Reduction Of Manganese Oxides Research Articles

Reduction of Manganese Oxides: Thermodynamic, Kinetic and Mechanistic Considerations for One- Versus Two-Electron Transfer Steps

Manganese oxides, typically similar to δ-MnO2, form in the aquatic environment at near neutral pH via bacterially promoted oxidation of Mn(II) species by O2, as the reaction of [Mn(H2O)6]2+ with O2 alone is not thermodynamically favorable below pH of ~ 9. As manganese oxide species are reduced by the triphenylmethane compound leucoberbelein blue (LBB) to form the colored oxidized form of LBB (λmax = 623 nm), their concentration in the aquatic environment can be determined in aqueous environmental samples (e.g., across the oxic–anoxic interface of the Chesapeake Bay, the hemipelagic St. Lawrence Estuary and the Broadkill River estuary surrounded by salt marsh wetlands), and their reaction progress can be followed in kinetic studies. The LBB reaction with oxidized Mn solids can occur via a hydrogen atom transfer (HAT) reaction, which is a one-electron transfer process, but is unfavorable with oxidized Fe solids. HAT thermodynamics are also favorable for nitrite with LBB and MnO2 with ammonia (NH3). Reactions are unfavorable for NH4+ and sulfide with oxidized Fe and Mn solids, and NH3 with oxidized Fe solids. In laboratory studies and aquatic environments, the reduction of manganese oxides leads to the formation of Mn(III)-ligand complexes [Mn(III)L] at significant concentrations even when two-electron reductants react with MnO2. Key reductants are hydrogen sulfide, Fe(II) and organic ligands, including the siderophore desferioxamine-B. We present laboratory data on the reaction of colloidal MnO2 solutions (λmax ~ 370 nm) with these reductants. In marine waters, colloidal forms of Mn oxides (< 0.2 µm) have not been detected as Mn oxides are quantitatively trapped on 0.2-µm filters. Thus, the reactivity of Mn oxides with reductants depends on surface reactions and possible surface defects. In the case of MnO2, Mn(IV) is an inert cation in octahedral coordination; thus, an inner-sphere process is likely for electrons to go into the empty e g * conduction band of its orbitals. Using frontier molecular orbital theory and band theory, we discuss aspects of these surface reactions and possible surface defects that may promote MnO2 reduction using laboratory and field data for the reaction of MnO2 with hydrogen sulfide and other reductants.

Arsenic and other water‐quality issues affecting groundwater, Indus alluvial plain, Pakistan

AbstractGroundwater beneath the alluvial plain of the Indus River, Pakistan, is reported to be widely polluted by arsenic (As) and to adversely affect human health. In 79 groundwaters reported here from the lower Indus River plain in southern Sindh Province, concentrations of As exceeded the WHO guideline value for drinking water of 10 μg/L in 38%, with 22% exceeding 50 μg/L, Pakistan's guideline value. The As pollution is caused by microbially‐mediated reductive dissolution of sedimentary iron oxyhydroxides in anoxic groundwaters; oxic groundwaters contain &lt;10 μg/L of As. In the upper Indus River plain, in Punjab Province, localized As pollution of groundwater occurs by alkali desorption as a consequence of ion exchange in groundwater, possibly supplemented by the use for irrigation of groundwater that has suffered ion exchange in the aquifer and so has values &gt;0 for residual sodium carbonate. In the field area in southern Sindh, concentrations of Mn in groundwater exceed 0.4 mg/L in 11% of groundwaters, with a maximum of 0.7 mg/L, as a result of reduction of sedimentary manganese oxides. Other trace elements pose little or no threat to human health. Salinities in groundwaters range from fresh to saline (electrical conductivity up to 6 mS/cm). High salinities result from local inputs of waste water from unsewered sanitation but mainly from evaporation/evapotranspiration of canal water and groundwater used for irrigation. The process does not concentrate As in the groundwater owing to sorption of As to soils. Ion exchange exerts a control on concentrations of Na, Ca, and B but not directly on As. High values of Cl/Br mass ratios (most ≫288, the marine value) reflect the pervasive influence on groundwater of sewage‐contaminated water from irrigation canals through seepage loss and deep percolation of irrigation water, with additional, well‐specific, contributions from unsewered sanitation.

An Electrochemical Quartz Crystal Microbalance Investigation of Manganese Oxide Deposition and Dissolution in Sulfuric Acid Relevant for Zinc Electrowinning

Manganese(II)-ions are present in varying amounts in the electrolyte used for zinc electrowinning, based on the manganese content in the ore. The manganese ions are oxidized at the anode during electrowinning to form manganese dioxide alongside the oxygen evolution reaction. This has both positive and negative consequences for the lead-based anodes used in the industry today. The manganese dioxide prevents corrosion of the lead and hence lowers the amount of lead as an impurity in the produced zinc. Manganese dioxide also increases the selectivity of the oxygen evolution reaction to the chlorine evolution reaction. On the other hand, the manganese dioxide layer grows in thickness over time leading to an increase in cell resistance in addition to a possibility of flaking off and short circuiting the cell. The anodes therefore have to be removed from the cells at regular intervals to physically remove the MnO2 layer, which is a labor intensive task. Lead-based electrodes are not ideal as anode materials for several reasons. They are not very active for oxygen evolution and they introduce lead impurities to the produced metal. Lead is also unwanted for environmental reasons. It is desirable to replace them with electrochemically active dimensionally stable anodes (DSAs). The aim of this study is to investigate manganese oxide deposition and dissolution on noble metals by using conventional electrochemical methods and electrochemical quartz crystal microbalance (EQCM). In this work EQCM was used to provide additional information about the manganese oxide deposition and dissolution from a sulfuric acid solution. The electrode materials studied were platinum and gold. The electrolyte consisted of 0.1 M H2SO4 with 5 mM Mn2+-ions and the experiments were conducted at room temperature. Cyclic voltammograms were recorded for a number of different upper reversal potentials and sweep rates. In addition, sweep-hold and step-hold experiments were conducted. In these experiments the potential was swept or stepped to various potentials at which manganese oxide deposition occurred and held for a given time. These sequences were followed by a negative-going sweep from the hold potential. To complement the EQCM experiments, manganese oxide was deposited on a platinum disc at two different potentials (1.45 V and 1.55 V) and studied by scanning electron microscopy (SEM) as a function of deposition time. The cyclic voltammograms showed an oxidation peak commencing at about 1.42 V and peaking around 1.46 V in the positive-going scan (Fig 1), as observed elsewhere [1]. The oxidation peak coincided with an increase in mass. Clear indications of a nucleation, growth and collision mechanism were observed when the upper reversal potential was chosen from the rising part of the oxidation peak, i.e. below 1.46 V. This peak occurs below the onset of oxygen evolution and is thus related to the onset and continuous formation of manganese oxide. The shape and size of the oxidation peak were similar on gold and platinum. Subsequently, a cathodic peak (reduction of manganese oxide) was observed in the negative-going scan at potentials just positive of 1.3 V, which also appeared similarly on both electrode materials. Interestingly, a second reduction peak at around 1.2 V was observed at both electrodes, but only when the upper limit was sufficiently positive (above 1.5 V). Voltammograms recorded at different sweep rates showed no clear trend in the peak current for the oxidation peak. However, the mass deposited did show a very clear dependence and was inversely proportional to the sweep rate. The SEM images showed that the deposit formed at the higher potential had more dendrites than the deposit formed at the lower potential although almost the same amount of charge was passed in the deposition experiments. The change from one to two reduction peaks when reversing the sweep at potentials above 1.55 V indicates a change in the reduction and deposition mechanism. The lack of sweep-rate dependence of the oxidation peak shows that it is at least not entirely governed by bulk diffusion of Mn2+. To be able to implement DSAs in zinc electrowinning it is important to understand the mechanism of manganese oxide deposition and dissolution. W.H. Kao and V.J. Weibel, Electrochemical oxidation of manganese(II) at a platinum electrode, J. Appl. Electrochem., 22 (1992) 21-27. Figure 1

Impact of an historic underground gas well blowout on the current methane chemistry in a shallow groundwater system

Blowouts present a small but genuine risk when drilling into the deep subsurface and can have an immediate and significant impact on the surrounding environment. Nevertheless, studies that document their long-term impact are scarce. In 1965, a catastrophic underground blowout occurred during the drilling of a gas well in The Netherlands, which led to the uncontrolled release of large amounts of natural gas from the reservoir to the surface. In this study, the remaining impact on methane chemistry in the overlying aquifers was investigated. Methane concentrations higher than 10 mg/L (n = 12) were all found to have δ13C-CH4 values larger than -30‰, typical of a thermogenic origin. Both δ13C-CH4 and δD-CH4 correspond to the isotopic composition of the gas reservoir. Based on analysis of local groundwater flow conditions, this methane is not a remnant but most likely the result of ongoing leakage from the reservoir as a result of the blowout. Progressive enrichment of both δ13C-CH4 and δD-CH4 is observed with increasing distance and decreasing methane concentrations. The calculated isotopic fractionation factors of εC = 3 and εD = 54 suggest anaerobic methane oxidation is partly responsible for the observed decrease in concentrations. Elevated dissolved iron and manganese concentrations at the fringe of the methane plume show that oxidation is primarily mediated by the reduction of iron and manganese oxides. Combined, the data reveal the long-term impact that underground gas well blowouts may have on groundwater chemistry, as well as the important role of anaerobic oxidation in controlling the fate of dissolved methane.

Temperature-dependent oxide removal from manganese- and iron oxide-coated soil redox bars

Soil temperature is a fundamental parameter affecting not only microbial activity but also manganese (MnIII,IV) and iron (FeIII) oxide reduction rates. The relationship between MnIII,IV oxide removal from oxide-coated redox bars is missing at present. This study investigated the effect of variable soil temperatures on oxide removal by MnIII,IV and FeIII oxide-coated redox bars in water-saturated soil columns in the laboratory. The Mn coatings contained the mineral birnessite, whereas the Fe coatings contained a mixture of ferrihydrite and goethite. Additionally, platinum (Pt) electrodes designed to measure the redox potential (EH) were installed in the soil columns, which were filled with either a humic topsoil with an organic carbon (Corg) content of 85 g kg−1 (pH 5.8) or a subsoil containing 2 g Corg kg−1 (pH 7.5). Experiments were performed at 5, 15, and 25 °C. Although elevated soil temperatures accelerated the decrease in EH after water saturation in the topsoil, no EH decreases regardless of soil temperature occurred in the subsoil. Besides soil temperature, the importance of soil organic matter as an electron donor is highlighted in this case. Complete removal of the MnIII,IV oxide coating was observed after 28, 14, and 7 days in the soil columns filled with topsoil at 5, 15, and 25 °C, respectively. Along the Fe redox bars, FeIII reducing conditions first appeared at 15 °C and oxide removal was enhanced at 25 °C because of lower EH, with the preferential dissolution of ferrihydrite over goethite as revealed by visual differences in the FeIII oxide coating. Oxide removal along redox bars followed the thermodynamics of the applied minerals in the order birnessite > ferrihydrite > goethite. In line with Van’t Hoff’s rule, turnover rates of MnIII,IV and FeIII oxide reduction increased as a result of increased soil temperatures. Taking into account the stability lines of the designated minerals, EH-pH conditions were in accordance with oxide removal. Soil temperature must therefore be considered a master variable when evaluating the oxide removal of redox bars employed for the monitoring of soil redox status.

Comparative structure-potentio-spectroscopy of the Shewanella outer membrane multiheme cytochromes

Many species of bacteria can generate energy in the anoxic subsurface by directly coupling intracellular oxidative reactions to the reduction of extracellular metal oxides. Coupling these processes requires electron transfer networks that extend from the inside of the cell, across the outer membrane to the extracellular terminal electron acceptors. The best described of these networks is from Shewanella oneidensis MR-1, where four structures of outer membrane multiheme cytochromes (OMMCs) have been determined. These OMMCs contain 10–11 bis-histidine-ligated c-type hemes and are directly involved in the reduction of iron and manganese oxides at the cell surface. The heme ligands for some of these structures have been characterised using electron paramagnetic resonance (EPR), the redox properties have been mapped by protein film electrochemistry (PFE) and more recently molecular dynamic simulations have been used to obtain microscopic redox potentials for individual heme groups. This review maps these different experimental techniques onto the structures, providing insight into the intramolecular electron transfer pathways of OMMCs, revealing future directions for study.

Potentially Active Iron, Sulfur, and Sulfate Reducing Bacteria in Skagerrak and Bothnian Bay Sediments

ABSTRACTIn many marine surface sediments, the reduction of manganese (Mn) and iron (Fe) oxides is obscured by sulfate reduction, which is regarded as the predominant anaerobic microbial respiration process. However, many dissimilatory sulfate and sulfur reducing microorganisms are known to utilize alternative electron acceptors such as metal oxides. In this study, we tested whether sulfate and sulfur reducing bacteria are linked to metal oxide reduction based on biogeochemical modeling of porewater concentration profiles of Mn2+ and Fe2+ in Bothnian Bay (BB) and Skagerrak (SK) sediments. Steady-state modeling of Fe2+ and Mn2+ porewater profiles revealed zones of net Fe (0–9 cm BB; ∼10 and 20 cm SK) and Mn (0–5 cm BB; 2–8 cm SK) species transformations. 16S rRNA pyrosequencing analysis of the in-situ community showed that Desulfobacteraceae, Desulfuromonadaceae and Desulfobulbaceae were present in the zone of Fe-reduction of both sediments. Rhodobacteraceae were also detected at high relative abundance in both sediments. BB sediments appeared to harbor a greater diversity of potential Fe-reducers compared to SK. Additionally, when the upper 10 cm of sediment from the SK was incubated with lepidocrocite and acetate, Desulfuromonas was the dominant bacteria. Real-time quantitative polymerase chain reaction (qPCR) results showed decreasing dsrA gene copy numbers with depth coincided with decreased Fe-reduction activity. Our results support the idea that sulfur and sulfate reducing bacteria contribute to Fe-reduction in the upper centimeters of both sediments.

Kinetics of Silicothermic Reduction of Manganese Oxide for Advanced High-Strength Steel Production

The kinetics of silicothermic reduction of manganese oxide from MnO–SiO2–CaO–Al2O3 slags reacting with Fe-Si droplets were studied in the temperature range of 1823 K to 1923 K (1550 °C to 1650 °C). The effects of initial droplet mass, initial droplet silicon content, and initial slag manganese oxide content were studied. Data obtained for 15 pct silicon showed agreement with control by mass transport of MnO in the slag with a mass transfer coefficient (k s) of 4.0 × 10−5 m/s at 1873 K (1600 °C). However, when this rate-determining step was tested at different initial silicon contents, the agreement was lost, suggesting mixed control between silicon transport in the metal and manganese oxide transport in the slag. Increasing the temperature resulted in a decrease in the rate of reaction because of an increase in the favorability of SiO as a product. Significant gas generation was found during all experiments, as a result of silicon monoxide production. The ratio of silicon monoxide to silica formation was increased by factors favoring silicon transport over that of manganese, further supporting the conclusion that the reaction is under mixed control by transports of both silicon and manganese oxide.

Effect of nitrogen forms on reduction of manganese oxides in an Oxisol by plant root exudates

ABSTRACTReductive dissolution of soil manganese (Mn) oxides increases potential toxicity of Mn2+ to plants. In order to examine the effect of nitrogen forms on reduction of Mn oxides in rhizosphere soil, a rhizobox experiment was employed to investigate the reduction of Mn oxides due to the growth of soybean and maize in an Oxisol with various contents of NO3–-N and NH4+-N and a total N of 200 mg kg−1. The results showed that exchangeable Mn2+ in rhizosphere soil was 9.6–32.7 mg kg−1 higher than that in bulk soil after cultivation of soybean and maize for 80 days, which suggested that plant root exudates increased reduction of soil Mn oxides. Application of ammonium-N promoted reduction of Mn oxides in rhizosphere soil compared to application of nitrate and nitrate together with ammonium. Soybean cultivation led to a higher reduction in soil Mn oxides than maize cultivation. Application of single ammonium enhanced Mn uptake by the plants and led to more Mn accumulating in plant leaves, especially for soybean. Therefore, application of ammonium-based fertilizer can promote reduction of soil Mn oxides, while application of nitrate-based fertilizer can inhibit reduction of soil Mn oxides and thus reduce Mn2+ toxicity to plants.

Practical reduction of manganese oxide

Manganese is an important metal used in steel industry. It is abundant in steel as an alloying element. Additionally, it is used as a deoxidiser in steel production. In steel industry, manganese metal is used as an intermediate product of ferromanganese. Ferromanganese is generally produced by reduction of oxidised manganese. Reduction is in the form of either metalothermic reduction or carbothermic reduction. Practically, metallographic reduction is performed with silicon or aluminium which form more stable oxides than magnesium. Carbothermic reduction means reduction with carbon. All of the reduction reactions are highly endothermic and a high amount of thermal energy is required for the accomplishment of these reactions [1, 2]. The most abundant forms of the manganese oxides are MnO2, Mn2O3, Mn3O4 and MnO. These compounds dissociate during heating.

Elemental mercury adsorption and regeneration performance of sorbents FeMnOx enhanced via non-thermal plasma

Recent laboratory experiments and field tests have demonstrated the potential for modified regenerable materials to be a cost-effective alternative to expensive activated carbon for mercury control in coal-fired power plants. To develop a competitive and commercially viable technology, better mercury capture and regeneration performances of regenerable sorbents are required. For this purpose, non-thermal plasma was used to treat the magnetic sorbents FeMnOx synthesized by co-precipitation. The mercury adsorption tests showed that the treated sorbents with non-thermal plasma had higher mercury removal efficiency than raw sorbents, and longer treatment time resulted in higher efficiency. The main reason was that the content of the high valence manganese oxides and lattice oxygen were greatly increased by non-thermal plasma treatment, and this played a significant role in the mercury removal process. Further analysis showed that the lattice oxygen coming from the stepwise reduction of manganese oxide (MnO2→Mn2O3→MnO) served as an oxidant in the reaction with Hg0. The NO, SO2 and H2O inhibited the mercury adsorption while O2 and HCl promoted mercury removal. High valence of manganese oxides and lattice oxygen were consumed during mercury adsorption, while non-thermal plasma treatment could replenish it and increase mercury removal regeneration performance better than without non-thermal plasma treatment. In summary, the combination of magnetic separation, thermal regeneration and plasma treatment makes FeMnOx an excellent recyclable sorbent for mercury emission control.

Carbonation and deactivation kinetics of a mixed calcium oxide–copper oxide sorbent/oxygen carrier for post-combustion carbon dioxide capture

Removing carbon dioxide from industrial effluents via solid carbonate sorbents is a potential greenhouse gas mitigation strategy that can also produce hydrogen from the effluent in water gas shift reactors downstream. However, to regenerate the calcium oxide (from the carbonate) requires high temperature that is provided via an oxidation step. To avoid costly oxycombustion to regenerate the CaCO3, it is possible to use the heat generated during the reduction of copper oxide and manganese oxides as chemical looping agents. Unfortunately, sorbents sinter thereby and consequently the surface area drops in the regeneration step. A mixed CuO–CaO sorbent with 10% calcium aluminate binder lost 84% of its initial BET surface area during the first 100min on stream. Homogeneous calcium–copper–cement oxide sorbent deactivate to the 0.5 order with respect to the remaining surface area and a decay constant equal to 2.86min−1. An empirical equilibrium power law model characterizes the carbonation rate from 425°C to 665°C with 10% to 20% CO2 in a 45mm fixed bed. The apparent activation energies of the forward and reverse reaction are 220 and 120kJmol−1, respectively. In addition to sintering, the reaction rate of the regeneration step decreases with time on stream following a zero order process. We attributed this gradual irreversibility to increased crystallinity, reducing the diffusion rate of CO2 to the active carbonate phase. By integrating this resistance to the kinetic model, we are able to explain 99% of the variance in the gas profiles measured by a mass spectrometer.

Manganese Deposits of Africa

Africa hosts just over 80 percent (%) of the world's known land-based ore resources of manganese metal and produced some 41.1% of the 18 million tonnes (Mt) of manganese metal in ores, that was mined during 2014. The deposits are mainly of sedimentary and supergene origin comprising four major types, namely banded iron formation (BIF)-hosted, black shale-hosted, oolitic and supergene/karst-hosted deposits. The BIF-hosted Kalahari Manganese Field (KMF) is by far the largest of these deposits holding some 4,200 Mt of manganese metal that represents about 77% of the world's known land-based resource. Other major deposits are present in the Francevillian of Gabon, represented by high-grade supergene manganese oxide ores derived from weathering of rhodochrosite-bearing black shale source rock, and manganese carbonates interbedded with black shale and greywackes associated with volcanicdominated orogenic belts in the Birimian of West Africa and Lukoshi Complex at Kisenge in the Democratic Republic of the Congo (DRC). High-grade supergene manganese oxide ores cap the manganese carbonate beds. All these major ore deposits formed in the interval 2.0-2.2 Ga. Additional deposits that contain only relatively small resources but that hold important scientific information about the Earth's history include the ~ 2.4 Ga BIF-hosted Rooinekke deposit of the Transvaal Supergroup, which formed prior to the ~ 2.32 Ga Great Oxidation Event (GOE), karst-hosted deposits of the Postmasburg area that formed along a 2.0 Ga palaeoweathering profile below red beds of the Gamagara/Mapedi succession in South Africa, the ~1.9 Ga Tolwe deposit that represents the oldest known example of oolitic manganese ore in the world and the BIF-hosted Otjosondu deposit in Namibia, related to the Neoproterozoic Sturtian Snowball Earth Event (an extremely cold period of time in geologic history when Earth was covered by a virtual global ice cap). The manganese carbonates present in most of these deposits are all highly enriched in light organic carbon isotopes indicating that they were derived from reduction of original manganese oxide precipitates by organic carbon during diagenesis. This implies that the water columns from which the sedimentary manganese beds originally precipitated, as well as the floors of the depositories, were oxygenated. Post Archaean Australian Shale (PAAS)-normalized cerium (Ce) anomalies in Rare Earth Element (REE) data, support the presence of oxic conditions in the basins. It is thus not surprising that all the major 2.0-2.2 Ga deposits formed after the GOE. In this regard the relatively small BIF-hosted Rooinekke manganese deposit is very significant in that it formed prior to the GOE suggesting that at least some free oxygen was present in ocean basins before that time. There also appears to have been a major tectonosedimentary control on the distribution of the major 2.0- 2.2 Ga deposits. All of them, including the KMF, apparently formed in back-arc basinal settings immediately prior to amalgamation of cratonic blocks along Eburnian orogenic belts and formation of the supercontinent Columbia.

Real-Time Manganese Phase Dynamics during Biological and Abiotic Manganese Oxide Reduction.

Manganese oxides are often highly reactive and easily reduced, both abiotically, by a variety of inorganic chemical species, and biologically during anaerobic respiration by microbes. To evaluate the reaction mechanisms of these different reduction routes and their potential lasting products, we measured the sequence progression of microbial manganese(IV) oxide reduction mediated by chemical species (sulfide and ferrous iron) and the common metal-reducing microbe Shewanella oneidensis MR-1 under several endmember conditions, using synchrotron X-ray spectroscopic measurements complemented by X-ray diffraction and Raman spectroscopy on precipitates collected throughout the reaction. Crystalline or potentially long-lived phases produced in these experiments included manganese(II)-phosphate, manganese(II)-carbonate, and manganese(III)-oxyhydroxides. Major controls on the formation of these discrete phases were alkalinity production and solution conditions such as inorganic carbon and phosphate availability. The formation of a long-lived Mn(III) oxide appears to depend on aqueous Mn(2+) production and the relative proportion of electron donors and electron acceptors in the system. These real-time measurements identify mineralogical products during Mn(IV) oxide reduction, contribute to understanding the mechanism of various Mn(IV) oxide reduction pathways, and assist in interpreting the processes occurring actively in manganese-rich environments and recorded in the geologic record of manganese-rich strata.

Kinetics of the formation reaction of manganese carbide under various gases

Carbothermic reduction of manganese oxide under various gases in the temperature range of 1273-1573 K in order to understand the formation reaction of manganese carbide is investigated. The experimental results indicate that the formation reaction of manganese carbide depends on the temperature, the CO partial pressure, and the activity of carbon. The reaction rate for Mn7C3 formation is shown to be correlated with the CO gas diffusivity of the atmosphere in that the CO partial pressure acts as the driving force for the formation reaction in question. Furthermore, the results indicate that the reaction rate of Mn7C3 formation under H2 gas is significantly faster than that under He gas; that is because methane gas that is formed by the reaction between H2 with carbon accelerates the overall reduction process. On the other hand, a high CO partial pressure is shown to retard the formation reaction of Mn7C3.

A direct electrochemical route from oxides to TiMn2 hydrogen storage alloy

This study is for investigating the direct electro-deoxidation of mixed TiO2–MnO2 powder to prepare TiMn2 alloy in molten calcium chloride. The influences of process parameters, such as sintering temperature, cell voltage, and electrolysis time, on the electrolysis process were examined to investigate the mechanism of alloy formation. The composition and morphology of the products were analyzed by XRD and SEM, respectively. The electrochemical property of TiMn2 alloy was investigated by cyclic voltammetry measurements. The results show that pure TiMn2 can be prepared by direct electrochemical reduction of mixed TiO2/MnO2 pellets at a voltage of 3.1V in molten calcium chloride of 900°C for 7h. The electro-deoxidation proceeds from the reduction of manganese oxides to Mn, which is reduced by TiO2 or CaTiO3 to form TiMn2 alloy. The cyclic voltammetry measurements using powder microelectrode show that the prepared TiMn2 alloy has good electrochemical hydrogen storage property.

Sulfur-Tolerant Mn-Ce-Ti Sorbents for Elemental Mercury Removal from Flue Gas: Mechanistic Investigation by XPS

The influence of structural interaction between Mn, Ce, and Ti on the elemental mercury adsorption properties of Mn-Ce-Ti sorbents has been investigated. For this purpose, Mn-Ce-Ti sorbents have been synthesized by coprecipitation, codeposition-precipitation, coimpregnation, sequential impregnation, and deposition-impregnation to get different structural properties. All the sorbents were evaluated for elemental mercury capture in the absence and presence of SO2. Among the sorbents tested, the Mn-Ce-Ti sorbent synthesized by coimpregnation exhibits better sulfur resistance. X-ray diffraction measurements show that the samples prepared using impregnation methods exhibit intense reflections due to the cubic fluorite structure of cerium. Temperature-programmed desorption measurements reveal that the type and strength of acid sites depend on the synthesis method. Temperature-programmed reduction measurements suggest that precipitating Mn and Ce together over the surface of titania promotes surface ceria and manganese oxide reduction. However, the simultaneous precipitation of Mn, Ce, and Ti inhibits surface ceria and manganese oxide reduction. X-ray photoelectron spectroscopy (XPS) measurements suggest that the mechanism for Hg adsorption over Mn-Ce-Ti sorbents is the same regardless of the presence or absence of SO2. The Ce 3d XPS spectra show that all the samples exhibit peaks due to both Ce4+ and Ce3+ oxidation states. Surface atomic ratios determined from XPS measurements show that the sorbent synthesized by coimpregnation yields a higher concentration of cerium atoms on the surface and exhibits better sulfur resistance than the other sorbents. The sorbent synthesized by coprecipitation yields more titanium atoms on the surface and exhibits the least sulfur resistance.

Reduction of mixed Mn-Zr oxides: in situ XPS and XRD studies.

A series of mixed Mn-Zr oxides with different molar ratios Mn/Zr (0.1-9) have been prepared by coprecipitation of manganese and zirconium nitrates and characterized by X-ray diffraction (XRD) and BET methods. It has been found that at concentrations of Mn below 30 at%, the samples are single-phase solid solutions (MnxZr1-xO2-δ) based on a ZrO2 structure. X-ray photoelectron spectroscopy (XPS) measurements showed that manganese in these solutions exists mainly in the Mn(4+) state on the surface. An increase in Mn content mostly leads to an increase in the number of Mn cations in the structure of solid solutions; however, a part of the manganese cations form Mn2O3 and Mn3O4 in the crystalline and amorphous states. The reduction of these oxides with hydrogen was studied by a temperature-programmed reduction technique, in situ XRD, and near ambient pressure XPS in the temperature range from 100 to 650 °C. It was shown that the reduction of the solid solutions MnxZr1-xO2-δ proceeds via two stages. During the first stage, at temperatures between 100 and 500 °C, the Mn cations incorporated into the solid solutions MnxZr1-xO2-δ undergo partial reduction. During the second stage, at temperatures between 500 and 700 °C, Mn cations segregate on the surface of the solid solution. In the samples with more than 30 at% Mn, the reduction of manganese oxides was observed: Mn2O3 → Mn3O4 → MnO.

Mn/TiO2 and Mn–Fe/TiO2 catalysts synthesized by deposition precipitation—promising for selective catalytic reduction of NO with NH3 at low temperatures

Mn/TiO2 and Mn–Fe/TiO2 catalysts have been prepared by impregnation (IMP) and deposition-precipitation (DP) techniques and characterized by N2 physisorption, XRPD, NH3-TPD, H2-TPR, XPS and TGA. 25wt% Mn0.75Fe0.25Ti-DP catalyst, prepared by deposition precipitation with ammonium carbamate (AC) as a precipitating agent, showed superior low-temperature SCR (selective catalytic reduction) of NO with NH3. The superior catalytic activity of the 25wt% Mn0.75Fe0.25Ti-DP catalyst is probably due to the presence of amorphous phases of manganese oxide, iron oxide, high surface area, high total acidity, acid strength and ease of reduction of manganese oxide and iron oxide on TiO2 in addition to formation of an SCR active MnOx phase with high content of chemisorbed oxygen (Oα). The optimum catalyst might be used as tail-end SCR catalysts in, e.g., biomass-fired power plants and waste incineration plants.

Preparation of ZrMn2 hydrogen storage alloy by electro-deoxidation in molten calcium chloride

ZrMn2 alloy was electro-synthesized directly from cathode pellets compacted with powdered mixture of MnO2 and ZrO2 in molten calcium chloride. Sintering temperature, cell voltage and electrolysis time were the dominant factors that affected the characteristics of the final product. The results confirmed the formation of pure ZrMn2 alloy through the electro-deoxidation of the mixed oxide pellets at 3.1 V for 12 h in 900 °C CaCl2 melt. The X-ray diffraction (XRD) and cyclic voltammetry analysis suggested that the electro-deoxidation proceeded from the reduction of manganese oxides to Mn, followed by ZrO2 or CaZrO3 reduction on the pre-formed Mn to ZrMn2 alloy. The cyclic voltammetry measurements using powder microelectrode showed that the prepared ZrMn2 alloy has a good electrochemical hydrogen storage property.