Sum-frequency vibrational spectroscopy of CO adsorption on Pt(111) and Pt(110) electrode surfaces in perchloric acid solution: Effects of thin-layer electrolytes in spectroelectrochemistry

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 aqu eous electrolyte. For potentials slightly above the potential of hydrogen e volution, CO adsorbs on terminal and 3-fold hollow sites of the Pt(111) sur face 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 wit h the spectroscopic signature of terminal and bridge-like coordinations sim ilar to 2065 cm(-1) and 1850 cm(-1). respectively). Only terminal adsorptio n is observed for the (110) surface. Although these frequencies of the CO s tretching vibrations are in good agreement with previous infrared reflectio n-absorption studies, our sum-frequency generation (SFG) spectra indicate a complete oxidation and removal of the CO adlayer at potentials of similar to 0.55 V for Pt(111) and similar to 0.45 V for Pt(110), well below the res pective main oxidation potentials in cyclic voltammetry. We show that the l ower oxidation potentials observed in SFG are specific for a GO-depleted th in-layer electrolyte and that the CO depletion results from the combination of CO oxidation in the layer and limited diffusion of CO from the bulk ele ctrolyte. We do not find indications for CO adsorbed in a state which is in visible to SFG, as reported in a recent study of CO adsorption on Pt(111) i n 0.5 M H2SO4 (S. Baldelli et al. J. Phys. Chem. B 1999, 103, 8920.