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

Alves, O. B. and Hoster, H. E. and Behm, R. J. (2011) Electrochemistry at Ru(0001) in a flowing CO-saturated electrolyte-reactive and inert adlayer phases. Physical Chemistry Chemical Physics, 13 (13). pp. 6010-6021. ISSN 1463-9076

Abstract

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 O(ad) formation on the CO(ad) saturated surface in the positive-going scan and of O(ad) reduction in the negative-going scan. O(ad) formation proceeds via nucleation and 2D growth of high-coverage Oad islands in a surrounding CO(ad) phase, and it is connected with CO(ad) oxidation at the interface between the two phases. In the negative-going scan, mixed (CO(ad) + O(ad)) phases, most likely a (2 x 2)-(CO + 2O) and a (2 x 2)-(2CO + O), are proposed to form at E <0.57 V by reduction of the O(ad)-rich islands and CO adsorption into the resulting lower-density O(ad) 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 CO(ad) coverages due to CO adsorption in the CO-saturated electrolyte, which lowers the CO adsorption energy and thus the barrier for CO(ad) oxidation during CO bulk oxidation.