Microstructural Characterization of Highly HDS-Active Co₆S₈-Pillared Molybdenum Sulfides

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Abstract

In this work, we have used transmission electron microscopy (TEM) to study Co6S8(PPh3)x-pillared MoS2 and have directly observed that the Co clusters can either intercalate into the bulk or can bind to defect sites at the edges of MoS2. A distribution of discrete 0.87 nm scattering centers has been assigned to remnants of the Co6S8(PPh3)6 clusters. On the basis of X-ray diffraction (XRD) studies, a lattice expansion of 1.48 nm was expected if the triphenylphosphine ligands remained intact. The distribution of Co scattering centers, however, was consistent with that expected for a Co6S8 core (0.8 nm). The expansion of the {00l}-MoS2 planes was almost always localized about a single Co cluster, and the degree of lattice expansion (0.78 nm) was also consistent with the dimensions of the Co6S8 cores, confirming that the ligands had been nearly completely removed. The organic ligands were removed either during the vacuum pumpdown in the TEM specimen chamber or almost immediately upon electron-beam exposure. Additional electron-beam exposures of up to 1 h caused no further structural changes. The inorganic framework remained intact throughout the duration of the experiment. The Co6S8-pillared MoS2 materials possessed dibenzothiophene (DBT) hydrodesulfurization (HDS) activities that were slightly better than that of a commercial sulfided Co−Mo/Al2O3 catalyst under low-severity HDS conditions. However, the selectivity to biphenyl was somewhat poorer than that of commercial Co−Mo and Ni−Mo catalysts under all conditions examined. The HDS and TEM results suggest that the role of Co in sulfided Co−Mo catalysts is to prop apart MoS2 layers, creating a higher percentage of "rim" sites (in the Daage−Chianelli terminology) and enhancing hydrogenation capability.

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