Host‐guest tuning of the CO 2 reduction activity of an iron porphyrin cage

Efficient electrocatalytic CO 2 reduction requires developing catalysts with high selectivities and high activities, which is simultaneously difficult to achieve. Here, we present a new approach to tune the CO 2 reduction activity based on host‐guest chemistry enabled by an iron porphyrin cage catalyst. The cage design allows the hosting of alkali metals in the side walls causing a change in the electrostatic potential inside the cage cavity. Density functional theory calculations show that the guest potassium ions assist the reduction of CO 2 by inverting the two‐electron transfer from iron(0) to CO 2 from endothermic to exothermic. Accordingly, electrochemical experiments with the cage catalyst show that in the presence of the potassium ions, the overpotential for the CO 2 reduction decreases, and the catalytic activity increases while the high selectivity of the cage is retained. A novel coupling between the electrochemical cell and a mass spectrometer allowed the trapping of the key intermediates. Cryogenic ion spectroscopy characterization of the intermediates showed the details of the potassium ions hosting in the reduced cage and of the stabilization of the Fe‐COOH intermediates by the interaction with the potassium ions at the single‐molecule level. Key pointsHost‐guest chemistry of iron porphyrin‐cage catalysts in electrocatalytic CO 2 reduction results in an increase in the activity and the selectivity of the catalysis. Electrochemistry—mass spectrometry coupling allowed studying of the reaction intermediates in CO 2 reduction by mass spectrometry and by helium tagging infrared photodissociation spectroscopy. DFT calculations showed the details of the CO 2 reduction pathway inside of the cage cavity and the working principles of the reactivity enhancement by the host‐guest chemistry.