A concerted assessment of potential-dependent vibrational frequencies for nitric oxide and carbon monoxide adlayers on low-index platinum-group surfaces in electrochemical compared with ultrahigh vacuum environments: Structural and electrostatic implications

Abstract

Electrode potential-dependent intramolecular stretching frequencies, ${\nu }_{\mathrm{NO}},$ for nitric oxide adlayers on ordered low-index Pt, Rh, Ir, and Pd electrodes in acidic aqueous solution measured by infrared reflection-absorption spectroscopy (IRAS) are compared with corresponding data obtained in ultrahigh vacuum (UHV) environments in order to assess the manner and degree to which the chemisorbate vibrational properties are controlled by electrostatic factors. For most of the seven surfaces for which corresponding UHV-based data are also available, the coverage-dependent ${\nu }_{\mathrm{NO}}$ spectral fingerprints observed in the corresponding electrochemical case are closely comparable, suggesting the occurrence of the same (or similar) binding sites and adlayer structures. The ${\nu }_{\mathrm{NO}}$ frequencies at a given coverage are typically 10–15-fold more sensitive to the electrostatic potential (or field) at the Pt-group electrodes than for isolated (gas-phase) NO, highlighting the importance of potential-dependent surface bonding. The ${\nu }_{\mathrm{NO}}$ frequencies in the electrochemical and UHV interfacial environments are compared at an equivalent metal-vacuum “surface potential,” ${\phi }_v^M,$ deduced from work-function data for the latter systems, together with the “absolute” potential of the hydrogen reference electrode, $E_{\mathrm{ab}}\mathrm{(ref)}.$ This procedure is facilitated by the stability of adsorbed NO toward electro-oxidation even at high electrode potentials in acidic media. In view of the piecemeal analyses along these lines undertaken previously for carbon monoxide, vibrational data for CO as well as NO are included so as to provide a comprehensive compilation for 14 surface-adsorbate pairs. Most systems selected exhibit a single dominant chemisorbate binding site as gleaned from the spectral form. For saturated adlayers, where the adsorbate coverages and binding geometries are known (or are likely) to be similar within the two environments, the vibrational frequencies are found to be remarkably concordant, within 5–10 cm⁻¹, when the comparison is undertaken at equivalent ${\phi }_v^M$ values deduced by presuming that $E_{\mathrm{ref}}\mathrm{(abs)}$ is 4.8 (±0.1) V. This value is consistent with the “higher” $E_{\mathrm{ref}}\mathrm{(abs)}$ estimates quoted in the literature. Although the ${\nu }_{\mathrm{NO}}$ and ${\nu }_{\mathrm{CO}}$ values are anticipated to be sensitive only to the potential drop across the chemisorbed adlayer, the inclusion of corrections to the electrochemical potentials for ${\phi }_v^M$ components located spatially outside this region does not substantially alter this conclusion. Some adlayer structural implications are considered, along with the value of such well-defined electrochemical systems to explore systematically electrostatic-field effects on surface bonding.