Redox Behavior of Pt/Co₃O₄(111) Model Electrocatalyst Studied by X-ray Photoelectron Spectroscopy Coupled with an Electrochemical Cell

Abstract

Achieving high stability of supported noble metal nanoparticles with respect to sintering is one of the major challenges in electrocatalysis. In this study, we explored the role of metal–support interaction in stabilizing the morphology of a well-defined model electrode consisting of Pt nanoparticles supported on well-ordered Co3O4(111) films on Ir(100). We employed X-ray photoelectron spectroscopy coupled with an electrochemical cell to analyze changes in the oxidation states of both the supported Pt nanoparticles and Co3O4(111) support as a function of electrode potential. We found that immersion into the aqueous electrolyte at pH 10 (phosphate buffer) has no effect on the integrity and chemical composition of the Co3O4(111) film in a potential window between 0.5 and 1.4 VRHE. At lower potentials, reduction of the Co3O4(111) to Co­(OH)2 and metallic Co is accompanied by rapid dissolution of the film. In the presence of supported Pt particles, metal–support interaction gives rise to the formation of partially oxidized Ptδ+ species at the metal/oxide interface. Under electrochemical conditions, these species are readily oxidized yielding platinum oxide at the Pt/Co3O4(111) interface at potentials as low as 0.5 VRHE. The appearance of interfacial platinum oxide is accompanied by the formation of surface and bulk platinum oxides at potentials above 1.0 and 1.1 VRHE, respectively. While the formation and decomposition of surface and bulk platinum oxides depend on the electrode potential, the interface platinum oxide is stable between 0.5 and 1.4 VRHE. We propose that the high stability of supported Pt nanoparticles with respect to sintering is associated with the presence of platinum interface oxide stabilized by the metal–support interaction.

Self-limited growth of an oxyhydroxide phase at the Fe3O4(001) surface in liquid and ambient pressure water

“…Transition-metal (oxy)hydroxides have been shown to be highly active for the oxygen evolution reaction (OER), and iron specifically is hypothesized to be involved in the active site on several mixed oxides. [44][45][46][47] The formation of (oxy)hydroxides under OER conditions has been described for several systems, [48][49][50][51][52][53][54][55] and the first direct atomic-scale study of such a system was recently published by Fester et al, who used STM and XPS to show that exposure to ambient pressures of water transforms monolayer CoO x films supported on Au(111) to a cobalt (oxy)hydroxide. 15 However, the present work is the first time the conversion to (oxy)hydroxide structures have been imaged at an atomic scale on a bulk oxide.…”

Cobalt Oxide-Supported Pt Electrocatalysts: Intimate Correlation between Particle Size, Electronic Metal–Support Interaction and Stability

“…In previous work, it was demonstrated that model electrocatalysts based on Co 3 O 4 (111) thin films can be prepared and transferred into an EC environment while maintaining atomic-level control. ,, Recently, we identified the EMSI between nanometer-sized Pt particles and Co 3 O 4 (111) by means of ex situ emersion X-ray photoelectron spectroscopy (XPS) . In the subnanometer regime, however, Pt particles show different particle size effects and different chemical behavior. − …”

“…Two spin–orbit doublets emerge in the Pt 4f spectra of as-prepared conventional Pt NPs at 71.5 (Pt 4f 7/2 ) and 72.3 eV (Pt 4f 7/2 ) which are associated with metallic Pt 0 and a partially oxidized Pt δ+ species at the metal/oxide interface, respectively. , In contrast to conventional NPs, no metallic Pt 0 was detected in the Pt 4f spectra of the subnanometer Pt aggregates. Instead, two spin–orbit doublets emerge at 72.4 (Pt 4f 7/2 ) and 73.9 eV (Pt 4f 7/2 ) associated with partially oxidized Pt δ+ and Pt 4+ species, respectively. , The absence of metallic Pt 0 suggests that EMSI leads to a modification of the electronic structure of all Pt atoms within the subnanometer particles (see Supporting Information, Chapter 3.1 for a more detailed discussion on the growth mode).…”

“…The increase of the Pt δ+/2+ signal at the expense of Pt 0 indicates the onset of oxidation of conventionally sized Pt NPs at 1.1 V RHE . Progressive oxidation of conventional Pt NPs above 1.1 V RHE gives rise to a new component at 74.0 eV (Pt 4f 7/2 ) associated with the Pt 4+ state. , Upon returning to 0.5 V RHE , the contribution from Pt δ+/2+ significantly decreases and the signal from the Pt 4+ state vanishes. This observation indicates that the oxidation of Pt NPs is chemically reversible.…”

Anisotropy of Pt nanoparticles on carbon- and oxide-support and their structural response to electrochemical oxidation probed by in situ techniques

“…They found an interface Pt oxide which is stable between 0.5 and 1.4 V and believed this to be the reason for the catalyst's high stability towards particle sintering. 39 Studies on model electrodes made from doped-SnO 2 thin films supporting Pt nanoparticles showed that electronic metal-support interactions are the cause for superior specific ORR activity when compared to Pt on carbon. 40,41 Recently, we showed that the density of oxidizable Pt sites is an important property governing catalytic activity for ORR as we investigated the degradation pathways of ITO-supported Pt nanoparticles under simulated fuel cell conditions using combined in situ X-ray diffraction and scattering techniques.…”