EXTENDING SURFACE RAMAN-SPECTROSCOPY TO TRANSITION-METAL SURFACES FORPRACTICAL APPLICATIONS .1. VIBRATIONAL PROPERTIES OF THIOCYANATE AND CARBON-MONOXIDE ADSORBED ON ELECTROCHEMICALLY ACTIVATED PLATINUM SURFACES

By using confocal microprobe Raman spectroscopy and a unique electroch emical pretreatment procedure for the Pt surface, we are able to exten d the detailed surface Raman studies, for the first time, to bare Pt e lectrodes in a wide potential region (e.g. -1.0 to +1.4 V vs SCE) and to more general adsorbates such as SCN- and CO having small Raman scat tering cross sections. Taking advantage of being capable of observing the adsorbate-metal vibrational bands in the low-frequency region over IR and SFG techniques, the surface Raman spectroscopy has demonstrate d the virtues of yielding detailed information on the surface bending affected by surface coverage, coadsorbate, electrolyte ions, and elect rode potential. The potential-dependent Raman spectra of SCN- reveal t hat the N-bound adsorbate is favored at the more negative potentials. The orientation conversion to the S-bound species occurs in the more p ositive region, depending on the SCN- concentration. In a solution of 10(-5) M NaSCN and 0.1 M NaClO4, a new C-N stretching band at ca. 2010 cm(-1) was assigned to the bridge-bound SCN- due to the low surface c overage. A preliminary study on CO irreversibly adsorbed on Pt, from - 1.4 to 0.2 V in 0.1 M NaOH, infers that the interactions of CO with ad jacent coadsorbed oxygen-containing species and with the surface are e ven more complex. At -0.8 V, the hydroxyl species starts to coadsorb w ith CO to the surface, then is partially oxidized at more positive pot entials, and finally forms the oxidized sites at potentials positive o f 0.0 V. In comparison with the roughened Ag, Cu, and Au electrodes, t he dispersed Pt surface by electrochemical activation has shown better stability and reproducibility for the Raman measurements. This furthe r provides a good reason to be optimistic that surface Raman spectrosc opy will contribute on an increasingly broad frontier to the developme nt of a true molecular-level probe of interfaces, particularly for pra ctical applications. Finally, it is worth emphasizing that the present results could promote new efforts to sort out the surface selection r ule and SERS mechanism(s).