Ho Yeung H. Chan

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 potential, φ (“Stark-tuning” effect), observed in electrochemical systems is exploited for the same metal surfaces in contact with ambient-pressure gases so to estimate φ values in the latter environment. Saturated CO adlayers on palladium and platinum films are examined along these lines by using surface-enhanced Raman spectroscopy (SERS) to obtain frequencies for both the C–O (νCO) and metal–carbon stretching (νM--CO) vibrations in CO-containing aqueous electrochemical and gaseous environments. The effective gas-phase surface potentials extracted by matching the vibrational frequencies with the corresponding potential-dependent electrochemical spectra are substantially (ca. 1–1.5 V) lower than the work functions for such interfaces under “clean” (ultrahigh vacuum) conditions. These disparities are ascribed to the occurrence of electrochemical-like redox half-reactions in the ambient-pressure gas-phase environment, leading to surface charging, and, hence, marked alterations in the surface potential as controlled by potential-dependent redox kinetics. The possible implications of these and related findings to ambient-pressure adsorption and catalysis are discussed.