Keller Daphne E, de Groot Frank M F, Koningsberger Diederik C, Weckhuysen Bert M
Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, PO Box 80083, 3508 TB Utrecht, The Netherlands.
J Phys Chem B. 2005 May 26;109(20):10223-33. doi: 10.1021/jp044539l.
Vanadium oxide (1 wt %) supported on gamma-Al(2)O(3) was used to investigate the interface between the catalytically active species and the support oxide. Raman, UV-vis-NIR DRS, ESR, XANES, and EXAFS were used to characterize the sample in great detail. All techniques showed that an isolated VO(4) species was present at the catalyst surface, which implies that no V-O-V moiety is present. Surprisingly, a Raman band was present at 900 cm(-1), which is commonly assigned to a V-O-V vibration. This observation contradicts the current literature assignment. To further elucidate on potential other Raman assignments, the exact molecular structure of the VO(4) entity (1 V=O bond of 1.58 A and 3 V-O bonds of 1.72 A) together with its position relative to the support O anions and Al cation of the Al(2)O(3) support has been investigated with EXAFS. In combination with a structural model of the alumina surface, the arrangement of the support atoms in the proximity of the VO(4) entity could be clarified, leading to a new molecular structure of the interface between VO(4) and Al(2)O(3). It was found that VO(4) is anchored to the support oxide surface, with only one V-O support bond instead of three, which is commonly accepted in the literature. The structural model suggested in this paper leaves three possible assignments for the 900 cm(-1) band: a V-O-Al vibration, a V-O-H vibration, and a V-(O-O) vibration. The pros and cons of these different options will be discussed.
负载在γ-Al₂O₃上的氧化钒（1 wt%）用于研究催化活性物种与载体氧化物之间的界面。拉曼光谱、紫外-可见-近红外漫反射光谱（UV-vis-NIR DRS）、电子顺磁共振（ESR）、X射线吸收近边结构（XANES）和扩展X射线吸收精细结构（EXAFS）被用于详细表征该样品。所有技术均表明，催化剂表面存在孤立的VO₄物种，这意味着不存在V-O-V部分。令人惊讶的是，在900 cm⁻¹处出现了一个拉曼峰，该峰通常被归因于V-O-V振动。这一观察结果与当前文献的归属相矛盾。为了进一步阐明其他可能的拉曼归属，利用EXAFS研究了VO₄实体的确切分子结构（1个键长为1.58 Å的V=O键和3个键长为1.72 Å的V-O键）及其相对于Al₂O₃载体的O阴离子和Al阳离子的位置。结合氧化铝表面的结构模型，可以阐明VO₄实体附近载体原子的排列，从而得到VO₄与Al₂O₃之间界面的新分子结构。研究发现，VO₄仅通过一个V-O载体键锚定在载体氧化物表面，而不是文献中通常认为的三个。本文提出的结构模型为900 cm⁻¹处的峰留下了三种可能的归属：V-O-Al振动、V-O-H振动和V-(O-O)振动。将讨论这些不同选项的优缺点。
