ELSEVIER Journal of Electroanalytical Chemistry 379 (1994) 329-337 JOURNAL OF In-situ FTIR study of the electrocatalytic oxidation of ethanol at iridium and rhodium electrodes N.R. de Tacconi a, R.O. Lezna a B. Beden b, F. Hahn b, C. Lamy b a Universidad de La Plata, INIFTA, Sucursal 4, Casilla de Correo 16, 1900 La Plata, Argentina b Laboratoire de Chimie I, Electrochimie et Interactions, URA-CNRS 350, Universit~ de Poitiers, 40 avenue du Recteur Pineau, 86022 Poitiers C#dex, France Received 21 December 1993; in revised form 21 March 1994 Abstract In-situ Fourier transform IR (FTIR) reflectance spectroscopy has been used to study the electroadsorption and oxidation of ethanol at polycrystalline Ir and Rh electrodes in HC10 4 solutions. The formation of surface and solution species has been followed optically during slow voltammetric scans. Ethanol electroadsorption leads to the formation of linearly bonded and bridge-bonded CO on Rh surfaces but only to linearly bonded CO on Ir. In the oxidation potential region, Ir electrodes show different catalytic properties from Rh electrodes, with Ir being a more selective catalyst than Rh. Oxidized states at early potentials play an activating role in ethanol electro-oxidation. Acetaldehyde is probably an intermediate product, but the major product on Ir electrodes is acetic acid whereas CO 2 is the main product at Rh electrodes. Keywords: FTIR spectroscopy; Electrocatalysis; Oxidation; Ethanol; Iridium; Rhodium 1. Introduction The electrocatalytic oxidation of small organic molecules on noble metals is a current topic of interest in electrochemistry [1-7]. In the case of ethanol, which is a model organic molecule with two C atoms, most of the studies have been carried out on Pt electrodes [7-13]. The catalytic activities of the noble metals Rh and Ir have not been widely tested, although there have been attempts to compare them with Pt [8,14]. Furthermore, for formic acid oxidation, it has been reported that single-crystal Ir faces have different activities for poi- son formation and for the main oxidation reaction [15]. In addition, the catalytic activity of Ir electrodes may depend on the properties and thickness of the surface oxide film [14,16] which, in turn, change with pH [17]. The aim of the present paper is to compare the catalytic activities Of Rh and Ir, taking the oxidation of ethanol as a test reaction. As they have the same number of unpaired d electrons per atom, they may have similar adsorptive properties and their surface oxides may have comparable stability. Our investigation was mostly focused on ethanol 0022-0728/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0022-0728(94)03454-B adsorption and oxidation at Ir electrodes. To our knowledge, no extensive optical studies of the electro- chemical interface have been published so far. Fourier transform IR (FTIR) spectroscopy was used to analyse the potential-dependent electroadsorption and oxida- tion of ethanol during a potential scan, using the single-potential alteration IR reflectance spectroscopy (SPAIRS) technique [18]. Several important new results are reported in this work. First, it was found that the oxidation efficiency depends on the electrode material, with Ir being a better electrocatalyst than Rh from the point of view of selectivity. Second, ethanol was generally shown to be oxidized to acetic acid on Ir, in contrast with Rh electrodes where CO 2 is the main product. Third, most of the CO 2 produced arises from the oxidation of adsorbed CO species. 2. Experimental The working electrodes, which were polycrystalline Ir and Rh discs (high purity metals from Johnson Matthey), were polished with alumina of particle size