Electrochemical and spectroelectrochemical studies of nickel- and copper-based catalysts for the reduction of carbon dioxide

Rebecca Elizabeth Wittrig, Purdue University

Abstract

The trinickel complex, (Ni$\sb3(\mu\sb3$-CNMe)($\mu\sb3$-I)(dppm)$\sb3$) (PF$\sb6$), 1, and the dicopper complex, (Cu$\sb2(\mu$-PPh$\sb2$bipy)$\sb2$(MeCN)$\sb2$) (PF$\sb6\rbrack\sb2,$ 3, are both electrocatalysts for the reduction of carbon dioxide. Both complexes are able to catalyze the reduction of CO$\sb2$ at potentials that are much more favorable than the potential required for the uncatalyzed reduction. Complex 1 exhibits a reversible one-electron reduction at $-$1.09 V vs. SCE. When the reduction is carried out under a CO$\sb2$ atmosphere, CO(g) and CO$\sb3\sp{2-}$ are produced. Complex 3 exhibits two reversible one-electron reductions, at $-$1.35 V and $-$1.53 V vs. SCE. CO(g) and CO$\sb3\sp{2-}$ are produced under a CO$\sb2$ atmosphere. The rates of the heterogeneous electron transfers in both systems have been studied using rotating disk voltammetry. The rates of the homogeneous electron transfers were studied by performing computer simulations of the electrocatalytic systems. We found that the rate-determining step between the reduced nickel complex and CO$\sb2$ occurs with a rate constant of k$\sb{\rm CO\sb2} = 1.6 \pm 0.3$ M$\sp{-1}$ s$\sp{-1}$. In system 3, the rate-determining step was found to occur with a rate constant of k$\sb{\rm CO\sb2} = 18 \pm 4$ M$\sp{-1}$ s$\sp{-1}.$ The rate-determining step for both systems has been found to be first order in both (catalyst) and (CO$\sb2$), with a second order dependence overall. The mechanisms of the electrocatalytic reactions have been explored using infrared spectroelectrochemistry (IR SEC). The design and testing of a thin-layer specular reflectance SEC cell is described. The reductions of the catalysts were effected in the absence and presence of CO$\sb2$ within the SEC cell, and the IR spectra taken over time. Changes in the catalyst upon addition of an electron(s) and the formation of CO$\sb2$ reduction products were monitored. The infrared spectroelectrochemical technique has also been applied to the study of the electrochemical reactions of metal carbonyl compounds. These reactions include metal-metal bond homolysis reactions, such as electrochemical cross-coupling reactions between (M$\sb2$(CO)$\sb{10}$) and ($\rm CpM\sp\prime(CO)\sb3\rbrack\sb2$ (where M = Mn or Re and M$\sp\prime$ = W or Mo). Some ligand substitution reactions involved with metal-metal bond homolysis reactions are also described. Finally, IR SEC was used to follow the electron transfer chain catalysis reaction between ($\rm(CH\sb3CN)\sb3M(CO)\sb3$) (M = W, Mo) and isocyanide ligands.

Degree

Ph.D.

Advisors

Kubiak, Purdue University.

Subject Area

Analytical chemistry

COinS