Maximizing Active Site Concentrations at Ni-Substituted WS2 Edges for Hydrogenation of Aromatic Molecules.

High concentrations of Ni in bimetallic sulfide catalysts lead to the formation of segregated Ni sulfides (NiSx), 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 H2 and CO2 activation processes. Treatment of Ni-WS2/γ-Al2O3 catalysts in aqueous acids, particularly concentrated HCl, results in a significant reduction of NiSx 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 H2. 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 NiSx does not influence electronic properties.