Hachem Dridi

In the framework of modern energy challenges, the reduction of CO₂ into fuels calls for electrogenerated low-valent transition metal complexes catalysts designed with considerable ingenuity and sophistication. For this reason, the report that a molecule as simple as protonated pyridine (PyH⁺) could catalyze the formation of methanol from the reduction of CO₂ on a platinum electrode triggered great interest and excitement. Further investigations revealed that no methanol is produced. It appears that CO₂ is not really reduced but rather participates, on the basis of its aquation into carbonic acid, in hydrogen evolution. Actually, the situation is not that straightforward, as revealed by scrutinizing what happens at the platinum electrode surface. The present study confirms the lack of methanol formation upon bulk electrolysis of PyH⁺ solutions at Pt and provides a detailed account of the Faradaic yield for H₂ production as a function of the electrode potential, but the main finding is that CO₂ reduction is accompanied by a strong inhibition of the electrode process taking place when it is carried out in the presence of acids such as PyH⁺ and AcOH. Cyclic voltammetry and in situ infrared spectroscopy were closely combined to investigate and understand the nature and consequences of the inhibition process. Constant comparison between the two acids was required to decipher the course of the reaction owing to the fact that the IR responses are perturbed by PyH⁺ adsorption. It finally appears that inhibition is caused by the reduction of CO₂ into CO, whose high affinity with platinum triggers the formation of a Pt–CO film that prevents the reaction process. Thus, a paradoxical situation develops in which the high affinity of Pt for CO helps to decrease the overpotential for the reduction of CO₂ and therefore blocks the electrode, preventing the reaction process.

Despite decades of active attention, important problems remain pending in the catalysis of dioxygen reduction by iron porphyrins in water in terms of selectivity and mechanisms. This is what happens, for example, for the distinction between heterogeneous and homogeneous catalysis for soluble porphyrins, for the estimation of H2O2/H2O product selectivity, and for the determination of the reaction mechanism in the two situations. With water-soluble iron tetrakis(N-methyl-4-pyridyl)porphyrin as an example, procedures are described that allow one to operate this distinction and determine the H2O2/H2O product ratio in each case separately. It is noteworthy that, despite the weak adsorption of the iron(II) porphyrin on the glassy carbon electrode, the contribution of the adsorbed complex to catalysis rivals that of its solution counterpart. Depending on the electrode potential, two successive catalytic pathways have been identified and characterized in terms of current–potential responses and H2O2/H2O selectivi...

Molecular catalysis of H₂ production from the electrochemical reduction of acids by transition-metal complexes is one of the key issues of modern energy challenges. The question of whether it proceeds heterolytically (through reaction of an initially formed metal hydride with the acid) or homolytically (through symmetrical coupling of two molecules of hydride) has to date received inconclusive answers for a quite simple reason: the theoretical bases for criteria allowing the distinction between homolytic and heterolytic pathways in nondestructive methods such as cyclic voltammetry have been lacking heretofore. They are provided here, allowing the distinction between the two pathways. The theoretical predictions and the diagnosing strategy are illustrated by catalysis of the reduction of phenol, acetic acid, and protonated triethylamine by electrogenerated iron(0) tetraphenylporphyrin. Rather than being an intrinsic property of the catalytic system, the occurrence of either a heterolytic or a homolytic pathway results from their competition as a function of the concentrations of acid and catalyst and the rate constants for hydride formation and H₂ evolution by hydride protonation or dimerization.