Structure and Electronic Properties of Solid Acids Based on Tungsten Oxide Nanostructures

UV-visible diffuse reflectance spectroscopy was used to probe the electronic structure and domain size of tungsten oxide species in crystalline isopolytungstates, monoclinic WO3, and dispersed WOx species on ZrO2 surfaces. UV-visible absorption edge analysis, CO2 chemisorption, and Raman spectroscopic results show that three distinct regions of WOx coverage on ZrO2 supports appear with increasing WOx surface density: submonolayer region (0-4 W nm-2), polytungstate growth region (4-8 W nm-2), and polytungstate/crystalline WO3 coexistence region (>8 W nm-2). The structure and catalytic activity of WOx species on ZrO2 is controlled only by WOx surface density (W nm-2), irrespective of the WOx concentration, oxidation temperature, and ZrO2 surface area used to obtain a particular density. The submonolayer region is characterized by distorted octahedral WOx species that are well dispersed on the ZrO2 surface. These species show a constant absorption edge energy, they are difficult to reduce, and contain few acid sites where o-xylene isomerization can occur at 523 K. At intermediate WOx surface densities, the absorption edge energy decreases, WOx domain size increases, WOx species become easier to reduce, and o-xylene isomerization turnover rates (per W atom) increase with increasing WOx surface density. At high WOx surface densities, a polytungstate monolayer coexists with monoclinic WO3 crystallites. The growth of monoclinic WO3 crystallites results in lower o-xylene isomerization turnover rates because WOx species become inaccessible to reactants. In the presence of H2 at typical catalytic reaction temperatures (∼523 K), strong acid sites form on WOx-ZrO2 catalysts with polytungstate domains by a slight reduction of the cluster and delocalization of an electron from an H atom resulting in H+δ (Brønsted acid site).