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

Infrared spectroscopy is applied to probe qualitative structural featu res of the adlayers formed by CO at step sites and on terrace planes o f Pt(335){Pt(S)-[4(111)x(100)]} in the aqueous electrochemical environ ment. The C-O stretching vibrational features are reported for adlayer s formed from (CO)-C-12/(CO)-C-13 isotopic mixtures over a wide range of CO surface coverages. At saturation, the predominant spectral featu res are associated with the vibrational modes of terrace-CO in termina l (atop) coordination environments. The position of the (CO)-C-12 and (CO)-C-13 spectral features and their relative intensity are examined for several (CO)-C-12/(CO)-C-13 fractions, and they are shown to displ ay the characteristics of a strongly coupled system. In comparison wit h corresponding mixed isotope spectra for CO at Pt(lll) electrodes, in termolecular 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 wi th edge-CO are discerned, and the intermolecular coupling for atop CO is weaker than for corresponding coverages of CO at Pt(lll). 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, v alues are determined for the dynamic dipole-dipole coupling parameter (Delta nu(d)) and the chemical (static-dipole) shift parameter (Delta nu(s)). Values for Delta nu(s) are generally small at all coverages. V alues for Delta nu(d) are small (<8 cm(-1)) at low coverages, where CO forms one-dimensional structures along the step edges, and they incre ase to large values (similar to 42 cm(-1)) at coverages that coincide with the growth of two-dimensional structures on the terrace planes. T he majority of measurements were made for the Pt(335) electrode at pot entials in the classical double-layer region, although dipole coupling parameters are also reported for Pt(335)/CO at potentials in the hydr ogen adsorption region, where Delta nu(d) approaches zero at low cover ages.