The influence of reducing and sulfiding conditions on the properties of unsupported MoS 2-based catalysts

Abstract

Unsupported MoS 2 catalysts were obtained from the decomposition of ammonium tetrathiomolybdate (ATM) at variable temperatures (400–700 °C) and under different gas compositions, from pure H 2S to pure H 2. The catalysts were further studied in the non-promoted state or promoted by Ni and Co. Catalytic activity and selectivity were studied in the model reaction of thiophene hydrodesulfurization (HDS). Surface areas, crystalline phase and particle size distributions were determined by Brunauer–Emmet–Teller (BET), X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. A comparison of average values calculated from these techniques has enabled the understanding of the morphology of the solids. The catalysts were characterized before and after catalytic tests by X-ray photoelectron spectroscopy (XPS), laser Raman spectroscopy (LRS) and temperature-programmed reduction (TPR). Comparison of catalytic activity trends with the results of the characterizations show that over-stoichiometric sulfur, present in the fresh catalysts in the form of edge-located S 2 2 - species, plays a key role for the activity of unsupported MoS 2 and for its ability to be promoted. Direct hydrogenation (HYD) of thiophene to butane occurs presumably with the participation of –SH groups, produced from the opening of S–S bridges by hydrogen. Whatever the gas atmosphere, any treatment leading to the removal of over-stoichiometric sulfur leads to a decrease in HYD selectivity. Thus, very similar catalytic properties were observed for MoS 2 annealed at 700 °C in pure H 2, H 2S or N 2 gases. Ni and Co introduced by means of reflux with acetylacetonates, gave identical promotion trends for all the MoS 2 samples. The solids treated in pure H 2S could not take up promoter atoms at the edges, whereas for the H 2-reduced samples high promotion levels were achieved. The degree of stacking does not seem to have a significant impact on the thiophene HDS activity and selectivity of the unsupported MoS 2 catalysts.