Synthesis, characterization and thermal stability of a crystalline niobium oxysulfate

The reaction of Nb2O5 with 96% H2SO4 was studied at 200–300 °C for 3 h. No reaction occurred at 200 °C. An amorphous niobium oxysulfate was formed when the reaction temperature was 250 °C. However, the crystaline niobium oxysulfate was produced when the temperature reached 300 °C. XRD analysis shows that the crystal form of the product is stable when it is calcinated at 550 °C. The product almost completely decomposed to Nb2O5 when the calcination temperature reached 750 °C. TG analyses show that a 23.05% mass loss is observed from 560 to 850 °C, due to the release of SO3 and the formation of Nb2O5. The chemical formula of the product was estimated as Nb2O4(SO4) by the results of TG analysis. The product Nb2O4(SO4) has a different crystal form compared with the reported β-Nb2O4(SO4) (JCPDs 16-671). Based on powder XRD analysis, it was found that the compound crystallizes in the orthorhombic system with a space group Pmma (no. 51), and the corresponding lattice parameters were calculated. The crystal form was estimated as a single-crystal and multcrystalline with sizes in the micron range by SEM and TEM analysis. The molar ratio of H2SO4 to Nb2O5 (4.8:1–120.7:1) and reaction time (3–20 h) did not make any difference for the reaction at 300 °C.

► The reaction of concentrated H2SO4 and Nb2O5 is systematically studied.
► Crystal form of Nb2O4(SO4) is obtained.
► The crystallographic data of Nb2O4(SO4) is proposed.