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Tuning the Electronic and Molecular Structures of Catalytic Active Sites with Oxide Nanoligands

Speaker: Israel Wachs, Lehigh
Date & Time: April 5, 2007 - 11:00am
Location: Chem 260

Tuning the Electronic and Molecular Structures of Catalytic Active Sites with Oxide Nanoligands
Laboratory for Surface Modification

Israel Wachs, Lehigh
11:00 AM, Chem 260

The catalytic active sites in supported metal oxides are significantly affected by the charge transfer between the surface metal oxide active component and the underlying oxide support ligand. The charge transfer can modify the electron density of the surface metal oxide catalytic active sites and dramatically influence the resultant catalytic performance. Thus, oxide support nanoligands can assist in the design of new and novel catalytic materials.

A SiO2 material was used as the support material to synthesize ~1-10 nm titanium oxide nanodomains on the relatively inert and amorphous SiO2 matrix by varying the titania content. Catalytically active surface redox vanadium oxide and acidic tungsten oxide sites were synthesized by impregnation onto the supported TiO2/SiO2 substrates. Raman spectroscopy established that crystalline V2O5 and WO3 nanoparticles were not formed and that only surface vanadia and tungsten oxide species were present. Furthermore, these catalytic active sites were found to preferentially self-assemble on the titania nanoligands as surface VOx/WOx species.

The reactivity of the supported redox surface VOx and acidic surface WOx catalytic active sites was significantly influenced by the domain size of the titania nanoligand. Whereas, surface redox sites experience enhanced catalytic activity with increasing domain size, the inverse occurs with surface acid sites. The activity of the surface WOx decreased with increasing domain size indicating that smaller titania domain size resulted in more acidic surface WOx sites. This reveals that surface acidic sites are more active when coordinated to oxide support nanoligands with higher band gaps (higher local electron density and less electron delocalization). In contrast, the activity of the redox surface VOx sites increased with increasing domain size of the titania nanoligands. This reveals that surface redox sites are more active when coordinated to oxide support nanoligands with lower band gaps (lower electron density and more electron delocalization). Thus, different types of catalytic active sites and reaction mechanisms have different requirements in the nanodomain range.

These model studies with titania nanoligands in a SiO2 matrix have demonstrated for the first time how oxide support nanoligands can tune the catalytic activity of surface redox and surface acidic catalytic active sites. These new insights can assist in the molecular engineering of novel supported metal oxide catalysts by tuning the redox/acidic surface functionalities.

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