THE STRUCTURE OF ADSORBED BROMIDE CONCURRENT WITH THE UNDERPOTENTIAL DEPOSITION (UPD) OF CU ON PT(111)

The adsorption of bromide anions concurrent with the underpotential de position (UPD) of Cu on Pt(111) was examined utilizing ex-situ LEED an d in-situ surface X-ray scattering for structure determination, in com bination with coverage determination by rotating ring disk flux measur ements with the Pt(111) single crystal as the disk electrode. The resu lts show definitively that Cu UPD on Pt(111) in the presence of bromid e is a two-step process, with the total amount of Cu deposited at unde rpotentials similar to 0.95 +/- 5% ML (1 ML=1 adatom per Pt atom). The results also indicate that the surface coverage by adsorbed bromide u ndergoes only a small (<0.05 ML) change upon the deposition of Cu, eve n up to a nominal monolayer. We propose a model wherein the first stag e of deposition occurs by displacement of the close-packed hexagonal l ayer of Pr adatoms by Cu adatoms through a ''turn-over'' process, in w hich Cu is sandwiched between the Pt surface and the Pr overlayer, lea ding to the formation of an ordered Pt(111)-Cu-Br bilayer intermediate phase which closely resembles the (111) planes of the Cu(I)Br crystal . The coverage of both Cu and Pr in this intermediate phase is similar to 0.5 ML. The second stage is the filling-in of the Cu plane of the bilayer to form a pseudomorphic (1 x 1) Cu monolayer and a disordered Pr adlayer with a coverage of similar to 0.4 ML. The same mechanism is suggested for Cu UPD on Pt(111) in solutions containing chloride anio ns.