Basic Introduction of Molybdenum Oxides and Sulfides

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Molybdenum oxides

Molybdenum can generate a range of oxides; of which, molybdenum oxide (MoO3) and molybdenum dioxide (MoO2) are the most stable oxides. In addition, it can generate several intermediate oxides, with MonO3n-1 (Mo9O26, and Mo8O23, and Mo4O11) as corresponding components. Because the outer electronic structure of molybdenum is 4d55s1, its compounds are mainly +6 states, as well as +2, +3, +4, +5 states.

Molybdenum trioxide

Molybdenum trioxide is indispensable intermediate compounds to produce molybdenum metal, with great significance in the production. It can be made by molybdenum metal oxidization, molybdenum suboxides oxidization, or by oxidizing and roasting molybdenite (MoS2). Molybdenum trioxide, white powder with slightly green, turns into yellow when heating, with density of 4.69 g/cm3, melting point of 795°C and boiling point of 1,155°C. In the vapor of 800~1,000°C, it mainly exists in the form of (MoO3)3, and significantly sublimates over 600°C. Besides, its heat of formation is 745±6.3 kJ/mol, and it can be reduced to molybdenum metal by hydrogen within 800~900°C. Molybdenum trioxide is mainly acidic oxides, but its acidity is weaker than that of tungsten trioxide. Since it has a certain amphoteric nature, it can react with alkali and some strong acid. And solubility of molybdenum trioxide is 0.4~2 g/L in water at 20°C, and the solution is acidic (pH=4~4.5). Molybdenum trioxide reacts with acid and alkali as follows:

To react with acid:  MoO3+H2SO4=MoO2SO4+H2O

MoO3+2HCI=MoO2Cl2+H2O

MoO3+HNO3=MoO2(NO3)2+OH

To react with alkali: 2MeOH+MoO3=MeMoO4+H2O(Me represents for K, Na and NH4)

Molybdenum trioxide dissolves in the aqueous solution of alkali and ammonia and generates molybdate. In addition, it cannot generate solid solution with tungsten trioxide.

Molybdenum dioxide

Molybdenum dioxide (MoO2), dark brown powder, density of 6.34 g/cm3, heat formation of 590 kJ/mol, is made by the reduction of molybdenum trioxide by hydrogen at the temperature within 450~470°C. Actually, molybdenum dioxide is insoluble in water, alkali and non-oxidizing acid solution, but nitric acid can oxidize MoO2 into MoO3. Molybdenum dioxide crystals are rutile crystal lattice, with lattice constants of a=4086A and C=2.79A.

Intermediate oxides

Intermediate oxides may be generated by reducing MoO3 with hydrogen, oxidizing MoO2, heating the mixture of MoO2 and MoO3 in an inert atmosphere (nitrogen), or mixing MoO3 and molybdenum powder at a certain proportion. Mo4O11 is bluish violet, while Mo8O23 and Mo9O26 are bluish black. Mo4O11 is slightly soluble in water, sulfuric acid, hydrochloric acid and dilute alkali solution.

Molybdenum sulfides

Molybdenum reacts with sulfur generating three kinds of sulfides—-MoS3, MoS2 and Mo2S3, but only MoS2 and MoS3 are useful. MoS2, which is the main raw material for the preparation of molybdenum, exists in the form of molybdenite in the nature. And it insoluble in water, ammonia, soda, and reducing inorganic acid solutions with its melting point of 1,180°C.In addition, it can be oxidized to MoO3 when heating to 450~550°C in air. In the absence of air, synthetic molybdenum disulfide can be obtained by heating high sulfides, the reaction of sulfur vapore and molybdenum powder, and melting molybdenum trioxides with soda and sulfur. Heat of formation of molybdenum disulfide is 77.4 kilocalories at 805°C, while 80 kilocalories at 1,005°C. At temperatures higher than 1,000°C in a vacuum, MoS2 dissociates into molybdenum metal and sulfur vapor. As for molybdenum trisulfide, it is little in the nature. Hydrogen sulfide can precipitate out MoS3 through reacting with heated acidic ammonium molybdate solution. MoS3, which looks dark brown, is easily soluble in ammonium sulfide and all kinds of sodium sulfides to generate thio-molybdate that is easily soluble in water and can be crystallized and separated out from the solution in the form of vermeil crystals that can be dissolved into molybdenum trisulfide by all kinds of acids. In addition, MoS3 dissolves in ammonium sulfite or sodium sulfite solution and thus generates (NH4)2MoS4 and (NH4)2MoOx·S4-x. (NH4)2MoS4 is soluble in water and can resolve into MoS3 when solution acidizing. Molybdenum trisulfide precipitation is used not only for the chemical analysis of molybdenum, but also for molybdenum extraction from solution. This chemical reaction can be used to determine the separation of molybdenum and tungsten-molybdenum in chemical analysis.