Sample preparation: The Ru(0001) electrodes were prepared under UHV conditions in an experimental set-up described elsewhere. Clean and atomically flat surfaces were obtained by several cycles of Ar ion sputtering (p Ar = 3 × 10 -5 mbar, I = 4 µA cm -2 , t = 30 min) and flash annealing to 1600 K, followed by seven cycles of flash annealing to 1600 K and adsorption of 10 L of O 2 at T< 600 K during sample cooling to remove carbon impurities, finished by three flash annealing cycles to 1600 K without O 2 adsorption. Pt was deposited by means of electron beam physical vapour deposition with an Omicron EFM-3 evaporator at a rate of ca. 0.1 ML min -1 . During the deposition process, the sample was held at 670 K to improve the Pt layer-by-layer growth. Higher deposition temperatures must be avoided to prevent surface alloy formation. The deposition rate was frequently tested by depositing submonolayer amounts of Pt on a freshly prepared Ru(0001) surface.