Electrochemical infrared studies of monocrystalline iridium surfaces. Part 2: Carbon monoxide and nitric oxide adsorption on Ir(110)

The adsorption of carbon monoxide and nitric oxide on an ordered Ir(110) electrode surface in aqueous 0.1 M HClO4 has been probed by voltammetry together with in-situ infrared reflection-absorption spectroscopy (IRAS). Exclusively atop coordination of both CO and NO is suggested from the relatively high C-O and N-O stretching (nu(CO), nu(NO)) frequencies observed, 1980-2060 and 1820-1840 cm(-1) respectively, that upshift with increasing coverage. Adsorption of NO as well as CO is essentially molecular, with near-unity saturation coverages, as deduced from voltammetry as well as infrared spectrophotometry. The potential-dependent nu(CO) frequencies for the saturated CO adlayer are closely compatible with that for the corresponding Ir(110)/CO interface in ultrahigh vacuum (UHV) once the differences in surface potential are taken into account. In contrast to the case of the latter system, however, the electrochemical lr(110)/CO interface exhibits a pair of nu(CO) bands at intermediate CO coverages (theta(CO)), suggestive of a difference in substrate-induced adlayer domains in the two environments. Closely similar theta(CO)-dependent vco spectra and voltammetric oxidation profiles were obtained for adlayers formed by either partial electroxidative stripping from a saturated adlayer or by direct dosing from a dilute CO solution. This unusual behavior indicates that extensive CO "islands" are not formed by partial adlayer electrooxidation, in contrast to the behavior of most ordered low-index Pt-group electrodes, suggesting that the substrate morphology features nanoscale domains rather than large terraces. The nu(CO) and nu(NO) frequencies for saturated adlayers on Ir(110) and (111) are similarly red-shifted from the gas-phase nu(CO) and nu(NO) values. However, the nu(CO)-E and especially the nu(NO)-E dependences ("Stark-tuning" slopes) are markedly larger than the predicted gas-phase values. The larger d nu(NO)/dE values are ascribed to more extensive potential-dependent d pi-2 pi* back-donation for adsorbed atop NO compared with CO.