Abstract
Although significant progress has been made over the last couple of years, acidic polymer electrolyte membrane fuel cells (PEMFC) still require a substantial amount of the precious metal platinum, which renders them too costly for their global commercialization. State-of-the-art electrocatalysts are Pt nanoparticles supported on low-cost and electron conducting carbon substrates, such as Vulcan XC-72. However, these Pt-C catalysts are not sufficiently stable during long-term fuel cell operation due to carbon corrosion at the cathode side and poisoning with carbon monoxide contained as a trace gas in the industrial fuel at the anode side. That is why a lot of effort has been dedicated to develop alternative support materials such as oxides, e.g. titania or Sb- and In-doped tin oxide. In contrast to carbon supports, oxides provide an inert substrate, which can also tune the catalysts’ CO tolerance, as indicated by a strong negative shift of the oxidation peak in classical electrochemical CO stripping experiments. The lowered CO poisoning tendency can be due to various reasons, for instance a) support material induced changes in the electronic structure of the catalytically active Pt centers reducing the CO binding strength, or b) a so-called bifunctional effect, in which the support material provides oxygen-containing functional groups accelerating the oxidative removal of adsorbed CO. It is, however, not an easy task to unravel whether an electronic and/or a bifunctional effect is responsible for the enhanced CO tolerance. In this paper, we use CO as a probe molecule to study the Pt nanoparticle-support interaction of various oxide-supported catalysts. We attempt to separate the electronic and the bifunctional contribution by using electrochemical CO stripping experiments and in-situ gas phase diffuse reflectance infrared absorption Fourier transform spectroscopy (DRIFTS). Hence, we compare the CO vibrational frequency of gas phase adsorbed carbon monoxide, sensitive to electronic changes, with the position of the CO stripping peak. Furthermore, we suggest an alternative solution to the peak multiplicity puzzle occurring in electrochemical CO stripping [1] by considering the presence of bridge- bonded CO, known from in-situ gas phase spectroscopy [2-4], also in an aqueous environment. [1] P. Urchaga, S. Baranton, C. Coutanceau, and G. Jerkiewicz, “Electro-oxidation of CO chem on Pt nanosurfaces: Solution of the peak multiplicity puzzle,” Langmuir, vol. 28, no. 7, pp. 3658–3663, 2012. [2] A. Bourane, O. Dulaurent, and D. Bianchi, “Heats of Adsorption of Linear and Multibound Adsorbed CO Species on a Pt/ Al2O3 Catalyst Using in Situ Infrared Spectroscopy under Adsorption Equilibrium,” vol. 125, pp. 115–125, 2000. [3] J. Xu, J. T. Y. Jr, J. Xu, and J. T. Yates, “Catalytic oxidation of CO on Pt ( 335 ): A study of the active site,” vol. 725, no. 335, 1998. [4] A. M. B. Heyden, B. E., “The Adsorption Of CO On Pt(111) Studied By Infrared Reflection-Absorption Spectroscopy,” Surf. Sci., vol. 125, pp. 787–802, 1983. Figure 1
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