Site-dependent vibrational coupling of CO adsorbates on well-defined step and terrace sites of monocrystalline platinum: Mixed-isotope studies at Pt(335) and Pt(111) in the aqueous electrochemical environment

Abstract

Infrared spectroscopy is applied to probe qualitative structural features of the adlayers formed by CO at step sites and on terrace planes of Pt(335){Pt(S)-[4(111)×(100)]} in the aqueous electrochemical environment. The C–O stretching vibrational features are reported for adlayers formed from 12CO/13CO isotopic mixtures over a wide range of CO surface coverages. At saturation, the predominant spectral features are associated with the vibrational modes of terrace-CO in terminal (atop) coordination environments. The position of the 12CO and 13CO spectral features and their relative intensity are examined for several 12CO/13CO fractions, and they are shown to display the characteristics of a strongly coupled system. In comparison with corresponding mixed isotope spectra for CO at Pt(111) electrodes, intermolecular coupling for terrace-CO on the (111) surface planes of Pt(335) is observed to be significantly stronger, reflecting the higher CO surface coverages on the edge sites and the terrace sites of the Pt(335) surface plane. At low coverages, spectral features associated with edge-CO are discerned, and the intermolecular coupling for atop CO is weaker than for corresponding coverages of CO at Pt(111). The weak coupling at low coverages is attributed to the exclusive CO occupation at the step edges, which confines the intermolecular coupling to one dimension, in the direction along the step edges. For all coverages, values are determined for the dynamic dipole–dipole coupling parameter (Δνd) and the chemical (static–dipole) shift parameter (Δνs). Values for Δνs are generally small at all coverages. Values for Δνd are small (<8 cm−1) at low coverages, where CO forms one-dimensional structures along the step edges, and they increase to large values (∼42 cm−1) at coverages that coincide with the growth of two-dimensional structures on the terrace planes. The majority of measurements were made for the Pt(335) electrode at potentials in the classical double-layer region, although dipole coupling parameters are also reported for Pt(335)/CO at potentials in the hydrogen adsorption region, where Δνd approaches zero at low coverages.