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

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• 作者：Lars Giebeler ; Andreas Wirth ; Johan A. Martens ; Herbert Vogel ; Hartmut Fuess
• 关键词：Vanadium molybdenum oxides ; Oxidation catalysis ; Acrolein ; Acrylic acid ; Phase transition ; Deactivation ; X-ray powder diffraction ; Electron diffraction
• 刊名：Applied Catalysis A: General
• 出版年：2010
• 出版时间：15 May 2010
• 年：2010
• 卷：379
• 期：1-2
• 页码：155-165
• 全文大小：1916 K

文摘

V-Mo-W mixed oxides are prepared by a hydrochemical preparation route with a stoichiometry of V₂Mo₈W_xO_y 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/re-oxidation 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.