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