A Pt/Ru nanoparticulate system to study the bifunctional mechanism of electrocatalysis C. Roth a, * , A.J. Papworth b , I. Hussain a , R.J. Nichols a , D.J. Schiffrin a a Centre for Nanoscale Science, Department of Chemistry, The University of Liverpool, Liverpool L69 7ZD, United Kingdom b Department of Engineering, The University of Liverpool, Liverpool L69 3GH, United Kingdom Received 30 November 2004; received in revised form 29 March 2005; accepted 19 April 2005 Available online 31 May 2005 Abstract The reduced sensitivity of Pt–Ru alloys towards CO poisoning results from two effects: the ligand effect and the bifunctional mechanism. Although these have been known for many years, their applicability to nanoparticle electrocatalysts remains unclear. Furthermore, it is not known if the formation of a Pt–Ru alloy is necessary to improve the catalyst tolerance to CO or if the presence of Ru in immediate proximity to Pt nanoparticles (non-alloy systems) brings about a significant change. A new approach to the detailed investigation of the underlying mechanisms is presented by using mixtures of surfactant-stabilised Pt and Ru nanoparticles attached to an oxidised glassy carbon electrode. After CO activation the particle-decorated carbon surfaces become active for the methanol oxidation reaction as a result of the removal of part or all of the surfactant shell. However, despite this activation the onset potential and the peak maximum for both CO and methanol oxidation remain unchanged, independently of the mixture composi- tion, whereas the maximum current decreases with increasing Ru content. Scanning transmission electron microscopy (STEM) investigations confirmed the close proximity between Pt and Ru nanoparticles on the electrode surface. However, no enhancement of activity was observed which may be due to the presence of small amounts of capping ligands preventing direct metal contact between the Ru and Pt nanoparticles. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Bifunctional mechanism; CO activation; Cyclic voltammetry; Methanol oxidation; Pt; Ru; Nanoparticles 1. Introduction Pt–Ru alloys are prominent catalysts in low tempera- ture fuel cell anodes, since they exhibit a reduced sensi- tivity towards CO poisoning compared with pure platinum [1–4]. Their outstanding performance for the electrooxidation of both H 2 containing CO and metha- nol has been discussed mainly in relation to a ligand ef- fect and a bifunctional mechanism [5–8]. For the former, the presence of ruthenium is reported to alter the elec- tronic properties of Pt causing CO to adsorb less strongly on the catalyst surface. For the latter, it has been proposed that ruthenium is able to provide oxy- gen-containing species at more negative potentials than platinum. These can then oxidise CO on adjacent plati- num sites. Although these two effects have been known for many years, their significance in nanoparticle elect- rocatalysis remains unclear, in particular in relation to the mechanism of oxidation of H 2 /CO or methanol [9]. An important issue of contemporary discussion is whether a Pt–Ru alloy is needed to obtain the desired improvement in CO sensitivity or if the mere presence of Ru next to electrocatalytically active Pt centres is suf- ficient to improve fuel cell performance in the presence 0022-0728/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2005.04.014 * Corresponding author. Present address: Institute for Materials Science, Petersenstrasse 23, D-64287 Darmstadt, Germany. Tel.: +49 6151 16 5498; fax: +49 6151 16 6023. E-mail addresses: chrisi@liv.ac.uk, c_roth@tu-darmstadt.de (C. Roth). www.elsevier.com/locate/jelechem Journal of Electroanalytical Chemistry 581 (2005) 79–85 Journal of Electroanalytical Chemistry