Infrared spectroscopy of carbon monoxide and nitric oxide on palladium(111) in aqueous solution:: unexpected adlayer structural differences between electrochemical and ultrahigh-vacuum interfaces

Coverage-dependent infrared reflection-absorption spectra (IRAS) measured for carbon monoxide and nitric oxide adlayers dosed onto Pd(111) in acidic aqueous solution are examined in comparison with IRAS and other adlayer structural data available on Pd(111) in ultrahigh vacuum (UHV). Unlike most other ordered Pt-group surfaces: the coverage-dependent spectral fingerprints for both electrochemical adlayers on Pd(111) are markedly different from their UHV-based counterparts. These disparities are magnified after allowance is made for the differences in surface potentials, and hence interfacial electrostatic fields, between the electrochemical and UHV-based systems. Specifically, at low dosed CO coverages (theta(CO)< 0.4) a single C-O stretching (nu(CO)) band is obtained at ca. 1830-1880 cm(-1). consistent with threefold-hollow coordination, which is replaced at higher coverages up to saturation (theta(CO) = 0.75) by a pair of bands at 1940-1960 and 1890-1920 cm(-1), suggestive of 'bridging-like' along with hollow CO binding. Unusually similar theta(CO)-dependent infrared spectra were obtained upon sequential partial electrooxidation of a saturated layer, indicating that any CO islands thereby forming are largely dissipated. While the infrared spectra are qualitatively similar to the behavior in UHV at low coverages, the electrochemical nu(CO) frequencies are significantly (30-40 cm(-1)) higher than expected, and the higher-frequency nu(CO) feature seen for theta(CO) > 0.4 has no obvious counterpart at the Pd(111)/UHV interface. These behavioral differences can be partly, yet not entirely, rationalized in terms of water coadsorption. Dosed NO adlayers exhibit largely a single N-O stretching (nu(NO)) band blueshifting by ca. 30 cm(-1) up to ca. 1750 cm(-1) at saturation, suggestive of largely atop NO coordination. The absence of the markedly lower-frequency nu(NO) bands seen at low dosages on Pd(111) in UHV is ascribed to partial segregation of NO and coadsorbed water, yielding higher local NO coverages. These unexpected nu(CO) and nu(NO) spectral properties at Pd(111) electrodes with respect to the corresponding UHV-based systems are also discussed in comparison with corresponding IRAS behavior for CO and NO adlayers on other low-index Pt-group surfaces. The coadsorption of CO and NO on Pd(111) is also briefly considered. The form of the nu(CO)/nu(NO) spectra indicate the presence of some CO segregation as well as CO/NO molecular intermixing. (C) 1999 Elsevier Science S.A. All rights reserved.