Quantitative relationship between the nature of surface species and the catalytic activity of tungsten oxides supported on crystallized titania

A series of WOx/TiO2 catalysts containing between 0.6 and 3.4 W at./nm2 was prepared by pore volume impregnation using ammonium metatungstate solution on crystallized TiO2. Characterization of the solids by X-ray diffraction, Raman and infrared spectroscopy indicated that the W phase was present essentially as a surface W species. The results also suggested an increased degree of condensation of the W phase with increasing W surface density. The most condensed W species was characterized by a ν(WO) vibration at 1017 cm−1.The IR or Raman bands attributed to specific surface species of the solids were correlated with the abundance of relatively strong Brønsted acid sites and isopropanol dehydration activity. A direct correlation was observed between propene formation rate and the area of the band at 1017 cm−1, attributed to the most condensed W species. Furthermore, the abundance of this species appeared to be directly related to that of Brønsted acid sites capable of retaining 2,6-dimethyl pyridine (lutidine) following desorption at 573 K. The behavior of the WOx/TiO2 system was compared with that reported for the corresponding WOx/ZrO2 solids. Higher intrinsic activity (activity per W atom) was observed for WOx/TiO2 system.

The surface species of WOx/TiO2 catalysts, were correlated with the abundance of relatively strong Brønsted acid sites and isopropanol dehydration activity. A direct correlation was observed between the abundance of these acid sites, the intensity of the infrared band attributed to the most condensed W species and propene formation rate.Figure optionsDownload high-quality image (109 K)Download as PowerPoint slide