The Homogeneous Reduction of CO2 by Ni(cyclam)+: Increased Catalytic Rates with the Addition of a CO Scavenger

The homogeneous electrochemical reduction of CO2 by the molecular catalyst Ni(cyclam)(2+) is studied by electrochemistry and infrared spectroelectrochemistry. The electrochemical kinetics are 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 are a Ni(I) carbonyl, Ni(cyclam)(CO)(+), and a Ni(II) coordinated bicarbonate, Ni(cyclam)(CO2OH)(+). The rate-limiting step during electrocatalysis is determined to be CO loss from the deactivated species, Ni(cyclam)(CO)(+), to produce the active catalyst, Ni(cyclam)(+). Another macrocyclic complex, Ni(TMC)(+), is deployed as a CO scavenger in order to inhibit the deactivation of Ni(cyclam)(+) by CO. Addition of the CO scavenger is shown to dramatically increase the catalytic current observed for CO2 reduction. Evidence for the Ni(TMC)(+) acting as a CO scavenger includes the observation of Ni(TMC)(CO)(+) by IR. Density functional theory (DFT) calculations probing the optimized geometry of the Ni(cyclam)(CO)(+) species are also presented.