We describe the preparation and characterization of unusual
pentavalent molybdenum oxide stabilized by
water molecules, Mo2O5·2H2O.
Ultrasound irradiation of a slurry of molybdenum hexacarbonyl,
Mo(CO)6,
in Decalin for 3 h under ambient air yields blue-colored
Mo2O5·2H2O. Infrared
(FT-IR) spectrum analysis
of the resulting blue product reveals that the Mo ions possess
molybdenyl bonds (MoO) and Mo−O character
and also shows the presence of hydrogen-bonded, as well as coordinated,
water molecules. The DSC profile
of the blue oxide shows the presence of two endothermic peaks at around
100 °C and 140 °C, corresponding
to the elimination of hydrogen-bonded and coordinated water molecules,
respectively. The amount of water
molecules was determined by thermogravimetric analysis (TGA).
Characterization using powder X-ray
diffraction (XRD) and transmission electron microscopy (TEM) with
selected area electron diffraction (SAED)
shows the amorphous nature of the blue product. The TEM picture
shows that the blue oxide is composed
of spongy platelet nanoparticles (∼20 nm). Heating the initial
blue powder at 300 °C for 2 days under an
oxygen, hydrogen, and nitrogen atmosphere yields X-ray crystalline
MoO3, MoO2, and a mixture of
MoO3
and MoO2, respectively. X-ray photoelectron
spectroscopy (XPS), along with the potentiometric
titration
analysis of the blue oxide, confirms the formation of pentavalent
molybdenum oxide. UV−visible absorption
studies of the blue product demonstrate that the characteristic
absorption of the Mo(V) (d-cation) oxide
system and the Mo ions probably consists of two types of coordination
symmetry (T
d
and
O
h
). Electron spin
resonance (ESR) experimental results revealed an unusual doublet
pattern, which is ascribed to superhyperfine
coupling of pentavalent molybdenum with a proton of coordinated water.
The nanostructured amorphous
pentavalent molybdenum oxide (blue oxide) thus formed has also been
successfully deposited on Stober's
silica micropheres (250 nm) ultrasonically. The TEM images of
silica-supported blue oxide reveal uniform
distribution and strong adhering nature of the blue oxide. FT-IR
spectroscopy illustrated the structural changes
that occur when the amorphous SiO2 is coated sonochemically
with the blue oxide.