Platinum is an important catalyst widely used in energy conversion and storage, especially in the hydrogen-oxygen polymer electrolyte membrane fuel cell, where it is used at the anode and cathode¹. However, potential excursions can lead to surface oxidation and reduction, which restructures the surface and can lower the efficiency of the catalyst and lead to dissolution². Platinum atoms leave their original lattice sites during oxidation. Surface X-ray diffraction (SXRD) has determined that on Pt(111) the initial stage of Pt extraction is in a place-exchange process, in which the Pt atom moves directly above its original site, which is now occupied by an oxygen atom³. However, on Pt(100) recent work by our team has shown that the initial oxidation gives a “stripe” structure comprising 1D chains of raised Pt and oxygen atoms⁴.

Presented here is an analysis of combined simultaneous SXRD and electrochemistry data for Pt(111) and Pt(100) oxidation in HClO₄ and H₂SO₄. The correlation between the charge transfer and Pt extraction processes is investigated by in-situ cyclic voltammetry, potential step, and potential sweep-hold measurements.

The prior structural analysis from a complete set of crystal truncation rods is used find calibration curves for the X-ray intensity of selected anti-Bragg reflections as a function of the coverages θ_PE of different types of extracted Pt atoms. Integration of charge during electrochemical measurements was converted to electrons passed per surface Pt atoms, to give a notional “electron coverage” θ_e. In this way, simultaneous measurement of intensity with current during the selected potential program can be converted to θ_e vs θ_PE curves. The slopes of these curves are compared with suggested stoichiometric reactions, based on the known SXRD-determined structures. There is some uncertainty insofar as SXRD cannot distinguish O from OH, but we are able to estimate Pt oxidation states. On Pt(111), the electron-transfer and Pt extraction coverage ratios vary with sweep rate, indicating that the rates of adsorption and extraction are kinetically determined and changing during the first oxidation peak. On Pt(100), the stripe structure and stoichiometry suggest a Pt(II) oxide component in the initial stages.

We thank other team members involved in various phases of this work: Natalie Stubb (University of Victoria), Valentin Briega-Martos, Daniel Sandbeck (Forschungszentrum Jülich GmbH), Martin Ruge, Ole Fehrs (Kiel University), Federico Calle-Vallejo (Universistat de Barcelona), and the ID31 beamline staff at ESRF. Financial support from NSERC and DFG is appreciated.

Reference

1. R. Stamenkovic, D. Strmcnik, P. P. Lopes, N. M. Markovic, Energy and fuels from electrochemical interfaces, Nature materials, 16 (2017) 57-69.
2. J. Sandbeck, O. Brummel, K. J. Mayrhofer, J. Libuda, I. Katsounaros, S. Cherevko, Dissolution of platinum single crystals in acidic medium, ChemPhysChem, 20 (2019) 2997.
3. Drnec, M. Ruge, F. Reikowski, B. Rahn, F. Carlà, R. Felici, J. Stettner, O.M. Magnussen, D.A. Harrington, Initial stages of Pt(111) Electrooxidation: Dynamic and Structural Studies by Surface X-ray Diffraction, Electrochim. Acta, 224 (2017) 220-227.
4. Fuchs, J. Drnec, F. Calle-Vallejo, N. Stubb, D. J. Sandbeck, M. Ruge, S. Cherevko, D. A. Harrington, O. M. Magnussen, Structure dependency of the atomic-scale mechanisms of platinum electro-oxidation and dissolution, Nature Catalysis, 3 (2020) 754-761.