title: X-ray absorption fine structure combined with x-ray fluorescence spectrometry. Part 15. Monitoring of vanadium site transformations on titania and in mesoporous titania by selective detection of the vanadium K alpha1 fluorescence.
abstract: X-ray absorption fine structure combined with X-ray fluorescence spectrometry was applied to various V+TiO2 hybrid samples. Emitted V K alpha1 fluorescence from the sample was selectively counted by using a high-energy-resolution (0.4 eV) spectrometer equipped with a Ge(331) crystal. Two advantages of this method, extremely high signal/background ratio and the compatibility of measurements in the atmosphere of reaction gas (in situ study in relation to heterogeneous catalysis), were effective at the V K-edge. Structure transformation of the V sites was spectroscopically followed for the V/TiO2 catalyst. The monooxo tetrahedral vanadate site was demonstrated to exist at 473 K. It transformed into dispersed species of 5-fold coordination in ambient air and further into polymeric VO(x) species in 0.85 kPa of water at 290 K. In the presence of 3.2 kPa of 2-propanol, dissociative adsorption of 2-propanol on the dispersed V species was strongly suggested at 290-473 K. In situ structure changes of V sites on TiO2 were reported by means of XAFS for the first time. The V(V) sites for the V/TiO2 catalysts were essentially identical with those for V supported on mesoporous (high-surface-area) TiO2 and V-TiO2 sample prepared by the sol-gel method. However, predominant V(IV) sites were found for mesoporous V-TiO2. The V(IV) sites substituted on the Ti sites of TiO2. When the molar ratio of V/Ti increased from 1/100 to 1/5.0, major octahedral V sites in the TiO2 matrix looked to transform into tetrahedral ones.
authors.affiliation: Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta 4259-G1-16, Midori-ku, Yokohama 226-8502, Japan. yizumi@chemenv.titech.ac.jp
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Izumi
authors.forename: Fumitaka
authors.initials: F
authors.lastname: Kiyotaki
authors.forename: Nobuhiro
authors.initials: N
authors.lastname: Yagi
authors.forename: Aurel-Mihai
authors.initials: AM
authors.lastname: Vlaicu
authors.forename: Atsushi
authors.initials: A
authors.lastname: Nisawa
authors.forename: Sei
authors.initials: S
authors.lastname: Fukushima
authors.forename: Hideaki
authors.initials: H
authors.lastname: Yoshitake
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Iwasawa