The isothermal isotopic exchange reaction of O-18(2) With O-16 Of CrO3, MoO
3, Nb2O5, WO3, V2O5, and Re2O7 supported on ZrO2 has been investigated with
in situ laser Raman spectroscopy. Isotopic exchange of the oxygen atoms of
the supported transition metal oxides with O-18(2) is difficult and requir
es several successive reduction-O-18(2) reoxidation cycles at relatively hi
gh temperatures. The Raman spectroscopy data reveal that all the supported
transition metal oxides an present as a monooxo species on ZrO2. This findi
ng 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 refe
rence compounds. On this basis, coordination models of the molecular struct
ures are proposed for CrO3/ZrO2, MoO3/ZrO2, Nb2O5/ZrO2, WO3/ZrO2, V2O5/ZrO2
, and Re2O7/ZrO2 catalysts under dehydrated conditions.