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

Abstract

UV−visible diffuse reflectance spectroscopy was used to probe the electronic structure and domain size of tungsten oxide species in crystalline isopolytungstates, monoclinic WO 3, and dispersed WO x species on ZrO 2 surfaces. UV−visible absorption edge analysis, CO 2 chemisorption, and Raman spectroscopic results show that three distinct regions of WO x coverage on ZrO 2 supports appear with increasing WO x surface density: submonolayer region (0−4 W nm -2 ), polytungstate growth region (4−8 W nm -2 ), and polytungstate/crystalline WO 3 coexistence region (>8 W nm -2 ). The structure and catalytic activity of WO x species on ZrO 2 is controlled only by WO x surface density (W nm -2 ), irrespective of the WO x concentration, oxidation temperature, and ZrO 2 surface area used to obtain a particular density. The submonolayer region is characterized by distorted octahedral WO x species that are well dispersed on the ZrO 2 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 WO x surface densities, the absorption edge energy decreases, WO x domain size increases, WO x species become easier to reduce, and o -xylene isomerization turnover rates (per W atom) increase with increasing WO x surface density. At high WO x surface densities, a polytungstate monolayer coexists with monoclinic WO 3 crystallites. The growth of monoclinic WO 3 crystallites results in lower o -xylene isomerization turnover rates because WO x species become inaccessible to reactants. In the presence of H 2 at typical catalytic reaction temperatures (∼523 K), strong acid sites form on WO x −ZrO 2 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).