Infrared spectroscopy of model electrochemical interfaces in ultrahigh vacuum: Surface–cation solvation in the Pt(111)/K+–methanol system

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Abstract
Infrared reflection–absorption spectroscopic (IRAS) measurements are reported for methanol dosed onto Pt(111) in ultrahigh vacuum (UHV) in both the presence and absence of adsorbed potassium atoms at 90 K with the objective of elucidating the nature of sequential cation solvation at this model electrochemical interface. Corresponding variations in the metal‐UHV work‐function (Φ), evaluated with a Kelvin probe, yield additional insight into the interfacial electrostatic environment as a function of the alkali and methanol dosages. Methanol forms a particularly suitable solvent for such a ‘‘double‐layer modeling’’ study since both the O–H stretching (νOH) and C–OH stretching (νC–OH) vibrations are sensitive to the local coordination environment. In addition, comparisons are made with the detailed infrared spectral data available for progressive methanol solvation of gas‐phase alkali cations [(a) A. J. Draves, Z. Luthey‐Schulten, W.‐L. Liu, and J. M. Lisy, J. Chem. Phys. 93, 4589 (1990); (b) T. J. Selegue, N. Moe, J. A. Draves, and J. M. Lisy, ibid. 96, 7268 (1992)], allowing unprecedented insight into the manner and extent to which cation solvation is affected by the metal surface. The initial stage of methanol solvation of interfacial K+ is signaled by substantially downshifted and relatively sharp νOH and νC–OH bands at ∼3100 and 1010 cm−1, respectively, which are not observed in the absence of K+. This spectral behavior is consistent with the formation of a primary solvation shell featuring methanol–cation coordination via the oxygen along with –OH hydrogen bonding to the metal surface. The significant (∼0.5–1 eV) Φ increases observed under these conditions support the presence of primary solvation methanol with a negative‐outward δ−O–Hδ+ dipole orientation. The second solvation stage, referring to K+–methanol stoichiometries above ∼3, is accompanied by the appearance of markedly upshifted νOH and νC–OH bands, at ∼3300 and 1050 cm−1, respectively, suggesting the occurrence of extensive first–second shell H‐bonding. Marked Φ decreases are observed in this dosage regime, more closely akin to the behavior observed in the absence of adsorbed alkali. The methanol dosage‐dependent interfacial νC–OH behavior is markedly different to that observed in the gas phase, highlighting the role of the metal in modifying the nature of both the primary and second‐shell solvation structure. The structure of methanol on uncharged (i.e., K+‐free) Pt(111) is also addressed by combined IRAS and work‐function measurements. The H‐bonded structures even within multilayer methanol films differ significantly from the analogous bulk phases. The effects of competitive CO chemisorption on K+ solvation are also considered.