Activation of tetraalkylammonium thiotungstates for the preparation of Ni-promoted WS2 catalysts

A comparative study of the synthesis of unsupported Ni/WS2 catalysts by ex situ and in situ decomposition of thiosalts is herein reported. Ammonium thiotungstate (ATT), tetramethyl ammonium thiotungstate (TMATT), tetrapropylammonium thiotungstate (TPATT) and tetrabutylammonium thiotungstate (TBATT) and their nickel-promoted counterparts were used as precursors. Ex situ activation was performed under a H2S/H-2 flow whereas the in situ activation consists of the decomposition of the thiosalt in the presence of a hydrocarbon solvent during the hydrodesulfurization (HDS) of dibenzothiophene (DBT). The catalytic activity depends on both the method of activation and the nature of the precursor. The tetraalkylammonium thiotungstate precursors have been characterized by thermogravimetric analysis and the final catalysts by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and BET specific surface area measurements. Compared to ATT, the in situ activation of tetraalkylammonium thiotungstates has a negative impact on the HDS activity of the as-formed NiW catalysts while a moderate gain in activity was observed for ex situ activated catalysts. The presence of carbon in the tetraalkylammonium precursors accelerates the crystallization rate Of WS2-based catalysts leading to well-organized WS2 structure but with low specific surface areas. The accelerating rate of crystallization also limits the accommodation of Ni on the WS2 edges and then hinders the synergetic effect. This situation differs strikingly from MoS2-based catalysts for which no accelerating effect of carbon on the crystallization rate was observed. (C) 2004 Elsevier B.V. All rights reserved.