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 (V 2 O 5, Nb 2 O 5, CrO 3, MoO 3, WO 3, Re 2 O 7 ) supported on SiO 2 were investigated with time-resolved 18 O− 16 O exchange in situ Raman spectroscopy measurements. The supported group 5−7 dehydrated surface transition metal oxides were exclusively present as isolated species on SiO 2 because of the absence of bridging M−O−M vibrations. The SiO 2 -supported group 5 (VOx and NbOx) surface metal oxides exhibit band splitting into two Raman vibrations (M 16 O and M 18 O), which is consistent with monoxo surface O M(−O−Si) 3 species. The SiO 2 -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( 16 O) 2, M( 18 O) 2, and 18 O M 16 O. Identification of the intermediate surface 18 O M 16 O struc-ture was guided by recent DFT calculations. The SiO 2 -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( 16 O) 3, 18 O Re( 16 O) 2, ( 18 O ) 2 Re 16 O, and ( 18 O ) 3 Re) during isotopic oxygen exchange. Excellent prediction was also achieved for the isotopic shifts for the completely 18 O-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 18 O− 16 O exchange.