In Situ Raman and FTIR Spectroscopy of Molybdenum(VI) Oxide Supported on Titania Combined with 18O/16O Exchange: Molecular Structure, Vibrational Properties, and Vibrational Isotope Effects

Molybdenum oxide deposited on anatase with Mo surface densities in the range 1.8−17.0 Mo/nm 2 was studied by in situ vibrational (Raman and FTIR) spectroscopies and 18 O/ 16 O isotopic exchange experiments combined with in situ Raman spectra at 450 °C. The vibrational isotope effects and the combined interpretation of the observed Raman fundamental, IR overtone, as well as calculated zero-order band wavenumbers and characteristics suggest a mono-oxo configuration for the deposited molybdena phase at low (up to 3 Mo/nm 2 ) as well as at high (4−6 Mo/nm 2 ) coverage, irrespective of the extent of association (polymerization). The Raman band due to Mo═O stretching is observed at 994 cm −1 for all samples with surface densities up to 6 Mo/nm 2 . Isolated mono-oxo monomolybdates in C 3 v O═Mo(−O−Ti) 3 configuration predominate at low loadings, while the presence of mono−oxo polymolybdates is evidenced at high surface coverage. A “next-nearest-neighbor 18 O/ 16 O substitution” vibrational isotope effect is observed, resulting in red shifts (6−7 cm −1 ) of the Mo═ 16 O Raman band wavenumber. All observations are addressed and discussed, and consistent band assignments and interpretations are made. A mechanism accounting for the 18 O/ 16 O exchange process is proposed and discussed at the molecular level.