Phases in vanadium pentoxide–tungsten trioxide catalysts
The X-ray diffraction (XRD) patterns of the V2O5–WO3 catalysts containing 0–100 mol% WO3, obtained from a solution of ammonium metavanadate and ammonium polytungstate in oxalic acid, are discussed. It has been found that (i) the V–O catalyst consists of V2O5 and traces of V2O4 phases, (ii) the W–O catalyst contains a monoclinic WO3 phase structurally related to the orthorhombic one (MrO) and (iii) mixed V2O5–WO3 catalysts are composed of V2O5 and of a monoclinic WO3 phase structurally related to the tetragonal (MrT) one. The MrT polymorph of tungsten trioxide is shown to be formed as a result of introducing vanadium ions into the WO3 lattice. The amount of vanadia dissolved in tungsten (VI) oxide under the conditions of catalyst preparation has been estimated to be not lower than 10 mol%. The significant decrease in the grain size of V2O5 and small decrease in the grain size of WO3 due to WO3–V2O5 and V2O5–WO3 solid solution formation, are discussed. The creation of mixed crystals due to epitaxial growth of (i) vanadia on MrT tungsten trioxide crystals in tungsten-rich catalysts and (ii) MrT WO3 on vanadia crystals in vanadium-rich catalysts is postulated.