BIMETALLIC ELECTROCHEMICAL INTERFACES - CARBON-MONOXIDE COADSORBED WITH COPPER AND LEAD ON ORDERED LOW-INDEX RHODIUM SURFACES

The influence of predosed underpotential deposited (UPD) copper and lead upon carbon monoxide adsorbed on ordered Rh(111), (100), and (110) electrodes in aqueous 0.1M HClO4 has been examined by means of in-situ FTIR spectroscopy in the C-O stretching (nu(CO)) frequency region. The presence of submonolayer coverages of Cu and Pb results in roughly commensurate decreases in the saturated CO coverage. Nevertheless, even low copper coverages on Rh(100) diminish markedly the intensities of the terminal and bridging nu(CO) features, attributed to screening of neighboring C-O dipoles by the metal adlayer atoms. On Rh(110), copper attenuates preferentially the bridging nu(CO) feature, signaling the formation of a microscopically intermixed Cu-CO adlayer. While the behavior of the Pb-CO and Cu-CO adlayers on Rh(111) does not differ greatly, the presence of lead on Rh(100) and especially (110) blocks multifold sites in a highly selective fashion as discerned from the loss of the bridging nu(CO) feature. The observed greater tendencies to form intermixed adlayers for Pb-CO than for Cu-CO, and in the crystallographic sequence Rh(111) < (100) < (110), are consistent with steric and electronic expectations.