Solvothermal Synthesis and Supported Catalysis of Polyanion-derived Metal Oxide Nanoparticles

Supported metal oxides (SMOs) are important catalytic materials that find numerous applications in important industrial processes. Improving the structural properties of SMOs is a challenging objective due to material synthesis and characterization limitations. Recent developments in the characterization of SMOs, specifically tungstated zirconia (WOx/ZrO2), have revealed structural information that renewed scientific interest in developing more sophisticated synthetic protocols for SMOs.;The current work aims to provide a robust characterization of WO x/ZrO2 by using different characterization techniques and probe reactions. Conventional and non-conventional synthetic methods are investigated to cover the whole spectrum of published methods in order to understand the properties and limitations of these techniques. In the second part of this work, a new synthetic approach is presented that successfully produces ultrasmall (smaller than 2 nm) tungsten oxide nanoparticles (WOx NPs). By using conventional tungsten precursors and oleylamine, WOx NPs are synthesized, characterized, and finally supported to test their propene metathesis activity.;Conventional WOx/ZrO2 catalysts were prepared and extensively studied by probing their n-pentane isomerization activity and methanol dehydration activity. WOx/ZrO2 prepared via incipient wetness impregnation shows maximum n-pentane isomerization turnover rates (TOR) at intermediate surface densities (rhosurf). This method delivers the most active n-pentane isomerization WOx/ZrO 2 catalysts since it maximizes the number density of the active sub-nm slightly distorted Zr-WOx sites at rhosurf between 5.2-6.2 W/nm2. By comparing the n-pentane isomerization activity with the methanol dehydration activity of WOx /ZrO2, n-pentane isomerization is shown to be an excellent probe reaction for qualitatively identifying the relative (to the other species) population density of Zr-WOx clusters. Bimolecular n-pentane isomerization is the prevailing mechanism and requires a higher population density of Zr-WOx clusters than methanol dehydration.;In the second part of this work, a new solvothermal synthesis route for the preparation of ultrasmall tungsten oxide nanoparticles (WOx NPs) is introduced. By using ammonium polyanionic salts and oleylamine, high yields (92+/-5%) of oleylaminecoated WOx NPs were consistently synthesized. The co-addition of an organic oxidant during the synthesis led to smaller WOx NPs thereby providing insight into the NP synthesis mechanism. Deposition and activation of the NPs on SiO2 support by removal of oleylamine allows better control over the WOx domain size than conventional methods. Oleylamine suppresses WOx NP sintering during calcination and prevents the formation of larger polytungstates present in conventional catalysts. The supported WOx NPs were found to be up to 3 times more selective for metathesis products than conventionally prepared tungstated silica likely due to their controlled structure.