Characterization of Sonochemically Prepared Unsupported and Silica-Supported Nanostructured Pentavalent Molybdenum Oxide

Abstract

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) (d1-cation) oxide system and the Mo ions probably consists of two types of coordination symmetry (Td and Oh). 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.