Investigation of the peroxovanadate sol-gel process and characterization of the gels.

摘要

Vanadium oxide gels derived from aqueous solution of V₂O ₅ and H₂O₂ have been investigated using in situ 51V NMR and laser Raman spectroscopic techniques. On the basis of this characterization, a pathway for peroxovanadate decomposition has been proposed, including the presence of two peroxovanadate dieters. New Raman bands and assignments for these species are reported. Experimental 51V NMR evidence suggested the VO₂+ species was directly involved in the formation of the gel. The vanadia xerogels were studied using 51V and 17O MAS NMR, 17 O 3QMAS NMR, TGA, DSC, XRD, SEM and laser Raman spectroscopy. Based primarily on the 51V MAS NMR and TGA results, the coordination of five distinct vanadia sites has been detailed, which possibly include a previously unreported dieter. The relative concentration of these sites changed as dehydration progressed, and a model of this process has been proposed based on the numerical analysis of the NMR MAS spectra. Depending on sample treatment, it was possible to synthesize both layered and non-layered materials. The laser Raman spectra revealed differences between layered and non-layered materials. These differences have been attributed to the interaction of coordinated water molecules, which were trapped between layers and held firmly in place, thus restricting or altering certain Raman-active vibrations. The environments of oxygen sites in crystalline V₂O₅ and in vanadia produced via sol-gel synthesis were also investigated using 17O MAS and 3QMAS NMR. For crystalline V₂O₅, three sites were observed: V=O (vanadyl), V₂O (bridging), and V ₃O (corner sharing). Line shape parameters for these sites were determined from numerical simulation of the MAS spectrum. For the vanadia gel, assignments have been proposed for several oxygen sites including bridging and corner sharing oxygen, along with several vanadyl sites. Based on the 17O MQMAS NMR results, the coordination of the water sites has been detailed. Upon re-hydration of the layered gel, a preferred site for initial water re-adsorption was observed. Surprisingly, the oxygen atoms of these re-adsorbed water molecules readily exchanged into all of the oxygen sites of the gel even at room temperature. These oxygen-exchanged sites in the gel persisted upon thermal treatment to 550°C.