Enzyme-like reactivity for increasing selectivity in CO2 electrochemical reduction

The development of selective catalysts for the reduction of CO2 mostly focuses on electrocatalytic approaches and aims at increasing the selectivity of the reaction while keeping a high activity, which is difficult to achieve. Metalloporphyrins are good catalysts for CO2 reduction because they have favorable electronic properties and offer the possibility to make use of secondary coordination sphere effects. Here, we present a new approach to CO2 reduction, which is based on host-guest chemistry enabled by an iron porphyrin cage catalyst. When this iron porphyrin cage catalyst is immobilized on a conducting carbon support the selectivity for CO2 reduction to CO stays above 90 % in a wide range of overpotentials. The hosting of potassium ions in the cage of the catalyst decreases the overpotential of the reduction and increases the catalytical activity while retaining the high selectivity. DFT calculations show that the potassium ions assist the reduction of CO2 by making the 2-electron transfer from iron(0) to CO2 exothermic. Upon protonation, the Fe-COOH intermediates have been trapped by combining an electrochemical cell with an electrospray ionization mass spectrometer and their structure has been characterized by cryogenic ion spectroscopy.