Rh electrodeposition on Pt in acidic medium: a study using cyclic voltammetry and an electrochemical quartz crystal microbalance R.T.S. Oliveira, M.C. Santos, L.O.S. Bulh~ oes, E.C. Pereira * Laborat orio Interdisciplinar de Eletroqu  ımica e Cer^ amica, Departamento de Qu  ımica, Centro Multidisciplinar para o Desenvolvimento de Materiais Cer^ amicos, Universidade Federal de S~ ao Carlos, Caixa Postal 676, 13560-970 S~ ao Carlos, SP, Brazil Received 16 June 2003; received in revised form 20 February 2004; accepted 6 March 2004 Available online 14 May 2004 Abstract This paper describes Rh electrodeposition on Pt electrodes in HClO 4 solutions. Using cyclic voltammetry (CV), coupled with electrochemical quartz crystal microbalance (EQCM) studies, we determined the main characteristics related to oxide formation on rhodium deposited on platinum. During the deposition, the results demonstrated that Rh 0 ad-atoms are stable on Pt in the potential range investigated at different saturation coverages. Data obtained using EQCM and CV show that the number of electrons transferred in the oxidation of Rh ads on Pt is 3 and a Rh(OH) 3 species was proposed as the adsorbed product. At potentials higher than 1.0 V, Rh(OH) 3 is oxidized to RhO(OH) 3 . This species is reduced to Rh(OH) 3 and then to Rh 0 during the negative potential sweep. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Rhodium; EQCM; Pt electrodes; Electrodeposition 1. Introduction Although rhodium is a very expensive metal, its thin films have received some attention [1] and its com- pounds have been widely used as electron mediators for the development of new sensors [2,3]. The most com- monly used materials for this purpose are metals, metal complexes and some organic dyes [4]. The structure of metal atoms deposited on a metal surface is itself an interesting subject [5] due to the properties of the deposited metal layers, their alloys and multilayers [6–8]. Therefore, they have received great attention as a potential new class of magnetic and su- perior alloy catalysts [9–17]. Multi-component catalyst systems consisting of two or more active metal compo- nents or both metal and oxide constituents [18,19] are often used to carry out electrocatalytic reactions. Studies of the deposition of rhodium clusters are motivated by the well-known catalytic properties of this metal [20–22]. In fact, rhodium is a common component of the three-way catalyst used for the simultaneous conversion of nitrogen oxides, carbon monoxide, and hydrocarbons in automobile exhausts [23,24]. However, there are few studies detailing the electrodeposition of rhodium on polycrystalline electrodes [1,25]. Vukovic [25] studied the electrochemical behavior of rhodium in HClO 4 and demonstrated that the surface roughness depends on the current density and electro- deposition time. To calculate the roughness factor the author used the charge density of one H monolayer on Rh (211 lC cm 2 ), which is an average of the charges at the (1 1 1), (1 0 0) and (1 1 0) index planes. Jerkiewicz and Borodzinski [26] investigated the re- lationship between the surface states of oxide films on Rh electrodes and the kinetics of the oxygen evolution reaction. Oxide formation was studied on polycrystal- line Rh electrodes in aqueous 0.5 mol dm 3 of H 2 SO 4 in the potential range of 0.8–2.4 V versus RHE. From the results obtained, the authors proposed the formation of * Corresponding author. Tel.: +55-16-261-5215/+55-16-260- 8214x213; fax: +55-16-260-8214/+55-16-260-8214x208. E-mail address: decp@power.ufscar.br (E.C. Pereira). 0022-0728/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2004.03.006 Journal of Electroanalytical Chemistry 569 (2004) 233–240 www.elsevier.com/locate/jelechem Journal of Electroanalytical Chemistry