© Copyright 1997 American Chemical Society
OCTOBER 15, 1997 VOLUME 13, NUMBER 21
Letters Underpotential Deposition of Lead onto Pt(001): Interface Structure and the Influence of Adsorbed Bromide C. A. Lucas,*,†,‡ N. M. Markovic,† and P. N. Ross† Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, and Department of Physics, Oliver Lodge Laboratory, University of Liverpool, Liverpool, L69 7ZE, U.K. Received March 13, 1997. In Final Form: June 30, 1997X Using in-situ X-ray diffraction, we have studied the underpotential deposition of Pb onto a Pt(001) electrode both in pure perchloric acid and in the presence of bromide anions. In pure perchloric acid, the Pb adlayer forms monolayer-high islands which fully cover the Pt surface but have no long range order in the surface plane. The presence of strongly adsorbed Br atoms at step edges on the Pt(001) surface and the competitive nature of the Pb-Br interaction cause a rapid exchange of Pb and Br on the Pt terraces, the Pb then forming a strongly bonded c(2×2) adlayer.
1. Introduction The underpotential deposition (UPD) of Pb onto Pt(hkl) electrodes has been studied in some detail due to the enhancement of the catalytic activity of certain reactions produced by submonolayer Pb deposits.1,2 Most studies have concentrated on the Pt(111) surface. A number of submonolayer structures at varying Pb coverage were observed by ex-situ low-energy electron diffraction (LEED)3 for Pt(111) pretreated with iodine. More recently, Adzic et al.4 studied the UPD of Pb onto Pt(hkl) in-situ, combining X-ray diffraction and scanning tunneling microscopy * To whom correspondence may be addressed at the University of Liverpool. † University of California. ‡ University of Liverpool. X Abstract published in Advance ACS Abstracts, September 15, 1997. (1) Adzic, R. R. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Ed.; Wiley: New York, 1994; Vol. 13. (2) Borup, R. L.; Sauer, D. E.; Stuve, E. M. Surf. Sci. 1993, 293, 10. Clavilier, J.; Orts, J. M.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1990, 293, 197. (3) Schardt, B. C.; Stickney, J. L.; Stern, D. A.; Wieckowski, A.; Zapien, D. C.; Hubbard, A. T. Surf. Sci. 1996, 175, 520. (4) Adzic, R. R.; Wang, J.; Vitus, C. M.; Ocko, B. M. Surf. Sci. 1993, 293, L976.
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(STM) measurements. In pure perchloric acid, they found that for a sufficiently high coverage of Pb on Pt(111), the Pb monolayer formed a (3×x3) rectangular structure with four Pb atoms in the unit cell. As the electrode potential was scanned positively, the (3×x3) structure underwent an order-disorder transition and no other ordered structures were observed. Using X-ray diffraction and rotating ring disk electrode (RRDE) measurements, we also studied the UPD of Pb onto Pt(111) but in solution containing bromide ions.5 Our results showed that in the potential range of Pb UPD, adsorbed bromide remained on the surface and in some cases could induce the (slow) formation of a p(2×2) structure.6 Pb UPD onto Pt (001) has been less studied than Pt(111). Aberdam et al.7 reported exsitu LEED and Auger analysis of Pb UPD onto Pt(001) in 0.1 M HClO4 in which they observed a c(2×2) superstructure, although the LEED pattern had a strong diffuse background which they attributed to Pb vacancies. The more recent in-situ X-ray diffraction and STM study by (5) Markovic, N. M.; Grgur, B. N.; Lucas, C. A.; Ross, P. N. J. Electroanal. Chem., in press. (6) At present we do not have a definitive structural model for the p(2×2) structure. (7) Aberdam, D.; Traore, S.; Durand, R.; Faure, R. Surf. Sci. 1987, 190, 319.
© 1997 American Chemical Society
5518 Langmuir, Vol. 13, No. 21, 1997
Letters
Adzic et al.4 did not confirm the ex-situ LEED structures. Adzic et al. were unable to detect X-ray scattering from any ordered superstructures on Pt(001) and attributed the LEED results to the loss of potential control and/or the desorption of water. The corresponding STM results showed that for a monolayer coverage of Pb, the Pb adatoms form islands which cover the entire Pt surface. In this Letter we present new X-ray scattering results for the UPD of Pb onto Pt(001). In pure 0.1 M HClO4 supporting electrolyte, we confirm the finding by Adzic et al.4 of an absence of any ordered superstructures for the Pb adlayer. From crystal truncation rod (CTR) measurements we derive the Pb coverage and Pt-Pb bond length. With bromide ions in solution we report a “surfactant effect”8 of the adsorbed Br, which induces the formation of an ordered commensurate c(2×2) structure for the Pb monolayer. 2. Experimental Section The Pt(001) crystal (miscut
