Study of super-efficient defective MoVSbO catalysts used for ethane oxidative dehydrogenation by HAADF-STEM and of their thermal evolution by environmental electron microscopy and tomography†

Abstract

Scanning transmission electron microscopy (STEM) and in situ electron tomography (ET) is used to characterize the structure and morphology of a M1 phase based MoVSbO oxide catalyst. The catalyst, prepared through a new method involving the controlled combustion of an amino–organic compounds added as a structuring agent, has recently been shown to be very efficient in the oxidative dehydrogenation of ethane. The study shows that the M1 active phase has a specific morphology consisting of short nanorods particles with rounded cross-section and internal and external pores similarly shaped. Such a morphology contributes to several planes on the lateral facets of the nanorods and on the wall of the external pores exposing the sites commonly accepted as the catalytic active sites. While some of these planes, i.e., like {120} and {210}, are already identified as generating active sites, this study proposes that additional planes, namely, {130} and {110} are formed. The evolution of the morphology with the thermal treatments of the catalysts is followed in situ. This study indicates that the nanorods are already shaped and porous after the hydrothermal synthesis step and drying, while the heat-treatments under O₂ and N₂ increase both the internal and external porosity of the nanorods. This study also highlights that there is an optimal heat treatment beyond which the texture of the catalyst evolves detrimentally.