In-situ spectroscopy of cyanide vibrations on Pt(111) and Pt(110) electrode surfaces: potential dependencies and the influence of surface disorder

The stretching vibrations of cyanide (CN^-) ions adsorbed on single-crystalline Pt electrodes in aqueous electrolytes were measured in-situ by optical sum-frequency generation (SFG). Adsorption of CN^- on well-ordered Pt(111) and Pt(110) surfaces is characterized by a stretching vibration with a strongly potential-dependent frequency, ranging from about 2090 cm^-1 near the potential of hydrogen evolution to values of about 2150 cm^-1 (Pt(111)) and 2130 cm^-1 (Pt(110)) for potentials close to Pt oxidation. At negative potentials, a second band with a frequency around 2090 cm^-1 is resolved on Pt(111). A vibrational band around 2147 cm^-1 with very small potential dependence is related to cyanide on microscopically disordered surfaces. This band, previously observed on polycrystalline Pt electrodes as well as on Pt(110) and Pt(100), appears after application of oxidation-reduction cycles to single-crystalline electrode surfaces and is assigned to a surface intermediate state of [Pt(CN)₄]^2- complex formation.