ELECTROCHEMICAL INFRARED STUDIES OF MONOCRYSTALLINE IRIDIUM SURFACES - 3 - ADSORBED NITRIC-OXIDE AND CARBON-MONOXIDE AS PROBES OF IR(100) INTERFACIAL STRUCTURE

The adsorption of carbon monoxide and nitric oxide on Ir(100) in acidi c aqueous solutions has been probed by voltammetry together with in-si tu infrared reflection-absorption spectroscopy, with the objective of assessing the substrate as well as adlayer structure and bonding. The ordered Ir(100) surface was prepared by flame annealing followed by co oling in a H-2/Ar stream. Similar to, and consistent with, the reporte d behavior in ultrahigh vacuum (UHV), at potentials E greater than or equal to 0 V vs SCE (i.e., in the presence of coadsorbed water) the C- O stretch (nu(CO)) for adsorbed CO occurs at frequencies, 1970-2040 cm (-1), that are indicative of atop (or near-atop) surface coordination. At lower potentials where coadsorbed hydrogen is present, however, th e nu(CO) spectra at intermediate CO coverages (0.2 < theta(co) < 0.5) include a lower-frequency band suggestive of bridge-bonded CO. The vol tammetric features for hydrogen adsorption-desorption, dominated by a sharp current-potential peak at -0.04 V, are consistent with the forma tion of localized H islands in the presence as well as absence of coad sorbed CO. The nu(CO) spectral form at intermediate CO coverages, howe ver, indicates that some CO/H intermixing is also present. The occurre nce of partial NO dissociative chemisorption is evident from the volta mmetry for irreversibly adsorbed layers. This is also consistent with the N-O stretching (nu(NO)) infrared spectra which exhibit relatively weak bands at frequencies ca. 1620-1650 and 1790-1810 cm-' suggestive of the presence of bridging as well as atop NO. The infrared spectral as well as the voltammetric findings are diagnostic of the presence of an unreconstructed Ir(100) surface by comparison with the reported be havior for the (1 x 1) and hexagonal reconstructed forms of Ir(100) in UHV.