Solid-state ‘‘V NMR Structural Studies on Supported Vanadlum(V) Oxide Catalysts: Vanadium Oxide Surface Layers on Alumina and Titania Supports

Solid-state wide-line, magic-angle spinning (MAS) and pulse excitation (nutation) NMR techniques are applied in a study of local environmeots in two-dimensional vanadium(V) oxide surface layers on titania and alumina supports. Two main surface vanadium oxide species with different bonding environments are detected, which are, on the basis of their anisotropic chemical shift and nuclear electric quadrupolar coupling properties, assigned to 4and 6-cdordinate V-O environments. In all materials, the relative amount of 6-coordinate surface species increases monotonically as a function of the vanadium oxide surface coverage. However, the results indicate a marked dependence of the surface vanadium oxide structure on the metal oxide support material. Vanadium(V) oxide on Ti02 (anatase) substrates displays the highest tendency to be 6-coordinated down to very low surface coverages. Hydrous species (OH and H20) participate in this environment, and upon dehydration in vacuo, a new tetrahedral species is formed. These results illustrate the suitability of 5’V NMR as a unique quantitative spectroscopic tool in the structural analysis of vanadium(V) oxide catalytic materials.