The role of interfacial potential in adsorbate bonding: electrode potential-dependent infrared spectra for saturated CO adlayers on Pt(110) and related electrochemical surfaces in varying solvent environments

Abstract

Infrared spectra in the CO stretching (νCO) region are reported for saturated CO adlayers on Pt(110) in various solvent media over wide (∼3 V) ranges of electrode potential in order to explore the influence of the surface potential in various ionic double-layer environments on the adsorbate bonding. The solvents chosen - acetonitrile, tetrahydrofuran, dichloromethane, methanol, and water — span a wide range of polarity and other properties. The double-layer cations, constituting the ionic countercharge to the negative surface electronic charges encountered here, are tetrabutylammonium (TBA + ), tetraethylammonium (TEA + ) and sodium, present as the perchlorate salts. The tetra-alkylammonium ions provide noninteracting cations of differing size, enabling the effects of altering the inner-layer thickness to be examined. In such media, a single terminal ν CO feature (referring to a coverage θ CO ≈ 1.0) is obtained on Pt(110) throughout the potential range of about −2 to 1 V versus aqueous saturated calomel electrode (SCE), with frequencies ( ν CO t ) in the range 2050–2100 cm −1, decreasing linearly with potential ( E). While the ν CO t − E behavior is approximately independent of the solvent, the slopes observed in TEA + (20–21 cm −1 V −1) ar significantly greater than in TBA + (15–17 cm −1 V −1). These differences can be interpreted by using a simple double-layer model where the inner-layer thickness is determined by the CO adlayer plus the cation crystallographic radius. The potential of zero charge, E pzc ≈ 0.8 V versus SCE, as determined by the intersection point of these ν CO t − E plots, is consistent with the surfac potential of the (necessarily uncharged) Pt(110)/CO interface in ultrahigh vacuum (UHV) as determined by work-function measurements. Similar findings were obtained for saturated CO adlayers on Rh(111), although bridging as well as terminal CO coordination is observed and a potential-induced site conversion is obtained in some solvating environments. In acetonitrile with sodium perchlorate electrolyte, terminal to bridging site conversion was induced at all three surfaces at increasingly negative electrode potentials in the sequence Pt( l 10) < Pt(111) < Rh(111); such effects arise apparently from specific Na +-CO adlayer interactions. The dependencies of the ν CO frequencies on the potential drop across the CO adlayer as deduced here are compared with related results obtained for Pt(111)/CO in UHV.