Oxide supports can modify and stabilize platinum nanoparticles (NPs) in electrocatalytic materials. We studied related phenomena on model systems consisting of Pt NPs on atomically defined Co3O4(111) thin films. Chemical states and dissolution behavior of model catalysts were investigated as a function of the particle size and the electrochemical potential by ex situ emersion synchrotron radiation photoelectron spectroscopy and by online inductively coupled plasma mass spectrometry. Electronic metal–support interaction (EMSI) yields partially oxidized Ptδ+ species at the metal/support interface of metallic nanometer-sized Pt NPs. In contrast, subnanometer particles form Ptδ+ aggregates that are exclusively accompanied by subsurface Pt4+ species. Dissolution of Co x+ ions is strongly coupled to the presence of Ptδ+ and the reduction of subsurface Pt4+ species. Our findings suggest that EMSI directly affects the integrity of oxide-based electrocatalysts and may be employed to stabilize Pt NPs against sintering and dissolution.

Particle size and shape effects control the oxidation behavior of nanostructured electrocatalysts. We investigated the oxidation state of Pd nanoparticles supported on Ar + -sputtered highly oriented pyrolytic graphite (HOPG) and well-ordered Co 3 O 4 (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 Ar + -sputtered HOPG and Co 3 O 4 (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 Co 3 O 4 (111). Specifically, we detected the formation of partially oxidized Pd δ+ aggregates in combination with atomically dispersed Pd 2+ 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 V RHE has no significant effect for Pd/ Co 3 O 4 (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 Co 3 O 4 (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.

The oxidation state and stability of Pd nanoparticles supported on well-ordered Co 3 O 4 (111) films prepared on Ir(100) have been investigated in UHV and under both oxidizing and humid conditions by means of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near ambient X-ray photoelectron spectroscopy (NAP−XPS). After preparation, the supported Pd nanoparticles (size 4 nm) were found to be predominantly metallic. Small amounts of Pd 2+ , resulting from the electronic metal support interaction (EMSI), were found in the form of PdO and as ionic species dissolved in Co 3 O 4 (111). Annealing of the Pd/Co 3 O 4 (111) model catalyst in UHV triggers sintering of the Pd nanoparticles but leaves the oxidation states of Pd and the substrate largely unaffected. The oxidation of Pd/Co 3 O 4 (111) is coupled with dissolution of Pd 2+ species into Co 3 O 4 (111) and underlying Ir(100) resulting in a significant loss of Pd from the surface. The corresponding phenomenon occurs largely under oxidizing and, to a minor extent, under humid conditions. The reverse oxygen spillover is facilitated in the presence of Pd 2+ species dissolved in Co 3 O 4 (111) yielding Pd nanoparticles supported on CoO(111). The CoO(111) support remains stable under humid conditions but is reversibly converted to Co 3 O 4 (111) under oxidizing conditions.