Infrared spectroscopy of model electrochemical interfaces in ultrahigh vacuum: Coupled influence of double layer anion/cation solutes upon acetone solvent chemisorption on Pt(111)

Abstract

Abstract The effects of adsorbing chlorine and/or potassium on the infrared spectra of acetone dosed onto Pt(111) in ultrahigh vacuum (UHV) are reported with the objective of ascertaining the manner and extent to which the oxygen bound ( η 1 ) chemisorbed state of acetone is influenced by ‘specific anion’ adsorption, along with the cation countercharge in this ‘UHV electrochemical model’ system. Acetone was chosen as a model solvent in view of its dipolar properties along with the presence of an infrared chromophore, the carbonyl stretching vibration, the frequency as well as the intensity of which is sensitive to the dipolar orientation and coordination of adsorbed acetone. The presence of co-adsorbed chlorine atoms stabilizes substantially the perpendicular solvent dipole orientation, as deduced from a marked intensity increase in the ca. 1640 cm −1 band due to the η 1 chemisorbed state of acetone. This effect, which can be understood most simply in terms of an attractive electrostatic interaction between the Cl δ− metal image and δ+ CO δ− oriented dipoles, is opposite to that observed in the presence of adsorbed K + alone. In the latter case, even very low fractional coverages (around 0.02) of K + are sufficient to eliminate essentially the η 1 chemisorbed acetone state as deduced from the removal of the ca. 1640 cm −1 band; this finding indicates that long range (over two to three solvent diameters) cation-solvent interactions are operative [cf. I. Villegas and M.J. Weaver, J. Am. Chem. Soc., 118 (1996) 458]. In the presence of co-adsorbed chlorine + potassium mixtures, the η 1 acetone state is also largely absent even for marked (five- to 10-fold) stoichiometric deficiencies of the cation relative to the ‘anion’ solute component. This latter result can be rationalized on the basis of the anticipated greater ionicity of the adsobed cation (K + ) relative to the chemisorbed anion (Cl δ− ) species. These findings are corroborated by means of measurements of the work-function changes induced on Pt(111) by dosing acetone in the presence of varying coverages of chlorine and/or potassium. The implications to our understanding of the interplay between interfacial ion-solvent and solvent-surface interactions in electrochemical systems are noted on a more general basis.