Molybdenum Dioxide, MoO₂

Molybdenum Dioxide, MoO₂, is produced by a number of reactions involving either the reduction of the trioxide or the oxidation of the sesquioxide. Thus it may be prepared by the action of hydrogen, or of a mixture of carbon monoxide and carbon dioxide, upon molybdic acid or mercurous molybdate; by heating sodium or potassium trimolybdate to redness in hydrogen and washing the residue with water; by heating sodium trimolybdate with zinc and washing the residue with potassium hydroxide solution and then hydrochloric acid; by reduction of ammonium molybdate with colloidal palladium at ordinary temperatures; by heating ammonium molybdate, a mixture of sodium molybdate and ammonium chloride, or a mixture of molybdenum trioxide and ammonium molybdate, and suitably washing the residue with ammonia and then hydrochloric acid; and by electrolysis of fused molybdic anhydride.

Molybdenum dioxide forms violet-blue monoclinic crystals, pseudo-tetragonal,

a:b:c = 0.9869:1:0.5765; β = 91° 34',

of density 6.34. It is more strongly magnetic than the metal. The dioxide is insoluble in water, in hydrochloric acid, and in caustic potash solution, but is attacked by nitric acid and by fused potash. It reduces ammoniacal solutions of salts of silver, but not those of mercury or copper.

Reduction of a solution of molybdic acid by means of long digestion with the powdered metal causes the dark blue solution to assume a reddish colour; on addition of ammonia a reddish-brown precipitate is obtained, originally thought to be the hydrate of molybdenum dioxide, but more recently considered to be a hydrate of the oxide Mo₂O₅. The red solution before precipitation contains a salt of this oxide; it is considered probable (Guichard) that the dioxide does not form salts. A similar red solution may be obtained by electrolysis of a sulphuric acid or oxalic acid solution of molybdic acid, or by heating a hydrochloric acid solution of an alkali molybdate with potassium iodide. The hydrate MoO₂.H₂O, obtained in each case by precipitation with ammonia, is soluble in water, but is reprecipitated by the addition of salts, thus behaving as a colloid; the aqueous solution upon evaporation yields first a gel, and finally a dark brown insoluble hydrate. Though soluble in ammonium carbonate solution, the hydrate in question is insoluble in aqueous caustic potash; in the air it is readily oxidised, giving the blue oxide. The salts obtained by dissolving the freshly precipitated hydroxide in acids (the anhydrous oxide being insoluble) are, when anhydrous, black in colour; when hydrated they are reddish brown; several of these salts are known.