SURFACE-POTENTIALS OF METAL-GAS INTERFACES COMPARED WITH ANALOGOUS ELECTROCHEMICAL SYSTEMS AS PROBED BY ADSORBATE VIBRATIONAL FREQUENCIES

The dependence of adsorbate vibrational frequencies on the surface pot ential, phi (''Stark-tuning'' effect), observed in electrochemical sys tems is exploited for the same metal surfaces in contact with ambient- pressure gases so to estimate phi values in the latter environment. Sa turated CO adlayers on palladium and platinum films are examined along these lines by using surface-enhanced Raman spectroscopy (SERS) to ob tain frequencies for both the C-O (nu(CO)) and metal-carbon stretching (nu(M-CO)) vibrations in CO-containing aqueous electrochemical and ga seous environments. The effective gas-phase surface potentials extract ed by matching the vibrational frequencies with the corresponding pote ntial-dependent electrochemical spectra are substantially (ca. 1-1.5 V ) lower than the work functions for such interfaces under ''clean'' (u ltrahigh vacuum) conditions. These disparities are ascribed to the occ urrence of electrochemical-like redox half-reactions in the ambient-pr essure gas-phase environment, leading to surface charging, and, hence, marked alterations in the surface potential as controlled by potentia l-dependent redox kinetics. The possible implications of these and rel ated findings to ambient-pressure adsorption and catalysis are discuss ed.