Electrochimica Acta 56 (2011) 2443–2449 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Oxidation of carbon monoxide on poly-oriented and single-crystalline platinum electrodes over a wide range of pH Ruben Gisbert a,b , Gonzalo García a,c , Marc T.M. Koper a,∗ a Leiden Institute of Chemistry, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands b Instituto de Electroquimica de la Universidad de Alicante, Apt. 99, E-03080 Alicante, Spain c Departamento de Química Física, Facultad de Química. Universidad de La Laguna, Astrofísico F. Sánchez s/n, 38071 La Laguna, Tenerife, Spain article info Article history: Received 6 July 2010 Received in revised form 10 November 2010 Accepted 11 November 2010 Available online 18 November 2010 Keywords: Carbon monoxide oxidation Platinum pH Carbonate Phosphate abstract The oxidative stripping of pre-adsorbed carbon monoxide has been studied on poly-oriented platinum, and on Pt(1 1 1), Pt(1 0 0) and Pt(1 1 0) single-crystal electrodes in phosphate buffer solutions as a function of pH, both stripping voltammetry and chronoamperometry. It was found that the stripping peak potential has a tendency to decrease as a function of pH until a pH of ca. 10–11, which is ascribed to a weaker adsorption of phosphate on platinum with increasing pH. Above a pH of ca. 11, the stripping peak appears to stay constant or increase, depending on the surface structure. We hypothesize that this may be due to the fact that above a pH of ca. 10–11, the main product of carbon monoxide oxidation is carbonate, which may be block active sites on the surface. By comparison with the stripping on the single-crystalline Pt, the stripping voltammetry on the poly-oriented Pt electrode appears as a convolution of the oxidation on the different facets. A similar conclusion is drawn for the chronoamperometry experiments, and it is suggested that this may be the main reason for the asymmetrical transients observed on the poly- oriented Pt electrodes, as well as on other strongly heterogeneous Pt electrodes that have been studied in the literature. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction The electrochemical adsorption and oxidation of carbon monox- ide on platinum electrodes are among the most studied reactions in the electrocatalysis field during the last decades [1–13]. One of the principal reasons for the interest in this reaction is that CO results from the incomplete oxidation of small organic molecules (methanol, ethanol, etc.) in low temperature fuel cells. The CO pro- duced is adsorbed strongly, poisoning the platinum surface and thereby severely limiting the catalytic oxidation activity [14,15]. Also, CO is present as an impurity in hydrogen fuel produced from hydrocarbons through the reformate process. Although the CO per- centage in the reformate gas is very low, the platinum based catalyst still becomes poisoned and consequently the performance of the fuel cell is compromised [14,15]. For these reasons, one of the major objectives in fuel cell catalysis is to develop catalysts with higher catalytic activity towards CO oxidation or with a higher specificity towards a fuel oxidation path in which the formation of poisoning species is avoided [14–19]. ∗ Corresponding author. E-mail addresses: ruben.gisbert@ua.es (R. Gisbert), ggarcia@ull.es (G. García), m.koper@chem.leidenuniv.nl (M.T.M. Koper). Besides the practical interest, the oxidation of carbon monox- ide on a platinum electrode is also an important model reaction [20]. It is well known that the kinetics of CO oxidation on plat- inum does not only depend on the structure of the platinum surface [2–7,21–23], but also depend on the properties of the electrolyte solution [4–7,24–27]. In fact, there is consensus in the literature about the higher relative catalytic activity for CO oxidation on plat- inum in alkaline solution compared to acidic solution, but there is little molecular-level understanding as to why this is. In acidic media, it is now well-established that step and defect sites pos- sess a unique activity for CO oxidation, even to the extent that on a series of stepped Pt electrodes, it has been concluded that all CO, both those initially adsorbed at or near the step and those initially adsorbed on the terrace, react to CO 2 at the step sites [3,23]. CO oxi- dation on the Pt(1 1 1) terrace is essentially negligible, and all CO will diffuse rapidly to the step or defect sites and be oxidized there. On the other hand, more recently, we have studied CO oxidation on Pt[n(1 1 1) × (1 1 0)] stepped electrodes in alkaline solution, and indeed observed that the CO oxidation potential is lower than in perchloric acid solution. Surprisingly, four different active oxida- tion sites on the surface, i.e. sites with (1 1 1), (1 1 0) and (1 0 0) orientation, and kink sites can be observed during CO stripping voltammetry [4]. It was found that the catalytic activity towards the CO oxidation increases in the following way: kink > step > terrace sites [4]. Moreover, from Fourier transform infrared spectroscopy 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.11.032