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Electrochemical Infrared Studies of Monocrystalline Iridium Surfaces. Part 2: Carbon Monoxide and Nitric Oxide Adsorption on Ir(110)

1998; American Chemical Society; Volume: 14; Issue: 9 Linguagem: Inglês

10.1021/la9711692

1520-5827

Roberto Gómez, Michael J. Weaver,

Advanced Materials Characterization Techniques

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 (νCO, νNO) 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 νCO 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 Ir(110)/CO interface exhibits a pair of νCO bands at intermediate CO coverages (θCO), suggestive of a difference in substrate-induced adlayer domains in the two environments. Closely similar θCO-dependent νCO 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 νCO and νNO frequencies for saturated adlayers on Ir(110) and (111) are similarly red-shifted from the gas-phase νCO and νNO values. However, the νCO−E and especially the νNO−E dependences ("Stark-tuning" slopes) are markedly larger than the predicted gas-phase values. The larger dνNO/dE values are ascribed to more extensive potential-dependent dπ−2π* back-donation for adsorbed atop NO compared with CO.