DFT study of the mechanism of hydrogen evolution catalysed by molecular Ni, Co and Fe catalysts containing a diamine–tripyridine ligand†
Abstract
Electrolysis of water to obtain hydrogen is a practical way to transform surplus electrical power into clean and sustainable hydrogen fuels. A series of molecular [M(bztpen)(sol)]2+ (M = Ni, Co, and Fe) complexes (bztpen = N-benzyl-N,N′,N′-tris(pyridine-2-ylmethyl)ethylenediamine) have been reported to be efficient electrocatalysts for H2 production from water reduction. Density functional calculations were performed to explore the reaction mechanisms of water reduction, catalyzed by these three catalysts. The calculations showed that [NiII(bztpen)]2+ and [FeII(bztpen)]2+ have quite similar mechanisms, involving four major steps, namely, one electron reduction, proton-coupled electron transfer, protonation of a pyridine ligand, and H–H bond formation. However, in contrast with [FeII(bztpen)]2+, [NiII(bztpen)]2+ prefers to be hexa-coordinated with a water molecule bound, and the dissociation of this water molecule is required to generate a more reactive penta-coordinated species. For [CoII(bztpen)]2+, the mechanism involves three steps, namely, two sequential proton-coupled electron transfer processes, followed by H–H bond formation. For all three catalysts, the first reduction is the rate-limiting step, and the H–H bond formation has a very facile barrier.