Sonochemical Synthesis of Molybdenum Oxide- and Molybdenum Carbide-Silica Nanocomposites

A novel sonochemical approach for the preparation of molybdenum oxide (Mo₂O₅·xH₂O) and molybdenum carbide (Mo₂C) clusters coated on silica carriers, in which Mo(CO)₆ precursor serves as an in situ source for the coating phase of both materials (molybdenum oxide and molybdenum carbide), has been described. Ultrasonic irradiation of a slurry of molybdenum hexacarbonyl, Mo(CO)₆, and silica microspheres in decane for 3 h, under ambient air and argon, yields diphasic molybdenum oxide-silica (MOS) and molybdenum carbide-silica (MCS) composites, respectively. Characterization using powder X-ray diffraction and transmission electron microscopy, with selected area electron diffraction, shows the amorphous nature of the nanocomposites. The phase evolution by long-term thermal reduction of MOS under H₂ shows that the Mo₂O₅ undergoes stepwise reduction following the sequence, Mo^V → Mo^IV → Mo^o. The TEM image of MOS and MCS shows that the sonochemical decomposition products of Mo(CO)₆ attached on the silica carrier as clusters, as thin layers, or as nanoparticles depends upon the precursor composition. UV-visible absorption studies on the sonochemically produced MOS demonstrate that the characteristic absorption of the Mo^V (d¹ cation) oxide system and the Mo ions are likely to possess two types of coordination symmetry (T_d and O_h). Considerable changes in the characteristic UV-visible absorption of MOS, compared to that of bulk molybdenum oxide and bare silica, was observed, perhaps associated with the chemical interaction, to form a desirable interfacial bond. FT-IR spectroscopy illustrated the structural changes that occur when the amorphous SiO₂ is coated sonochemically. It has been proposed that the coating takes place via ultrasonic-cavitation-induced decomposition of the precursor into the required coating phase and the breaking of the strained siloxane link of the silica; subsequently, the coating phase and silica particles collide with each other and deposit themselves into the coating.