COLL 487 |
| V. Stamenkovic1, M. Arenz1, Philip N. Ross2, and Nenad Markovic2. (1) Lawrence Berkeley National Laboratory, Berkeley, CA 94720, (2) Material Sciences, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 |
| In the present paper, we attempt to provide the insight into the macroscopic level descriptions of the Pt3Sn(111) surface in the electrochemical environment by a combination of ex-situ low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and low energy ion scattering (LEIS) and in-situ surface X-ray scattering (SXS) and Fourier transform infrared (FTIR) spectroscopy. In ultra high vacuum (UHV) the clean-annealed surface produces a p(2 x 2) LEED pattern consistent with the surface composition, determined by LEIS, of 25 at% Sn. SXS results show that the p(2 x 2) structure can be “transferred” from UHV into 0.5 M H2SO4 and that the surface structure remains stable from 0.05 to 0.8 V. At 0.05 V the expansion of Pt surface atoms, ca. +2% from the bulk lattice spacing, is induced by adsorption of underpotential deposited (UPD) hydrogen. At 0.5 V, where Pt atoms are covered by (bi)sulfate anions, the topmost layer is contracted relative to 0.05 V, although Sn atoms expend significantly, ca 8.5%. The p(2 x 2) structure is stable even in solution containing CO. In contrast to the Pt(111)-CO system, no ordered structures of CO are formed on the Pt3Sn(111) surface and the topmost layer expands relatively little (ca. 1.5% ) from the bulk lattice spacing upon the adsorption of CO. The binding site geometry of CO on Pt3Sn(111) is determined by FTIR. In contrast to the near invariant band of a-top CO on Pt(111), changes in bandshape (splitting of the band) and frequency (increase in the frequency mode) are clearly visible on the Pt3Sn(111) surface. In order to explain the lineshape of the CO bands we suggest that in addition to alloying effects, other factors, such as intermolecular repulsion between co-adsorbed CO and OH species, are controlling segregation of CO into cluster domains where the local CO coverage is different from the coverage expected for the CO-CO interaction on an unmodified Pt(111) surface |
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Electrochemistry and Surface Science Symposium in Memory of Mike Weaver
Division of Colloid and Surface Chemistry |