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 O-18/O-16 isotopic exchange experiments combined with in situ Raman spectra at 450 degrees 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 nu 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 O-18/O-16 substitution" vibrational isotope effect is observed, resulting in red shifts (6-7 cm(-1)) of the Mo=O-16 Raman band wavenumber. All observations are addressed and discussed, and consistent band assignments and interpretations are made. A mechanism accounting for the O-18/O-16 exchange process is proposed and discussed at the molecular level.