INFRARED-SPECTROSCOPY OF MODEL ELECTROCHEMICAL INTERFACES IN ULTRAHIGH-VACUUM - SURFACE-CATION SOLVATION IN THE PT(111) K+-METHANOL SYSTEM/

Infrared reflection-absorption spectroscopic (IRAS) measurements are r eported for methanol dosed onto Pt(lll) in ultrahigh vacuum (UHV) in b oth 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 (Phi), evaluated with a Kelvin probe, yiel d additional insight into the interfacial electrostatic environment as a function of the alkali and methanol dosages. Methanol forms a parti cularly suitable solvent for such a ''double-layer modeling'' study si nce both the O-H stretching (v(OH)) and C-OH stretching (v(C-OH)) vibr ations are sensitive to the local coordination environment. In additio n, comparisons are made with the detailed infrared spectral data avail able for progressive methanol solvation of gas-phase alkali cations [( a) A. J. Draves, Z. Luthey-Schulten, W.-L. Liu, and J. M. Lisy, J. Che m. Phys. 93, 4589 (1990); (b) T. J. Selegue, N. Moe, J. A. Draves, and J. M. Lisy, ibid. 96, 7268 (1992)], allowing unprecedented insight in to 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 v(OH) and v(C-OH) bands at similar to 3100 and 1010 cm(-1), respectively, w hich are not observed in the absence of K+. This spectral behavior is consistent with the formation of a primary solvation shell featuring m ethanol-cation coordination via the oxygen along with -OH hydrogen bon ding to the metal surface. The significant ((similar to 0.5-1 eV) Phi increases observed under these conditions support the presence of prim ary solvation methanol with a negative-outward O-delta--H-delta+ dipol e orientation. The second solvation stage, referring to K+-methanol st oichiometries above similar to 3, is accompanied by the appearance of markedly upshifted v(OH) and v(C-OH) bands, at similar to 3300 and 105 0 cm(-1), respectively, suggesting the occurrence of extensive first-s econd shell H-bonding. Marked Phi, decreases are observed in this dosa ge regime, more closely akin to the behavior observed in the absence o f adsorbed alkali. The methanol dosage-dependent interfacial v(C-OH) b ehavior is markedly different to that observed in the gas phase, highl ighting the role of the metal in modifying the nature of both the prim ary and second-shell solvation structure. The structure of methanol on uncharged (i.e., K+-free) Pt(lll) is also addressed by combined IRAS and work-function measurements. The H-bonded structures even within mu ltilayer methanol films differ significantly from the analogous bulk p hases. The effects of competitive CO chemisorption on K+ solvation are also considered. (C) 1995 American Institute of Physics.