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

Abstract

Na⁺ and TMA⁺ intercalated M_0.3V₂O₅·1.5H₂O (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 Na_0.3V₂O₅·1.5H₂O phase can be thermodynamically stable or can evolve with time. At a pH > 3, the Na_0.3V₂O₅·1.5H₂O 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.