The Effects of Bromide Adsorption on the Underpotential Deposition of Copper at the Pt(111)-Solution Interface

The Pt(111)-Cu-Br system was examined by utilizing ex-situ UHV and in-situ surface x-ray scattering (SXS) spectroscopic techniques in combination with the rotating ring-disk measurements. In copper free solution at 0.2 V, we observe a hexagonal, close-packed bromide monolayer that is incommensurate, but aligned with the underlying platinum lattice. The nearest neighbor distance is a continuous function of the electrode potential; bromide adsorption begins at ca. -0.15 V and reaches a coverage of ≈ 0.42 ± 5% ML at 0.5 V. Cu UPD on Pt(111) in the presence of bromide is a two-step process, with the total amount of Cu deposited at underpotentials ≈ 0.95 ± 5% ML. The RRDE analysis shows that Br coverage changes relatively little (< 0.05 ML) over the entire Cu UPD region. We propose a model wherein the first stage of deposition occurs by displacement of close packed Br ad-species by Cu adatoms through a "turn-over" process in which Cu is sandwiched between the Pt surface and the Br overlayer, leading to the formation of an ordered Pt(111)-Cu-Br bilayer intermediate phase that closely resembles the (111) planes of the Cu(I)Br crystal. The coverage of both Cu and Br in this intermediate phase is ≈ 0.5 ML. The second stage is the filling-in of the Cu plane of the bilayer to form a pseudomorphic (1 × 1) Cu monolayer and a disordered Br adlayer with a coverage of ≈ 0.4 ± 5% ML.