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 [Ni^II(bztpen)]²⁺ and [Fe^II(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 [Fe^II(bztpen)]²⁺, [Ni^II(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 [Co^II(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.