Publication
756
J. Am. Chem. Soc., 136 (39), 13727-13734, 2014
DOI:10.1021/ja505845t
|
|
|
|
|
|
|
Molecular catalysis of H2 evolution. Diagnosing heterolytic vs. homolytic pathways |
|
|
|
Cyrille Costentin, Hachem Dridi, and Jean-Michel Savéant
Université Paris Diderot, Sorbonne Paris Cité, Laboratoire d’Electrochimie Moléculaire, Unité Mixte de Recherche Université - CNRS No 7591, Bâtiment Lavoisier, 15 rue Jean de Baïf, 75205 Paris Cedex 13, France
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. |