Tuning the electronic and molecular structures of catalytic active sites with oxide nanodomains

An extensive series of supported WO3/Zrx(OH) 4-2x catalysts (WZrOH) were synthesized by standard aqueous impregnation of ammonium metatungstate into an amorphous Zrx(OH)4-2x metastable support followed by high temperature calcination (773-1173 K). The supported WZrOH catalysts were also compared to well-defined model supported WO3/ZrO2 catalysts (WZrO2) consisting of a thermally stable crystalline m-ZrO2 support. The comparative study revealed that the Zr-stabilized distorted WO3 NPs are the catalytic active sites in supported WZrOH catalysts. The current findings represent a new model for the origin of the enhanced solid acidity of supported WZrOH catalysts.;Model supported WZrO2 supported catalysts, containing sub-monolayer quantities of surface WOx species on the crystalline m-ZrO2 support, were impregnated with either WOx, ZrO x or WOx + ZrOx precursors, dried and calcined at high temperatures (973 K). The TEM analysis revealed that the supported (WOx+ZrOx)/WZrO2 catalysts consisted of both the m-ZrO2 and t-ZrO2 support phases, with the later possessing a porous structure. Raman spectroscopy detected a decrease in formation of crystalline WO3 nanoparticles (NPs) and an increase in Zr-stabilized WO3 NPs. The corresponding UV-vis DRS Eg values were just slightly higher than that for the model supported WZrO 2 catalysts at the same surface tungsten oxide density. The TOF for methanol dehydration to dimethyl ether over the surface acid sites was found to increase by as much as ∼102 only when WOx and ZrOx were co-impregnated on the model supported WZrO2 catalysts. The enhanced TOF is shown to be related to the formation of the Zr-stabilized distorted WO3 NPs from the co-impregnation process, confirmed that the Zr-stabilized WO3 NPs are the active site found in the super-active WZrOH catalysts.;A series of supported 1-50% ZrO2/SiO2 catalysts were synthesized and subsequently used to anchor surface VOx redox and surface WOx acid sites. The supported ZrO2 phase was present as fully dispersed surface ZrOx species at low zirconia loadings and crystalline monoclinic-ZrO2 nanoparticles (∼0.5-3.5 nm) at higher loadings. The domain size variation in the supported ZrO 2 phase with zirconia content gives rise to a decreasing UV-vis edge energy decrease from ∼5.7 to ∼5.2 eV that reflects the increasing electron delocalization for the supported zirconia phase. The CH3OH chemical probe revealed that the surface VOx sites are redox in nature and the surface WOx sites contain acidic character. For the redox surface VOx sites anchored onto the zirconia component, the redox TOF increased with increasing domain size of the zirconia phase. For the acidic surface WOx sites anchored onto the zirconia component, the acidic TOF decreased with increasing domain size of the zirconia phase. The opposite dependencies of the redox surface VOx sites and acidic surface WOx sites reflect the different electronic requirements of redox and acidic catalytic active sites. This study demonstrates that the catalytic activity of surface redox and acidic sites can be tuned by varying the domain size of oxide support nanoligands.;A series of supported 1-50% TiO2/SiO2 catalysts were synthesized and subsequently used to anchor surface VOx redox and surface WOx acid sites. The electronic and molecular structures of titania nanoligands on a relatively inert SiO2 substrate were continuously varied by successfully controlling the titania domain size. The specific catalytic activity of surface redox (VO4) and acidic (WO 5) sites coordinated to the titania nanoligands are extremely sensitive to the degree of electron delocalization of the titania nanoligands. With decreasing titania domain size, <10 nm, acidic activity increases and redox activity decreases due to their inverse electronic requirements. This is the first systematic study to demonstrate the ability of oxide nanoligands to tune the structure and reactivity of surface metal oxide catalytic active sites. (Abstract shortened by UMI.).