Oxides of molybdenum

At least nine oxides of molybdenum have been reported , but of these the monoclinic dioxide MoOj and the orthorhombic trioxide are the most common and most stable. The others, with formulae lying between M0O2 and M0O3 are metastable mixed valency oxides with intense colour and metallic lustre, formed from one or both 

Molybdenum trioxide, or molybdic oxide , melts at 795 C and boils at 1155°C at normal atmospheric pressure. It has a fairly high hardness and in many early publications was described as a harmful abrasive product from the oxidation of molybdenum disulphide in service. In fact the trioxide is not highly abrasive, being possibly less abrasive in some circumstances than molybdenum disulphide itself , and it has been recommended for use as a lubricant at elevated temperatures to 700°C. 

Molybdenum dioxide, although generally less Important than the trioxide, is more abrasive . It has been suggested that abrasion associated with synthetic or 

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The first-stage catalysts for the oxidation to methacrolein are based on complex mixed metal oxides of molybdenum, bismuth, and iron, often with the addition of cobalt, nickel, antimony, tungsten, and an alkaU metal. Process optimization continues to be in the form of incremental improvements in catalyst yield and lifetime. Typically, a dilute stream, 5—10% of isobutylene tert-huty alcohol) in steam (10%) and air, is passed over the catalyst at 300—420°C. Conversion is often nearly quantitative, with selectivities to methacrolein ranging from 85% to better than 95% (114—118). Often there is accompanying selectivity to methacrylic acid of an additional 2—5%. A patent by Mitsui Toatsu Chemicals reports selectivity to methacrolein of better than 97% at conversions of 98.7% for a yield of methacrolein of nearly 96% (119). 

The crystal structures observed during the oxidation of molybdenum consist of stable molybdenum dioxide in contact with the metal throughout the range 300-700°C. As the film thickens in the low-temperature range, the trioxide predominates on the surface. At 400°C, molybdenum trioxide is no longer observed and molybdenum dioxide is the only oxide observed. 

Cp = 7 -C5H5 m-CPBA = m-chloroperbenzoic acid Scheme 45 Oxidation of molybdenum polysulfido complexes... 

Reduction of mixed oxides of molybdenum iron briquettes with hydrogen... 

The methods of measuring the amount of cuprous oxide formed are numerous but the one most frequently used involves the reduction of either phosphomolybdic acid or arsenomolybdic acid by the cuprous oxide to lower oxides of molybdenum. The intensity of the coloured complex produced is related to the concentration of the reducing substances in the original sample. The colour produced with arsenomolybdic acid is more stable and the method is more sensitive than with phosphomolybdic acid. 

Alternatively, the oxide may be prepared by heating a molybdate salt, such as ammonium molybdate, with a reducing agent such as zinc. The dioxide also may be obtained along with other oxides of molybdenum when molybdenum metal is heated in air. 

Molybdenum(VI) oxide is the most stable oxide of molybdenum. It reacts with alkali solutions, forming molybdates ... 

Finally, there are two special cases in which the rectilinear law is observed when the rate-controlling factor is the rate of supply of O2 and when the metal oxide is volatile at the temperature of oxidation. The latter case occurs in the high temperature oxidation of molybdenum, since M0O3 is quite volatile, and in this case dw/dt is negative. 

This paper presents the more important data and conclusions from three reports which describe the uptake behavior of the vaporized oxides of molybdenum, tellurium, and rubidium by molten and solid substrates at high temperatures (I, 2, 3). These oxides were used as the vapor species because of their relatively high volatility and because of their importance as radioactive constituents, or the precursors of important constituents, of radioactive fallout particles. 

The thermal decomposition of alpha-12-molybdosilicic acid, H SiMo O ] and its two and four electron reduction products, H6[SiMo12040] and H8[SiMoi204o] has been studied in detail173-177). The reduced products decompose at higher temperatures than the oxidized species, the oxides of molybdenum that result from such decomposition have surface areas 10 m2/g. The reduced product yield Mo4Ou upon decomposition. 

Since the reaction involves irreversible oxidation of molybdenum, it would not be expected to be catalytic. 

As a kind of extension of this particular type of trinuclear structure, there are numerous compounds that contain two such units fused together on a common edge to give either a discrete molecule as in W4(OEt)i6 (Fig. 16-23), and Mo408(OPr )4py4 or extended arrays of such a unit, joined by shared nonmetal atoms, as in MNb4Cln (Fig. 18-B-13) or certain mixed oxides of molybdenum such as Bai.i4Mo80i6. 

Oxidation of molybdenum hexacarbonyl with acetic acid produces the carboxylate-bridged quadruple-bonded dimers, as shown inequation (35). This complex can undergo a number of substitution reactions that leave the metal metal bond intact. ... 

Typical analyses of three commercial powders are shown in Table 2.3 . They differ mainly in that the fine grinding of the Superfine powder increased the acidity and ailowed it to pick up some oii and water contamination. The acid insoiubies include siliceous material, and these may possibly be abrasive, but there is no direct correlation between silica content and abrasiveness. The molybdic oxide, formed by oxidation of molybdenum disulphide, is less abrasive than most technical grades of the disulphide itself, as shown in Table 2.4 and it is only the... 

Ross, S. and Sussman, A., Surface Oxidation of Molybdenum Disulphide, J. Phys. Chem., 59, 889, (1955). 

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. 

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. 

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