Reactivity of V2O5, MoO3 and WO3 in molten KNO3, studied by mass spectrometry

The reactivity of transition metal oxides MoO3, WO3 and V2O5 in molten KNO3 was studied by mass spectrometry of the gases evolved during heating, and X-ray diffraction of the solidified melts. The reaction stoichiometry showed that multistep reactions of the polyoxoanions occur. Intermediate products identified in the reactions of the oxides are hexa- and pentavanadates, tri- and dimolybdates, and ditungstate. After prolonged heating at 550 degreesC, the reactions proceeded to form the monooxoanions: VO3-, MoO42- and WO42-. The reactivity of the oxides decrease in the sequence: V2O5 > MoO3 > WO3, the inverse of the oxoacidity sequence deduced from the equilibria in ionic melts (WO3 > MoO3 > V2O5). The first reaction peak is determined by kinetics of the solid-liquid reaction, whereas further transformations of polyoxospecies are determined by their oxoacidity. (C) 2001 Elsevier Science BN: All rights reserved.