The article addresses the critical molecular structural issue of differentiating between the mono-oxo (MoO) and di-oxo [Mo(O)2] configurations as well as the most plausible structures for the oxo-molybdenum [(MoOx)n] sites (including aspects related to coordination number of Mo and extent of association/polymerization) deposited on typical catalyst supports such as γ-Al2O3, monoclinic ZrO2, TiO2-anatase and SiO2. The issue is of topical character and has been the subject of persistent post-2005 research endeavors comprising both theoretical (mainly DFT) work as well as careful experimental/spectroscopic studies (Raman, IR, DR-UV/Vis) that in some cases have also been combined with isotopic labeling experiments. The pertinent vibrational properties are discussed in relation to site configuration (mono-oxo vs. di-oxo), structure and extent of association/polymerization of dispersed oxomolybdates. Vibrational isotope effects and mechanisms for 18O/16O exchange at the molecular level are given special attention.
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