Particle Size and Shape Effects in Electrochemical Environments: Pd Particles Supported on Ordered Co3O4(111) and Highly Oriented Pyrolytic Graphite
Abstrakt
Particle size and shape effects control the oxidation behavior of nanostructured electrocatalysts. We investigated the oxidation state of Pd nanoparticles supported on Artsputtered highly oriented pyrolytic graphite (HOPG) and well-ordered Co3O4(111) films on Ir(100) as a function of electrode potential by means of synchrotron radiation photoelectron spectroscopy coupled with an ex situ emersion electrochemical (EC) cell.
Scanning tunneling microscopy revealed the growth of hemispherical and flat Pd nanoparticles on Artsputtered HOPG and Co3O4(111), respectively. The oxidation state of Pd nanoparticles is controlled by electronic metal support interaction (EMSI) associated with charge transfer at the interface.
We found that the Pd nanoparticles are largely metallic on HOPG and partially oxidized on Co3O4(111). Specifically, we detected the formation of partially oxidized Pd delta+ aggregates in combination with atomically dispersed Pd2+ species.
The latter species dominate at small Pd coverage and form the metal/oxide interface at high Pd coverage. Immersion into an alkaline electrolyte (pH 10, phosphate buffer) at potentials between 0.5 and 1.1 VRHE has no significant effect for Pd/Co3O4(111) but yields traces of surface Pd oxide at 0.9 and 1.1 V-RHE for Pd/HOPG.
Formation of PdO was observed at 1.3 and 1.5 V-RHE. Quantitative analysis suggests nearly one monolayer and nearly two monolayers of PdO on the surfaces of the Pd nanoparticles supported on HOPG and Co3O4(111) at 1.5 V-RHE, respectively.
The differences in the oxidation behavior reveal the decisive role of the EMSI in the stability of the metal/oxide interfaces in an EC environment.