Identification of Ru phases in PtRu based electrocatalysts and relevance in the methanol electrooxidation reaction J.L.Go ´ mez de la Fuente a, *, F.J. Pe ´ rez-Alonso b , M.V. Martı ´nez-Huerta b , M.A. Pen ˜a b , J.L.G. Fierro b , S. Rojas b a Department of Materials Science and Engineering, Norges Teknisk-Naturvitenskapelige Universitet (NTNU), C/Sem Sælandsvei 12, NO-7491 Trondheim, Norway b Instituto de Cata ´lisis y Petroleoquı´mica, CSIC, C/Marie Curie 2, 28049 Madrid, Spain 1. Introduction Bimetallic surfaces have been known for their especial catalytic properties, which often exceed those of their individual compo- nents. This is particularly important for the development of highly active and stable catalysts for electrocatalytic reactions such as methanol or CO electrooxidation and oxygen reduction reaction which are core processes of the so called direct methanol fuel cells (DMFCs). Platinum ruthenium bimetallic catalysts show superior performance (either in terms of activity or stability) than Pt alone in both CO and methanol electrooxidation reactions. Therefore PtRu catalysts are benchmark catalysts for DMFCs. Although still under debate, the bifunctional mechanism in which Ru provides OH ad species at less positive potentials than Pt [1] is the most accepted. The role of Ru in weakening the strength of the CO bond to Pt (ligand effect) cannot be neglected either [2]. RuO x (OH) y is a well known Ru species since is the active component in the chloralkali industry and an excellent energy storage material for supercapacitor applications [3]. This species conducts both electrons and protons and it has a high affinity for oxygen nucleation as OH at potentials below 0.4 V. These properties endow this Ru species with very interesting properties as electrocatalysts for CO and methanol electrooxidation [4,5]. It has been recognized that PtRu black are not single-phase materials, but bulk mixtures of Pt metal, Pt hydrous oxides and hydrous and dehydrated RuO 2 [6]. In the same line, hydrous ruthenium oxide has been reported to be a mixture of non- stoichiometric oxoruthenium states, involving at least three oxygen-containing species and H 2 O. In fact, H 2 O is the source of both oxygen and hydroxyl groups [7,8]. The archetypal composition of the anodic electrocatalysts for DMFC consists of carbon supported PtRu nanoparticles. However, whether a real PtRu alloy is formed or not, and what is the oxidation state of their components, especially that of Ru is still under debate. To this end, X-ray photoelectron spectroscopy (XPS) technique is a powerful tool since it could disclose the actual oxidation state species on the catalyst surface. Unfortunately, the binding energy of C 1s core-level falls in the same energy region of Ru 3d levels (the principal photoelectronic contribution to the XPS spectrum of Ru). This feature may account to the lack of reliable references of the photoelectron binding energy for ruthenium compounds in electrocatalysts for DMFCs. In this work, a comprehensive XPS study with special attention to the oxidation state of Ru is given. The aim of this study is to identify the most active Ru species during the electrooxidation reaction. It must be recalled that the XPS study has been undertaken on fresh samples. Catalysis Today 143 (2009) 69–75 ARTICLE INFO Article history: Available online 25 March 2009 Keywords: Electrocatalyst Methanol PtRu Amorphous ruthenium oxide XPS ABSTRACT A relationship between the chemical state of Ru on bimetallic PtRu bulk samples and their performance in both CO and methanol electrooxidation has been established. The nature of the Ru species in the bimetallic samples has been scrutinized by means of XPS, XRD and TPR. The following Ru species were detected; reduced ruthenium (Ru 0 ), anhydrous RuO 2 and hydrous Ru oxide. The actual nature of the latter species consists of two amorphous oxides of general formula RuO 2 ÁxH 2 O and RuO x (OH) y as determined from the XPS analysis. Irrespective of the Ru phases, all PtRu catalyst studied display a similar CO oxidation pattern. However, methanol electrooxidation was found dependent on the Ru phases. Thus, catalysts containing Ru 0 are more active in the methanol oxidation reaction, at least during the early stages of the reaction. More stable catalyst are obtained if amorphous Ru oxide phases are the predominant ones. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +47 735 94 127. E-mail addresses: gomez@nt.ntnu.no, gofuen@gmail.com (J.L.Go ´ mez de la Fuente). Contents lists available at ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod 0920-5861/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2008.10.052