Monomeric vanadium oxide on a θ-Al ₂O ₃ support: A combined experimental/theoretical study

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 H ₂O.