The influence of MoS2 slab 2D morphology and edge state on the properties of alumina-supported molybdenum sulfide catalysts.
Résumé
To study the influence of MoS2 particles morphology on their catalytic properties, a series of MoS2/Al2O3 catalysts was prepared, by means of treatment of oxide precursor under variable conditions. The conditions of treatment were selected to generate the MoS2 crystallites exposing preferentially M-edges or M- and S-edges, as predicted by the theory phase diagrams. The MoS2 particles size was estimated by electron microscopy and the degree of sulfidation to MoS2 was determined from X-ray photoelectron spectroscopy. The edge state of MoS2 particles was probed by FT-IR of adsorbed CO and by temperature programmed reduction (TPR). Hydrogenation (HYD) selectivity and hydrodesulfurization (HDS) selectivity were estimated from the catalytic tests using hydroconversion of 2,3-dimethylbut-2-ene and 3-methylthiophene. It seems that the variations of 2D morphology of MoS2 particles influence the HYD/HDS selectivity. However, a drastic drop of activity was observed when the preparation temperature increased from 250 to 700 °C which cannot be accounted by the decrease of the amount of exposed edges due to thermal sintering. At the same time a drastic loss of ability to adsorb CO and harder reducibility of sulfide edges were observed for the samples prepared at high temperatures. The results suggest that well crystallized edges of MoS2 bearing no excess sulfur have low activity whatever their symmetry type.