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

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 ∼600 K the participation of oxygen from the MoO3 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 MoO3 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 MoO3 in the partial oxidation of propene.