On the morphological properties of tungsta-titania de-NO_xing catalysts

Tungsta-titania samples with different W loadings up to 15 wt.% and calcined at different temperatures have been prepared and characterized by surface area measurements, mercury porosimetry, x-ray diffraction, microstructural analysis, and laser Raman spectroscopy. It has been found that W inhibits the initial sintering of TiO₂ (anatase) and the anatase → rutile transformation. The morphological and structural properties of the samples (surface area, porosity, and phase composition) have been related to the microscopic properties of the materials such as crystallite dimensions and defects concentration. A model for the sintering of TiO₂ is discussed. This model is based on the diffusion of surface hydroxyls, formed upon adsorption of water on surface oxygen vacancies. A role for W is proposed in terms of stabilization of both material defects and surface hydroxyls.