​Recycling carbon dioxide would reduce its atmospheric accumulation, one of the main causes of global warming. To turn CO₂ into fuel or building blocks for organic compounds of interest, we must first break the O=C=O bonds, a very energy-consuming process. For this, researchers can draw inspiration from nature, which posess particularly effective metalloenzymes, such as carbon monoxide dehydrogenase, which reversibly reduces CO₂ to CO.

In 2019, the I2BC team developed a new iron porphyrin* type catalyst, directly inspired by the active site of carbon monoxide dehydrogenase and particularly promising for reduction of carbon dioxide. More precisely, the researchers had modified the iron porphyrin by introducing, on its second coordination sphere, urea functions, acting as molecular pillars providing hydrogen bonds to immobilize the CO₂ on the metal center, as in the natural enzyme (Gotico et al., Angew.Chemie, 2019).

In order to enhance the energy and kinetic performance of this catalyst, the researchers studied here the impact of the topology of urea groups on the electro-catalytic reduction of CO₂. For this, two isomers of the catalyst comprising two urea groups were synthesized and their catalytic properties characterized: in the first isomer, the two urea groups face each other on the same side of the porphyrin cycle (alpha alpha), whereas in the second, the two urea groups are in trans, on either side of the porphyrin cycle (alpha beta).

The obtained results show that the modification of the second coordination sphere of the catalyst impacts its functional properties (figure). Indeed, if the alpha alpha isomer possesses a better affinity for CO₂ than the alpha beta isomer, the latter has better catalytic activity with the best rate constant reported in the literature for the electro-catalytic reduction of CO₂ to CO. The authors attribute this observation to a more efficient protonation step of CO₂ during its reduction. In fact, when the CO₂ molecule is trapped between the two urea groups in the case of the alpha alpha isomer, this step is very slowed down whereas in the case of the alpha beta, isomer, the CO₂ molecule is held by only one hydrogen bonded arm thus allowing the proton access for the conversion of CO₂ into CO.

In conclusion, this study tells us that a better catalysis of CO₂ reduction requires a subtle control of its fixation at the catalytic site and of access of protons, an essential ingredient for the rupture of the C=O bond.

Contact : winfried.leibl@cea.fr

* An iron porphyrin is a molecule with a macrocyclic structure, like the heme of hemoglobin, which is used to fix oxygen in red blood cells.