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.[8] It is a yellow solid, although impure samples can appear blue or green.
A section of the chain comprising edge-sharing octahedra. Oxygen atoms in back and front of the chain link to other chains to build the layer.[9]
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.[9][10] Also known is a metastable (β) form of MoO3 with a WO3-like structure.[11][2]
Preparation and principal reactions
Molybdite on molybdenite, Questa molybdenum mine, New Mexico (size: 11.0×6.7×4.1 cm).
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:[12]
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:
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)."[13] 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,[14] especially when forming an ohmic contact to organic semiconductors.[15]
^Åsbrink, S.; Kihlborg, L.; Malinowski, M. (1988). "High-pressure single-crystal X-ray diffraction studies of MoO3. I. Lattice parameters up to 7.4 GPa". J. Appl. Crystallogr. 21 (6): 960–962. doi:10.1107/S0021889888008271.