Abstract
We present and discuss sum-frequency vibrational spectra of CO adsorbed on (111)- and (110)-oriented Pt single-crystal electrodes in a 0.1 M HClO4 aqueous electrolyte. For potentials slightly above the potential of hydrogen evolution, CO adsorbs on terminal and 3-fold hollow sites of the Pt(111) surface with frequencies of 2066 cm-1 and 1788 cm-1, respectively. Around 0.4 V versus RHE, a structural transition of the CO adlayer takes place with the spectroscopic signature of terminal and bridge-like coordinations (∼2065 cm-1 and 1850 cm-1, respectively). Only terminal adsorption is observed for the (110) surface. Although these frequencies of the CO stretching vibrations are in good agreement with previous infrared reflection−absorption studies, our sum-frequency generation (SFG) spectra indicate a complete oxidation and removal of the CO adlayer at potentials of ∼0.55 V for Pt(111) and ∼0.45 V for Pt(110), well below the respective main oxidation potentials in cyclic voltammetry. We show that the lower ox...
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