( <i>Invited</i> ) Correlation between Surface Structure and Structural Changes of Single Crystal Model Electrodes during Electrocatalytic Reactions by Ex Situ STM Imaging

Over the last decades, fundamental research on the electrochemical/-catalytic properties of low-index single crystals has played an essential role in understanding and improving industrially relevant and more complex (electro-)catalyst materials. Some of the fundamental questions deal with the elucidation of the activity of specific sites on a catalyst material, which should ultimately reveal the most active site and the catalyst structure under reaction conditions, including possible restructuring phenomena, which can significantly alter the electrocatalytic activity. In this context, mono- or bimetallic single-crystal electrodes provide suitable substrates since the number of different sites on their surfaces is generally limited. A central technique to elucidate catalytic sites and sites prone to restructuring is scanning tunnelling microscopy (STM). The technique can be employed under various conditions (ambient, ultrahigh vacuum (UHV) and in situ in the presence of an electrolyte). The choice depends on the method and environment used to prepare the electrodes and the respective reaction conditions. While in situ STM imaging provides information on the potential dependent surface structural properties, such measurements are challenging under reaction conditions, and results are scarce. In this work, we approach this aspect by performing STM imaging before and after the electrochemical or -catalytic investigation. Electrodes studied are Pt(111), Ru(0001) and Pt-modified Ru(0001). These electrodes can be prepared under ambient and UHV conditions, yielding different initial structural properties depending on the preparation conditions. Hence, they also provide different degrees of freedom for nanostructure formation, as illustrated by STM images recorded under different conditions. The electrodes' initial structure determines the electrodes' activity for potentials where the electrodes are stable under electrochemical and -catalytic conditions. Possible changes observed in the electrochemical and electrocatalytic properties can be related to structural changes observed on these electrodes by STM imaging (under UHV or ambient conditions) after these measurements. Specifically, we focus on electrochemical measurements in pure or MeOH and CO-containing acid electrolytes. The electrooxidation of these molecules was investigated rather vividly for Pt and bimetallic PtRu catalysts over the last decades due to their application in MeOH and H 2 polymer electrolyte membrane fuel cells. First, we will show that for Pt submonolayer-modified Ru(0001) electrodes prepared under UHV conditions, surface structural changes are hardly visible in the electrochemical measurements in pure electrolytes (performed in an electrochemical flow cell attached to the UHV system) and can only be revealed by additional STM imaging under UHV conditions. We show that Pt attached to the descending Ru step edges shifts the onset potential for Ru step flow corrosion to more positive potentials until it corrodes before the Pt islands restructure. [1,2] In contrast, during the CO and methanol oxidation reaction, significant changes in activity are observed, and the origin of activity change can be attributed to these local structural changes.[1,2,3] Similar experiments were performed for Pt multilayer modified Ru(0001) electrodes prepared under UHV conditions, with Pt layer thicknesses ranging between one to four layers.[4] For surfaces with a layer distribution of one to four layers, electrochemical measurements indicate surface restructuring, where additional STM imaging reveals that restructuring only occurs in regions with one to two Pt layers, while thicker layers are stable. For electrodes with three to five layers, we found that the electrodes are stable up to 1.4 V vs RHE, hence, ate significantly higher potentials than Pt(111) (irreversible restructuring occurs at about 1.17 V).[5] From additional surface X-ray diffraction (SXRD), we can also show that the onset of the reversible place exchange for Pt(111) surfaces shifts to more positive potentials during the oxidation of MeOH, which can not be inferred from electrochemical or STM imaging. Based on these combined techniques, including differential electrochemical mass spectrometry measurements (DEMS), we will discuss reasons for the difference in stability of these electrodes, also in comparison with results obtained on Pt(hkl) electrodes studied by STM under laboratory conditions and DEMS. In total, this work will show the limitations but, more importantly, the benefits of performing STM imaging before and after electrochemical and catalytic studies to elucidate local structural changes that are not accessible by global probes, such as electrochemistry and SXRD. References [1] Engstfeld, Brimaud, Behm, AngewChemIntEd. , 53 (2014) 12936 [2] Engstfeld, Klein, Brimaud, Behm, SurfSci , 631 (2015) 248 [3] Engstfeld, Klein, and S. Brimaud, ChemPhysChem , 22 (2021) 828 [4] Engstfeld, Forschner, Löw, Pithan, Beyer, Jusys, Bansmann, Behm, and Drnec, ChemCatChem (2025) e202401913 [5] Fuchs, Drnec, Calle-Vallejo, Stubb, Sandbeck, Ruge, Cherevko, Harrington, Magnussen , Nature Catalysis , 3 (2020) 754