Brázdová Veronika, Ganduglia-Pirovano M Verónica, 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):23532-42. doi: 10.1021/jp0539167.
The structure, stability, and vibrational properties of isolated V2O5 clusters on the Al2O3(0001) surface have been studied by density functional theory and statistical thermodynamics. The most stable structure does not possess vanadyl oxygen atoms. The positions of the oxygen atoms are in registry with those of the alumina support, and both vanadium atoms occupy octahedral sites. Another structure with one vanadyl oxygen atom is only 0.12 eV less stable. Infrared spectra are calculated for the two structures. The highest frequency at 922 cm(-1) belongs to a V-O stretch in the V-O-Al interface bonds, which supports the assignment of such a mode to the band observed around 941 cm(-1) for vanadia particles on alumina. Removal of a bridging oxygen atom from the most stable cluster at the V-O-Al interface bond costs 2.79 eV. Removal of a (vanadyl) oxygen atom from a thin vanadia film on alpha-Al2O3 costs 1.3 eV more, but removal from a V2O5(001) single-crystal surface costs 0.9 eV less. Similar to the V2O5(001) surface, the facile reduction is due to substantial structure relaxations that involve formation of an additional V-O-V bond and yield a pair of V(IV)(d1) sites instead of a V(III)(d2)/V(V)(d0) pair.
通过密度泛函理论和统计热力学研究了Al2O3(0001)表面孤立的V2O5团簇的结构、稳定性和振动特性。最稳定的结构不含有钒氧基氧原子。氧原子的位置与氧化铝载体的氧原子位置对齐，两个钒原子均占据八面体位置。另一种含有一个钒氧基氧原子的结构稳定性仅低0.12 eV。计算了这两种结构的红外光谱。922 cm(-1)处的最高频率属于V-O-Al界面键中的V-O伸缩振动，这支持了将该模式归属于氧化铝上钒氧化物颗粒在941 cm(-1)左右观察到的谱带。从V-O-Al界面键处最稳定的团簇中去除一个桥连氧原子需要2.79 eV。从α-Al2O3上的薄钒氧化物膜中去除一个（钒氧基）氧原子成本要高1.3 eV，但从V2O5(001)单晶表面去除则成本低0.9 eV。与V2O5(001)表面类似，这种容易还原是由于大量的结构弛豫，包括形成一个额外的V-O-V键并产生一对V(IV)(d1)位点，而不是V(III)(d2)/V(V)(d0)对。
