IN-SITU FOURIER-TRANSFORM INFRARED SPECTROELECTROCHEMICAL STUDY OF BISULFATE AND SULFATE ADSORPTION ON GOLD, WITH AND WITHOUT THE UNDERPOTENTIAL DEPOSITION OF COPPER

In situ surface infrared (IR) spectroelectrochemistry is used to investigate the adsorption of sulfate (SO42-) and bisulfate (HSO4-) ions on polycrystalline gold surfaces in sodium sulfate and sulfuric acid, and also during copper underpotential deposition in sulfuric acid medium. In sodium sulfate solution, IR peaks due to surface sulfate and bisulfate are observed at potentials within the double-layer region on gold, and the ratio of IR peak intensities for sulfate to bisulfate increases as the applied potential is made more negative. In sulfuric acid, surface IR spectra indicate that adsorbed sulfate is favored at more positive potentials, while adsorbed bisulfate is prevalent at more negative voltages; also, a potential-dependent reorientation of water is observed in the spectra. IR spectroslectrochemical data from a sulfuric acid system containing copper sulfate indicate that adsorbed sulfate is present on gold at more positive potentials, and its coverage increases when underpotentially deposited (UPD) copper is present on the gold substrate surface. When the applied potential is sufficiently negative to effect full discharge of UPD copper, IR spectroelectrochemistry shows a loss of adsorbed sulfate and an increase in surface bisulfate species, with a concomitant reorientation of adsorbed water molecules. The results are explained in terms of surface electrostatic considerations during UPD of copper, and also in terms of potential-dependent pH changes at the interface. To our knowledge, this is the first reported infrared spectroelectrochemical study of interfacial changes occurring during metal underpotential deposition processes.