Molybdenum trioxide

Molybdenum trioxide describes a family of inorganic compounds with the formula MoO3(H2O)n where n = 0, 1, 2. The anhydrous compound is produced on the largest scale of any molybdenum compound since it is the main intermediate produced when molybdenum ores are purified. The anhydrous oxide is a precursor to molybdenum metal, an important alloying agent. It is also an important industrial catalyst. It is a yellow solid, although impure samples can appear blue or green.

Molybdenum trioxide occurs as the rare mineral molybdite.

Structure
In the gas phase, three oxygen atoms are bonded to the central molybdenum atom. In the solid state, anhydrous MoO3 is composed of layers of distorted MoO6 octahedra in an orthorhombic crystal. The octahedra share edges and form chains which are cross-linked by oxygen atoms to form layers. The octahedra have one short molybdenum-oxygen bond to a non-bridging oxygen. Also known is a metastable (β) form of MoO3 with a WO3-like structure.

Preparation and principal reactions
MoO3 is produced industrially by roasting the mineral molybdenite (molybdenum disulfide), the chief ore of molybdenum:
 * 2 MoS2 + 7 O2   →  2 MoO3  +  4 SO2

Similar procedures apply to the recovery of molybdenum from spent catalysts. The resulting trioxide can be purified by sublimation. The laboratory synthesis of the dihydrate entails acidification of aqueous solutions of sodium molybdate with perchloric acid:


 * Na2MoO4 + H2O  +  2 HClO4 →  MoO3*2H2O  +  2 NaClO4

The dihydrate loses water readily to give the monohydrate. Both are bright yellow in color. Molybdenum trioxide dissolves slightly in water to give "molybdic acid". In base, it dissolves to afford the molybdate anion.

Uses
Molybdenum trioxide is used to manufacture molybdenum metal:
 * MoO3 +  3 H2  →   Mo  +  3 H2O

Molybdenum trioxide is also a component of the co-catalyst used in the industrial production of acrylonitrile by the oxidation of propene and ammonia.

Because of its layered structure and the ease of the Mo(VI)/Mo(V) coupling, MoO3 is of interest in electrochemical devices and displays. It has been described as "the most commonly used TMO [transition metal oxide] in organic electronics applications ... it is evaporated at relatively low temperature (~400 °C)." It has favourable electronic and chemical properties for use as interfacing layers, p-type dopants and hole transport materials in OLEDs, organic solar cells and perovskite solar cells, especially when forming an ohmic contact to organic semiconductors.