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.
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