In situ infrared spectra for dilute (i.e., low-coverage) carbon monoxide and nitric oxide adlayers on (111) and (100) platinum, rhodium, iridium and palladium surfaces in acidic aqueous solution are compared with vibrational spectra and other structural information for the corresponding systems in ultrahigh vacuum (UHV), with the objective of assessing the roles of the solvated double layer on the preferred binding sites and vibrational frequencies. Results are considered for eight CO and five NO adlayers, involving C-O (nu(CO)) and N-O (nu(NO)) stretching bands, respectively. The vibrational frequencies in the electrochemical and UHV environments are compared at equivalent surface potentials by adjusting (or extrapolating) the former data to electrode potentials equivalent to the work function (Phi) of the latter interfaces, presuming that the absolute potential of the standard hydrogen electrode is 4.8 V. Unlike the excellent agreement between such electrochemical and UHV-based chemisorbate frequencies, nu(Phi), obtained previously in this fashion for saturated CO and NO adlayers, the majority of dilute adlayer systems exhibit significant (greater than or equal to 20 cm(-1)) disparities in the nu(Phi) values. In some cases, notably for Pt(100)/CO, Pt(111)/CO, Rh(111)/CO and Pd(111)/NO, the nature and magnitude of the discrepancies indicate that the energetically preferred binding site is altered by the double-layer environment, most likely by chemisorbate-coadsorbed water interactions, along with the formation of segregated chemisorbate-water domains. The milder, yet still significant, disparities between electrochemical and UHV nu(Phi) values that are evident for other systems, prominently for Pd(111)/CO and Pd(100)/CO, suggest the occurrence of solvation-induced perturbations on the chemisorbate binding-site population and 'local' surface potentials. New electrochemical infrared data are also presented for the archetypical Pt(111)/CO system. At higher electrode potentials, bordering the onset of CO electrooxidation, the pair of low-coverage nu(CO) bands diagnostic of both atop and bridging coordination, reflecting the influence of water coadsorption, are replaced by a lone atop nu(CO) feature, the latter being similar to infrared spectra observed on clean Pt(111) in UHV. This unusual potential-dependent behavior is ascribed to the preferential electrooxidation of bridging CO. (C) 1999 Elsevier Science B.V. All rights reserved.
