Enhancing hydrogenation activity of Ni-Mo sulfide hydrodesulfurization catalysts.

Unsupported Ni-Mo sulfides have been hydrothermally synthesized and purified by HCl leaching to remove Ni sulfides. Unblocking of active sites by leaching significantly increases the catalytic activity for dibenzothiophene hydrodesulfurization. The site-specific rates of both direct (hydrogenolytic) and hydrogenative desulfurization routes on these active sites that consist of coordinatively unsaturated Ni and sulfhydryl groups were identical for all unsupported sulfides. The hydrogenative desulfurization rates were more than an order of magnitude higher on unsupported Ni-Mo sulfides than on Al2O3-supported catalysts, while they were similar for the direct (hydrogenolytic) desulfurization. The higher activity is concluded to be caused by the lower average electronegativity, i.e., higher base strength and polarity, of Ni-Mo sulfides in the absence of the alumina support and the modified adsorption of reactants enabled by multilayer stacking. Beyond the specific catalytic reaction, the synthesis strategy points to promising scalable routes to sulfide materials broadly applied in hydrogenation and hydrotreating.

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.

Ionic Conductivity Measurements-A Powerful Tool for Monitoring Polyol Reduction Reactions.

The reduction of metal precursors during the polyol synthesis of metal nanoparticles was monitored by ex situ ionic conductivity measurements. Using commonly used platinum precursors (K2PtCl6, H2PtCl6, and K2PtCl4) as well as iridium and ruthenium precursors (IrCl3 and RuCl3), we demonstrate that their reduction in ethylene glycol at elevated temperatures is accompanied by a predictable change in ionic conductivity, enabling a precise quantification of the onset temperature for their reduction. This method also allows detecting the onset temperature for the further reaction of ethylene glycol with HCl produced by the reduction of chloride-containing metal precursors (at ≈120 °C). On the basis of these findings, we show that the conversion of the metal precursor to reduced metal atoms/clusters can be precisely quantified, if the reaction occurs below 120 °C, which also enables a distinction between the stages of metal particle nucleation and growth. The latter is demonstrated by the reduction of H2PtCl6 in ethylene glycol, comparing ionic conductivity measurements with transmission electron microscopy analysis. In summary, ionic conductivity measurements are a simple and straightforward tool to quantify the reduction kinetics of commonly used metal precursors in the polyol synthesis.