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Reactivity of acetaldehyde at platinum and rhodium in acidic media. A DEMS study

Abstract The electrochemical behaviour of acetaldehyde at platinum and rhodium electrodes in acid solutions was studied applying differential electrochemical mass spectrometry (DEMS). In an acetaldehyde-containing solution, CO2 was the sole electrooxidation product detected at both electrodes, whereas only methanewas recorded in the hydrogen region. The production of acetic acid was indirectly established at platinum from DEMS. Residues were studied through the design of flow cell experiments. As for the acetaldehyde-containing solution, the adsorbates produce CO2 and methane during the oxidation and reduction processes, respectively. However, ethane was also observed at platinum during the cathodicstripping of the adspecies. The yield of these substances depends on the adsorption potential (Ead). These results suggest that the fragmentation of acetaldehyde occurs during adsorption and reduction reactions, and the extent of the C C bond scission is influenced by the Ead and the nature of the electrode, being favoured at rhodium. © 2002 Elsevier Science Ltd. All rights reserved.
Keywords:Acetaldehyde; Platinum; Rhodium; Differential electrochemical mass spectrometry

1. Introduction Acetaldehyde can be considered as a model substance for the electrochemical behaviour of aldehydes at noble metal electrodes. Moreover, it is an intermediate in the electrooxidation of ethanol to acetic acid, which competes with the complete oxidation of the alcohol to CO2, decreasing the efficiency of thelatter process. It has to be taken in mind that the oxidation to CO2 is the reaction of interest for fuel cell applications of this alcohol. However, scarce studies can be found in the literature concerning the electroreactivity of acetaldehyde, and these refer mainly to platinum [1 – 7]. In-situ Fourier transform infrared spectroscopy has shown that CO2 was the sole electrooxidation product atplatinum in a 0.01 M acetaldehyde solution, whereas both CO2 and acetic acid were obtained from 0.1 and 1
* Corresponding author. Fax: +34-922-318-002. E-mail address: epastor@ull.es (E. Pastor). 1 On leave from Laboratorio de Electroquımica, Universidad de ´ Panama, Panama. ´ ´ 2 ISE member. 3 On leave from Laboratorio de Electroquımica, Universidad de la ´ Republica, Montevideo, Uruguay. ´

Msolutions [2]. The amount of CO2 evolved was similar for the three concentrations, suggesting that in the reaction pathway responsible for the formation of carbon dioxide, the amount of adsorbate limits the production of this gas [2]. Only COad was apparent in the spectra containing acetaldehyde in the solution [2–5], but the presence of other adsorbed species has been demonstrated applying a flow cellprocedure [5]. Structural effects were studied using single-crystal platinum electrodes [3–6]. On the other hand, the analysis of the gas phase formed during a potential-controlled electrolysis of a 0.7 M acetaldehyde acid solution at 0.038 V versus RHE, has been performed by gas chromatography [7]. The production of ethane and methane (46 and 54%, respectively) was established from theelectroreduction of acetaldehyde at this potential [7]. In a preliminary DEMS experiment, only methane was detected during reduction of the adsorbates [1]. To our knowledge, no investigations have been performed on the reactivity of acetaldehyde on rhodium. The aim of this work is the study of acetaldehyde reactions (reduction, oxidation and adsorption) at platinum and rhodium in acid solution, by usingcyclic

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J. Sil6a-Chong et al. / Electrochimica Acta 47 (2002) 1441–1449

voltammetry combined with differential electrochemical mass spectrometry (DEMS). Strongly adsorbed species were studied applying a flow cell procedure under potential control of the...