Spectro-Microscopy of Individual Pt–Rh Core–Shell Nanoparticles during Competing Oxidation and Alloying

TL;DR

This study uses high-resolution spectro-microscopy to observe how Rh behaves on Pt–Rh nanoparticles during oxidation and alloying processes.

Contribution

The paper introduces a correlative spectro-microscopy approach to distinguish Rh oxidation, alloying, and sintering in real-time.

Findings

Rh oxidation is more intense on the Rh layer over Pt-core than on nearby STO support.

Rh diffuses into Pt at high temperatures, competing with oxidation.

Rh/RhOx nanoparticles on STO sinter under oxidizing conditions.

Abstract

The surface chemical composition of supported single
Pt–Rh
core–shell nanoparticles was studied to understand the Rh behavior
in oxidizing and reducing gas environments using spectro-microscopy
with high spatial resolution. We combined in situ X-ray photoemission electron microscopy with ex situ scanning electron-, atomic force-, and scanning Auger-microscopy
to distinguish Rh oxidation–reduction, dewetting–sintering,
and alloying–segregation during the course of the experiment.
A more than 20% higher Rh 3d5/2 oxide to metal photoemission
intensity ratio for the Rh layer on top of the Pt-core was found as
compared to the bare strontium titanate (STO) oxide catalyst support
in close vicinity, where Rh/RhO
x
nanoparticles
are forming. At elevated temperatures, Rh diffuses into the Pt particle,
and this alloying at the Pt metal surface competes with Rh oxidation,
whereas the Rh/RhO
x
nanoparticles on the
STO support are observed to sinter under identical oxidizing and temperature
environments. A nanoparticle facet-dependent analysis of selected
Pt-core nanoparticles suggests that Rh oxidation is most advanced
on a small nanoparticle with a low coordination top facet that we
indexed by electron backscatter diffraction, demonstrating the strength
of our correlative approach.

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