Infrared Spectroscopy of Model Electrochemical Interfaces in Ultrahigh Vacuum: Evidence for Coupled Cation−Anion Hydration in the Pt(111)/K+,Cl- System

Infrared reflection−absorption spectroscopy (IRAS) measurements along with work-function (Φ) and temperature-programmed desorption (TPD) data are reported for deuterated water dosed onto Pt(111) in ultrahigh vacuum (uhv) in the presence of adsorbed chlorine and potassium in order to assess the nature and extent of hydration of a “chemisorbed” (or specifically adsorbed) anion in the absence and presence of a cation “countercharge”, and vice versa. As studied previously (ref 6a), potassium constitutes an ideal “cation” for this purpose since it induces large Φ decreases even for small K coverages (θ K ≤ 0.08) that are modified substantially by hydration; this primary solvation also yields downshifted O−D stretching (ν OD ) vibrations (ca. 2300−2500 cm -1 ) that are diagnostic of cation-induced water reorientation. Adsorption of atomic chlorine (dosed as Cl 2 ) yields milder yet nonmonotonic Φ changes, suggestive of only partial charge transfer to form Cl δ- . The presence of adsorbed Cl attenuates the large (ca. 1.2 eV) Φ decreases produced by water dosing onto clean Pt(111), yet in largely featureless fashion. An absence of strong Cl hydration, suggested by these data and from TPD measurements, is also indicated by lack of solute-induced ν OD bands in the corresponding infrared spectra, except at high Cl coverages (θ Cl ) and submonolayer D 2 O dosages. Markedly different behavior, however, is observed in the presence of both Cl and K adsorbates. Addition of roughly stoichiometric Cl coverages to adsorbed K in the absence of water yields substantial (ca. 1.5 eV) increases followed by only minor changes for higher θ Cl values, suggesting intimate electrostatic K/Cl interactions involving K + ···e - /Cl - ···e + dipole polarization. Furthermore, water addition to K/Cl mixtures where θ Cl ≥ θ K triggers only minor Φ changes, in contrast to the situation for the absence of Cl. This evidence that the interfacial hydration of both “ionic” solutes is being modified in their combined presence is further indicated by corresponding ν OD infrared fingerprints that differ substantially from corresponding spectra obtained for each solute separately. Thus, the K + “hydration shell” ν OD signature is largely removed in the presence of Cl, and a new feature (at 2620 cm -1 ) appears; the latter is most likely associated with solvation of Cl δ- /K + pairs. Further evidence for the coupled nature of this hydration is also gleaned from the marked cooperative influences of Cl and/or K adsorption upon the water desorption temperatures. The qualitative implications of these findings to the conventional picture of solvation in the electrochemical double layer are also noted.