Electrochemistry at Ru(0001) in a flowing CO-saturated electrolyte—reactive and inert adlayer phases

We investigated the electrochemical oxidation and reduction processes on ultrahigh vacuum prepared, smooth and structurally well-characterized Ru(0001) electrodes in a CO-saturated and, for comparison, in a CO-free flowing perchloric acid electrolyte by electrochemical methods and by comparison with previous structural data. Structure and reactivity of the adsorbed layers are largely governed by a critical potential of E = 0.57 V, which determines the onset of Oad formation on the COad saturated surface in the positive-going scan and of Oadreduction in the negative-going scan. Oad formation proceeds via nucleation and 2D growth of high-coverage Oad islands in a surrounding COad phase, and it is connected with COadoxidation at the interface between the two phases. In the negative-going scan, mixed (COad + Oad) phases, most likely a (2 × 2)-(CO + 2O) and a (2×2)-(2CO + O), are proposed to form at E < 0.57 V by reduction of the Oad-rich islands and CO adsorption into the resulting lower-density Oad structures. CO bulk oxidation rates in the potential range E > 0.57 V are low, but significantly higher than those observed during oxidation of pre-adsorbed CO in the CO-free electrolyte. We relate this to high local COad coverages due to CO adsorption in the CO-saturated electrolyte, which lowers the CO adsorption energy and thus the barrier for COadoxidation during CO bulk oxidation.