Bulk Structural Investigation of the Reduction of MoO 3 with Propene and the Oxidation of MoO 2 with Oxygen

Abstract

Reduction of MoO 3 in propene and oxidation of MoO 2 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 MoO 3 in propene and the oxidation of MoO 2, only crystalline MoO 3 and MoO 2 were detected by in situ XRD. The formation of a “Mo 18O 52”-type shear structure as intermediate during reduction of MoO 3 in propene and during oxidation of MoO 2 in oxygen was observed by in situ XAS. The solid-state kinetics of the reduction of MoO 3 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 MoO 2 is governed by three-dimensional diffusion. A schematic reaction mechanism for the reduction of MoO 3 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 MoO 3 bulk, (iii) formation of Mo 18O 52-type shear structures in the lattice, and (iv) formation and growth of MoO 2 nuclei. With respect to a redox mechanism for the partial oxidation of propene on MoO 3, three stages are distinguished. (i) at temperatures below ∼600 K the participation of oxygen from the MoO 3 bulk is negligible. (ii) At temperatures between ∼600 and ∼700 K oxygen vacancy diffusion in the bulk is sufficient to make a redox mechanism feasible, affording a partially reduced MoO 3 under reaction conditions. (iii) At temperatures above ∼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 MoO 3 in the partial oxidation of propene.