The Pt-3 electrode (Fig. ), in turn, remains almost unchanged during / after the COOR, i.e., it is highly stable under reaction conditions. This result is somewhat unexpected, considering that exposure of bare Pt(111) electrodes to potential cycles in CO-free electrolytes with a potential limit larger than 1.17 V leads to significant irreversible restructuring of the electrode after a few potential cycles. Additional STM measurements for Pt-3 electrodes restructured in a controlled way by consecutive potential cycles in pure 0.5 M H 2 SO 4 are shown in Fig. in the supporting information (SI). Under these conditions, the electrochemical data suggested that the surface restructured at around 1.3 V. STM images revealed that thicker Pt layers (three atomic layers or more) remained unchanged, while the remaining regions with mono-or bilayer Pt films were entirely restructured. Reasons for the layerdependent stability (first to third layer) are discussed below.