Periodic density functional study on structural and vibrational properties of vanadium oxide aggregates -: art. no. 165420

We present periodic density-functional calculations within the generalized gradient approximation (Perdew-Wang 91) on structures and vibrational properties of different vanadium oxide aggregates, namely, bulk V2O5 and its (001) surface, as well as thin vanadium oxide films supported by alpha-alumina. Vanadium is differently coordinated by oxygen in the different systems. The calculated vibrational frequencies of bulk V2O5 are in good agreement with observed IR and Raman frequencies, for stretching modes the rms deviation is 40 cm(-1). The calculations for the V2O5(001) surface suggest modifications of previous assignments of high-resolution electron-energy-loss spectroscopy (HREELS) data. In agreement with HREELS, vanadyl frequencies shift to higher wave numbers on surface formation. The calculated frequencies for bulk Al2O3 are systematically lower than the observed IR data (by about 30 cm(-1)). Models for V2O3 supported on Al2O3 are obtained when in the outermost layers of Al2O3(0001) slabs Al is replaced by V. These films do not show vibrations above 930 cm(-1). Oxygen adsorption on top of the vanadium sites on these supported films creates very stable vanadyl groups with binding energies of about 450 kJ/mol (1/2O(2)). Bond distances, vibrational frequencies, and oxygen binding energies are compared with those of vanadyl groups at the V2O5(001) surface and in (V2O5)(n) clusters (n=2,4). The relevance of the findings for experiments on vanadia particles supported on alumina is discussed.