Nitric oxide and carbon monoxide adsorption on polycrystalline iridium electrodes: A combined Raman and infrared spectroscopic study

The adsorption of nitric oxide and carbon monoxide on polycrystalline iridium in aqueous 0.1 M HClO4 was probed by in-situ surface-enhanced Raman spectroscopy (SERS) and infrared reflection-absorption spectroscopy (IRAS) with the primary objective of probing the molecular and dissociative chemisorption of the former adsorbate in this electrochemical environment. The latter adsorbate was examined partly as a means of characterizing the microscopic nature of the iridium electrode, given the well-documented sensitivity of the CO intramolecular vibration to the surface structure. The SERS vibrational technique was harnessed by utilizing ultrathin iridium films electrodeposited on a gold substrate. The availability of this method enables vibrational features associated with surface-adsorbate as well as intramolecular bonds to be scrutinized, facilitating detection of adsorbed NO fragments. Saturation adsorption of CO yields a single C-O stretching (nu(CO)) band at ca. 2040-2070 cm(-1) in both the Raman and infrared spectra, suggestive of exclusive atop (or near-atop) binding as on monocrystalline Ir surfaces. Significant (15-20 cm(-1)) discrepancies in the potential-dependent nu(CO) frequencies measured by SERS and IRAS are evident, however, indicating that the ensemble distribution of iridium surface microenvironments sensed by these techniques is dissimilar. However, the nu(CO) frequencies are approximately consistent with those evaluated for corresponding iridium-vacuum interfaces once the differences in surface potentials are taken into account. In contrast to the nu(CO) band, the frequency of the metal-CO (nu(M-CO)) vibration decreases with increasing electrode potential, in harmony with SERS findings on other transition-metal surfaces and theoretical bonding expectations. Adsorption of NO yields a weaker N-O stretching (nu(NO)) band at 1780-1810 cm(-1) in both the SERS and IRAS spectra, indicative of the presence of (probably atop) molecular NO chemisorption. The SERS lower-frequency region is dominated by a well-defined band at 570 cm(-1), attributed to a metal-oxygen stretch from the chemisorbed oxygen fragment formed by NO dissociation. This assignment arises in part from the markedly different potential-dependent stability as well as intensity of the 570 cm(-1) band in comparison with the nu(NO) feature. Supporting evidence includes the observation of a near-identical vibrational band upon oxygen dissociation (and NO adsorption) on Ir surfaces in vacuum and for the present Ir films upon gas-phase O-2 dosing. While the extent of NO dissociation on Ir cannot be estimated quantitatively, consideration of band intensities suggests that the coverages of molecular NO and the atomic-oxygen fragment are not greatly different. Comparisons are briefly made with NO adsorption on other transition-metal surfaces in electrochemical and vacuum environments.