Cyanate and thiocyanate adsorption at copper and gold electrodes as probed by in situ infrared and surface-enhanced Raman spectroscopy

Abstract

Vibrational spectroelectrochemical techniques have been used to investigate the structure and dynamics of the electrochemical double layer using pseudohalides as specifically adsorbed probe molecules. Infrared and surface-enhanced Raman spectra of thiocyanate and infrared spectra of the structurally related cyanate at the copper|water and gold|water interface are reported. In neutral solutions, N- and S-bound adsorbed thiocyanate was found at the gold electrode, whereas at copper only S-bound thiocyanate exists. In both cases, thiocyanate is adsorbed in the whole polarizable potential range. At both copper and gold electrodes, cyanate coordinates via the nitrogen atom, not being adsorbed at negative potentials and occupying bridged adsorption sites at lower coverages. In alkaline solution, no adsorption of cyanate occurs, whereas the thiocyanate adsorption is not affected by the coadsorption of hydroxyl ions. These results lead to the assumption that the binding of thiocyanate to the surface is stronger and more covalent, whereas the coordination of cyanate is more ionic. In addition, at negative sample potentials cyanate was found to adsorb as cyanide, which is formed from cyanate by reduction. This process seems to lead to an activation of the surface in the case of gold.