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Homogeneous Electrocatalytic Reduction of Carbon Dioxide to Carbon Monoxide by Ni(cyclam)


The homogeneous electrochemical reduction of CO₂ by the molecular catalyst [Ni(cyclam)]2+ was studied by electrochemistry and infrared spectroelectrochemistry. This catalyst has been previously shown to have increased CO₂ reduction activity when adsorbed on a mercury electrode. The homogeneous reactivity, without a mercury electrode, was often ignored in the literature. Ni(cyclam) was found to efficiently and selectively produce CO at moderate overpotentials in both aqueous and mixed organic solvent systems in a homogenous fashion at an inert glassy carbon electrode. Methylated analogs of Ni(cyclam) were also studied and observed to have more positive reduction potentials and attenuated CO₂ reduction activity. The electrochemical kinetics were probed by varying CO₂ substrate and proton concentrations. Products of CO₂ reduction are observed in infrared spectra obtained from spectroelectrochemical experiments. The two major species observed were a Ni(I) carbonyl, [Ni(cyclam)(CO)]⁺, and a Ni(II) coordinated bicarbonate, [Ni(cyclam)(CO₂OH)]⁺. The rate-limiting step during electrocatalysis was determined to be CO loss from the deactivated species, [Ni(cyclam)(CO)]+, to produce the active catalyst, [Ni(cyclam)]+. Another macrocyclic complex, [Ni(TMC)]⁺, was deployed as a CO scavenger in order to inhibit the deactivation of [Ni(cyclam)]⁺ by CO. Addition of the CO scavenger was shown to dramatically increase the catalytic current observed for CO₂ reduction by [Ni(cyclam)]⁺. Evidence for the [Ni(TMC)]⁺ acting as a CO scavenger includes the observation of [Ni(TMC)(CO)]⁺ by IR. Density functional theory calculations, probing the optimized geometry of the [Ni(cyclam)(CO)]⁺ species, are also presented. These findings have implications on the increased activity for CO₂ reduction when [Ni(cyclam)]⁺ is adsorbed on a mercury electrode. The [Ni(cyclam)(CO)]⁺ structure has significant distortion of the Ni center out of the plane of the cyclam nitrogens. This distortion strengthens the Ni-CO interaction by increasing back- bonding interactions. This leads to the hypothesis that the mercury surface, through Hg-Ni interactions, prevents the distorted geometry seen in solution leading to a more planar geometry. This helps to destabilize the carbonyl adduct which inhibits the extent of CO poisoning of the catalyst when adsorbed on a mercury electrode. Alternative approaches to prevent CO poisoning without using such a toxic substance as mercury are critical to improving this unique catalytic system

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