Molybdenum dioxide

Manufacturing Processes. Ammonium molybdate or molybdenum trioxide is reduced to molybdenum metal powder by hydrogen in a two-stage process. In the first stage, MoO or ammonium molybdate is reduced to molybdenum dioxide, M0O2, at temperatures around 600°C in the... 

Representative compounds for the +4 oxidation state are shown in Figure 4. The violet tetravalent molybdenum dioxide [18868-43 ] M0O2, is formed by the reduction of M0O3 with H2 at temperatures below which Mo metal is formed or M0O3 is volatile (ca 450°C). MoCl [13320-71 -3] is formed upon treatment of M0O2 at 250°C with CCl (see Fig. 1). 

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. 

A. K. Suri and C. K. Gupta, Electrolytic Production of Molybdenum from a Molten Chloride Bath Using Molybdenum Dioxide Carbon Anodes, J. Less Common Met., Vol. 37, p. 389,1973. 

Synonyms Soluble compounds include molybdenum trioxide, ammonium molybdate, and sodium molybdate insoluble compounds include molybdenum disulfide and molybdenum dioxide... 

Unlike the trioxide, molybdenum dioxide is stable towards nonoxidizing acids, alkalies or fused salts. 

Molybdenum Arsenates and Molybdo-arsenates.—Molybdous Arsenate, Mo(HAs04)2.wH20( ), is said1 to be formed as a grey precipitate when molybdous chloride is treated with sodium monohydrogen arsenate the precipitate first redissolves, hut afterwards becomes permanent. Molybdic arsenate, obtained in a similar manner from molybdic chloride, has been described by Berzelius, who also considered that an acid salt was produced on dissolving the hydrate of molybdenum dioxide in excess of arsenic acid, since the solution turned blue on standing.1... 

Molybdenum dioxide (Mo0a) is prepared4 6 by ignition of the trioxide MoOs in a stream of dry hydrogen at 450°C for three hours. A combustion tube and circular oven (see No. 14) are used. It is not necessary to remove the remain-... 

Eighty grams of the molybdenum dioxide and 21g of aluminum are used and worked up as under chromium (No. 2). A regulus of about 50g of metal is obtained with a purity of 98%. With smaller quantities the yield drops due to excessive volatilisation of the product. The metal contains traces of aluminum and silicon. 

The postulated existence of the oxysulfide MoOS can be ruled unlikely on the basis of the structures of Mo02 und MoS2. Molybdenum dioxide has a three-dimensional structure with distorted octahedral coordination (d2sp3 hybridization) about the molybdenum atoms12while MoS2 has a layer structure with a trigonal prismatic coordination about the molybdenum atoms (d4sp hydridization). Consequently, any existence of a MoOS species may be expected to be only transient. 

Molybdenum oxide trichloride has been prepared by heating molybdenum(VI) oxide or molybdenum dioxide dichloride with molybdenum(V) chloride. Molybdenum(VI) oxide tetrachloride is a by-product of the latter reaction.1 It was also prepared by the sealed-tube reaction of liquid sulfur dioxide with molybdenum (Y) chloride2 and by the thermal decomposition of molybdenum oxide tetrachloride in a stream of nitrogen. In the following procedure, it is prepared by reducing molybdenum oxide tetrachloride with refluxing chlorobenzene.4... 

FIGURE 13 Graded oxide nanoparticle. Mo02 was oxidized with air at 723 K to give a core-shell structure of molybdenum dioxide and possibly molybdenum trioxide that was identified by its different electron energy loss spectrum. No structural description of the highly disordered and catalytically relevant outer oxide shell could be determined, either with XRD or with TEM, (or even with EXAFS spectroscopy) as the signals are dominated by the core structure. 

When steam is passed over molybdenum at high temperatures, hydrogen and molybdenum dioxide are formed. The reaction is reversible, and has been studied from the equilibrium point of view over the temperature range 700° to 1100° C. 

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

Molybdenum dioxide is highly abrasive, and its formation would be a serious disadvantage to lubrication performance, but in practice it seems clear that the formation of the dioxide is transitory or exceptional, and that the oxidation process normally produces the more harmless trioxide. 

Suzuki et al ° made hot-pressed composites of molybdenum disulphide 80%, molybdenum dioxide 10% and niobium 10%. They were pressed at 25 MPa and 1500°C in carbon dies, and the flexural strength was 59 - 63 MPa and the elastic modulus 27.9 MPa. To improve the strength they added 5% of 304 stainless steel, and this gave a flexural strength of 69 - 80 MPa and elastic modulus 43.8 MPa. The coefficient of friction of the latter compact was 0.07 to 0.18 and the specific wear rate 2.2 x 10" mm Nm at 450°C in vacuum. 

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