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 Mo7−Mo12 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 (NMo) 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.