Hydrogenation on Mo and W sulfides occurs at the edges of the sulfide slabs. The rate of hydrogen addition is directly proportional to the concentration of sulfhydryl (SH) groups at the slab edge and the metal atom attached to it. Sulfhydryl groups vicinal to edge-incorporated Ni hydrogenate with much higher rates than SH close to Mo and W. Each subset of SH groups, however, exhibits nearly identical intrinsic activity and selectivity, independent of the sulfide composition. The higher activity of Ni-WS compared to Ni-MoS stems from a higher concentration of SH groups on the former sulfide associated with a higher tendency of its surface vacancies to react with H .
Hydrogenation on Mo and Wsulfides occurs at the edges of the sulfide slabs.T he rate of hydrogen addition is directly proportional to the concentration of sulfhydryl (SH) groups at the slab edge and the metal atom attached to it. Sulfhydryl groups vicinal to edge-incorporated Ni hydrogenate with muchh igher rates than SH close to Mo and W. Each subset of SH groups,however,exhibits nearly identical intrinsic activity and selectivity,independent of the sulfide composition. The higher activity of Ni-WS 2 compared to Ni-MoS 2 stems from ah igher concentration of SH groups on the former sulfide associated with ah igher tendency of its surface vacancies to react with H 2 .
High concentrations of Ni in bimetallic sulfide catalysts lead to the formation of segregated Ni sulfides (NiS x ), which are rather inactive alone as large crystallites and even impede the accessibility of active sites at the sulfide slab edges that catalyze a multitude of hydrogenation reactions and H 2 and CO 2 activation processes. Treatment of Ni-WS 2 /γ-Al 2 O 3 catalysts in aqueous acids, particularly concentrated HCl, results in a significant reduction of NiS x and ≤5-fold enhancement of the phenanthrene hydrogenation rate. Using infrared (IR) spectroscopy of probe molecules, we show that the acid-treated catalysts have a high concentration of accessible metal edge sites, a high degree of Ni substitution, and consequently a high sulfhydryl (SH) concentration at the slab edges in the presence of H 2 . The site-specific "turnover frequency" (based on SH concentrations determined by IR measurements) is identical for all parent and acid-treated sulfide catalysts studied, showing that excess NiS x does not influence electronic properties.
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