6010 Phys. Chem. Chem. Phys., 2011, 13, 6010–6021 This journal is c the Owner Societies 2011 Cite this: Phys. Chem. Chem. Phys., 2011, 13, 6010–6021 Electrochemistry at Ru(0001) in a flowing CO-saturated electrolyte—reactive and inert adlayer phases O. B. Alves, H. E. Hoster* and R. J. Behm* Received 27th June 2010, Accepted 27th January 2011 DOI: 10.1039/c0cp01001d 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 O ad 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  2)-(CO + 2O) and a (22)-(2CO + O), are proposed to form at E o 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. 1. Introduction The electrooxidation of carbon monoxide is a key process in the Polymer Electrolyte Fuel Cell (PEFC) technology, e.g., in PEFCs operated by CO-containing fuel gases resulting from steam reforming of hydrocarbons or alcohols 1 or in direct methanol fuel cells (DMFCs), where CO appears as surface blocking poison or reaction intermediate. 2 Compared to pure Pt, PtRu electrodes were found to have a much better tolerance for CO traces in the fuel gas and a higher activity towards methanol oxidation, 3 which over the years has made PtRu the state-of-the-art anode catalyst in these PEFC appli- cations. 4 Their better activity is largely attributed to the high affinity of Ru to oxygen, which enables dissociative H 2 O adsorption at low potentials. 5–7 This provides OH ad or O ad as a co-reactant for the oxidation of CO adsorbed on neigh- boring Pt sites in a bifunctional Langmuir–Hinshelwood mechanism. Even on pure Ru electrodes, CO oxidation starts at lower potentials than on Pt electrodes, 8 which on a first view might indicate that the surface is catalytically more active than Pt. However, at higher potentials, where mass transport limited oxidation currents are achieved at Pt electrodes, 9 the oxidation currents obtained at polycrystalline Ru 9 and Ru(10  10) 10 were both found to be orders of magnitude lower than on Pt. On the smooth Ru(0001) surface, CO oxidation is even less efficient. 11,12 At not too high anodic potential limits and finite potential scan rates, the complete oxidation of pre-adsorbed CO adlayers requires several potentiodynamic cycles in the CO-free electrolyte (‘‘CO ad stripping’’), 8,11,13,14 whereas on Pt electrodes 15,16 and even on polycrystalline Ru electrodes 9 all CO ad is removed in a single cycle. Qualitatively, the low activity for CO oxidation can be explained by the strong adsorption not only of O ad (and/or OH ad ), but also of CO ad on Ru, which in combination makes CO 2 formation less favourable than on Pt. 8–10,17 The much lower CO oxidation activity of the smooth Ru(0001) surface may result from different effects, such as the lack of subsurface oxygen 12 or the low density of adsorption sites involving low-coordination substrate atoms. Sputter roughening of a Ru(0001) electrode surface indeed caused a significant increase in activity. 11 A low CO oxidation activity of Ru(0001) was reported also for the solid|gas interface, where mixed adlayers of CO ad and O ad on Ru(0001) were found to be largely inert towards CO 2 formation under ultrahigh vacuum (UHV) conditions. 18,19 In that case, higher oxidation rates require high O 2 pressures and elevated temperatures, where active, oxygen-rich surface phases can be formed. 20–23 In a comprehensive book chapter about the catalytic effects of Pt surface modifications on the electrochemical properties Institute of Surface Chemistry and Catalysis, Ulm University, D-89069 Ulm, Germany. E-mail: harry.hoster@uni-ulm.de, juergen.behm@uni-ulm.de PCCP Dynamic Article Links www.rsc.org/pccp PAPER Downloaded by Technische Universitaet Muenchen on 21 October 2011 Published on 21 February 2011 on http://pubs.rsc.org | doi:10.1039/C0CP01001D View Online