Vittadini Andrea, Casarin Maurizio, Sambi Mauro, Selloni Annabella
CNR-ISTM and INSTM, via Marzolo 1, I-35131 Padova, Italy.
J Phys Chem B. 2005 Nov 24;109(46):21766-71. doi: 10.1021/jp0536910.
Periodic density functional calculations have been used to investigate the structure and stability of epitaxial vanadium oxide films grown on the TiO(2)(001) anatase surface. The formation energy of films of V(2)O(5) stoichiometry, initially low, is found to rapidly increase with the film thickness, at variance to what is obtained for reduced pseudomorphic VO(2) films. This is in tune with results of oxygen-assisted molecular beam epitaxy. The oxidation of thick, viz. >2 monolayers (ML), VO(2) films yields a c(2 x 2) reconstructed surface, in agreement with low energy electron diffraction. These films are composed by partially reduced inner V atoms in a distorted-octahedral environment, and by isolated surface dioxovanadium centers exhibiting a distorted trigonal-bipyramidal coordination. Single scattering simulations of X-ray photoelectron diffraction patterns have also been performed, taking both 2- and 3-ML surface surface-oxidized films as models. Results are in fair agreement with experiments referring to films grown in oxidizing conditions, which suggests that coherent vanadia ultrathin films could be formed in vanadia-titania catalysts. The electronic structure of the films has been finally studied, finding that the terminal oxygens carried by the surface dioxovanadium species have a strong nucleophilic character, which makes them potential active centers for selective oxidation catalysis.
周期性密度泛函计算已被用于研究在TiO(2)(001)锐钛矿表面生长的外延氧化钒薄膜的结构和稳定性。发现化学计量比为V(2)O(5)的薄膜的形成能最初较低，但随着薄膜厚度的增加而迅速增加，这与还原的赝晶VO(2)薄膜的情况不同。这与氧辅助分子束外延的结果一致。厚的即大于2个单层(ML)的VO(2)薄膜的氧化产生c(2 x 2)重构表面，这与低能电子衍射结果一致。这些薄膜由处于扭曲八面体环境中的部分还原的内部V原子以及呈现扭曲三角双锥配位的孤立表面二氧钒中心组成。还对X射线光电子衍射图案进行了单散射模拟，以2-ML和3-ML表面氧化薄膜作为模型。结果与关于在氧化条件下生长的薄膜的实验结果相当吻合，这表明在钒-钛催化剂中可以形成相干的超薄氧化钒薄膜。最后研究了薄膜的电子结构，发现表面二氧钒物种携带的末端氧具有很强的亲核特性，这使其成为选择性氧化催化的潜在活性中心。
