Adsorption of CN at the Pt(111)/liquid interface by dissociation of acetonitrile and the potentiality of CO contamination: A sum-frequency generationstudy

The adsorption of cyanide, CN, on the Pt(111) surface by dissociation of ac etonitrile, CH3CN, has been investigated by sum-frequency generation (SFG) as surface vibrational spectroscopy and by cyclic voltammetry. The adsorpti on procedure involved cooling of the flame-annealed Pt crystal above a dilu te aqueous solution of acetonitrile (0.1 M HClO4 + 0.025 M CH3CN), followed by immersion of the sample into the electrolyte. The adsorbed species is i dentified as cyanide by the characteristic potential dependence of the stre tching frequency, which increased from 2087 cm(-1) at 0.1 V to 2135 cm(-1) at 0.95 V, as well as by cyclic voltammetry. Comparison with the voltammogr am in a 1 x 10(-3) M solution of CH3CN indicates that the dissociation take s place in the gas phase above the solution. A second band in our spectra w ith a frequency near 2155 cm(-1) is tentatively assigned to a Pt-cyano surf ace complex. We also report on experiments in which the same annealing/cool ing procedure has been applied using neat liquid acetonitrile. Bands near 1 860 and 2073 cm(-1) in SFG spectra of these samples are shown to originate from adsorbed CO, most likely produced by oxidation of CH3CN vapor at the h ot Pt crystal. CN adsorption at the Pt(111)/liquid acetonitrile interface, performed by deposition of cyanide ions from solutions of tetrabutylammoniu m cyanide in acetonitrile, is characterized by a single band near 2110 cm(- 1).