In this work, we present atomic-scale images of Co promoted MoS2 slabs supported on γ-Al2O3 obtained by High Resolution Scanning Transmission Electron Microscopy equipped with High Angular Annular Dark Field detector (HR STEM-HAADF). These images allow to study the effect of citric acid (CA) addition during preparation, on the slab length and its stability during thiophene hydrodesulfurization (HDS) reaction. Thus, the observations, obtained for sulfide catalysts prepared without or with citric acid as chelating agent, evidenced strong decrease in size of the CoMoS nano-slabs as detected indirectly by the adsorption of CO followed by Infrared spectroscopy (IR/CO). Quantitative dispersion values were calculated from the images taking into account the true shape of the particles instead of the classical hexagonal shape hypothesis. Characterization of the used catalyst shows that these nanoclusters are stable under model thiophene HDS reaction.. Such observations allow a better understanding of the effect of chelating agent addition on promoted MoS2 samples in order to explain their catalytic activity.