Spectroscopic and electrochemical studies of ionic and molecular adsorption at the Au(111) electrode surface
The thermodynamics of the so-called perfectly polarized electrode and the chronocoulometric technique were employed to study the adsorption of I- and OH- anions at the Au(111) electrode. The Gibbs excess, the Gibbs energy of adsorption, the electrosorption valency and the dipole formed by the adsorbed anion and its image charge in the metal were determined. The experimental results indicate that the strength of anion adsorption increases in the order SO42- < Cl- < OH- < Br - < I-. Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) was developed during this project, and it was applied to study hydroxide adsorption at the Au(111) surface. The SNIFTIRS results show that oxide formation occurs when the coverage by hydroxide reaches 1/3 monolayer. The integrated infrared intensity of the O-H stretching band correlates very well with the Gibbs excess of hydroxide determined by chronocoulometry. The SNIFTIRS technique was also employed to study adsorption of benzonitrile (BN) and 4-cyanopyridine (4-CNPy) at the Au(111) electrode. For BN adsorption, my studies show that the BN molecules are initially oriented flat ([pi]-bonded) on the electrode surface and progressively reorient from the flat to a vertical (N-bonded) state when the electrode potential approaches the potential of zero charge (pzc). When the potential is greater than 0.05 V(SCE), the adsorbed molecules partially hydrolyze to form benzamide (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. For 4-CNPy adsorption, the a1 bands were used to study the orientation of 4-CNPy and the b 2 bands were used to extract quantitative information about the surface concentration of adsorbed species. Our IR studies show that the adsorbed 4-CNPy molecules are hydrolyzed to form isonicotinamide (INA) at positive electrode potentials. The IR spectra acquired at either very negative or very positive potentials indicate that 4-CNPy is reduced at E < -1.2 V(SCE) and it is oxidized at E > 0.6 V(SCE).