ELECTROCHEMICAL AND FOURIER-TRANSFORM INFRARED-SPECTROSCOPY STUDIES OF BENZONITRILE ADSORPTION AT THE AU(111) ELECTRODE

The subtractively normalized interfacial Fourier transform infrared te chnique has been employed to study the adsorption of benzonitrile (BN) at the Au(lll) electrode surface. The vibrational spectra have been u sed to study (i) the dependence of the band intensity on the surface c overage, (ii) the character of surface coordination, and (iii) the sta bility of adsorbed BN molecules at positive potentials. Our studies sh ow that BN molecules are totally desorbed from the Au(lll) surface at potentials more negative than -0.6 V (SCE) and they adsorb at the gold surface at more positive potentials. At potentials more negative than 0.05 V (SCE), the adsorption has an associative character. The BN mol ecules are initially oriented flat (pi-bonded) on the electrode surfac e and progressively reorient from the flat to a vertical (N-bonded) st ate when the electrode potential approaches the potential of zero char ge. This change of the surface coordination is gradual and apparently involves a progressive change of the tilt angle. When the potential is greater than 0.05 V (SCE), the character of BN adsorption becomes dis sociative and the adsorbed molecules partially hydrolyze to form benza mide (BA). The adsorbed layer becomes a mixture of BN and BA molecules . The ratio of BN to BA molecules decreases as the electrode potential increases. The optical properties of CaF2 prisms and flat windows hav e also been investigated. Our results show that flat windows should no t be used to study physical features such as orientation and coordinat ion of adsorbates. However, they are useful to extract quantitative in formation about the surface concentration of adsorbed species and to d etermine the composition of the interfacial region.