A vibrational and spectroscopic study of WO3/TiO2-Al2O3 catalyst precursors

WO3/TiO2-Al2O3 catalyst powders with titania-alumina mixed oxides synthesized by a sol-gel procedure have been prepared by dry impregnation. The surface structure of the resulting materials has been investigated by In and Raman spectroscopies in the skeletal region and IR spectra of adsorbed water and ammonia; Iris concluded that the largely predominant tungsten species, when these are below the monolayer coverage slid in dry conditions, on all the supports studied, are constituted by monooxo wolframyl species which are coordinatively unsaturated and act as strong Lewis acid sites. The overall coordination around tungsten is consequently 4 and/or 5. By adsorption of water, the overall coordination of tungsten grows and the site behaves as a strong Bronsted acid site. The vibrational behavior of such species suggests that the W=O vibrators: are uncoupled, which means that they belong to isolated molecular units anchored to the surface by W-O-(Ti,Al) bonds, without significant extent of W-O-W bridges. The nature of the support surface significantly modifies the strength of the W=O bond, which is indicative of the electronic state of tungsten. This is attributed to the different basicity of the surface oxide ions that act as the ligands of the wolframyl ion. The support surface hydroxy groups apparently do not play an important role in anchoring the wolframyl species. The "monolayer capacity" of the supports apparently depends quite strongly on the support nature, and it seems that care should be taken with its calculation, to compare on the same basis the results arising from different laboratories.