Experiment and Theory of Fuel Cell Catalysis: Methanol and Formic Acid Decomposition on Nanoparticle Pt/Ru Matthew A. Rigsby, † Wei-Ping Zhou, †,⊥ Adam Lewera, ‡ Hung T. Duong, † Paul S. Bagus, § Wolfram Jaegermann, | Ralf Hunger, | and Andrzej Wieckowski* ,† Department of Chemistry, UniVersity of Illinois at Urbana-Champaign, Urbana, Illinois 61801, Department of Chemistry, UniVersity of Warsaw, ul. Pasteura 1, 02-093 Warsaw, Poland, Department of Chemistry, UniVersity of North Texas, Denton, Texas 76203-5070, and Surface Science DiVision, Institute of Materials Science, Darmstadt UniVersity of Technology, D-64287 Darmstadt, and Bessy-II, Berlin, Germany ReceiVed: June 18, 2008; ReVised Manuscript ReceiVed: July 29, 2008 This study seeks to explore the effects of the electronic structure of Pt/Ru alloy nanoparticles on reactivity of small organic molecules of relevance to fuel cell applications through the combined use of synchrotron radiation photoelectron spectroscopy and electrochemistry. Platinum core-level binding energies were found to increase linearly with the addition of ruthenium. This effect is a product of lattice strain and charge transfer, and is explained in terms of the d-band center theory proposed by Nørskov and co-workers (Hammer, B.; Nørskov, J. K. Surf. Sci. 1995, 343, 211). In the course of the study of electrooxidation of methanol we have found that it is very difficult, if not impossible, to separate the effects of the bifunctional mechanism and the electronic structure effects that might play a role in the activity. However, data for electrooxidation of formic acid, when studied at a short reaction time (where the indirect reaction pathways and poisoning intermediates are assumed to play a negligible role), demonstrate a definitive contribution from electronic structure on the reactivity. The reactivity of the Pt/Ru nanoparticles toward formic acid electrooxidation is discussed in terms of the d-band center theory. 1. Introduction As a result of motivating factors such as cost, increasing energy demand, and environmental concerns, there is much interest in developing fuel cell technology, which has the potential to provide a cleaner and cheaper source of power. Direct liquid fuel cells, using methanol, ethanol, or formic acid, could be used as replacements for power sources in cell phones or laptop computers. 1-7 However, the high cost of platinum catalysts, slow reaction kinetics, poor selectivity, and catalyst poisoning have so far precluded the widespread use of fuel cells. Various bimetallic anode catalysts have been studied over the years in an effort to eliminate or reduce the negative effects of these issues. While not providing much benefit in terms of cost, Pt/Ru 8 has been shown to be effective in regards to the other factors, and is therefore one of the most commonly studied anode catalysts. It is generally accepted that in bimetallic catalysts, enhance- ment of catalytic activity, as compared to monometallic catalysts, can be attributed to a bifunctional mechanism 9,10 and/or an electronic effect. 11,12 Via enhanced oxidation, the bifunctional mechanism of bimetallic catalysts assists in the removal of surface poisoning species, such as carbon monoxide, with each metal in the alloy playing a separate role. The effects of the electronic structure, on the other hand, are still not completely understood. A fundamental insight, including theoretical un- derstanding of these mechanisms is necessary in order to design novel catalysts that can be used in new fuel cells. The density functional theory approaches developed by Nørskov et al. 13-20 have, for instance, indicated that changes in adsorption energies and activation barriers for reaction are directly linked to changes in position of the center of the metal/alloy d-band with respect to the Fermi level. The strength of binding of an atom or molecule to a surface depends on the degree of filling of the antibonding states between the two interacting species, and this, in turn, is dependent on the density of metal d states near the Fermi level. Consequently, one would expect to be able to tune the activity of a catalyst by inducing changes in its surface electronic structure. In an alloy, the surface electronic structure is affected by lattice strain 12,21-23 and charge transfer. 24 Ac- cording to Nørskov’s d-band center theory, these effects lead to narrowing or widening of the d-band and a subsequent shift in its center of gravity toward or away from the Fermi level to conserve energy and maintain a constant filling of the d-band. Conveniently, these shifts in the position of the center of the d-band are linked to shifts in core-level binding energies, making it possible to use core-level photoelectron spectroscopy to observe changes in surface electronic structure. 15,25-27 There has been much effort to verify the d-band center theory through experimental techniques. While the theory has been found to not hold for some systems, 28 there have been major successes in connecting changes in adsorption to changes in position of the center of the d-band. One such example is the oxidation (desorption) of surface CO, which could be a useful measure of catalytic activity. In the study of Pt/Ru nanoparticles containing CO, Tong et al. 29 found that the addition of ruthenium to platinum led to a decrease in the local density of states of platinum. This change in electronic structure, due to the addition of ruthenium, reduces the 2π* back-donation, and thereby * To whom correspondence should be addressed. E-mail: andrzej@ scs.uiuc.edu. † University of Illinois at Urbana-Champaign. ‡ University of Warsaw. § University of North Texas. | Darmstadt University of Technology and Bessy-II. ⊥ Present address: Chemistry Department, Brookhaven National Labora- tory, Upton, NY, 11973. J. Phys. Chem. C 2008, 112, 15595–15601 15595 10.1021/jp805374p CCC: $40.75  2008 American Chemical Society Published on Web 09/05/2008 Downloaded by UNIV OF PENN on September 16, 2009 | http://pubs.acs.org Publication Date (Web): September 5, 2008 | doi: 10.1021/jp805374p