Bulk Structural Investigation of the Reduction of MoO₃ with Propene and the Oxidation of MoO₂ with Oxygen

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• 作者：Ressler ; T. ; Wienold ; J. ; Jentoft ; R. E. ; Neisius ; T.
• 关键词：molybdenum oxide ; MoO₃ ; MoO₂ ; in situ ; time resolved ; XAFS ; XAS ; EXAFS ; XRD ; X-ray ; diffraction ; propene ; catalysis ; solid-state kinetics ; reduction ; oxidation ; redox mechanism ; Mars&ndash ; van Krevelen
• 刊名：Journal of Catalysis
• 出版年：2002
• 出版时间：August 15, 2002
• 年：2002
• 卷：210
• 期：1
• 页码：67-83
• 全文大小：681 K

文摘

Reduction of MoO₃ in propene and oxidation of MoO₂ in oxygen are investigated by in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Temperature-programmed and isothermal experiments (573–773 K) are performed to elucidate the structural evolution of phases present during the reactions and, in addition, to reveal the solid-state kinetics of the processes involved. During the reduction of MoO₃ in propene and the oxidation of MoO₂, only crystalline MoO₃ and MoO₂ were detected by in situ XRD. The formation of a “Mo₁₈O₅₂”-type shear structure as intermediate during reduction of MoO₃ in propene and during oxidation of MoO₂ in oxygen was observed by in situ XAS. The solid-state kinetics of the reduction of MoO₃ in propene exhibits a change in the rate-limiting step as a function of both temperature and extent of reduction. The solid-state kinetics of the oxidation of MoO₂ is governed by three-dimensional diffusion. A schematic reaction mechanism for the reduction of MoO₃ in propene and reoxidation in oxygen is proposed that consists of (i) generation of oxygen vacancies at the (100) or (001) facets by reaction with propene, (ii) vacancy diffusion in the MoO₃ bulk, (iii) formation of Mo₁₈O₅₂-type shear structures in the lattice, and (iv) formation and growth of MoO₂ nuclei. With respect to a redox mechanism for the partial oxidation of propene on MoO₃, three stages are distinguished. (i) at temperatures below ter border=0 SRC=/images/glyphs/BQ1.GIF>600 K the participation of oxygen from the MoO₃ bulk is negligible. (ii) At temperatures between ter border=0 SRC=/images/glyphs/BQ1.GIF>600 and ter border=0 SRC=/images/glyphs/BQ1.GIF>700 K oxygen vacancy diffusion in the bulk is sufficient to make a redox mechanism feasible, affording a partially reduced MoO₃ under reaction conditions. (iii) At temperatures above ter border=0 SRC=/images/glyphs/BQ1.GIF>700 K sufficiently fast oxygen diffusion in the lattice combined with rapid formation and annihilation of crystallographic-shear planes permits the participation of a considerable amount of the lattice oxygen of MoO₃ in the partial oxidation of propene.