MICROSTRUCTURAL CHARACTERIZATION OF HIGHLY HDS-ACTIVE CO6S8-PILLARED MOLYBDENUM SULFIDES

In this work, we have used transmission electron microscopy (TEM) to s tudy 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 scatte ring-centers has been assigned to remnants of the Co6S8(PPh3)(6) clust ers. On the basis of X-ray diffraction (XRD) studies, a lattice expans ion of 1.48 nm was expected if the triphenylphosphine ligands remained intact. The distribution of Co scattering centers, however, was consi stent with that expected for a Co6S8 care (0.8 nm). The expansion of t he {00l}-MoS2 planes was almost always localized about a single Co clu ster, and the degree of lattice expansion (0.78 nm) was also consisten t with the dimensions of the Co6S8 cores, confirming that the ligands had been nearly completely removed. The organic ligands were removed e ither during the vacuum pumpdown in the TEM specimen chamber or almost immediately upon electron-beam exposure. Additional electron-beam exp osures of up to I h caused no further structural changes. The inorgani c framework remained intact throughout the duration of the experiment. The Co6S8-pillared MoS2 materials possessed dibenzothiophene (DBT) hy drodesulfurization (HDS) activities that were slightly better than tha t 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 Go-Mo and Ni-Mo catalysts under all conditions examined. The HDS and TEM results suggest that the role of Co in sulf ided Go-Mo catalysts is to prop apart MoS2 layers, creating a higher p ercentage of ''rim'' sites (in the Daage-Chianelli terminology) and en hancing hydrogenation capability.