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.

Raman spectroscopy was used to demonstrate that the lattice dynamics of SrTiO 3 (STO) nanoparticles strongly depends on their microstructure, which is in turn determined by the synthetic approach employed. First-order Raman modes are observed at room temperature in STO single-crystalline nanocubes with average edge lengths of 60 and 120 nm, obtained via sol-precipitation coupled with hydrothermal synthesis and a molten salt procedure, respectively. First-order Raman scattering arises from local loss of inversion symmetry caused by surface frozen dipoles, oxygen vacancies, and impurities incorporated into the host lattice. The presence of polar domains is suggested by the pronounced Fano asymmetry of the peak corresponding to the TO2 polar phonon, which does not vanish at room temperature. These noncentrosymmetric domains will likely influence the dielectric response of these nanoparticles.

A combined experimental and theoretical study of vanadium oxide monomers on a θ-alumina surface under different environments has identified four different structures. Deep UV Raman results suggest that vanadia is attached predominantly to an aluminum site that was an isolated terminal Al−OH group on the θ-alumina surface. The preresonance Raman spectra for vanadium oxide supported on θ-alumina with a very low VO x surface density show three distinct VO bands under dehydrated conditions. The observed frequencies match well with the calculated stretching frequencies from B3LYP density functional theory for tridendate, bidendate, and molecular structures of vanadium oxide monomers on a dehydrated surface. The free energies calculated for these three structures from density functional theory as a function of temperature suggest that all three could exist on the surface with the tridentate structure being the most stable of the three on the dehydrated surface. Different structures and different degrees of vibrational coupling of V−O to V=O modes may cause the appearance of three VO bands in the preresonance Raman spectra. On the hydrated surface, the Raman spectra show a V−O band, in agreement with the calculated frequency for a monodentate structure on this surface. Finally, the calculated free energies of hydrated and dehydrated surfaces indicate a transition from a hydrated to a dehydrated θ-alumina surface occurs at around 600 K at 10−6 atm pressure of H2O.

We present detailed resonance Raman spectroscopic results excited at 220 and 287 nm for alumina-supported VO(x) catalysts. The anharmonic constant, harmonic wavenumber, anharmonic force constant, bond dissociation energy, and bond length change in the excited state for double bonded VO and single bonded V-O were obtained from fundamental and overtone frequencies. Totally symmetric and nontotally symmetric modes could be discerned and assigned on the basis of the overtone and combination progressions found in the resonance Raman spectra. Selective resonance enhancement of two different vibrational modes with two different excitation wavelengths was observed. This allowed us to establish a linear relationship between charge transfer energy and VO bond length and, consequently, to assign the higher-energy charge transfer band centered around 210-250 nm in the UV-vis spectra to the VO transition.