Degradation of Bimetallic Model Electrocatalysts – an in situ XAS Study

We have studied the potential-induced degradation of Pt monolayer model electrocatalysts on Rh(111) and Au(111) singlecrystal substrates. The anodic formation of Pt oxides was monitored using in situ high energy resolution fluorescence detection x-ray absorption spectroscopy (HERFD XAS). Although Pt was deposited on both substrates in a three-dimensional island growth mode, we observed remarkable differences during oxide formation that can only be understood in terms of strong Pt– substrate interactions throughout the Pt islands. Anodic polarization of Pt/Rh(111) up to +1.6 V vs. RHE (reversible hydrogen electrode) leads to formation an incompletely oxidized passive layer, whereas formation of PtO2 and partial Pt dissolution is observed for Pt/Au(111). SLAC-PUB-14485 SLAC National Accelerator Laboratory, Menlo Park, CA 94025 Work supported in part by US Department of Energy contract DE-AC02-76SF00515. Fuel Cell Catalysis DOI: 10.1002/anie.200((will be filled in by the editorial staff)) Degradation of Bimetallic Model Electrocatalysts – an in situ XAS Study Daniel Friebel*, Daniel J. Miller, Dennis Nordlund, Hirohito Ogasawara, Anders Nilsson* Proton exchange membrane fuel cells (PEMFC) could be an important building block for a renewable energy infrastructure, converting chemically stored energy – e.g., from solar peak production – back into electricity for electric vehicles or stationary off-the-grid applications. An unaccomplished prerequisite for such a development is the availability of cost-efficient electrocatalyst materials, in particular for the oxygen reduction reaction (ORR). Ptfree catalysts made from earth-abundant materials would be desirable, but exhibit too high overpotentials. Nevertheless, the cost of Pt-based catalysts can be reduced by tuning both the morphology and electronic structure to maximize activity. Significant enhancements can be achieved with bimetallic systems where the Pt 5d-band is shifted due to strain and ligand effects. However, highly active carbon-supported Pt and Pt-alloy nanoparticles have been successfully tested only on short time scales, whereas degradation occurs under long-term operating conditions through sintering, Pt dissolution, carbon corrosion and nanoparticle-support detachment. Furthermore, the enhanced catalytic activity of bimetallic nanoparticles is often achieved through a specific “coreshell” distribution of constituents, which also lacks long-term stability. 2 Here, we present a study on the anodic oxidation of small Pt islands supported on single-crystal Rh(111) and Au(111) substrates, using in situ x-ray absorption spectroscopy (XAS) in the high energy resolution fluorescence detection (HERFD) mode. By depositing ultrathin Pt layers onto a M(111) substrate, we mimic the strain and vertical ligand effects that also occur in Pt alloys, but with better control of structure and element distribution and the highest possible surface sensitivity of the bulk-penetrating hard x-ray probe. Metallic Pt and different Pt oxides can be clearly identified by the shape and intensity of the characteristic maximum (“white-line”) near the Pt L3 absorption edge due to 2p→5d transitions. The spectral resolution in conventional XAS is limited by the Pt 2p core hole lifetime broadening (~5.2 eV), but significantly sharpened spectral features can be obtained with the HERFD technique. The ORR activity for Pt overlayers on various transition metal substrates has been studied in detail with rotating disk electrode (RDE) measurements, and a volcano plot using the adsorption strength of atomic oxygen as descriptor has been established using density functional theory (DFT). While the ORR activities of the two systems studied here are of the same order of magnitude, they lie on opposite sides of the volcano, exhibiting weaker (Pt/Rh(111)) and stronger (Pt/Au(111)) O adsorption than pure Pt. However, there is an apparent discrepancy between the theoretically predicted trend and experimentally determined ORR activities for a number of Pt/M(111) systems prepared via redox displacement of underpotential-deposited Cu. This disagreement can be explained if, instead of the uniform twodimensional (2D) monolayers assumed in DFT calculations, redox displacement yields three-dimensional (3D) Pt islands. Indeed, 3D island growth has been confirmed with in situ scanning tunneling microscopy for electrochemically deposited Pt/Au(111). On Rh(111), a 2D Pt layer can be grown by UHV evaporation onto a heated substrate, and we recently studied electrochemical surface oxide formation on such a 2D Pt/Rh(111) sample with in situ HERFD XAS and EXAFS. In contrast, the redox-displacement technique results in small 3D islands also for Pt/Rh(111); this is evident from in situ EXAFS (Supporting Information). We use this 3D Pt/Rh(111) sample in our comparison with Pt/Au(111) in order to provide a similar Pt morphology. Interestingly, not only the d-band shifts and corresponding oxygen affinities for Pt/Au(111) and Pt/Rh(111), but also the surface energies of the substrate metals in these systems differ substantially. Au has a significantly lower surface energy than Pt, which explains why Pt cannot be grown on Au in a layer-by-layer mode. Rh, in contrast, has a higher surface energy than Pt. We find that surface and cohesion energies strongly influence the redox chemistry of Pt islands at potentials above 1.0 V (RHE) where Pt oxides and hydrated Pt cations become thermodynamically stable. Such conditions can occur in various fuel cell operating scenarios and contribute significantly to catalyst degradation. In situ HERFD XAS measurements on Pt/Rh(111) (Fig. 1a) and Pt/Au(111) (Fig. 1b) show significant changes in the white-line region as the potential is increased above 1.0 V. On both samples we initially observe a transition from a narrow absorption maximum at 11566 eV to a much broader peak around 11567 eV; similar changes were observed in our previous study of 2D Pt/Rh(111). However, while this new feature saturates on both 2D Pt/Rh(111) and 3D Pt/Rh(111) upon further increasing the potential, a second transition occurs on Pt/Au(111) after 1.4 V is reached. At this potential, a strong increase of the white-line intensity develops during a timescale of ~40 min and the absorption maximum shifts to 11568 eV. [∗] Dr. D. Friebel, D. J. Miller, Dr. Dennis Nordlund, Dr. H. Ogasawara, Prof. A. Nilsson SLAC National Accelerator Laboratory 2575 Sand Hill Rd, Menlo Park, CA 94025, USA Fax: (+1) 650 926 4100 Email: [email protected], [email protected] u [∗∗] This work is supported by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under contract DE-AC02-76SF00515. This research was partly carried out at the Stanford Synchrotron Radiation Lightsource, a National User Facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. By comparison with ab initio multiple-scattering calculations using FEFF8.4 for various Pt oxides (Fig. 2), it is clear that the high white-line intensities observed for Pt/Au(111) above 1.4 V can only be explained with the formation of Pt(IV). The broader appearance and weaker peak intensity in the measurement can be attributed to either additional Pt in lower oxidation states, or to a disordered PtO2 structure. 1.0 V 1.2 V 1.3 V 1.4 V, time: 6 min 27 min 50 min 1.6 V 0.9 V 1.0 V 1.1 V 1.2 V 1.3 V 1.4 V 1.5 V 1.6 V Rh Pt Au Pt a) Pt/Rh(111) b) Pt/Au(111)