Sulfide molybdenum nanoslabs are an important class of catalysts with various applications as in hydrotreatment, hydrocracking, Fischer-Tropsch synthesis, water gas shift and more recently in hydrogen evolution reaction. Since the catalytic reactions usually proceed on the edge sites of MoS2 slabs, the nature, structure and number of edge exposed directly impact the catalytic performances. MoS2 crystallographic (100) plane exposes principally two types of edges: the Mo terminated edge (M-edge) and the sulfur terminated edge (S-edge). Scanning tunneling microscopy and DFT calculations show on model systems, that MoS2 can adopt various shapes from a triangle exposing only one edge to a hexagon with both M-edge and S-edge. These two types of edges presenting different reactivity, and different stability for promoter accommodation, it appears crucial to account for these two types of edges on real-type sulfided catalysts (i.e. supported on high surface area Al2O3-type supports) and to identify the parameters that modify the relative proportion of M-edge and S-in order to control the catalytic properties of MoS2-based catalysts Low temperature CO adsorption followed by IR spectroscopy (IR/CO) is used to account for the preparation parameters that can modify the MoS2 morphology of real type MoS2/Al2O3 catalysts. It is found that the Mo loading as well as organic molecules addition during catalyst preparation affect the sulfide slab morphology. Conditions of sulfidation also change the MoS2 shape on Al2O3 support. The effect of Mo loading, citric acid addition and severization of sulfidation conditions on MoS2 morphology can be explained by the modification of MoS2-support interactions. In the case of strong interactions between MoS2 and Al2O3 support, MoS2 presents mainly M-edge rather than S-edge. By contrast when interaction between sulfide phase and support decreases a better balance between M- and S-edges can be obtained. Parallel between IR/CO study and HDS tests reveals that S-edge has a higher intrinsic activity than M-edge in thiophene HDS reaction. Hence, modification of nanoslabs MoS2 shape is a way to tune the catalyst activity.

Tuning surface properties of sulfide Mo-based nanoslabs to control catalytic properties

251st National Meeting (2016)

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