INTERACTIONS WITHIN MIXED NO CO ADLAYERS AT THE PT(100)-AQUEOUS ELECTROCHEMICAL INTERFACE AS PROBED BY INFRARED-SPECTROSCOPY/

In situ infrared reflection-absorption spectra are reported for NO/CO adlayers on Pt(100) in aqueous 0.1 M HClO4 as a function of the coadso rbate composition and electrode potential, E, and compared with corres ponding 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 str etching (nu(NO)) band at 1590-1625 cm(-1), the frequency upshifting wi th increasing coverage, theta(NO). The nu(NO) frequencies at a given t heta(NO) value are sensitive to the electrode potential, d nu(NO)/dE d ecreasing towards higher coverage, from 90 cm(-1) V-1 (at theta(NO) = 0.2) to 40 cm(-1) V-1 at saturation (theta(NO) approximate to 0.5) The frequencies as well as the form of the electrochemical nu(NO) spectra are closely similar to corresponding published infrared data on Pt(10 0) in ultrahigh vacuum at 300 K when the comparison is made at equival ent surface potentials. As reported previously, CO adsorption on Pt(10 0) in 0.1 M HClO4 yields C-O stretching (nu(CO)) bands located at 2015 -2060 and 1800-1870 cm(-1), attributed to atop and bridging coordinati on, respectively, the relative band intensities being sensitive to bot h the CO coverage and electrode potential. The mixed adlayers were for med by exposing a saturated NO layer to solution CO at 0.2-0.3 V vs. A g/AgCl. They are stable over the potential range ca. 0.1-0.35 V, highe r and lower potentials yielding CO electrooxidation and NO electroredu ction, respectively. Replacement of NO by CO at a given potential yiel ds progressive downshifts in the nu(NO) frequency, the nu(NO)-theta(NO ) dependence being similar to that observed for NO adsorption alone. T his coadsorbate behavior, along with related changes in the nu(CO) spe ctra, a strongly increased preference for atop CO binding, and intensi ty-transfer from the nu(NO) to the nu(CO) bands, is indicative of micr oscopic intermixing of the NO and CO components. Some aspects of the n u(NO) and nu(CO) spectral behavior, however, suggest that small NO agg regates may form at intermediate mixed NO/CO compositions. Unexpectedl y small (<10 cm(-1) V-1) nu(NO)-E and nu(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. (C) 1998 Elsevier Science B .V. All rights reserved.