Infrared Spectroscopy of Mixed Nitric-Oxide−Carbon-Monoxide Adlayers on Ordered Iridium(111) in Aqueous Solution:  A Model Study of Coadsorbate Vibrational Interactions

In-situ infrared reflection−absorption spectra are reported for mixed nitric-oxide−carbon-monoxide adlayers along with the constituent chemisorbates separately as a function of coverage on ordered Ir(111) at 0.4−0.45 V vs standard hydrogen electrode in aqueous 0.1 M HClO<INF>4</INF>, with the objective of assessing the composition-dependent nature of the coadsorbate vibrational interactions. This substrate−coadsorbate combination provides an informative model system since both chemisorbates appear to bind exclusively in atop (or near-atop) surface sites on the basis of their simple N−O (ν<INF>NO</INF>) and C−O (ν<INF>CO</INF>) vibrational fingerprints, and composition-dependent mixed adlayers can readily be formed via partial replacement of saturated irreversible adsorbed NO layers by exposure to dilute CO solutions. Increasing the CO coverage, θ<INF>CO</INF>, both in the absence and presence of coadsorbed NO, yields marked progressive blueshifts in the ν<INF>CO</INF> band frequency (ca. 2020−2075 cm^-1), attributable chiefly to enhanced dipole−dipole coupling. While adsorption of NO alone exhibited virtually coverage (θ<INF>NO</INF>)-independent ν<INF>NO</INF> frequencies, ca. 1835 cm^-1, indicative of chemisorbate island formation, dilution within mixed CO/NO adlayers yields progressive ν<INF>NO</INF> redshifts (down to ca. 1790 cm^-1). The composition-dependent ν<INF>CO</INF> and ν<INF>NO</INF> frequencies within the CO/NO adlayers are consistent with molecular intermixing, as supported by comparison with numerical simulations extracted from conventional dipole-coupling theory, although the observed nonlinear ν<INF>CO</INF>−θ<INF>CO</INF> dependence suggests the formation of locally enriched CO regions at intermediate compositions. Evidence supporting coadsorbate intermixing is obtained by comparing the composition-dependent CO and NO band absorbances with the dipole-coupling predictions. In particular, the presence of coadsorbed CO yields marked (up to 3-fold) decreases in the NO band absorbance, especially toward lower ν<INF>NO</INF> values, which arise from band-intensity transfer to neighboring higher-frequency (CO) oscillators. Despite the large (ca. 250 cm^-1) difference in ν<INF>NO</INF> and ν<INF>CO</INF> singleton frequencies, this striking effect is in approximate agreement with dipole-coupling theory, again presuming molecular CO/NO intermixing. The observed marked increases in the ν<INF>CO</INF> bandwidth toward lower θ<INF>CO</INF> values, along with pronounced asymmetric band shapes, are in good agreement with theoretical predictions that include stochastic fluctuations of the local adsorbate population density. Moreover, the larger intermediate-θ<INF>CO</INF> ν<INF>CO</INF> bandwidths observed in the presence of coadsorbed NO are also quantitatively accounted for on this basis in terms of coadsorbate intensity transfer. The more broad-based utility of such dipole-coupling analyses for elucidating local interactions within mixed adlayers is considered in light of these findings.