In Situ Raman Spectroscopy of Supported Transition Metal Oxide Catalysts:  ¹⁸O₂−¹⁶O₂ Isotopic Labeling Studies

The isothermal isotopic exchange reaction of 18 O 2 with 16 O of CrO 3, MoO 3, Nb 2 O 5, WO 3, V 2 O 5, and Re 2 O 7 supported on ZrO 2 has been investigated with in situ laser Raman spectroscopy. Isotopic exchange of the oxygen atoms of the supported transition metal oxides with 18 O 2 is difficult and requires several successive reduction− 18 O 2 reoxidation cycles at relatively high temperatures. The Raman spectroscopy data reveal that all the supported transition metal oxides are present as a monooxo species on ZrO 2 . This finding is consistent with the shifts calculated from the isotopic ratios for a simple diatomic oscillator, with the corresponding infrared spectra of the same catalysts and with the vibrational frequencies of several monooxo reference compounds. On this basis, coordination models of the molecular structures are proposed for CrO 3 /ZrO 2, MoO 3 /ZrO 2, Nb 2 O 5 /ZrO 2, WO 3 /ZrO 2, V 2 O 5 /ZrO 2, and Re 2 O 7 /ZrO 2 catalysts under dehydrated conditions.