Coadsorbate vibrational interactions within mixed carbon monoxide-nitric oxide adlayers on ordered low-index platinum-group electrodes

• Published in: Journal of electroanalytical chemistry (Lausanne, Switzerland) Vol. 467; no. 1; pp. 92 - 104
• Main Authors: Tang, Catherine, Zou, Shouzhong, Chang, Si-Chung, Weaver, Michael J
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 07.06.1999; Elsevier Science
• Subjects: Adsorption; Carbon monoxide; Chemistry; Electrochemistry; Exact sciences and technology; General and physical chemistry; IR spectroscopy; Nitric oxide; Platinum-group electrodes; Study of interfaces; Vibrational interactions; Platinum-group electrodes; Carbon monoxide; Vibrational interactions; IR spectroscopy; Nitric oxide; Crystal orientation; Coadsorption; Transition metal; Experimental study; Single crystal; Electrodes; Infrared spectrometry; Reflection spectrometry; Absorption spectrometry; Adsorption; Platinoid
• ISSN: 1572-6657; 1873-2569
• DOI: 10.1016/S0022-0728(98)00421-5

In-situ infrared reflection-absorption spectra are described for saturated mixed carbon monoxide-nitric oxide adlayers in comparison with CO and NO adsorbed separately on selected ordered Pt-group electrodes in aqueous solution in order to assess the nature of the coadsorbate vibrational interactions and hence the adlayer structure and bonding. Both chemisorbates exhibit pronounced intramolecular vibrational fingerprints ( ν CO, ν NO bands) that are sensitive to the local vibrational environment as well as the surface bonding geometry, enabling a microscopic-level assessment of the coadsorbate interactions in relation to CO and NO coadsorbed with water. The surfaces selected—Ir(110), Ir(111), Pt(100), Pt(111), Rh(100), Rh(111), and Pd(111)—yield near-exclusive molecular (rather than dissociative) NO adsorption, yet exhibit differing coverage-dependent NO, and especially CO, spectral and coordinative properties. Progressive displacement of NO by exposing NO-saturated electrodes to dilute CO solutions yielded chiefly molecularly intermixed CO/NO adlayers. This deduction is most straightforward (and quantitative) on Ir(110) and Ir(111), facilitated by the exclusively atop-like adsorption of CO and NO, as gleaned from the single ν CO and ν NO band frequencies. Both these features on Ir(110) redshift markedly (by 40–60 cm −1) upon dilution with either the other chemisorbate or with coadsorbed water. Such redshifts, along with the observed suppression of the ν NO band intensity in the CO/NO mixtures, arise chiefly from composition-dependent dynamic-dipole coupling within the intermixed dipolar adlayers, and are diagnostic of microscopic coadsorbate structure. Similar ν NO redshifts induced by dilution with coadsorbed CO are observed on each of the other surfaces. The composition-dependent analysis is facilitated by the observation of a lone coverage-dependent ν NO band, associated probably with multifold NO coordination, in both the absence and presence of coadsorbed CO in most cases. While corresponding effects upon the ν CO band frequencies are also observed, the spectra on Pt and Rh surfaces include a pair of ν CO bands with frequencies, ca. 1800–1850 and 2000–2070 cm −1, suggestive of bridging and atop-like CO coordination, respectively. On Rh(100), Rh(111), Pt(111) and especially Pt(100), NO coadsorption induces CO ‘bridging’ to ‘atop’ site switching. This is largely consistent with the formation of molecularly intermixed CO/NO adlayers where the strong preference of NO for multifold sites shifts CO molecules into neighboring atop-like configurations. On Pt(111), Rh(111), and Pd(111), however, the ν CO spectral fingerprints for the CO/NO adlayers suggest the additional presence of segregated compressed ‘CO-rich’ domains. Comparisons are made also with the behaviour of analogous CO/NO adlayers in ultrahigh vacuum.