High Resolution STEM-HAADF microscopy on γ-Al2O3 supported MoS2 catalyst – Proof by image of the changes in dispersion and morphology of the slabs with addition of citric acid.

Atomic-scale images of MoS2 slabs supported on γ-Al2O3 were obtained by High Resolution Scanning Transmission Electron Microscopy equipped with High Angular Annular Dark Field detector (HR STEM-HAADF). These observations, obtained for sulfide catalysts prepared without or with citric acid as chelating agent, evidenced variations in morphology (shape) and size of the MoS2 nanoslabs, as detected indirectly by the adsorption of CO followed by Infrared spectroscopy. Quantitative dispersion values and a morphology index (S-edge/M-edge ratio) were determined from the slabs observed. In this way, HR STEM-HAADF underlines that the addition of citric acid to Mo catalysts decreases the size of the particles and modifies the shape of the MoS2 nanoslabs from slightly truncated triangles to particles with a higher ratio of S-edge/M-edge. These finding are of great significance since tayloring the morphology of the slabs is a way to increase their catalytic activity and selectivy. Furthermore, this work demonstrated that the IR/CO method is a relevant approach to describe MoS2 morphology of supported catalysts used in hydrotreatment processes for clean fuel production.