The experiments were performed in a combined UHV and electrochemistry setup, which has been described in detail elsewhere. Materials: The acid solutions were prepared from high-grade chemicals and MilliQ water (18.2 MΩ cm). We used H 2 SO 4 Merck Suprapure 98% and HClO 4 Merck Suprapure 70%. The electrolytes were purged with N 2 (Westfalen 6.0) or O 2 (6.0, MTI) for the ORR measurements. The Ru(0001) single crystal electrodes were purchased from MaTecK GmbH (purity 99.99%, orientation accuracy <0.1°). The glassware was cleaned before each experiment by storage in highly concentrated KOH overnight and subsequently thoroughly boiled in and rinsed with hot MilliQ water.