Phases in vanadium pentoxide–tungsten trioxide catalysts

Abstract

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.