Molecular Structural Determination of Molybdena in Different Environments: Aqueous Solutions, Bulk Mixed Oxides, and Supported MoO3 Catalysts

UV-vis diffuse reflectance spectroscopy (DRS) and Raman spectroscopy were used to examine the electronic and molecular structures, respectively, of well-defined Mo(VI) bulk mixed oxide reference compounds ((i) isolated MoO4 or MoO6 monomers, (ii) dimeric O3Mo-O-MoO3, (iii) chains of alternating MoO4 and MoO6 units, (iv) MoO6-coordinated Mo-7-Mo-12 clusters, and (v) infinite layered sheets of MoO5 units), aqueous molybdate anions as a function of solution pH, and supported MoO3 catalysts (MoO3/SiO2, MoO3/Al2O3, and MoO3/ZrO2). Raman spectroscopy confirmed the identity and phase purity of the different bulk and solution molybdenum oxide structures. UV-vis DRS provided the corresponding electronic edge energy (Eg) of the ligand-to-metal charge transfer (LMCT) transitions of the Mo(VI) cations. A linear inverse correlation was found between Eg and the number of bridging Mo-O-Mo covalent bonds around the central Mo(VI) cation. A relationship between Eg and the domain size (N-Mo) for finite MoOx clusters, however, was not found to exist. Application of the above insights allowed for the determination of the molecular structures of the two-dimensional surface MoOx species present in supported MoO3 catalysts as a function of environmental conditions. The current electronic and molecular structural findings are critical for subsequent studies that wish to establish reliable structure-activity/selectivity relationships for molybdenum oxide catalysts, especially supported MoO3 catalysts.