The influence of reducing and sulfiding conditions on the properties of unsupported MoS2-based catalysts

Unsupported MoS2 catalysts were obtained from the decomposition of ammonium tetrathiomolybdate (ATM) at variable temperatures (400–700 °C) and under different gas compositions, from pure H2S to pure H2. 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 S22- species, plays a key role for the activity of unsupported MoS2 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 MoS2 annealed at 700 °C in pure H2, H2S or N2 gases. Ni and Co introduced by means of reflux with acetylacetonates, gave identical promotion trends for all the MoS2 samples. The solids treated in pure H2S could not take up promoter atoms at the edges, whereas for the H2-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 MoS2 catalysts.

Over-stoichiometric sulfur present at the edges of MoS2 in the form of S22- moieties plays a crucial role for the HDS activity and HYD selectivity, as well as for the promotion of unsupported MoS2 catalysts by nickel and cobalt.Figure optionsDownload high-quality image (181 K)Download as PowerPoint slide