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 (NiS), 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 NiS 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 NiS does not influence electronic properties.