THE ADSORPTION OF NI(CYCLAM)+ AT MERCURY-ELECTRODES AND ITS RELATION TO THE ELECTROCATALYTIC REDUCTION OF CO2

The adsorption of Ni(cyclam)2+ and Ni(cyclam)+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) at mercury electrodes from aqueous solutions was examined in the potential range between 0 and -1.6 V versus saturated calomel electrode (SCE) using single- and double-potential-step chronocoulometry. A detailed analysis of the chronocoulometric data is presented. Ni(cyclam)2+ is only weakly adsorbed over a limited potential range, and in quantities substantially less than one monolayer. In contrast, Ni(cyclam)+ is strongly adsorbed over a wide potential range, and substantial adsorption occurs at potentials much more positive than those where the complex can be generated by reduction of Ni(cyclam)2+ in solution. The adsorption occurs in two distinct stages. At potentials between -0.6 and -1.2 V up to half a monolayer can be adsorbed. At potentials between -1.2 and -1.3 V, a change in the structure of the adsorbed layer appears to occur which enables a full monolayer of the complex to be adsorbed at potentials negative of -1.3 V. The adsorption of Ni(cyclam)+ at potentials where it cannot be generated in solution is not diffusion controlled; the adsorption appears to require a rearrangement of the cyclam ligand. The kinetics of this process control the rate of adsorption. The altered ligand configuration in the adsorbed complex persists when the complex is oxidatively desorbed to generate a form of Ni(cyclam)2+ that can be reductively readsorbed at a diffusion-controlled rate. The role that the altered configuration of the adsorbed complex may play when it serves as a catalyst for the electrochemical reduction of CO2 is discussed.