In situ Raman spectroscopy of SiO2-supported transition metal oxide catalysts:: An isotopic 18O-16O exchange study

The molecular structures of dehydrated group 5-7 transition metal oxides (V2O5, Nb2O5, CrO3, MoO3, WO3, Re2O7) supported on SiO2 were investigated with time-resolved O-18-O-16 exchange in situ Raman spectroscopy measurements. The supported group 5-7 dehydrated surface transition metal oxides were exclusively present as isolated species on SiO2 because of the absence of bridging M-O-M vibrations. The SiO2-Supported group 5 (VOx and NbOx) surface metal oxides exhibit band splitting into two Raman vibrations (M=O-16 and M=O-18), which is consistent with monoxo surface O=M(-O-Si)(3) species. The SiO2-supported group 6 (CrOx, MoOx, and WOx) surface metal oxides consist of both monoxo O=M(-O-Si)(4) and dioxo (O=)(2)M(-O-Si)(2) structures. The dioxo surface species give rise to triplet band splitting corresponding to M(=O-16)(2), M(=O-18)(2), and O-18=M=O-16. Identification of the intermediate surface O-18=M=O-16 structure was guided by recent DFT calculations. The SiO2-supported group 7 (ReOx) metal oxide system exclusively contains trioxo surface (O=)(3)Re-O-Si species that give rise to quadruplet band splitting (Re(=O-16)(3), O-18=Re(=O-16)(2), (O-18=)(2)Re = O-16, and (O-18=)(3)Re) during isotopic oxygen exchange. Excellent prediction was also achieved for the isotopic shifts for the completely O-18-exchanged surface metal oxide structures with a simple diatomic oscillator model. The isotopic exchange studies reveal, for the first time, the exact number of Raman bands for surface monoxo, dioxo, and trioxo metal oxide structures, their positions, and their band splitting characteristics during isotopic O-18-O-16 exchange.