7 research outputs found
Size-Dependent Hydrogenation of <i>p-</i>Nitrophenol with Pd Nanoparticles Synthesized with Poly(amido)amine Dendrimer Templates
Monodisperse palladium nanoparticles
consisting of 10–200
atoms were prepared using polyÂ(amido)Âamine (PAMAM) dendrimer templates
and were evaluated as catalysts using the model reduction of <i>para</i>-nitrophenol. The use of dendrimer templates allows
for fine control of the average number of atoms per nanoparticle and
systematic investigation of the effect of size on the catalytic activity
of nanoparticles less than 2 nm in diameter. The palladium dendrimer-encapsulated
nanoparticles (DENs) were found to be highly active for the hydrogenation
of <i>para</i>-nitrophenol to <i>para</i>-aminophenol,
with surface area-normalized rate constants ranging from 0.87 to 1.65
L s<sup>–1</sup> m<sup>–2</sup> (which is greater than
any previously reported system). A near linear dependence of the observed
rate constant on the synthetic Pd<sup>2+</sup>:dendrimer ratio was
observed, suggesting that, within the size regime studied, most of
the atoms lie on the surface of the nanoparticle and contribute to
the catalytic activity. Interestingly, for Pd clusters containing
between 10 and 50 atoms, the rate constant normalized on a per atom
basis shows little variability, supporting the idea that all atoms
lie on the surface of these clusters. However, for particles containing
between 50 and 200 atoms, a decrease in per-atom activity is observed
with increasing particle size, suggesting that in this size regime
some atoms are located in the catalytically inactive core. Additionally,
the generation of the PAMAM dendrimer template was found to have a
significant effect on the observed rate constant due to steric crowding
at the periphery (whereas the choice of an amine- or hydroxyl-termination
on the dendrimer periphery did not)