Electrochemical Infrared Studies of Monocrystalline Iridium Surfaces. 3. Adsorbed Nitric Oxide and Carbon Monoxide as Probes of Ir(100) Interfacial Structure

Abstract

The adsorption of carbon monoxide and nitric oxide on Ir(100) in acidic aqueous solutions has been probed by voltammetry together with in-situ 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 cooling in a H2/Ar stream. Similar to, and consistent with, the reported behavior in ultrahigh vacuum (UHV), at potentials E ≥ 0 V vs SCE (i.e., in the presence of coadsorbed water) the C−O stretch (νCO) 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, the νCO spectra at intermediate CO coverages (0.2 < θCO < 0.5) include a lower-frequency band suggestive of bridge-bonded CO. The voltammetric features for hydrogen adsorption−desorption, dominated by a sharp current−potential peak at −0.04 V, are consistent with the formation of localized H islands in the presence as well as absence of coadsorbed CO. The νCO spectral form at intermediate CO coverages, however, indicates that some CO/H intermixing is also present. The occurrence of partial NO dissociative chemisorption is evident from the voltammetry for irreversibly adsorbed layers. This is also consistent with the N−O stretching (νNO) infrared spectra which exhibit relatively weak bands at frequencies ca. 1620−1650 and 1790−1810 cm-1, 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 behavior for the (1 × 1) and hexagonal reconstructed forms of Ir(100) in UHV.