V-51 NMR AND EPR STUDY OF REACTION-KINETICS AND MECHANISMS IN V2O5 GELATION BY ION-EXCHANGE OF SODIUM METAVANADATE SOLUTIONS

V2O5 gels have several potential uses in electronics and catalysis, bu t the rate and mechanism of the synthesis remain poorly understood. V- 51 NMR spectra show the consumption of the dioxovanadium cation (VVO2) to form vanadate polymers in solution. The only other vanadium(V) sp ecies in solution, decavanadic acid, acts primarily as a source of dio xovanadium cations for the polymerization. The oxovanadium cation (VIV O2+) may act as an initiator for the polymerization reaction with the dioxovanadium cation. The pH remains constant throughout this process, suggesting that hydrolysis of the polymer furnishes hydronium ions. V -51 MAS NMR of the resulting sol indicates that the vanadium environme nt in the polymer is similar to that of the dioxovanadium cation. Thes e results confirm some aspects of models previously proposed.1,2