In Situ Study of the Motion of Supported Gold Nanoparticles

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Abstract

Supported metal nanoparticles constitute an important class of materials in heterogeneous catalysts.
The active sites are mainly found on the surface of the nanoparticles so an often used typical design
parameter is to maximize the surface for a given volume of material, i.e. Optimize s/v ~d-1
. Thus, size distribution of nanoparticles becomes a key factor to evaluate a catalyst performance and
deactivation, although other factors can influence both of these parameters.

Particle growth by sintering is one of the main catalyst deactivation mechanism. This is the process
where the population of larger particles grow at the expense of smaller particles resulting in a loss of
active surface area. Studies of nanoparticles sintering [1] are often carried out ex situ and do not
provide information of how the process occurs. Especially the motion of the nanoparticles on the oxide
support and the dynamics at interface between nanoparticle and support. In this work, we will present
three different motions of nanoparticles on the oxide support: rigid-body sliding, rigid-body rotation
and layered movement via mass transport studied with atomic resolution.

Au/CeO2 is a widely studied system used for the catalytic conversion of carbon monoxide to carbon
dioxide. The typical interfacial relationship between nanoparticle and CeO2 support is {111}Au //
{111}CeO2 which can occur in two distinct configurations [2]. The lattice spacing is 0.235 and 0.312 nm
for {111} Au and {111} CeO2 respectively, giving a 25% mismatch in lattice spacing. To accommodate
this mismatch, a dislocation network with edge dislocations (T symbols in Figure 1) is formed, where
every four Au (111) layers match three CeO2 (111) layers (Figure 1). Our results indicate that gold
nanoparticles slide along the oxide support/nanoparticle interface in a rigid-body manner.
Interestingly, it seems as if the sliding process does not occur between the first interface gold layer
(Au1) and first oxide layer(s1), instead it happens at the Au1 and Au2 interface. The blue dotted line
shown on the figure indicates a point of reference. The Au nanoparticle moves upward by one Au
(111) lattice spacing (around 0.235 nm) between 38.4 s and 39.2 s. While the interface layers(Au1 and
s1) both remain at the same positions, suggesting a strong interactions between ceria and Au.

Another mechanism through which the nanoparticle can migrate on the support is through a mass
transport process. Figure 2 shows a sequence of HRTEM images indicating the motion a Au
nanoparticle moving on the oxide support through by this mechanism. Within 2 s, one (111) layer
(indicated by red arrow) at the bottom of the particle disappears and a new (100) layer (indicated by
blue arrow) populate the right corner (100) facet. The diffusion of atoms continues to move from the
new (100) facet to the other (111) facet (indicated by yellow arrow) of the nanoparticle. As a result
the whole particle moves laterally on the oxide substrate by 0.235 nm with respect to the substrate,
i.e. one (111) lattice space.

Apart from the above two migration processes, we also observed a rigid-body rotation of the
nanoparticle with the rotation axis parallel to the interface, which is different from the phenomenon
reported in [3]. Our atomic scale observation of nanoparticle movement on oxide substrates sheds
light on the underlying processes of sintering, especially through particle migration and coalescence.
Original languageEnglish
Publication date2020
Number of pages2
Publication statusPublished - 2020
EventMicroscopy and Microanalysis 2020 - Virtual, United States
Duration: 3 Aug 20207 Aug 2020

Conference

ConferenceMicroscopy and Microanalysis 2020
LocationVirtual
CountryUnited States
Period03/08/202007/08/2020

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