Structural determination of bulk and surface tungsten oxides with UV-vis diffuse reflectance spectroscopy and Raman spectroscopy

Combined UV-vis diffuse reflectance spectroscopy (DRS) and Raman spectroscopy were applied to examine the electronic and molecular structures, respectively, of well-defined W(VI) bulk mixed oxide reference compounds consisting of (i) isolated WO4 or WO6 monomers, (ii) dimeric O3W-O-WO3, (iii) polymeric chain of alternating WO4/WO6 units, and (iv) WO6-coordinated W-9-W-18 clusters. Raman spectroscopy was employed to confirm the identity and phase purity of the different tungsten oxide structures. UV-vis DRS provided the corresponding electronic edge energy (E-g) of the ligand-to-metal charge transfer (LMCT) transitions of the W(VI) cations. A correlation between the edge energy and the number of covalent bridging W-O-W bonds around the central W(VI) cation was found with E-g linearly decreasing with increasing number of bridging W-O-W bonds. However, a direct relationship between E-g and the domain size, N-w, for finite WOx, clusters does not exist. Subsequently, UV-vis and Raman spectroscopy information were applied to determine the local structures of the molecularly dispersed surface W(VI) species present in supported WO3/Al2O3, WO3/ZrO2, and WO3/SiO2 catalysts under ambient and dehydrated conditions.