On the configuration, molecular structure and vibrational properties of MoO_x sites on alumina, zirconia, titania and silica

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 [(MoO_x)_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.