Todorova Tanya K, Ganduglia-Pirovano M Veronica, Sauer Joachim
Humboldt-Universität zu Berlin, Institut für Chemie, Unter den Linden 6, D-10099 Berlin, Germany.
J Phys Chem B. 2005 Dec 15;109(49):23523-31. doi: 10.1021/jp053899l.
Using density functional theory and statistical thermodynamics, we obtained the phase diagram of thin VnOm films of varying thickness (approximately 2-6 A, 1-6 vanadium layers) supported on alpha-Al2O3(0001). Depending on the temperature, oxygen pressure, and vanadium concentration, films with different thickness and termination may form. In ultrahigh vacuum (UHV), at room temperature and for low vanadium concentrations, an ultrathin (1 x 1) O=V-terminated film is most stable. As more vanadium is supplied, the thickest possible films form. Their structures and terminations correspond to previous findings for the (0001) surface of bulk V2O3 [Kresse et al., Surf. Sci. 2004, 555, 118]. The presence of surface vanadyl (O=V) groups is a prevalent feature. They are stable up to at least 800 K in UHV. Vanadyl oxygen atoms induce a V(2p) core-level shift of about 2 eV on the surface V atoms. The reducibility of the supported films is characterized by the energy of oxygen defect formation. For the stable structures, the results vary between 4.11 and 3.59 eV per 1/2O2. In contrast, oxygen removal from the V2O5(001) surface is much easier (1.93 eV). This provides a possible explanation for the lower catalytic activity of vanadium oxides supported on alumina compared to that of crystalline vanadia particles.
利用密度泛函理论和统计热力学，我们得到了负载在α-Al2O3(0001)上不同厚度（约2 - 6埃，1 - 6个钒层）的VnOm薄膜的相图。根据温度、氧分压和钒浓度的不同，可能会形成不同厚度和终止结构的薄膜。在超高真空（UHV）中，室温下且钒浓度较低时，超薄的(1×1) O=V终止薄膜最稳定。随着供应更多的钒，会形成尽可能厚的薄膜。它们的结构和终止结构与之前关于块状V2O3(0001)表面的研究结果一致[Kresse等人，《表面科学》，2004年，555卷，118页]。表面钒氧基（O=V）基团的存在是一个普遍特征。在超高真空下，它们至少在800 K时是稳定的。钒氧基氧原子会使表面V原子的V(2p)芯能级发生约2 eV的位移。负载薄膜的还原性通过氧缺陷形成能来表征。对于稳定结构，每1/2O2的结果在4.11至3.59 eV之间变化。相比之下，从V2O5(001)表面去除氧要容易得多（1.93 eV）。这为氧化铝负载的钒氧化物催化活性低于结晶氧化钒颗粒提供了一种可能的解释。
