Catalysis of the conversion of carbon dioxide into to carbon monoxide.

Jean-Michel Savéant

Université Paris Diderot, Sorbonne Paris Cité, Laboratoire d'Electrochimie Moléculaire,

Bâtiment Lavoisier, 15 rue Jean de Baïf, 75205 Paris Cedex 13, France.

The reductive conversion of carbon dioxide into to carbon monoxide is an important issue

of contemporary energy and environmental challenges. Following the observation that direct

injection of one electron into the CO2 molecule requires highly negative potentials, several

electrogenerated low-oxidation state transition metal complexes have been proposed to serve

as homogeneous catalyst for this reaction in non-aqueous solvents such as N,N'-

dimethylformamide and acetonitrile. Among them, electrochemically generated Fe0

porphyrins have been shown to be good catalysts provided they are used in the presence of

Brönsted or Lewis acids. More recent investigations have extended the range of Brönsted

acids able to boost the catalysis of the CO2-to-CO conversion by electrogenerated Fe0TPP

without degrading the selectivity of the reaction. They have also provided a detailed analysis

of the reaction mechanism. This is notably the case with phenol, which gave rise to the idea of

installing prepositioned phenol groups right in the catalyst molecule. The result was indeed a

remarkably efficient and selective catalyst of the CO2-to-CO conversion. The internal phenol

moieties act both as H-bonding stabilizers of the initial Fe0-CO2 adduct and as high local

concentration proton donors. Association between electron transfer and proton transfer is

indeed essential in the breaking of one of the carbon - oxygen bonds of the CO2 molecule.

The fact that electron transfer, proton transfer, and C-O bond breaking are all concerted in this

instance renders the reaction particularly effective. This study was also the occasion to

propose and apply a rational way of benchmarking the various catalysts under the form of

catalytic Tafel plots (turnover frequency vs. overpotential plots), free from contingent factors

pertaining to the characteristics of the electrochemical cell.