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

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 3d_5/2 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.