Catalytically Active Vanadia Species on Silica: Effect of Oxygen and Water

Energies and free energies are calculated by density functional theory for the reaction of one and two water molecules with monomeric vanadyl species anchored on silica surface models. Whereas monohydrated species may be present depending on the local surface structure, dihydrated species are disfavored by entropy. Vibrational frequencies are calculated and compared with observed Raman spectra in the range between 900 and 1100 cm–1. Moreover, the oxidation of vanadyl species to vanadium peroxo species and the formation of vanadium oxo peroxo species anchored by one V–O–Si bond to the support (“umbrella model”) is studied. The calculated energies show that the formation of peroxo species is unlikely for surface models.