Potential-dependence of CO adlayer structures on Pt(111) electrodes in acid solution: Evidence for a site selective charge transfer

The oxidation of carbon monoxide on a Pt(111) electrode surface is studied under transient and stationary reaction conditions in a 0.1 M HClO4 aqueous solution as a function of the applied electrode potential. The optical res ponse is investigated by optical second harmonic generation (SHG) at 280 nm and infrared absorption spectroscopy (IRAS) starting at low potentials whe re the electrode is not reactive [e.g., 0.1 V vs the reversible hydrogen el ectrode (RHE)] towards more positive potentials where CO oxidation is initi ated. For transient reaction conditions, in the absence of CO dissolved in solution, the oxidation starts at about 0.5 V vs. RHE and consists of a fas t oxidation of about 10% of the adlayer and of a second reaction with slowe r kinetics involving the removal of the complete CO layer (overlayer stripp ing). Under steady-state conditions in CO-saturated solution the CO adlayer is stable up to 0.9 V vs RHE. At 0.63 V an overlayer phase transition is i ndicated by a 20% increase of the isotropic component of the second harmoni c (SH) intensity (pp-polarization). The same potential region for this phas e transition in the adlayer structure is derived from IRAS spectra after co rrecting for the effect of the thin layer electrolyte in IRAS measurements. The disappearance of hollow sites, the appearance of bridge sites, as well as an increased occupation of on-top sites at 0.63 V is interpreted as cor responding to the adlayer phase transition. The observations are consistent with a transition from the c(2x2) to the (root 19x root 19) adlayer struct ure of CO. The high sensitivity of SHG with regard to structural phase tran sitions of the CO adlayer is explained by distinct charge transfer contribu tions to the second-order surface susceptibility at different coordination sites. (C) 2000 American Institute of Physics. [S0021-9606(00)01540-3].