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/nm² was studied by in situ vibrational (Raman and FTIR) spectroscopies and ¹⁸O/¹⁶O 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/nm²) 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_3v 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 ¹⁸O/¹⁶O substitution" vibrational isotope effect is observed, resulting in red shifts (6-7 cm ^-1) of the Mo - ¹⁶O Raman band wavenumber. All observations are addressed and discussed, and consistent band assignments and interpretations are made. A mechanism accounting for the ¹⁸O/ ¹⁶O exchange process is proposed and discussed at the molecular level.