Electrochemical conversion of carbon dioxide (CO(2)) to carbon monoxide (CO) is a potentially useful step in the desirable transformation of the greenhouse gas to fuels and commodity chemicals. We have found that modification of iron tetraphenylporphyrin through the introduction of phenolic groups in all ortho and ortho' positions of the phenyl groups considerably speeds up catalysis of this reaction by the electrogenerated iron(0) complex. The catalyst, which uses one of the most earth-abundant metals, manifests a CO faradaic yield above 90% through 50 million turnovers over 4 hours of electrolysis at low overpotential (0.465 volt), with no observed degradation. The basis for the enhanced activity appears to be the high local concentration of protons associated with the phenolic hydroxyl substituents.

The direct and catalyzed electrochemistry of CO(2) partake in the contemporary attempts to reduce this inert molecule to fuels by means of solar energy, either directly, after conversion of light to electricity, or indirectly in that all elements of comprehension derived from electrochemical experiments can be used in the design and interpretation of photochemical experiments. Following reviews of the activity in the field until 2007-2008, the present review reports more recent findings even if their interpretation remains uncertain. It also develops useful notions that allow analyzing and comparing more rigorously the performances of existing catalysts when the necessary data are available. Among the general trends that transpire presently and are likely to be the object of active future work emphasis is put on the favorable role of acid addition in homogeneous catalytic systems and on the crucial chemical role of the electrode material in heterogeneous catalysis.

The search for efficient catalysts to face modern energy challenges requires evaluation and comparison through reliable methods. Catalytic current efficiencies may be the combination of many factors besides the intrinsic chemical properties of the catalyst. Defining turnover number and turnover frequency (TOF) as reflecting these intrinsic chemical properties, it is shown that catalysts are not characterized by their TOF and their overpotential (η) as separate parameters but rather that the parameters are linked together by a definite relationship. The log TOF-η relationship can often be linearized, giving rise to a Tafel law, which allows the characterization of the catalyst by the value of the TOF at zero overpotential (TOF(0)). Foot-of-the-wave analysis of the cyclic voltammetric catalytic responses allows the determination of the TOF, log TOF-η relationship, and TOF(0), regardless of the side-phenomena that interfere at high current densities, preventing the expected catalytic current plateau from being reached. Strategies for optimized preparative-scale electrolyses may then be devised on these bases. The validity of this methodology is established on theoretical grounds and checked experimentally with examples taken from the catalytic reduction of CO(2) by iron(0) porphyrins.

The starting point of this study of through-space substituent effects on the catalysis of the electrochemical CO-to-CO conversion by iron(0) tetraphenylporphyrins is the linear free energy correlation between through-structure electronic effects and the iron(I/0) standard potential that we established separately. The introduction of four positively charged trimethylanilinium groups at the para positions of the tetraphenylporphyrin (TPP) phenyls results in an important positive deviation from the correlation and a parallel improvement of the catalytic Tafel plot. The assignment of this catalysis boosting effect to the Coulombic interaction of these positive charges with the negative charge borne by the initial Fe-CO adduct is confirmed by the negative deviation observed when the four positive charges are replaced by four negative charges borne by sulfonate groups also installed in the para positions of the TPP phenyls. The climax of this strategy of catalysis boosting by means of Coulombic stabilization of the initial Fe-CO adduct is reached when four positively charged trimethylanilinium groups are introduced at the ortho positions of the TPP phenyls. The addition of a large concentration of a weak acid-phenol-helps by cleaving one of the C-O bonds of CO. The efficiency of the resulting catalyst is unprecedented, as can be judged by the catalytic Tafel plot benchmarking with all presently available catalysts of the electrochemical CO-to-CO conversion. The maximal turnover frequency (TOF) is as high as 10 s and is reached at an overpotential of only 220 mV; the extrapolated TOF at zero overpotential is larger than 300 s. This catalyst leads to a highly selective formation of CO (practically 100%) in spite of the presence of a high concentration of phenol, which could have favored H evolution. It is also very stable, showing no significant alteration after more than 80 h of electrolysis.

Current–potential responses in cyclic voltammetry and in preparative‐scale electrolysis are established for two‐electron two‐step homogeneous molecular catalysis after systematic categorization of the various possible reaction schemes. They allow the derivation of catalytic Tafel plots, reflecting the intrinsic properties of the catalysts independent of contingent electrolysis parameters. They serve as a rational basis for benchmarking catalysts of the same electrochemical reaction within the same log(turnover frequency) versus overpotential plane of merit.

and pursued his graduate studies under the guidance of Prof. Jean-Michel Save ´ant and Dr. Philippe Hapiot at the University of Paris-Diderot (Paris 7), where he received his Ph.D. in 2000. After a year as a postdoctoral fellow at the University of Rochester, working with Prof. J. P. Dinnocenzo, he joined the faculty at the University of Paris-Diderot as an associate professor. He was promoted to professor in 2007. His interests include mechanisms and reactivity in electron transfer chemistry with particular recent emphasis on electrochemical and theoretical approaches to proton-coupled electron transfer processes.

Two derivatives of iron tetraphenylporphyrin bearing prepositioned phenolic functionalities on two of the opposed phenyl groups prove to be remarkable catalysts for the reduction of CO2 to CO when generated electrochemically at the Fe(0) oxidation state. In one case, the same substituents are present on the two other phenyls, whereas in the other the two other phenyls are perfluorinated. They are taken as examples of the possible role of pendant acid-base groups in molecular catalysis. The prepositioned phenol groups incorporated into the catalyst molecule induce strong stabilization of the initial Fe(0)CO2 adduct through H-bonding, confirmed by DFT calculations. This positive factor is partly counterbalanced by the necessity, resulting from the same stabilization, to inject an additional electron to trigger catalysis. Thanks to the preprotonation of the initial Fe(0)CO2 adduct, the potential required for this second electron transfer is not very distant from the potential at which the adduct is generated by addition of CO2 to the Fe(0) complex. The protonation step involves an internal phenolic group and the reprotonation of the phenoxide ion thus generated by added phenol. The prepositioned phenol groups thus play both the role of H-bonding stabilizers and high-concentration proton donors. They play the same role in the second electron transfer step which closes the catalytic loop concertedly with the breaking of one of the two C-O bonds of CO2 and with proton transfer. It is also remarkable that reprotonation by added phenol is concerted with the three other events.

Most of the electrocatalytic processes of interest in the resolution of modern energy challenges are associated with proton transfer. In the cases where heavy atom bond cleavage occurs concomitantly, the question arises of the exact nature of its coupling with proton-electron transfer within the catalytic cycle. The cleavage of a C-O bond in the catalyzed electrochemical conversion of CO2 to CO offers the opportunity to address this question. Electrochemically generated iron(0) porphyrins are efficient, specific, and durable catalysts provided they are coupled with Lewis or Brönsted acids. The cocatalyst properties of four Brönsted acids of increasing strength, water, trifluoroethanol, phenol, and acetic acid, have been systematically investigated. Preparative-scale electrolyses showed that carbon monoxide is the only product of the catalytic reaction. Methodic application of a nondestructive technique, cyclic voltammetry, with catalyst and CO2 concentrations, as well as H/D isotope effect, as diagnostic parameters allowed the dissection of the reaction mechanism. It appears that the key step of the reaction sequence consists of an electron transfer from the catalyst concerted with the cleavage of a C-O bond and the transfer of one proton. This is the second example, and an intermolecular version of such a concerted proton-electron bond-breaking reaction after a similar electrochemical process involving the cleavage of O-O bonds has been identified. It is the first time that a proton-electron transfer concerted with bond breaking has been uncovered as the crucial step in a catalytic multistep reaction.