Infrared spectroscopic studies of bimetallic electrochemical interfaces: Carbon monoxide coadsorbed with metal overlayers on ordered low-index rhodium and platinum surfaces

Abstract

The influence of underpotential deposited (upd) copper on Rh(111) and (110) electrodes and of predosed antimony on Pt(111) electrodes upon coadsorbed carbon monoxide in aqueous 0.1 M HClO4 has been examined by means of in situ Fourier transform infrared spectroscopy in the C-O stretching (νCO) frequency region as illustrative examples of the use of infrared spectra to discern microscopic interactions in such binary electrochemical systems. Saturation dosages of CO in the presence of small upd copper coverages on Rh(111) yielded segregated coadsorbate domains as evidenced by νCO spectra. In contrast, the corresponding data for CO and copper on Rh(110) are consistent with the presence of intermixed adlayer structures as indicated by the marked decreases in the terminal νCO frequency and the selective removal of the CO bridge band. The irreversibly adsorbed nature of antimony adlayers on Pt(111) enabled the effect of known and adjustable metal coverages, θSb, upon the νCO spectra to be examined for various postdosed CO coverages. Increasing θSb yields approximately commensurate decreases in the saturation CO coverage, along with a progressive shift in site occupancy from twofold bridging to terminal coordination as deduced from the relative infrared band intensities. At the saturation antimony coverage, θSb=0.3, the CO binding is exclusively terminal. These results are consistent with adsorption of antimony chiefly in bridging sites, yielding a microscopically intermixed adlayer with CO. The observed greater tendencies to form intermixed adlayers for Sb–CO than for Cu–CO, and on (110) vs (111) surfaces are consistent with steric and electronic expectations.