Wu Zili, Kim Hack-Sung, Stair Peter C, Rugmini Sreekala, Jackson S David
Department of Chemistry, Center for Catalysis and Surface Science and Institute of Environmental Catalysis, Northwestern University, Evanston, Illinois 60208, USA.
J Phys Chem B. 2005 Feb 24;109(7):2793-800. doi: 10.1021/jp046011m.
Vanadia species on aluminas (delta- and gamma-Al2O3) with surface VOx density in the range 0.01-14.2 V/nm2 have been characterized by UV and visible Raman spectroscopy, UV-visible diffuse reflectance spectroscopy (UV-Vis DRS), and temperature-programmed reduction in hydrogen. It is shown that the alumina phase has little influence on the structure and reducibility of surface VOx species under either dehydrated or hydrated conditions. Three similar types of dispersed VOx species, i.e., monovanadates, polyvanadates, and V2O5, are identified on both aluminas under dehydrated conditions. Upon hydration, polymerized VOx species dominate on the surfaces of the two aluminas. The broad Raman band at around 910 cm(-1), observed on dehydrated V/delta-, gamma-Al2O3 at all V loadings (0.01-14.2 V/nm2), is assigned to the interface mode (V-O-Al) instead of the conventionally assigned V-O-V bond. The direct observation of the interface bond is of significance for the understanding of redox catalysis because this bond has been considered to be the key site in oxidation reactions catalyzed by supported vanadia. Two types of frequency shifts of the V=O stretching band (1013-1035 cm(-1)) have been observed in the Raman spectra of V/Al2O3: a shift as a function of surface VOx density and a shift as a function of excitation wavelength. The shift of the V=O band to higher wavenumbers with increasing surface VOx density is due to the change of VOx structure. The V=O stretching band in dispersed vanadia always appears at lower wavenumber in UV Raman spectra than in visible Raman spectra for the same V/Al2O3 sample. This shift is explained by selective resonance enhancement according to the UV-Vis DRS results. It implies that UV Raman has higher sensitivity to isolated and less polymerized VOx species while visible Raman is more sensitive to highly polymerized VOx species and crystalline V2O5. These results show that a multiwavelength excitation approach provides a more complete structural characterization of supported VOx catalysts.
通过紫外和可见拉曼光谱、紫外可见漫反射光谱（UV-Vis DRS）以及氢气程序升温还原对负载于氧化铝（δ - 和γ - Al₂O₃）上、表面VOx密度在0.01 - 14.2 V/nm²范围内的钒物种进行了表征。结果表明，在脱水或水合条件下，氧化铝相对表面VOx物种的结构和还原性影响很小。在脱水条件下，在两种氧化铝上都鉴定出了三种相似类型的分散VOx物种，即单钒酸盐、多钒酸盐和V₂O₅。水合后，聚合的VOx物种在两种氧化铝表面占主导。在所有V负载量（0.01 - 14.2 V/nm²）下，在脱水的V/δ - 、γ - Al₂O₃上观察到的约910 cm⁻¹处的宽拉曼带被归属于界面模式（V - O - Al），而非传统认为的V - O - V键。界面键的直接观察对于理解氧化还原催化具有重要意义，因为该键被认为是负载型钒氧化物催化氧化反应的关键位点。在V/Al₂O₃的拉曼光谱中观察到V = O伸缩带（1013 - 1035 cm⁻¹）有两种频率位移：一种是作为表面VOx密度的函数的位移，另一种是作为激发波长的函数的位移。随着表面VOx密度增加，V = O带向更高波数的位移是由于VOx结构的变化。对于相同的V/Al₂O₃样品，分散钒氧化物中的V = O伸缩带在紫外拉曼光谱中总是出现在比可见拉曼光谱更低的波数处。根据UV-Vis DRS结果，这种位移通过选择性共振增强来解释。这意味着紫外拉曼对孤立且聚合程度较低的VOx物种具有更高的灵敏度，而可见拉曼对高度聚合的VOx物种和结晶V₂O₅更敏感。这些结果表明，多波长激发方法为负载型VOx催化剂提供了更完整的结构表征。
