Phase transitions of V-Mo-W mixed oxides during reduction/re-oxidation cycles

V-Mo-W mixed oxides are prepared by a hydrochemical preparation route with a stoichiometry of V2Mo8WxOy with 0 <= x <= 5 via evaporation/crystallization and by spray-drying of ammonium polyoxometallate solutions combined with a subsequent calcination procedure. The phase composition has been examined by electron and X-ray powder diffraction. Significant changes are observed after reduction/reoxidation and after/during reactions in an air-acrolein atmosphere for all crystalline samples. Samples prepared from spray-dried precursors with tungsten contents x > 1 are amorphous/nanocrystalline. Independent of the preparation and reaction procedure, the formation of the thermodynamically metastable, tetragonal Mo5O14 phase is detected. Deactivation starts at the latest at 723 K in air with the formation of thermodynamically stable phases like orthorhombic MoO3. Smaller particles exhibit higher stability against deactivation or reduction. Reaction networks are established to illustrate the effect of various chemical reactions on crystalline and amorphous/nanocrystalline structures. The temperatures of the industrial processes are interpreted on the basis of the presented results. (C) 2010 Elsevier B.V. All rights reserved.