Influence of pH and ionic strength on vanadium(<scp>v</scp>) oxides formation. From V₂O₅·nH₂O gels to crystalline NaV₃O₈·1.5H₂O

Abstract

Na+ and TMA+ intercalated M0.3V2O5·1.5H2O (M = Na+ and TMA+) precipitates have been synthesized by adding chloride salts MCl or bases MOH to a metavanadate solution which was acidified using a proton exchange resin. As evidenced by X-ray diffraction, these vanadium oxides exhibit the layered structure of the V2O5·1.8H2O xerogels in which Na+ and TMA+ cations are intercalated between the layers. These precipitates result from the assembly of ribbon-like particles but in contrast to the V2O5·1.8H2O xerogels, the particles are no longer stacked on flat surfaces and their assembly is disorganized as evidenced by SEM and TEM. Depending on the final pH of the acidified metavanadate solution, the Na0.3V2O5·1.5H2O phase can be thermodynamically stable or can evolve with time. At a pH > 3, the Na0.3V2O5·1.5H2O phase disappears after a few days and is transformed to the crystalline NaV3O8·1.5H2O phase. The vanadium oxide phases were all characterized by X-ray diffraction, SEM, TEM, TGA, IR and 51V MAS NMR.