Mechanistic and electrochemical studies of the reduction of carbon dioxide as catalyzed by Ni(I)CYCLAM+

Citation

Balazs, Gabriel Bryan (1993) Mechanistic and electrochemical studies of the reduction of carbon dioxide as catalyzed by Ni(I)CYCLAM+. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/RJF2-WD05. https://resolver.caltech.edu/CaltechTHESIS:12042012-160347510

Abstract

A detailed electrochemical investigation of Ni(cyclam)^(2+) and its derivatives is described, especially with regard to the aqueous electrocatalytic reduction of CO_2 at mercury. Detailed chronocoulometric studies which quantify the extent of the adsorption of the active catalytic species Ni(cyclam)^+_(ads) are discussed. Ni(cyclam)^(2+) is only weakly adsorbed at mercury and in quantities substantially less than a monolayer. In contrast, Ni(cyclam)+ is adsorbed over a wide potential range and the adsorption process occurs in two potential dependent stages. An analysis of the kinetics of the adsorption process is discussed.

In the presence of CO, Ni(cyclam)^(2+) is electrochemically reduced to Ni(cyclam)^+-CO and Ni(cyclam)^0-CO. This latter species is insoluble and precipitates on the electrode surface. Both of these species are chemically unstable and slowly react to form Ni(cyclam)^(2+) in the presence of oxidizing agents.

Ni(cyclam)^+_(ads) catalyzes the reduction of CO_2 to exclusively CO. In unbuffered solutions, the OH- ion produced as a result of the reduction of CO_2 can decrease the flux of CO_2 molecules to the electrode surface by direct reaction with CO_2 to form HCO-^_3 or CO^(2-)_3,both of which are catalytically inactive towards reduction by Ni(cyclam)^+_(ads). In both buffered and unbuffered solutions, the precipitate Ni(cyclam)^0-CO which is formed on the electrode surface under all conditions where CO_2 is reduced causes a slow passivation of the electrode surface towards further catalytic reduction of CO_2.

The binding of CO_2 and CO to Ni(cyclam)^(2+), Ni(cyclam)^+, and Ni(cyclam)^+_(ads) is discussed. The active catalyst, Ni(cyclam) )^+_(ads), is able to coordinate CO_2, but not the product of the reduction, CO. Both Ni(cyclam)^(2+) and Ni(cyclam)+ are unable to coordinate CO_2 and thus solution species are not important in the catalytic cycle. A comparison of these results with previous studies is given and an overall mechanism for the electrocatalytic reduction of CO_2 is proposed. This mechanism contains several important modifications from earlier studies.

Thesis Files