MnO2 Surface Research Articles

Kinetics of colloidal MnO2 reduction by L-arginine in absence and presence of surfactants

The kinetics of the oxidation of L-arginine by water-soluble form of colloidal manganese dioxide has been studied using visible spectrophotometry in aqueous as well as micellar media. To obtain the rate constants as functions of [L-arginine], [MnO2] and [HClO4], pseudo-first-order conditions are maintained in each kinetic run. The first-order-rate is observed with respect to [MnO2], whereas fractional-order-rates are determined in both [L-arginine] and [HClO4]. Addition of sodium pyrophosphate and sodium fluoride enhanced the rate of the reaction. The effect of externally added manganese(II) sulphate is complex. It is not possible to predict the exact dependence of the rate constant on manganese(II) concentration, which has a series of reactions with other reactants. The anionic surfactant SDS neither catalyzed nor inhibited the oxidation reaction, while in presence of cationic surfactant CTAB the reaction is not possible due to flocculation of reaction mixture. The reaction is catalyzed by the nonionic surfactant TX-100 which is explained in terms of the mathematical model proposed by Tuncay et al. Activation parameters have been evaluated using Arrhenius and Eyring equations. On the basis of observed kinetic results, a probable mechanism for the reaction has been proposed which corresponds to fast adsorption of the reductant and hydrogen ion on the surface of colloidal MnO2.

Adsorption of dimethylamine from aqueous solution by manganese dioxide

Removal of precursors of N-nitrosodimethylamine (NDMA), such as the most direct precursor dimethylamine (DMA), might be an effective method to control NDMA formation during practical water treatment process. Adsorption of DMA onto manganese dioxide (MnO₂) from aqueous solution has been investigated using batch experiments in this study. Results indicate that DMA adsorption is initially rapid (in the first 5 h) and the adsorption process reaches a steady state after 15 h. The adsorption isotherms are well described by the Freundlich models. The desorption shows an irreversibility of DMA adsorption onto MnO₂. The effects of temperature, pH, ionic strength, humic acid, and the presence of other secondary aliphatic amines on adsorption processes are also examined. According to the experimental results, the amount of DMA adsorbed increases with an increase of temperature from 288 to 308 K, and with a decrease of ionic strength from 1 to 10 mM. The maximum adsorption appeared at pH 7.0. However, the amount of DMA adsorbed onto MnO₂ does not show obvious difference in the presence of humic acid. According to the results, it suggests that the adsorption is primarily brought about by electrostatic interaction between DMA and MnO₂ surface.

The Electrochemical Behavior and Catalytic Activity for Oxygen Reduction of MnO2/C– Toray Gas Diffusion Electrodes

Gas diffusion electrodes (GDE) composed of: Toray carbon paper gas diffusion layer (unteflonated and teflonated with 30%wt PTFE, respectively) and MnO2 supported on Vulcan XC72R catalyst layer with 20%wt PTFE, were investigated in 6 M KOH without and with O2, respectively. Four sources of MnO2 (Sigma-Aldrich, Tronox, Riedel and Merck Inc.) were comparatively studied by electrochemical methods accompanied by XRD characterization. Cyclic voltammetry scans of GDE in N2-purged electrolyte were used to estimate the Tafel slopes for Mn(IV) reduction and to identify active Mn(IV) sites that play an important catalytic role. Two oxygen reduction reaction (ORR) mechanisms were identified by porous rotating disk electrode (PRDE) polarization as a function of electrode potential. At high potentials (above −300 mV vs. MOE) the O2/HO2− step is mainly catalyzed by the quasi-unreduced MnO2 surface and active sites on the carbon support, while at potentials more negative than −300 mV, the redox catalysis by Mn(III) prevails. The main catalytic sites for the second step HO2−/OH−, are Mn(III) sites. The hydrophobic property of the porous electrode is important due to the effect on peroxide desorption/readsorption on the electrode surface. PRDE and flow cell experiments revealed the Sigma-Aldrich γ-MnO2 was the most active for ORR.

Impact of environmental conditions on the sorption behavior of radionuclide 60Co(II) on MnO2

The fate and transport of toxic metal ions and radionuclides in the environment is generally controlled by sorption reactions. The removal of 60Co(II) from wastewaters by MnO2 was studied as a function of various environmental parameters such as shaking time, pH, ionic strength, foreign ions, and humic substances under ambient conditions. The results indicated that the sorption of 60Co(II) on MnO2 was strongly dependent on pH and ionic strength. At low pH, the sorption of 60Co(II) was dominated by outer-sphere surface complexation and ion exchange with Na+/H+ on MnO2 surfaces, whereas inner-sphere surface complexation was the main sorption mechanism at high pH. The presence of HA/FA enhances 60Co(II) sorption at low pH values, whereas reduces 60Co(II) sorption at high pH values. The Langmuir and Freundlich models were used to simulate the sorption isotherms of 60Co(II) at three different temperatures of 298.15, 318.15 and 338.15 K. The thermodynamic parameters (ΔH0, ΔS0 and ΔG0) calculated from the temperature dependent sorption isotherms indicated that the sorption process of 60Co(II) on MnO2 was endothermic and spontaneous.

Gas Sensitivity of MnO<sub>2</sub> Doped Nano ZnO Coplanar Plate-Type Gas Sensor Arrays

Based on the integrated technologies of screen printing and hydrothermal method, this paper introduced a new integrative method with materials microstructure controlled growth and gas sensor fabrication. For example,based on space positioning using screen printing and ZnO nanwall with large specific surface area induced by hydrothermal method on the surface of ZnO film, the MnO2 doped nano ZnO coplanar plate-type gas sensor arrays with a good gas sensitivity to formaldehyde were fabricated successfully. The results show that the sensitivity to formaldehyde of MnO2 doped nano ZnO gas sensor arrays fabricated by hydrothermal method in 0.01mol/L Zn(NO3)2 solution, 95°C for 5h on the surface of MnO2 doped nano ZnO film and sintering at 500°C for 2h is higher than that of in distilled water. The existence of ZnO nanwall with large specific surface area induced by hydrothermal method on the surface of MnO2 doped nano ZnO film is the important reason of improvement for the sensitivity to formaldehyde.

PHOTO-REDUCTION KINETICS OF MnO<sub>2</sub> IN AQUATIC ENVIRONMENTS CONTAINING HUMIC ACIDS

The kinetics of photo-reduction of manganese(IV) oxide, MnO2 in aqueous medium, in the presence of humic acid has been investigated, including the possibility for the involvement of such radicals in the reduction. Results showed that in aqueous medium, the efficiency of the photo-reduction of MnO2 producing a soluble ionic Mn(II) was governed strongly by an internal electron transfer within an intermediate of HA-MnO2 anion complex, resulted in the chemical adsorption of humic acid on MnO2 surface. It was observed that under solar radiation and atmosphere containing oxygen, the rate of MnO2 reduction increased considerably, by factors of about 46 and 2 respectively. Moreover, there was evidence that some radicals certainly played a role in the photo-reduction of MnO2, since radical scavenger addition significantly decreased the reduction efficiency of MnO2. In the present of an excess of propanol-2, however, which was expected to remove all OH radicals present, only about 60 % decrease in reduction efficiency were observed, suggesting that radicals other than OH radical are involved in the reduction of MnO2. It was observed further that the presence of iron(III) in MnO2 up to 1 % (w/w) did not alter the reduction efficiency of MnO2. Keywords: MnO2 kinetics, photo-reduction, aquatic, humic acids

Macroporous MnO2 electrodes obtained by template assisted electrodeposition for electrochemical capacitors

Macroporous MnO2 electrodes prepared by template-assisted electrodeposition using spherical polystyrene colloidal particles are studied. The wettability of such electrodes by a LiClO4 aqueous electrolyte is measured by the contact angle technique. Cyclic voltammetry experiments are performed in order to evaluate the use of these electrodes for electrochemical capacitor applications. The specific capacity obtained is about 60% higher than that obtained for flat MnO2 surfaces showing that, in spite of the wettability being lower, some penetration of the electrolyte into the pores must occur, increasing the electroactive area with respect to the flat electrode. Furthermore, the macroporous electrode showed excellent electrochemical stability, with neither a capacitance decrease nor a loss of morphology, after 1000 cycles.

Bipolar resistive switching behavior in Ti/MnO2/Pt structure for nonvolatile memory devices

This study examined the electrical properties of Ti/MnO2/Pt devices with stable and reproducible bipolar resistive switching behavior. The dependency of the memory behavior on the cell area and operating temperature suggest that the conducting mechanism in the low resistance states is due to the locally conducting filaments formed. X-ray photoelectron spectroscopy showed that nonlattice oxygen ions form at the MnO2 surface. The mechanism of resistance switching in the system examined involves the generation and recovery of oxygen vacancies with the nonlattice oxygen ions.

Oxidative transformation of tetrachlorophenols and trichlorophenols by manganese dioxide

This study examined the transformation kinetics of three tetrachlorophenols (TeCPs) and three trichlorophenols (TCPs) in the presence of MnO2 under different solution chemistry conditions. The reaction rate measured for each CP decreased as a function of solution pH, and under the same solution chemistry conditions, the measured rates may depend primarily on both the adsorbability at the MnO2 surfaces and the isomeric structures of the CPs. Isomeric effects indicated that chloro substituent on ortho or para positions exhibited faster rates of transformation than on meta positions. Gas chromatography-mass spectrometry analysis with a derivatization method showed that dimers including polychlorinated phenoxyphenols and chlorinated polyhydroxybiphenyl were among the major products for all CPs. Monomeric products were among the major products of 2,4,6-TCP, 2,3,4-TCP, and 2,3,4,6-TeCP, whereas trimeric products also were among the major products of 2,3,4-TCP and 2,4,5-TCP. It appeared that hydroxylation of CPs and formation of dimeric or trimeric products via oxidative coupling were the major reaction mechanisms involved in the oxidation of CPs by MnO2.

Constraints on the utility of MnO2 cartridge method for the extraction of radionuclides: A case study using 234Th

Large volume (102–103 L) seawater samples are routinely processed to investigate the partitioning of particle reactive radionuclides and Ra between solution and size‐fractionated suspended particulate matter. One of the most frequently used methods to preconcentrate these nuclides from such large volumes involves extraction onto three filter cartridges (a prefilter for particulate species and two MnO2‐coated filters for dissolved species) connected in series. This method assumes that the extraction efficiency is uniform for both MnO2‐coated cartridges, that no dissolved species are removed by the prefilter, and that any adsorbed radionuclides are not desorbed from the MnO2‐coated cartridges during filtration. In this study, we utilized 234Th‐spiked coastal seawater and deionized water to address the removal of dissolved Th onto prefilters and MnO2‐coated filter cartridges. Experimental results provide the first data that indicate (1) a small fraction of dissolved Th (<6%) can be removed by the prefilter cartridge; (2) a small fraction of dissolved Th (<5%) retained by the MnO2 surface can also be desorbed, which undermines the assumption of uniform extraction efficiency for Th; and (3) the absolute and relative extraction efficiencies can vary widely. These experiments provide insight on the variability of the extraction efficiency of MnO2‐coated filter cartridges by comparing the relative and absolute efficiencies and recommend the use of a constant efficiency on the combined activity from two filter cartridges connected in series for future studies of dissolved 234Th and other radionuclides in natural waters using sequential filtration/extraction methods.

Sorption of Np(V), Pu(V), and Pu(IV) on colloids of Fe(III) oxides and hydrous oxides and MnO2

Sorption of Np(V), Pu(V), and Pu(IV) on colloids of synthetic goethite (α-FeOOH), hematite (α-Fe2O3), maghemite (γ-Fe2O3), and amorphous MnO2 was studied over wide ranges of solution pH and ionic strength by solvent extraction and X-ray photoelectron spectroscopy (XPS). Plutonium(V) is reduced upon sorption on α-FeOOH, but not on α-Fe2O3 and γ-Fe2O3. On the MnO2 surface, Pu occurs as Pu(VI). From the pH dependences of the actinide sorption, the equilibrium constants of the reactions of Np(V)O 2 + and Pu(V)O 2 + with the surface hydroxy groups of the investigated colloid particles and a set of the equilibrium constants of the reactions of Pu(IV) hydroxo complexes with α-FeOOH were obtained. If no redox reactions occur on the surface of the colloid particles, these constants decrease in the order \( K_{MnO_2 } > K_{\alpha - FeOOH} > K_{\alpha - Fe_2 O_3 } \sim K_{\gamma - Fe_2 O_3 } \).

Lithium contents and isotopic compositions of ferromanganese deposits from the global ocean

To test the feasibility of using lithium isotopes in marine ferromanganese deposits as an indicator of paleoceanographic conditions and seawater composition, we analyzed samples from a variety of tectonic environments in the global ocean. Hydrogenetic, hydrothermal, mixed hydrogenetic–hydrothermal, and hydrogenetic–diagenetic samples were subjected to a two-step leaching and dissolution procedure to extract first the loosely bound Li and then the more tightly bound Li in the Mn oxide and Fe oxyhydroxide. Total leachable Li contents vary from <1 ppm in hydrogenetic crusts to 1422 ppm (up to 1188 ppm measured here) in hydrothermal deposits. Hydrated Li ions in seawater and hydrothermal fluids are preferentially sorbed on the negatively charged surface of MnO2 by coulombic force. Hence, the abundant Li in hydrothermal deposits is mainly associated with the dominant phase, MnO2. The surface of amorphous FeOOH holds a slightly positive charge and attracts little Li, as demonstrated by data for hydrothermal Fe oxyhydroxide. Loosely sorbed Li in both hydrogenetic crusts and hydrothermal deposits exhibit Li isotopic compositions that resemble that of modern seawater. We infer that the hydrothermally derived Li scavenged onto the surface of MnO2 freely exchanged with ambient seawater, thereby losing its original isotopic signature. Li in the tightly bound sites is always isotopically lighter than that in the loosely bound fraction, suggesting that the isotopic fractionation occurred during formation of chemical bonds in the oxide and oxyhydroxide structures. Sr isotopes also show evidence of re-equilibration with seawater after deposition. Because of their mobility, Li and Sr in the ferromanganese crusts do not faithfully record secular variations in the isotopic compositions of seawater. However, Li content can be a useful proxy for the hydrothermal history of ocean basins. Based on the Li concentrations of the globally distributed hydrogenetic and hydrothermal samples, we estimate a scavenging flux of Li that is insignificant compared to the hydrothermal flux and river input to the ocean.

Magnetic order and valency at La0.7Sr0.3MnO3/SrTiO3 interfaces

We have performed first principles calculations of the electronic properties and the magnetic order at the interface of La0.7Sr0.3MnO3 and SrTiO3. The results are analysed in order to give the strength of the exchange between the surface MnO2 layer and the bulk and also the degree of the polarization of the bands. It is shown that these properties depend on the way the interface is grown. Two different interface terminations are identified and modelled. The results of the calculations compare favourably with recent experimental data.

Stability and Degradation Processes of 2,4,6-Trinitrotoluene (TNT) on Metal Oxide Surfaces

The chemical stability of 2,4,6-trinitrotoluene (TNT) when placed in physical contact with metal oxide surfaces is investigated. The TNT is deposited at 1–3 monolayers of coverage on the surface of microcrystalline MnO2, CuO, WO3, MoO3, Bi2O3, SnO2, and Fe2O3 by wet impregnation. The samples were placed in a 50°C oven and allowed to react over a period of 11 months. Periodically, samples were removed and analyzed chromatographically to determine whether products were being formed at the surface of the oxide. TNT proved to be inert to most oxides; however, both CuO and MnO2 effected a clean decomposition of the TNT into tri-nitrobenzene.

Oxidation kinetics of pentachlorophenol by manganese dioxide

This study examined the abiotic transformation kinetics of pentachlorophenol (PCP) by manganese dioxide (MnO2) at different solution chemistry and initial concentrations of PCP and MnO2. The measured PCP transformation rates were found to be on the order of 1.07 with respect to [PCP] and 0.91 and 0.87 with respect to [MnO2] and [H+], respectively. Dissolved Mn2+ and Ca2+ as background electrolytes considerably decreased the reaction rate because of their adsorption and hence blocking of active sites on MnO2 surfaces. The dechlorination number, 0.59 chloride ions per transformed PCP after a 1-h reaction, suggests that a fraction of the transformed PCP was not dechlorinated and may be coupled directly to dimeric products. Gas chromatography/ mass spectrometry and liquid chromatography/mass spectrometry/mass spectrometry techniques were used to identify two isomeric nonachlorohydroxybiphenylethers as major products and 2,3,5,6-tetrachloro-1,4-hydroquinone and tetrachlorocatechol as minor products. Product identification suggested that the reaction may include two parallel reactions to form either dimers or 2,3,5,6-tetrachloro-1,4-hydroquinone and tetrachlorocatechol via simultaneous dehydrochlorination and hydroxylation.

Investigation of the preparation and activity of gold catalysts in the total oxidation of n-hexane

The factors affecting the preparation of Au/TiO2 catalysts and their activity in the total oxidation of n-hexane were investigated. The mechanism of gold deposition–precipitation is discussed through comparison of the samples prepared by this method and others prepared by anion adsorption method. The influence of the pH and of the origin of TiO2 support used are additionally addressed. The difference of gold dispersion observed between the two methods is attributed to a difference of mobility of the gold precursors during the thermal treatment rather than to a difference of dispersion over the uncalcined samples. The mechanism of gold deposition–precipitation actually involves the reactions of gold hydroxy-chloride species with the surface. Another part of the work, thus, concerned the use of the deposition–precipitation method to prepare a Au/MnO2 catalyst. It is shown that the activity of γ-MnO2 is directly proportional to its surface area and that the deposition–precipitation procedure decreases the surface and activity of MnO2. However, the deposition of gold allows to avoid a too deep sintering of γ-MnO2 and, thus, helps to somehow preserve its activity.

Mechanism of the oxidation of d-glucose onto colloidal MnO2 surface in the absence and presence of TX-100 micelles

Aqueous colloidal manganese dioxide (MnO2) was prepared via titration by using potassium permanganate and sodium thiosulphate in aqueous neutral medium. The kinetics of oxidation of d-glucose onto the surface of colloidal MnO2 have been studied spectrophotometrically. The results show that the rate of initial stage (nonautocatalytic path) increases with increasing the [d-glucose], [H+], and temperature and also upon addition of nonionic surfactant Triton X-100 (TX-100), which indicates that the surfactant enhances the concentration of d-glucose at the surface of the colloidal MnO2. Hydrogen bonding interaction seemingly arises between –OH groups of d-glucose and oxygen of the ether linkages of polyoxyethylene chain of TX-100. A possible mechanism of the oxidative degradation of d-glucose is discussed in terms of d-glucose/TX-100 and colloidal MnO2 interaction.

Reduction of water-soluble colloidal manganese dioxide by thiourea: a kinetic and mechanistic study

Kinetic data for the colloidal MnO2–thiourea redox system are reported for the first time. The reduction of water-soluble colloidal MnO2 by thiourea (sulfur containing reductant) in aqueous perchloric acid medium has shown that it proceeds in two stages, i.e., a fast stage followed by a relatively slow second stage. The log (absorbance) versus time plot deviates from linearity. The kinetics of both the stages was investigated spectrophotometrically. The first-order kinetics with respect to [thiourea] at low concentration shifts to zero-order at higher concentration. The reaction rate increases with [HClO4] and the kinetics reveals complex order dependence in [HClO4]. Addition of P2O 7 4− and F− in the form of Na4P2O7 and NaF, respectively, has inhibitory effect on the reaction rate. The reaction proceeds through the fast adsorption of thiourea on the surface of the colloidal MnO2. A mechanism involving the protonated thiourea as the reactive reductant species is proposed. The observed results are discussed in terms of Michaelis–Menten/Langmuir–Hinshelwood model. From the observed kinetic data, colloidal MnO2–thiourea adsorption constant (K ad1) and rate constant (k 1) were calculated to be 1.25×1010 mol−1 dm3 and 3.1×10−4 s−1, respectively. The variation of temperature does not have any effect on the reaction rate.

Removal of Alachlor from Water by Catalyzed Ozonation in the Presence of Fe2+, Mn2+, and Humic Substances

The effects of Fe(II), Mn(II) and humic substances on the catalyzed ozonation of alachlor, an endocrine disruptor were investigated. Results revealed that small amounts of Fe(II), Mn(II), and humic substances could enhance the ozonation of alachlor, but larger amounts of them would retard the oxidation. These results were successfully identified by an electron paramagnetic resonance (EPR) spectroscopy/spin-trapping method that could quantify hydroxyl radicals. The production of hydroxyl radicals was obviously increased with the increasing of Fe(II) concentration, which contributed to enhance ozonation at low concentrations. But the excess Fe(II) consumed some of the radicals when it was added at a higher concentration (1.5 mg/L). However, no obvious radicals were observed when a different amount of Mn(II) was used, and the catalytic ozonation of alachlor by Mn(II) mainly followed the mechanism of “active sites created on the surface of MnO2.” The radical pathway was followed when alachlor was ozonated with different concentrations of humic substances because of its radical initiating, promoting, and inhibiting effects.

Navigated control in dental implantology

Monitoring and restoration activities at low-level radioactive waste disposal sites have identified complicated mixtures of inorganic and organic contaminants in soil and groundwater. Metallic contaminants are generally complexed with various chelating agents and organic acids which alter the geochemical behavior of the contaminants in subsurface media. The objective of this study was to provide an improved understanding of the geochemical processes controlling the subsurface transport of radioactive 60Co complexed with ethylenediaminetetraacetic acid (EDTA). Specifically, we investigated the kinetics and mechanisms of Co(II) EDTA2− oxidation to Co(III)EDTA− by the soil mineral pyrolusite (β-MnO2). A column displacement technique was utilized to investigate Co(II)EDTA2− reactivity and oxidation rates through packed beds of pyrolusite-coated SiO2. The interaction of Co(II)EDTA2− with the porous media was characterized by a MnO2-induced oxidation of the Co (II)EDTA2− to Co(III)EDTA−. The oxidation of Co(II)EDTA2− appeared to involve the reduction of Mn (IV) to both an aqueous Mn2+ species and a theorized Mn(III)-oxide solid phase. The redox reaction was catalytic since the reduction products were gradually reoxidized in the presence of dissolved O2 to form a Mn(IV)-oxide phase. Oxidation of surface-bound Mn2+ and the theorized Mn(III)-oxide was slow relative to Co(II)EDTA2− oxidation, and a reversible loss in the oxidative ability of the β-MnO2 occurred when exposed to CO(II)EDTA2−. The reduction in catalytic activity of the MnO2 was not the result of direct surface poisoning by Mn2+ but rather was believed to result from the formation of an intermediate Mn(III)-oxide solid phase whose oxidative potential was significantly less than MnO2. Thus, the kinetics of Co(II)EDTA2− oxidation to Co(III)EDTA− by MnO2 was dependent on the rate of MnO2 surface regeneration. The environmental implications of this redox reaction are pronounced, since any Co(III)EDTA− produced is extremely stable, and this enhances the persistence and transport of 60Co in subsurface environments.