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Isolated Effects of Surface Ligand Density on the Catalytic Activity and Selectivity of Palladium Nanoparticles

ABSTRACT: Alkanethiolate-capped palladium nanoparticles (PdNPs) have previously been synthesized by using a modified Brust–Schiffrin synthesis (using alkanethiosulfate instead of alkanethiol), in which the nanoparticle core size is established during alkanethiosulfate ligand passivation of the nanoparticle nucleation–growth initiated by borohydride reduction. Because of the dependence of core size on the amount of ligand present, surface ligand density decreases with increasing core size. Herein we present a method in which the core size is established independent of ligand addition, allowing the formation of PdNPs with similar core sizes yet different surface ligand densities. In this method, the core size is established during the temporary passivation of growing nanoparticles by borohydride and tetra-N-octylammonium bromide (TOAB), allowing nucleation to reach completion. Various molar equivalents of alkyl thiosulfate are then added, prompting the replacement of borohydride and TOAB and the formation of alkanethiolate-capped PdNPs. The resulting PdNPs were characterized by using 1H NMR, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The overall enhanced catalytic activity of hydrogenation/isomerization of alkenes and dienes was observed for PdNPs with a lower ligand density, proving the isolated effect of surface ligand density from other variations such as core size and shape. Surface ligand density is also shown to influence the hydrogenation/isomerization product selectivity of the catalytic reactions by regulating the formation of certain Pd–substrate intermediates and the kinetic diffusion of surface hydrogen/substrates. Graphical Abstract

SUBMITTER: Vargas K

PROVIDER: S-EPMC8171273 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Metal nanoparticle catalysts functionalized with small, well-defined organic ligands are important because such systems can provide a spatial control in the catalyst-substrate interactions. This article describes the synthesis, stability, and catalytic property evaluations of four different Pd nanoparticles capped with constitutional isomers of pentanethiolate ligands that have either a straight chain or an alkyl chain with one methyl group at different locations (α, β, or γ from the surface-bound sulfur). The structure and composition analyses of Pd nanoparticles confirm that they have similar average core sizes and organic ligand contents. The presence of methyl group at α position is found to lower the capping ability of short ligands and thus make Pd nanoparticles to lose their colloidal stability during the catalytic reactions. The overall activity and selectivity for hydrogenation and isomerization of pentene and allylbenzene derivatives are investigated for each combination of ligand and substrate. Catalysis results indicate that steric interactions between the ligands on the metal catalyst surface and the alkene substrates are a factor in controlling the activity of Pd nanoparticles. In particular, Pd nanoparticles capped with pentanethiolate isomer having a methyl group at α position exhibit poor and inconsistent catalytic activity due to its low colloidal stability. The presence of a methyl group at β position mildly interferes the adsorption of alkene group on the nanoparticle surface resulting in lower conversion yields. Interestingly, a methyl group at γ position only has a minimal effect on the catalytic property of Pd nanoparticles exhibiting similar catalysis results with Pd nanoparticles capped with straight chain pentanethiolate ligands. This indicates the proximity of steric group at the reactive site controls the nanoparticle activity for surface oriented reactions, such as hydrogenation and isomerization of alkenes in addition to their colloidal stability.

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Isolated Effects of Surface Ligand Density on the Catalytic Activity and Selectivity of Palladium Nanoparticles

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