Interactions within mixed NO/CO adlayers at the Pt(100)–aqueous electrochemical interface as probed by infrared spectroscopy

Abstract

In situ infrared reflection–absorption spectra are reported for NO/CO adlayers on Pt(100) in aqueous 0.1 M HClO 4 as a function of the coadsorbate composition and electrode potential, E, and compared with corresponding coverage-dependent spectra for pure NO and CO layers with the objective of elucidating the microscopic mixed adlayer structure. The spectra for irreversibly adsorbed NO at 0.3 V feature a single N–O stretching ( ν NO) band at 1590–1625 cm −1, the frequency upshifting with increasing coverage, θ NO. The ν NO frequencies at a given θ NO value are sensitive to the electrode potential, d ν NO/d E decreasing towards higher coverage, from 90 cm −1 V −1 (at θ NO=0.2) to 40 cm −1 V −1 at saturation ( θ NO≈0.5). The frequencies as well as the form of the electrochemical ν NO spectra are closely similar to corresponding published infrared data on Pt(100) in ultrahigh vacuum at 300 K when the comparison is made at equivalent surface potentials. As reported previously, CO adsorption on Pt(100) in 0.1 M HClO 4 yields C–O stretching ( ν CO) bands located at 2015–2060 and 1800–1870 cm −1, attributed to atop and bridging coordination, respectively, the relative band intensities being sensitive to both the CO coverage and electrode potential. The mixed adlayers were formed by exposing a saturated NO layer to solution CO at 0.2–0.3 V vs. Ag/AgCl. They are stable over the potential range ca. 0.1–0.35 V, higher and lower potentials yielding CO electrooxidation and NO electroreduction, respectively. Replacement of NO by CO at a given potential yields progressive downshifts in the ν NO frequency, the ν NO– θ NO dependence being similar to that observed for NO adsorption alone. This coadsorbate behavior, along with related changes in the ν CO spectra, a strongly increased preference for atop CO binding, and intensity-transfer from the ν NO to the ν CO bands, is indicative of microscopic intermixing of the NO and CO components. Some aspects of the ν NO and ν CO spectral behavior, however, suggest that small NO aggregates may form at intermediate mixed NO/CO compositions. Unexpectedly small (<10 cm −1 V −1) ν NO– E and ν CO– E “Stark-tuning” slopes are observed under these conditions. The broader significance of such coadsorption effects to the elucidation of chemisorbate properties at electrochemical interfaces is also noted.