SOLID-SOLID INTERACTIONS IN PURE AND LI2O-DOPED V2O5 AL2O3 SYSTEM

The effects of calcination temperature (500-1000-degrees-C) and lithium oxide-doping (4 and 10 mole %) on solid-solid interactions between Al2O3 and V2O5 were investigated using DTA, TG and their derivatives together with XRD techniques. The results obtained revealed that no solid-solid interaction between vanadium and aluminium oxides occurred at 500-degrees-C. These oxides interacted in part, at 650-degrees-C to produce AlVO4, the completion of this reaction requires a prolonged heating at this temperature. Li2O-doping much enhanced the interaction yielding AlV4, the presence of 4 mole % was found sufficient to stimulate the complete conversion of V2O5 into aluminium vanadate. Aluminium vanadate produced in the case of pure mixed solids decomposed entirely by heating at 750-degrees-C giving a well crystallized V2O5 and an amorphous alumina. On the other hand, lithium oxide-doping much increased the thermal stability of AlVO4 at 750-degrees-C and led also to the formation of AlVO3 and LiV10O25 phases. The heating of pure mixed oxides in air at 1000-degrees-C resulted in the formation of alpha-alumina (corrundum) together with V2O5. This treatment, in the case of doped mixed solids, effected the formation of alpha-Al2O3, V2O5 and LiV10O25 phases. The last compound was formed at 750-degrees-C and remained stable by heating at 1000-degrees-C. The promotion effect of Li2O in AlVO4 formation was tentatively attributed to dissolution of a small portion of Li2O in V2O5 lattice with subsequent increase in the mobility of V+5 ions.