Molybdenum anodic oxidation

These chemical reactions possibly precede the electrochemical reactions. Thus the electrochemical reactions in the case of molybdenum oxides may be taken to be similar to those which occur in electrorefining, i.e., electrochemical dissolution of molybdenum from the impure metallic molybdenum anode and subsequent deposition at the cathode. The combination of the chemical and the electrochemical reactions occurring at the anode can be represented in the following way ... 

Sulphides. MoS2 was prepared by electrolysis at 1000°C of a melt consisting of sodium tetraborate, sodium fluoride, sodium carbonate in which molybdenum (VI) oxide and sulphur were dissolved. The electrolysis was carried out at 1000°C with the melt contained in a graphite crucible also acting as anode. After electrolysis, the excess electrolyte was dissolved in water to obtain crystalline MoS2, containing however up to 2% carbon. A similar method was used for WS2 carbon was the principal impurity in the sulphides. 

Electrochemical method [54] Silicate is determined in sea water by four different electrochemical methods based on the detection of the silicomolybdic complex formed in acidic media by the reaction between silicate and molybdenum salts. The first two methods are based on the addition of molybdate and protons in a seawater sample in an electrochemical cell. A semiautonomous method was developed based on the electrochemical anodic oxidation of molybdenum, the complexation of the oxidation product with silicate and the detection of the complex by cyclic voltammetry. Finally a complete reagent-less method with a precision of 2.6% is described based on the simultaneous formation of the molybdenum salt and protons in a divided electrochemical cell. 

Previously we reported results of the study of mechanism and kinetics of anodic oxidation of titanium, tantalum, tungsten and molybdenum in nitrate melts [4j. 

In 1998, the anodic oxidation of molybdenum and tungsten in alcohols in the presence of LiCl (as electroconductive additive) was found to yield a variety of interesting oxo-metal alkoxide complexes, some of which have been authenticated by singlecrystal X-ray crystaUograpy. 

Okamoto H, Kawamura G, Ishikawa A, Kudo T. Activation of tungsten molybdenum carbide (W,Mo)C methanol anodic oxidation catalysts using alkaline solution. J Electrochem Soc 1987 134 1649-53. 

Numerous results show that in anodic oxidation reactions the activity of catalysts consistii of several metals is higher than that of the individual components [192, 237, 240, 241]. Bockris and co-workers [193] investigated a wide range of binary and ternary catalysts on the oxidation of methanol. The results (Fig. 12) show that alloys of platinum containing ruthenium and molybdenum are the most active Ru2Rhjo < Pt < Pt Ress = RuPt < PtYoRuisMojs. (The subscripts indicate the proportions of the components by weight.) It was noticed [240] that the catalytic properties of platinum are improved by addition of molybdenum. 

A thin layer deposited between the electrode and the charge transport material can be used to modify the injection process. Some of these arc (relatively poor) conductors and should be viewed as electrode materials in their own right, for example the polymers polyaniline (PAni) [81-83] and polyethylenedioxythiophene (PEDT or PEDOT) [83, 841 heavily doped with anions to be intrinsically conducting. They have work functions of approximately 5.0 cV [75] and therefore are used as anode materials, typically on top of 1TO, which is present to provide lateral conductivity. Thin layers of transition metal oxide on ITO have also been shown [74J to have better injection properties than ITO itself. Again these materials (oxides of ruthenium, molybdenum or vanadium) have high work functions, but because of their low conductivity cannot be used alone as the electrode. 

The solution should be free from the following, which either interfere or lead to an unsatisfactory deposit silver, mercury, bismuth, selenium, tellurium, arsenic, antimony, tin, molybdenum, gold and the platinum metals, thiocyanate, chloride, oxidising agents such as oxides of nitrogen, or excessive amounts of iron(III), nitrate or nitric acid. Chloride ion is avoided because Cu( I) is stabilised as a chloro-complex and remains in solution to be re-oxidised at the anode unless hydrazinium chloride is added as depolariser. 

Secondary lithium-metal batteries which have a lithium-metal anode are attractive because their energy density is theoretically higher than that of lithium-ion batteries. Lithium-molybdenum disulfide batteries were the world s first secondary cylindrical lithium—metal batteries. However, the batteries were recalled in 1989 because of an overheating defect. Lithium-manganese dioxide batteries are the only secondary cylindrical lithium—metal batteries which are manufactured at present. Lithium-vanadium oxide batteries are being researched and developed. Furthermore, electrolytes, electrolyte additives and lithium surface treatments are being studied to improve safety and recharge-ability. 

The electroextraction process for molybdenum involves the use of its oxides, carbides or sulfides as soluble anodes in a potassium chloride-potassium hexachloromolybdate (K3MoCl6) molten electrolyte. An inert atmosphere electrolytic cell, with a provision for semicontinuous electrolysis, is used for this purpose. The process operation consists of the following steps. 

Similarly to the case of direct-oxidation anode materials, sulfur-tolerant anode materials based on sulfides [6, 7] or double-perovskite oxides have special requirements for their processing into SOFC layers. For example, nickel sulfide-promoted molybdenum sulfide is tolerant to high sulfur levels [7], However, it has a low melting temperature [6] that has resulted in the development of cobalt sulfide as a stabilizer of the molybdenum sulfide catalyst [6], CoS-MoS2 admixed with Ag has an even higher performance in H2S-containing fuels than in pure H2 [6]. However, processing methods such as PS, infiltration, or sol-gel techniques that can process... 

The controlled oxidation of the low-valent metal alkoxides has in fact been used for the preparation of oxocomplexes. Thus on action of 02 onMo2(OBu )6 or the red-colored solutions obtained by anodic dissolution of molybdenum metal in alcohols (containing the derivatives of Mo(V)) were obtained the di-oxoalkoxides, Mo02(OR)2 [356,908],... 

New catalysts of hydrogen oxidation for low-temperature fuel cells are molybdenum and tungsten carbides [2, 3], For solid polymeric fuel cells the novel catalysts by plasma treatment of polymer membrane have been developed. The radicals at surface are generated. These radicals are catalysts of anodic reactions [4]... 

Molybdenum and tungsten are rendered passive more readily in acid than in alkaline solution this is the reverse of the behavior exhibited by chromium and the iron-group metals. Although oxidizing agents generally favor passivity, such is not the case with a tin anode in this instance, too, chloride ions do not have the inhibiting effect they have in other cases. It is apparent, therefore, that each metal requires its own specific conditions in order that it may be rendered passive. 

If oxide films are responsible for passivity, it is to be expected that an anode will become passive most readily in an electrolyte from which the oxide will separate most easily this expectation is realized in practice. The oxides of iron, cobalt, nickel and chromium are less soluble in alkaline than in acid solution, and passivity sets in more rapidly in the former. The oxides of molybdenum and tungsten, however, are more soluble in alkali than in acid, and so these metals are rendered passive most easily in acid electrolytes. 

Figure 11 (A) Stripping voltammetry (20 m Vs at 55 °C) of CO layers on humidified PEM fuel-cell anodes (1) platinum catalyst (2) platinum/molybdenum catalyst. Voltammetry in the absence of adsorbed CO on the platinum/molybdenum catalyst is shown in (3). Molybdenum-mediated electro-oxidation of adsorbed CO takes place on the alloy catalyst in the peak at 0.45 V and at lower overpotentials [79]. (B) Steady-state polarization curves of PEM fuel-cell anode at 85 °C for platinum (squares) and platinum/molybdenum catalysts in the presence of 100 ppm CO (filled points) and pure H2 (unfilled points). (From Ref 79.)... 

Figure 9.11 Emission spectra of the different types of sources in UV/Vis. A logarithmic scale accounts for the big differences of light intensity according to the wavelengths, notably for filamentless lamps. Below left and middle, general view of a lamp and that seen from above (reproduced courtesy of Oriel). Schematic presenting the circuit details for the lamp. The lamp is booted with a voltage of between 3 to 400 V. The anode is a molybdenum plate while the cathode is a filament of metallic oxide able of emitting electrons and connected to an electrical supply. The emission peaks of deuterium at 486 and 656.1 nm are often used to calibrate the spectrometer wavelength scale.

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