Identification of Mixed Valence Vanadium in ETS-10 Using Electron Paramagnetic Resonance, 51V Solid-State Nuclear Magnetic Resonance, and Density Functional Theory Studies

Microporous vanadium-substituted titanosilicate ETS-10 solids are promising photocatalysts for decomposition of organic molecules. The dopant vanadium metal modulates the electronic environment of the titanosilicate matrix and plays a major role in the enhancement of the photocatalytic activity. However, the local electronic and geometric structure of the vanadium sites in these materials is a subject of controversy. Using vanadium electron paramagnetic resonance (EPR) and 51V nuclear magnetic resonance (NMR) spectroscopy, we have characterized the local environments of the vanadium sites in vanadium-substituted ETS-10 samples with different vanadium loadings. The measurements reveal clearly the presence of V(IV) and V(V) oxidation states. The EPR results suggest that V(IV) is in octahedral sites and, therefore, must substitute for Ti in the framework. 51V NMR studies indicate that the V(V) species are adjacent to the V(IV) species in most cases on the basis of significant electron−nuclear dipolar interac...