Chemical techniques include determination of oxygen content while physical techniques, includes surface area and pore size analyses, particle size, XRD, FTIR and SEM analyses. The synthesized manganese oxide was shown to have octahedral layered structure. The catalytic activity of manganese oxide was studied by carrying out the oxidation of benzyl alcohol in liquid phase using n-octane as solvent at temperature (&lt;373) and oxygen pressure of up to one atmosphere. Benzaldehyde and benzoic acid were identified as the reaction products. However, benzoic acid appeared in reaction mixture after complete conversion of benzyl alcohol to benzaldehyde. Influence of different reaction parameters like comparison of reduced and unreduced catalyst, leaching of catalyst, effect of air, oxygen and nitrogen, effect of catalyst loading, life span of catalyst, effect of time and effect of partial pressure of oxygen on the reaction performance was studied. The catalyst can easily be separated from the reaction mixture by filtration, and can be regenerated by washing with water and ethanol and drying at 323 K.

Plasma-electrolytic formation, composition and catalytic activity of manganese oxide containing structures on titanium

In the present work, we report the data about formation of TiO 2-rutile or TiO 2 and Mn 2O 3, Mn 3O 4 containing oxide structures on titanium in aqueous electrolytes by means of plasma-electrolytic deposition. The layers formed are characterized by X-ray diffraction, electron probe microanalysis and scanning electron microscopy methods. The PEO coatings on titanium formed in sodium tetraborate solution contain the TiO 2 stabile rutile modification that is important when utilizing such a structure as a catalyst carrier. Manganese acetate adding into the electrolyte leads to formation of layers that contain Mn 2O 3, Mn 3O 4 and TiO 2-rutile in outer region. The manganese content in the surface layer depends on the formation conditions as well as on manganese acetate concentration in the electrolyte. Catalytic activity of the layers in CO → CO 2 reaction is studied in the static and flow conditions. The manganese-containing layers obtained possess the catalytic activity in CO → CO 2 oxidation reaction at the temperature range of 250–350 °C. The catalytic activity depends on the concentration and surface distribution of manganese as well as on the layers morphology.

Bifunctional catalytic activity of manganese oxide toward O 2 reduction: novel insight into the mechanism of alkaline air electrode

The mechanism of electro-catalytic reduction of oxygen (O 2) with manganese oxide was studied by cyclic voltammetry and rotating ring-disc electrode voltammetry using MnOOH-modified glassy carbon (GC) electrodes in alkaline media. The O 2 reduction at the MnOOH-modified GC electrodes was found to undergo an electrochemical process followed by sequent disproportionations of the electrochemical reduction intermediates, i.e., superoxide anion (O 2 − ) and hydrogen peroxide (HO 2 −, in 0.10 M KOH solution). The manganese oxides, which are used as the electrocatalysts in the alkaline air electrodes, were first found to possess an excellent catalytic activity not only for the disproportionation of the produced O 2 − into O 2 and HO 2 − but also for that of the produced HO 2 − into O 2 and OH −. Such bifunctional catalytic activities of MnOOH for the sequent disproportionations of O 2 − and HO 2 −, combined with electrochemical reduction of O 2 mediated by surface functional groups at carbon electrode surface, substantially effect a quasi-four-electron reduction of O 2 at the MnOOH-based alkaline air electrodes.

Electrochemical Characterization of Catalytic Activities of Manganese Oxides to Oxygen Reduction in Alkaline Aqueous Solution

The catalytic function and activity of manganese oxides (MnOx: and MnOOH) to the electrochemical reduction of in 0.10 M KOH aqueous solution have been investigated by cyclic voltammetry at MnOx/Nafion-modified gold electrodes. Two successive reduction current peaks were observed at Nafion-modified electrodes in the cyclic voltammograms, for a two-electron reduction of to hydrogen peroxide and for a two-electron reduction of to The peak current heights of and changed greatly depending on the kind of MnOx species incorporated into the MnOx/Nafion-modified gold electrodes; increased and decreased. On the assumption that produced in the first reduction step is chemically decomposed into and with a catalytic action of MnOx and that this regenerated is reduced again in the same first reduction step, we evaluated the catalytic activity of MnOx using the values of and MnOOH provided the highest catalytic activity to the electrochemical reduction of This result was supported by another experiment by using a chemical method where catalytic decomposition of with MnOx was estimated by measuring the concentration directly with a commercial oxygen sensor. © 2002 The Electrochemical Society. All rights reserved.

Effect of ZnO doping on surface and catalytic properties of manganese oxides supported on alumina

The effect of ZnO doping (1.42–12.58 wt.%) on surface and physicochemical properties of manganese oxides supported on alumina has been investigated using nitrogen adsorption at −196 °C and catalytic decomposition of H 2O 2 reaction at 30–50 °C. The pure and doped solids were subjected to thermal treatment at 400–1000 °C. The thermal products were characterized via X-ray diffraction (XRD)-technique. The results revealed that the specific surface areas of various samples decreased monotonically as a function of both the extent of zinc oxide doping and the precalcination temperature. Moreover, the activation energy of sintering, Δ E s, was found to decrease by increasing the concentration of ZnO to 5.44 wt.% and increased with increasing the amount of dopant to 12.58 wt.%. The pure and doped mixed solids preheated at 400 °C produced crystalline β-MnO 2 phase and its pattern intensity increased with increasing ZnO concentration up to 5.44 wt.%. Crystalline Mn 2O 3 phase was detected at 600 and 800 °C and its pattern intensity decreased with increasing the dopant concentration. Calcination of samples at 1000 °C showed the formation of crystalline γ-Al 2O 3, Mn 3O 4, ZnAl 2O 4, and ZnMn 2O 4 phases. The catalytic activities of the pure and doped mixed solids increased with increasing the reaction temperature and calcination temperature up to 600 °C. The ZnO doping led to the decrease of the catalytic activity of manganese oxides supported on alumina system. No measurable activity was observed for samples precalcined at 1000 °C because of formation of inactive compounds and/or sintering process. The changes in the catalytic activity of pure and doped solids were attributed to the change in oxidation states.