INFLUENCE OF COADSORBED BISMUTH AND COPPER ON CARBON-MONOXIDE ADLAYER STRUCTURES AT ORDERED LOW-INDEX PLATINUM AQUEOUS INTERFACES

The influence of predosed bismuth and underpotential deposited (UPD) copper on the adlayer structure of carbon monoxide on ordered Pt(111), (100), and (110) electrodes in 0.1 M HClO4 has been examined by means of in situ FTIR spectroscopy in the C-O stretching (nu(CO)) region. The irreversibly adsorbed and electroactive nature of the bismuth adlayers enabled the effect to be examined of known and adjustable metal coverages, theta-Bi, upon the nu-CO spectra for various dosed CO coverages, theta-CO. On Pt(111), increasing theta-Bi yields approximately commensurate decreases in the saturation CO coverage, along with a progressive shift in the CO site occupancy from twofold bridging to terminal coordination as deduced from the relative nu-CO band intensities. At the "saturation" bismuth coverage, theta-Bi(sat) = 0.21, the CO binding is exclusively terminal throughout the theta-CO range, 0.05 approximately-greater-than theta-CO < 0.37. These results are consistent with adsorption of bismuth chiefly in bridging sites, yielding a microscopically intermixed adlayer with CO. A similar effect of increasing theta-Bi upon theta-CO [up to theta-Bi(sat) = 0.43] is seen on Pt(100); the bridging nu-CO feature is again preferentially, yet incompletely, removed by theta-Bi(sat). These findings indicate that an intermixed layer is formed on Pt(100) with bismuth again present at twofold bridging sites. By contrast, saturated CO adlayers in the presence of UPD copper on Pt(111) form segregated domains as evidenced by nu-CO spectra. The latter results indicate the presence of compressed CO island structures even down to low theta-CO. Corresponding data for CO and copper on Pt(100), however, are consistent with the presence of intermixed structures, as deduced from the marked decreases in the terminal nu-CO frequency for larger copper coverages. Spectra for CO on Pt(110) with either coadsorbed bismuth or copper are also indicative of the presence of microscopically intermixed adlayers. The contrasting occurrence of segregated and intermixed copper/CO adlayers on Pt(111), and on Pt(100) and Pt(110), respectively, are consistent with the more open structure of the latter faces (cf. ref. [18]). For comparison, spectral data are also reported for CO/Bi adlayers on polycrystalline platinum.