Synthesis And Properties Of Molybdenum-Based Silica Nanocomposites

Nano molybdenum-based compounds, represented by nano molybdate and molybdenum oxide, have attracted much attention due to their favorable performance. For instance, the scheelite molybdates have photoluminescence (PL) properties, and molybdenum trioxide has high catalytic activities. However, the problems of low fluorescence quantum yield and poor stability limit their applications. One effective measure to solve such problems is taking advantage of nanocomposites by introducing another component. Silica is a suitable option due to its optical transparence and thermal stability. Combining molybdenum-based compounds with silica is expected to improve their properties like fluorescence and catalysis. Moreover, the structures can also affect the properties of the composites. Therefore, synthesizing molybdenum-based silica nanocomposites with various structures is significant for improving the properties of molybdenum-based compounds.In this dissertation, molybdenum-based silica nanocomposites with core-shell, raisin-bun, and hollow structures were synthesized via a reverse microemulsion method. The formation mechanisms and properties of each structure were investigated. The improvements for the properties of the molybdenum-based composites over those of the monocomponent materials were shown. Meanwhile, the methods developed were used to synthesize tungsten-based and vanadium-based silica nanocomposites with the same structures.Core-shell BaMoO4@SiO2nanocomposite was synthesized and the impact of the structure on the PL properties of BaMoO4was researched. It was found that silica coating could significantly enhance the PL intensity, and the PL intensity increased first and then decreased with increasing the shell thickness due to the competition between the reduction of surface defects and the absorption effect of the silica shells. The main PL emission peaks entered UV region due to quantum size effect. The coating method was extended to other scheelite molybdates and tungstates.Raisin-bun MoO3/SiO2nanocomposite (denoted as R-MoO3/SiO2, diameter23±2nm, MoO31.0+0.2nm) was prepared and its catalytic properties were evaluated by acetalization of benzaldehyde with ethylene glycol. The benzaldehyde conversion was as high as98.2%under the optimum condition of1.1mol%catalyst,110℃and2h. The TOF is44.6h-1.This catalyst can be repeatedly used5times without obvious deactivation. The catalytic performance improvement was attributed to the unique structure and ultrasmall size of the nanocomposites. The formation mechanism of the raisin-bun structure was investigated and a confinement effect determining the size of the raisins was proposed. Raisin-bun structured WO3/SiO2and V2O5/SiO2were synthesized following the same mechanism.Based on the electrostatic attraction, hollow MoO3(?)SiO2nanocomposite was fabricated and its catalytic properties were compared with R-MoO3/S1O2to show the impact of structures on the properties. For catalyzing the acetalization of benzaldehyde with ethylene glycol at1.1mol%,75℃and2h, the benzaldehyde conversion was85.0%and73.0%for hollow MoO3(?)SiO2and R-MoO3/SiO2, respectively. The corresponding TOF is38.6h-1and33.2h-1, respectively. The difference can be ascribed to the higher specific surface area and shorter diffusion channels of the hollow structure. An electrostatic self-assembly method was proposed and used to synthesize a series of hollow silica nanocomposites, including oxysalt(?)SiO2, metal(?)SiO2, metal oxide(?)SiO2, and qutumn dots(?)SiO2.