Sintering and Phase Transformation of V-Loaded Anatase Materials Containing Bulk and Surface V Species

Abstract

V-loaded anatase materials with V/Ti molar ratios in the range 0.02−0.06 have been prepared by a sol−gel procedure under inert atmosphere. Combining macroscopic (XRD, TEM, and BET) and spectroscopic (XPS, Raman, ESR, and 51V NMR) techniques, we show that most of the V species are incorporated within the anatase bulk (amount of surface V species below the sensitivity limit of XPS, i.e., lower than 0.5−1 at. %). As revealed by XPS and in situ XRD measurements, upon calcination below 600 °C in flowing oxygen, a migration of V species from the anatase bulk toward its surface, associated with a sintering of anatase particles, is induced. Above 600 °C, VxTi1-xO2 rutile solid solutions are formed. The threshold temperature at which anatase particles transform into rutile is affected by the V/Ti molar ratio. Located near 650 ± 50 °C in our experimental conditions for V-free titania, this temperature regularly increases with V loading and can be located at 750 ± 50 °C for a V/Ti molar ratio of 0.06. Furthermore, on a parallel experiment performed on a mechanical mixture of V-free titania and vanadia (V/Ti molar ratio of 0.05), we have observed that this temperature decreases to 550 ± 50 °C. These observations strongly suggest that the decrease in the threshold temperature at which V-loaded anatase materials are transformed into rutile, which has been previously observed, extensively discussed in the literature and attributed to vanadium, is not actually due to V species incorporated within the anatase bulk. Our results rather suggest that rutile formation is initiated near the surface of anatase particles and that a minimal amount of surface V species is required to decrease the threshold temperature at which rutile can be detected.