co2
Post on 09-Apr-2016
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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.
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