The electro-oxidation of carbon monoxide on ruthenium modified Pt(111)

The effect of adsorbed ruthenium sub-monolayers on Pill 1 1) on the electro-oxidation of CO has been investigated using ultra-hight vacuum-electrochemical transfer techniques, Overlayers of metal vapour deposited (MVD) ruthenium were characterised using a combination of LEED, LEISS and XPS. Nucleation and growth of ruthenium clusters on the Pt(I 1 1) surface takes place during deposition at 300 K from low coverages. Bulk dissolution occurs only after annealing the surface to temperatures in excess of 900 K. The hydrogen and anion adsorption/desorption behaviour, and the oxidative stripping of saturated CO overlayers, in 0.5 M sulphuric acid electrolyte on the ruthenium modified Pt(I 1 1) surfaces hits been investigated using cyclic voltammetry. The long range order characteristics of the voltammetry of the Pt( I I I) surface associated with the anion adsorption is destroyed by the lowest coverages of ruthenium. The hydrogen adsorption/desorption behaviour remains dominated by that of Pt(I 1 1), however, throughout the ruthenium sub-monolayer regime. At ruthenium coverages above one monolayer, the redox currents in the double layer region increase, and peaks we associate with hydrogen adsorption on ruthenium are apparent at 0.09 and 0.16 V vs reversible hydrogen electrode (R I I E). CO electro-oxidation on Pt(I 1 1) modified by ruthenium is characterised by two oxidation peaks (approximate to0.6 and 0.7 V RHE) in the stripping voltammetry, both at an over-potential significantly lower than that found oil Pt(I 1 1) under the same conditions. This overall promotion is associated with the bi-functional mechanism, and the reduction in over-potential is primarily associated with the promoted activation of water at the ruthenium atoms in the cluster. The first of the two promoted oxidation peaks (0.6 V) is a result of fast Langmuir Hinshelwood oxidation kinetics associated with platinum sites adjacent to the ruthenium cluster (Zone 1). The second oxidation peak (0.7 V) is associated with oxidation of CO in the remaining sites that lie at least one platinum atom away from the ruthenium sites (Zone 11). We show that the mobility of CO between these sites is relatively low in the presence of the sulphate anion: the diffusion of CO between sites is not responsible for the second peak in the stripping voltammetry, We suggest that the slower kinetics of the oxidation in the second peak is associated with the rate of spill-over and diffusion or (lie oxidant produced at the ruthenium sites to Zone I sites. (C) 2001 Elsevier Science PIN. All rights reserved.