The
charge state of palladium nanocatalysts can alter the catalytic
conversion process and is required for understanding the important
catalytic reactions in applications. However, the charge state of
palladium nanoparticles (NPs) has not been understood yet completely.
Here, we investigate the size dependence of the charge state of Pd
NPs deposited on an Al2O3/NiAl(110) surface
by noncontact atomic force microscopy and Kelvin probe force microscopy
at room temperature. We found that Pd NPs adsorbed on the line defects
on the surface were negatively charged and the charge state increased
with the NP size. Furthermore, Pd NPs show a different size variation
on two types of line defects. We consider the work function change
affected by the charging behavior mainly according to the classical
model for small metallic spheres. We demonstrate that the charge state
variation of the adsorbed Pd NPs on the line defects was affected
by its size change mainly, which results in the variation of charge
state difference. Our results provide insights into the mechanisms
of catalytic reactions.
Following
the chemical state evolution of a catalyst in the catalytic
cycle is crucial for the identification of the catalyst’s active
phase and reaction mechanism. However, it is difficult to ascertain
the oxidation state of a metal catalyst following oxygen exposure.
Here, we present a time-scale study of the charge state of Pd nanoclusters
on a model catalyst system, Pd/Al2O3/NiAl(110),
during an exposure of molecular oxygen by noncontact atomic force
microscopy and Kelvin probe force microscopy (KPFM). We speculate
that the Pd nanocluster can be oxidized by O2 at room temperature,
leading to the formation of metal-complex Pd
x
O
y
nanoclusters. Pd
x
O
y
shows a positive charge
on the alumina surface in KPFM images, and it is the major sintering
species in contrast with the stable Pd nanocluster. In addition, Pd
x
O
y
contains weak
binding oxygen that can be removed after annealing to a higher temperature.
Following the evolution from individual well-dispersed metal nanoclusters
to the oxidized state enables the identification of the key processes
that underlie gas-induced charge transition.
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