Temperature-Dependent evolution of the molecular configuration of oxo-tungsten(VI) species deposited on the surface of titania

The pH and temperature evolution of the molecular configuration of oxo-tungsten(VI) species deposited on TiO₂ by equilibrium deposition filtration and the temperature evolution of the samples texture are studied using in situ Raman spectroscopy, DR spectroscopy, N₂ adsorption-desorption measurements, and TGA. Prior to drying, WO ₄^2- monomers retained above a bridging surface hydroxyl of TiO₂ through H bonding (with a Ti ₂OH···O-WO₃ umbrella configuration) prevail at pH = 9. A pH decrease results in a gradual transformation to an inner sphere monosubstituted species located above terminal surface oxygen (Ti-O-WO₃). At pH = 7 both species coexist, whereas at pH = 5 the latter prevails with electrostatically retained polyoxotungstes also present. Heating to 100 °C causes the same effect resulted by the pH decrease at room temperature, that is, transformation of Ti₂OH·· ·O-WO₃ into Ti-O-WO₃, attributed to the removal of H₂O molecules upon heating, thereby liberating surface Ti atoms. Progressive heating to higher temperatures causes further removal of surface hydroxyls (confirmed also by TGA) liberating more neighbor surface Ti atoms and favoring further anchoring. This leads to formation of bisubstituted dioxo (Ti-O)₂-W(=O)₂ sites in the range 120-250 °C and to the prevalence of trisubstituted mono-oxo (Ti-O)₃-W=O sites at 430 °C. Polyoxotungstates are also detected in the sample with the highest W loading. The DRS spectra indicate a charge transfer effect between the surface O and Ti atoms proceeding via Ti-O-W surface bridges. Calcination of the support causes a decrease of the pore volume and pore area in the range of pores with small width and an increase in the range of pores with large width. This effect is not affected by the presence of the well dispersed "bi-dimensional" W species inferred by the Raman study.