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(11 0) electrode surface in aqueous 0.1 M HClO4 has been probed by voltamm etry 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)) frequ encies observed, 1980-2060 and 1820-1840 cm(-1) respectively, that ups hift with increasing coverage. Adsorption of NO as well as CO is essen tially molecular, with near-unity saturation coverages, as deduced fro m voltammetry as well as infrared spectrophotometry. The potential-dep endent nu(CO) frequencies for the saturated CO adlayer are closely com patible with that for the corresponding Ir(110)/CO interface in ultrah igh vacuum (UHV) once the differences in surface potential are taken i nto account. In contrast to the case of the latter system, however, th e 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 pro files were obtained for adlayers formed by either partial electroxidat ive stripping from a saturated adlayer or by direct dosing from a dilu te CO solution. This unusual behavior indicates that extensive CO ''is lands'' are not formed by partial adlayer electrooxidation, in contras t to the behavior of most ordered low-index Pt-group electrodes, sugge sting that the substrate morphology features nanoscale domains rather than large terraces. The nu(CO) and nu(NO) frequencies for saturated a dlayers on Ir(110) and (111) are similarly red-shifted from the gas-ph ase nu(CO) and nu(NO) values. However, the nu(CO)-E and especially the nu(NO)-E dependences (''Stark-tuning'' slopes) are markedly larger th an the predicted gas-phase values. The larger d nu(NO)/dE values are a scribed to more extensive potential-dependent d pi-2 pi back-donation for adsorbed atop NO compared with CO.