STRUCTURE OF MATERIALS

138 ESRF

Structure-dependency of the atomic-scale mechanisms of Pt electro-oxidation and dissolution, T. Fuchs (a), J. Drnec (b), F. Calle-Vallejo (c), N. Stubb (d), D.J. Sandbeck (e,f), M. Ruge (a), S. Cherevko (e), D.A. Harrington (d) and O.M. Magnussen (a), Nat. Catal. 3, 754-761 (2020); https://doi.org/10.1038/s41929- 020-0497-y.

(a) Institut für Experimentelle und Angewandte Physik, Christian-Albrechts- Universität zu Kiel (Germany) (b) ESRF (c) Departament de Ciència de Materials i Química Física & Institut de Química Teòrica i Computacional, Universitat de Barcelona (Spain) (d) Chemistry Department, University of

Victoria, British Columbia (Canada) (e) Helmholtz-Institute Erlangen-Nürnberg for Renewable Energy, Forschungszentrum Jülich (Germany) (f) Department of Chemical and Biological Engineering, Friedrich-Alexander- Universität Erlangen-Nürnberg (Germany)

[1] J. Drnec et al., Curr. Op. Electrochem. 4, 69-75 (2017). [2] M. Ruge et al., J. Electrochem. Soc. 164, H608-H614 (2017). [3] M.J. Eslamibidgoli & M.H. Eikerling, Electrocatalysis 7, 345-354 (2016). [4] H. Reichert et al., Physica B 336, 46-55 (2003). [5] J. Gustafson et al., Science 343, 758-761 (2014).

PRINCIPAL PUBLICATION AND AUTHORS

REFERENCES

potential is not too high, these atoms can move back to their original locations but at higher potentials, an irreversible restructuring of the surface occurs.

To elucidate the role of the Pt surface structure, in-depth comparative studies of Pt(111) and Pt(100) in perchloric acid solution were performed with in-situ SXRD, online inductively coupled plasma mass spectrometry (ICP-MS) and DFT. The early stages of oxidation of the two surfaces were found to be dramatically different and were correlated with differences in the dissolution behaviour (Figure 120a). Pt(100) not only starts to dissolve at lower potentials but its oxidation also immediately changes its surface structure in an irreversible manner. These observations clearly demonstrate that Pt dissolution is linked to the way Pt atoms are initially extracted from the surfaces during oxidation.

For an explanation of these differences, a better understanding of the place-exchange process on Pt(100) was required. To this end, high-energy SXRD (HESXRD) was performed at ID31 with photon energies of 70 keV, a method pioneered at the ESRF [4]. By capturing the scattered X-rays with a 2D X-ray detector, a large fraction of reciprocal space can be mapped in a short time. The feasibility of this approach for operando studies of catalytic surfaces has been demonstrated previously in the gas phase [5]. It was here employed for the first time to a solid

liquid interface and proved to be indispensable for clarifying the oxide s structure. Because the oxide slowly evolved with time, the large data sets necessary for a reliable determination of this poorly ordered surface structure could only be obtained by the HESXRD technique. The oxidation-induced changes, exemplified in Figure 120b for one out of 12 simultaneously measured crystal truncation rods, can be unambiguously assigned to a process in which the extracted Pt atom moves up and laterally away from its original site. This initiates the immediate extraction of a second atom, leading to the formation of atomic oxide stripes on the surface (Figure 120c). DFT calculations support this scenario and show that, differently from Pt(111), this mechanism produces unstable surface atoms at stripe ends that can dissolve during the oxidation as well as during the subsequent oxide reduction. Consequently, the extraction process on Pt(100) is irreversible from its inception.

These results show a high sensitivity of Pt electrocatalyst oxidation and degradation to the precise surface structure. Rational strategies for the design of catalysts with improved stability will have to take the different mechanisms into account. Further investigations will utilise the in- situ HESXRD method for studies of the oxidation of more open high-index surfaces. The goal is to develop a framework for the challenging task of understanding the factors determining the stability of real platinum catalyst particles.