Izumi Yasuo, Kiyotaki Fumitaka, Yagi Nobuhiro, Vlaicu Aurel-Mihai, Nisawa Atsushi, Fukushima Sei, Yoshitake Hideaki, Iwasawa Yasuhiro
Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta 4259-G1-16, Midori-ku, Yokohama 226-8502, Japan.
J Phys Chem B. 2005 Aug 11;109(31):14884-91. doi: 10.1021/jp052038+.
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.
X射线吸收精细结构与X射线荧光光谱联用技术被应用于各种V+TiO₂混合样品。使用配备Ge(331)晶体的高能量分辨率（0.4 eV）光谱仪选择性地计数样品发射的V Kα₁荧光。该方法的两个优点，即极高的信号/背景比以及在反应气体气氛中进行测量的兼容性（与多相催化相关的原位研究），在V K边处很有效。通过光谱法跟踪了V/TiO₂催化剂中V位点的结构转变。单氧四面体钒酸盐位点在473 K时被证明存在。在环境空气中它转变为五重配位的分散物种，在290 K、0.85 kPa的水存在下进一步转变为聚合的VO(x)物种。在3.2 kPa的2-丙醇存在下，强烈表明在290 - 473 K时2-丙醇在分散的V物种上发生解离吸附。首次通过XAFS报道了TiO₂上V位点的原位结构变化。V/TiO₂催化剂的V(V)位点与负载在介孔（高比表面积）TiO₂上的V以及通过溶胶-凝胶法制备的V-TiO₂样品的V(V)位点基本相同。然而，在介孔V-TiO₂中发现了主要的V(IV)位点。V(IV)位点取代了TiO₂的Ti位点。当V/Ti的摩尔比从1/100增加到1/5.0时，TiO₂基体中的主要八面体V位点似乎转变为四面体位点。
