¹⁷O MAs and 3QMAS NMR investigation of crystalline V₂O₅ and layered V₂O₅·nH₂O gels

The environments for oxygen sites in crystalline V₂O₅ and in layered vanadia gels produced via sol-gel synthesis have been investigated using ¹⁷O MAS and 3QMAS NMR. For crystalline V₂O₅, three structural oxygen sites were observed: V=O (vanadyl), V₂O (doubly coordinated), and V₃O (triply coordinated). Line-shape parameters for these sites were determined from numerical simulations of the MAS spectra. For the vanadia gels at various stages of dehydration, assignments have been proposed for numerous vanadyl, doubly coordinated, and triply coordinated oxygen sites. In addition, by correlating the ¹⁷O MAS and 3QMAS NMR, ⁵¹V MAS NMR, and thermogravimetric analysis data, the coordination of water sites has been established. On the basis of these results, the gel structure and its evolution at various stages of hydration have been detailed. Upon rehydration of the layered gel, we observed a preferred site for initial water readsorption. The oxygen atoms of these readsorbed water molecules readily exchanged into all types of oxygen sites even at room temperature.