On the structure of vanadium oxide supported on aluminas: UV and visible Raman spectroscopy, UV-visible diffuse reflectance spectroscopy, and temperature-programmed reduction studies

Vanadia species on aluminas (δ and γ-Al ₂O ₃) with surface VO _x density in the range 0.01-14.2 V/nm ² 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 VO _x species under either dehydrated or hydrated conditions. Three similar types of dispersed VO _x species, i.e., monovanadates, polyvanadates, and V ₂O ₅, are identified on both aluminas under dehydrated conditions. Upon hydration, polymerized VO _x species dominate on the surfaces of the two aluminas. The broad Raman band at around 910 cm ^-1, observed on dehydrated V/δ, γ-Al ₂O ₃ at all V loadings (0.01-14.2 V/nm ²), 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/Al ₂O ₃: a shift as a function of surface VO _x density and a shift as a function of excitation wavelength. The shift of the V=O band to higher wavenumbers with increasing surface VO, density is due to the change of VO _x 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/Al ₂O ₃ 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 VO _x species while visible Raman is more sensitive to highly polymerized VO _x species and crystalline V ₂O ₅. These results show that a multiwavelength excitation approach provides a more complete structural characterization of supported VO _x catalysts.