Controlling Surface Ligand Density and Core Size of Alkanethiolate-Capped Pd Nanoparticles and Their Effects on Catalysis

Abstract

This article presents systematic investigations on the relationship between the catalytic property and the surface ligand density/core size of thiolate ligand-capped Pd nanoparticles (PdNPs). The systematic variations in the two-phase synthesis of PdNPs generated from sodium S-dodecylthiosulfate were performed. The resulting PdNPs were characterized by transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and 1H NMR and UV–vis spectroscopy. The decrease in the molar equivalent of sodium S-dodecylthiosulfate (Bunte salts) resulted in the formation of nanoparticles with lower surface ligand density and larger particle core size. A decrease in the molar equivalent of tetra-n-octylammonium bromide or an increase in reaction temperature generated nanoparticles with higher surface ligand density and smaller particle core size. As the molar equivalent of NaBH4 decreased, the particle core size increased. The catalysis studies on various PdNPs with different surface ligand density and average core size showed a strong correlation between the PdNP composition and the turnover frequency (TOF) of the isomerization of allyl alcohol. Optimized “good” PdNPs with lower surface ligand coverage and larger core size catalyzed the isomerization of various allyl alcohols to carbonyl analogues with high activity and selectivity.

Hydrogen (H2) sensing and catalysis with organic-stabilized Pd and Pd alloy nanoparticles.

“…132 Comparing these results with those obtained with the Pd and PdAg NPs, we can confirm that our catalysts led to selective conversion of structurally different allyl alcohols.…”

“…51,67 Shon and co-workers recently described the same selectivity with alkanethiolate-capped Pd NPs prepared from Bunte salts. [130][131][132][133] With 12x C8NH 2 Pd NPs, the peak for allyl alcohol also decreased (Frame B), but two peaks appeared at 1.26 and 3.12 min, corresponding to the aldehyde and alcohol, respectively, in about a 50:50 ratio. Figure 6.3 shows the percent hydrogenation, percent isomerization, and total conversion of allyl alcohol at different times when 12x C8NH 2 Pd NPs were used as catalysts.…”

“…For C16NH 2 Pd and PdAg NPs, the best compromise between conversion and selectivity toward the formation of the isomerization product was reached at a constant H 2 flow rate of 1.69 ± 0.02 mL/min giving 100% conversion, [130][131][132][133] These Pd NPs were stable and highly selective toward the isomerization of allyl alcohol. In order to produce stable Pd NPs with different surface ligand densities and particle core sizes, they performed systematic variations in the synthesis of Pd NPs and observed that there is a strong correlation between the Pd NP composition and the rate of isomerization of allyl alcohol.…”

“…136,152 However, these NP catalysts exhibited higher activity (TOF = 700-1000), with C16NH 2 Pd 91 Ag 9 NPs being the best, compared to the activity of other homogeneous systems previously investigated including Pd and Pt NPs stabilized by dendrimers 30,123 and alkanethiolate monolayers (TOF < 500). 38,[130][131][132][133] Other studies have reported TOF >> 1000 when hollow polystyrene nanospheres 135 and collagen fiber 136 coated with Pd NPs were used to catalyze the hydrogenation of allyl alcohol. The reactions were carried out under different conditions, but these are clearly promising strategies for Pd catalysis.…”

“…133 Shon and co-workers showed the isomerization of allyl alcohol to the corresponding carbonyl compound with a relatively high selectivity (95%) compared to other Pd-based catalysts (<75%). [130][131][132][133] In our research we are the first to compare the catalytic activity of various alkylamine-, alkanethiolate-, and mixed ligand organicprotected Pd and PdAg NPs for the hydrogenation and isomerization of allyl alcohol in the presence of hydrogen gas.…”