UC San Diego

The electrocatalytic reduction of carbon dioxide (CO₂) to carbon monoxide (CO) is explored for both rhenium and manganese complexes. Electrochemistry, X-ray crystallography, Infrared spectroelectrochemistry, and stopped-flow kinetics are employed in order to identify catalysts and probe their mechanism and selectivity. Two catalysts in particular, Re(bipy-tBu)(CO₃(L) and Mn(bipy- tBu)(CO₃L (where bipy-tBu = 4,4'-di-tert-butyl-2,2'- bipyridine and L = Cl⁻, Br⁻ or (MeCN)(OTf)⁻), were studied extensively and displayed high activity, Faradaic efficiency, and selectivity for the reduction of CO₂ to CO. The Re-Cl catalyst exhibits a turnover frequency of > 200 s⁻¹, one of the fastest reported rates for a catalyst with appreciable turnover number. The Mn catalysts, when Brönsted acid sources are added to the electrochemical solution, exhibit current densities rivaling those of the Re-Cl catalyst. Amazingly, these catalysts showed high selectivity for CO₂ in both dry solvents and those with significant amounts of Brönsted acid added. Stopped-flow UV-Vis kinetics experiments showed that the reaction of the active form of the catalysts, [M(bipy-tBu)(CO)₃]⁻¹, is ca 50 times faster with CO₂ than they do with protons. Stopped-flow IR kinetics experiments comparing the reactions [Re(bipy-tBu)(CO)₃]⁻¹ with CO₂ and [Re(bipy)(CO)₃]⁻¹ with CO₂ shows, that at equal CO₂ concentrations, the bipy-tBu analog reacts ten times faster than the bipy analog. CO₂ also appears to react with both complexes via a concerted, two-electron oxidative addition of CO₂ to the metal center. The heterogenization of these catalysts was also explored with limited success. Intercalation, covalent bonds to gold, and covalent bonds to p-Si were all demonstrated, but none displayed activity towards the reduction of CO₂. Future experiments are suggested to solve this issue