Electrochimica Acta 45 (2000) 3305 – 3309 Electrocatalytic reactivity for oxygen reduction at epitaxially grown Pd thin layers of various thickness on Au(111) and Au(100) Hideo Naohara, Shen Ye, Kohei Uosaki * Physical Chemistry Laboratory, Diision of Chemistry, Graduate School of Science, Hokkaido Uniersity, Sapporo 060 -0810, Japan Received 10 September 1999; received in revised form 25 December 2000 Abstract Electrochemical reduction of oxygen was studied at the epitaxially grown ultra thin Pd layers of various thicknesses on Au(111) and Au(100) single crystal surfaces in 50 mM HClO 4 solution saturated by oxygen. The oxygen reduction at the Pd thin layers on the gold substrate started at more positive potential than that at the gold single crystal surfaces even only a submonolayer of Pd was deposited. The potentials where cathodic current of oxygen reduction started to increase were dependent on the potentials where the oxide of the ultra thin Pd layers was reduced. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Oxygen reduction; Palladium; Gold; Single crystal; Epitaxial layer www.elsevier.nl/locate/electacta 1. Introduction The electrochemical reduction of oxygen plays im- portant roles in many processes such as electrochemical energy conversion/storage, metal corrosion and electro- catalysis. It is especially important for the realization of highly efficient fuel cells, because the overpotential for oxygen reduction on noble metals such as Au and Pt was relatively large compared to that for hydrogen oxidation. It is also important in relation to the oxygen reduction in respiratory chain. Thus, the mechanism of the electrochemical reduction of molecular oxygen at wide varieties of electrode materials has been exten- sively studied [1]. Most of these studies were, however, carried out using polycrystalline electrodes. Because the electrochemical activity should be drastically affected by the surface structure of the electrode, it is essential to investigate the process at atomically well-defined electrode surfaces. Since Clavilier et al. reported that the single crystal surface of noble metals can be pre- pared by a flame annealing and quenching technique [2,3] the oxygen reduction reaction has been investi- gated using well-defined surfaces such as Au [4–7] and Pt [8 – 10] single crystal electrodes. It is also known that the electrocatalytic activities on an electrode surface can be modified and controlled by the deposition of the small amount of foreign metals. Motoo and Watanabe propose the idea of an ‘adatom’ in this respect [11,12]. The electrochemical deposition of various noble metals such as Ru, Rh and Pd on a Pt electrode were carried out to obtain a highly active electrode [13 – 15]. Recently, Kolb et al. investigated the adsorption and absorption of hydrogen and electro- chemical oxidation of formic acid on ultra thin Pd layers on single crystalline gold electrodes and found that the activities of the electrode depended on the * Corresponding author. Tel.: +81-11-7063812; fax: +81- 11-7063440. E-mail address: uosaki@pcl.sci.hokudai.ac.jp (K. Uosaki) 0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII:S0013-4686(00)00440-0