PH-dependent reduction potentials and proton-coupled electron transfer mechanisms in hydrogen-producing nickel molecular electrocatalysts

Samantha Horvath, Laura E. Fernandez, Aaron M. Appel, Sharon Hammes-Schiffer

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36 Citations (Scopus)

Abstract

The nickel-based P2Ph N2Bn electrocatalysts comprised of a nickel atom and two 1,5-dibenzyl-3,7-diphenyl-1, 5-diaza-3,7-diphosphacyclooctane ligands catalyze H2 production in acetonitrile. Recent electrochemical experiments revealed a linear dependence of the NiII/I reduction potential on pH with a slope of 57 mV/pH unit, implicating a proton-coupled electron transfer (PCET) process with the same number of electrons and protons transferred. The combined theoretical and experimental studies herein provide an explanation for this pH dependence in the context of the overall proposed catalytic mechanism. In the proposed mechanisms, the catalytic cycle begins with a series of intermolecular proton transfers from an acid to the pendant amine ligand and electrochemical electron transfers to the nickel center to produce the doubly protonated Ni0 species, a precursor to H2 evolution. The calculated Ni II/I reduction potentials of the doubly protonated species are in excellent agreement with the experimentally observed reduction potential in the presence of strong acid, suggesting that the catalytically active species leading to the peak observed in these cyclic voltammetry (CV) experiments is doubly protonated. The NiI/0 reduction potential was found to be slightly more positive than the NiII/I reduction potential, indicating that the NiI/0 reduction occurs spontaneously after the NiII/I reduction, as implied by the experimental observation of a single CV peak. These results suggest that the PCET process observed in the CV experiments is a two-electron/two-proton process corresponding to an initial double protonation followed by two reductions. On the basis of the experimental and theoretical data, the complete thermodynamic scheme and the Pourbaix diagram were generated for this catalyst. The Pourbaix diagram, which identifies the most thermodynamically stable species at each reduction potential and pH value, illustrates that this catalyst undergoes different types of PCET processes for various pH ranges. These thermodynamic insights will aid in the design of more effective molecular catalysts for H2 production.

Original languageEnglish
Pages (from-to)3643-3652
Number of pages10
JournalInorganic Chemistry
Volume52
Issue number7
DOIs
Publication statusPublished - Apr 1 2013

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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