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Title: Comment on 'New Insights in the Electrocatalytic Proton Reduction and Hydrogen Oxidation by Bioinspired Catalysts: A DFT Investigation'

Abstract

In the title paper, Vetere et al. reported a computational investigation of the mechanism of oxidation of H 2 / proton reduction using a model nickel complex for nickel-based electrocatalysts with cyclic phosphorous ligands incorporating pendant amines. These catalysts are attracting considerable attention owing to their high turn-over rates and relatively low overpotentials. These authors interpreted the results of their calculations as evidence for a symmetric bond breaking (forming) of H 2 directly to (from) two protonated amines in concert with a 2-electron reduction of the Ni(II) site to form a Ni(0) di-proton state. We show here that this interpretation is erroneous as we report the structure of an heterolytic cleavage transition state consistent with the presence of the Ni(II) center acting as a Lewis acid and of the pendant amines acting as Lewis bases. We determined the associated intrinsic reaction coordinate (IRC) pathway connecting the di-hydrogen (η 2-H 2) adduct and a hydride-proton state. We also characterize differently the nature of the transition state reported by these authors. H 2 oxidation / proton reduction with this class of catalysts is a heterolytic process.

Authors:
; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
1015257
Report Number(s):
PNNL-SA-76591
39981; KC0307010; TRN: US201111%%517
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Physical Chemistry A, 115(18):4861-4865; Journal Volume: 115; Journal Issue: 18
Country of Publication:
United States
Language:
English
Subject:
08 HYDROGEN; ADDUCTS; AMINES; CATALYSTS; CLEAVAGE; ELECTROCATALYSTS; HYDROGEN; LEWIS ACIDS; LEWIS BASES; NICKEL; OXIDATION; PROTONS; catalysis, nickel complexes; Environmental Molecular Sciences Laboratory

Citation Formats

Dupuis, Michel, Chen, Shentan, Raugei, Simone, DuBois, Daniel L, and Bullock, R Morris. Comment on 'New Insights in the Electrocatalytic Proton Reduction and Hydrogen Oxidation by Bioinspired Catalysts: A DFT Investigation'. United States: N. p., 2011. Web. doi:10.1021/jp111479z.
Dupuis, Michel, Chen, Shentan, Raugei, Simone, DuBois, Daniel L, & Bullock, R Morris. Comment on 'New Insights in the Electrocatalytic Proton Reduction and Hydrogen Oxidation by Bioinspired Catalysts: A DFT Investigation'. United States. doi:10.1021/jp111479z.
Dupuis, Michel, Chen, Shentan, Raugei, Simone, DuBois, Daniel L, and Bullock, R Morris. 2011. "Comment on 'New Insights in the Electrocatalytic Proton Reduction and Hydrogen Oxidation by Bioinspired Catalysts: A DFT Investigation'". United States. doi:10.1021/jp111479z.
@article{osti_1015257,
title = {Comment on 'New Insights in the Electrocatalytic Proton Reduction and Hydrogen Oxidation by Bioinspired Catalysts: A DFT Investigation'},
author = {Dupuis, Michel and Chen, Shentan and Raugei, Simone and DuBois, Daniel L and Bullock, R Morris},
abstractNote = {In the title paper, Vetere et al. reported a computational investigation of the mechanism of oxidation of H2 / proton reduction using a model nickel complex for nickel-based electrocatalysts with cyclic phosphorous ligands incorporating pendant amines. These catalysts are attracting considerable attention owing to their high turn-over rates and relatively low overpotentials. These authors interpreted the results of their calculations as evidence for a symmetric bond breaking (forming) of H2 directly to (from) two protonated amines in concert with a 2-electron reduction of the Ni(II) site to form a Ni(0) di-proton state. We show here that this interpretation is erroneous as we report the structure of an heterolytic cleavage transition state consistent with the presence of the Ni(II) center acting as a Lewis acid and of the pendant amines acting as Lewis bases. We determined the associated intrinsic reaction coordinate (IRC) pathway connecting the di-hydrogen (η2-H2) adduct and a hydride-proton state. We also characterize differently the nature of the transition state reported by these authors. H2 oxidation / proton reduction with this class of catalysts is a heterolytic process.},
doi = {10.1021/jp111479z},
journal = {Journal of Physical Chemistry A, 115(18):4861-4865},
number = 18,
volume = 115,
place = {United States},
year = 2011,
month = 5
}
  • In the title paper, Vetere et al. reported a computational investigation of the mechanism of H 2 oxidation/proton reduction using a model of nickel-based electrocatalysts that incorporates pendant amines in cyclic phosphorus ligands. These catalysts are attracting considerable attention owing to their high turnover rates and relatively low overpotentials. These authors interpreted the results of their calculations as evidence for a symmetric bond cleavage of H 2 leading directly to two protonated amines in concert with a two-electron reduction of the Ni(II) site to form a Ni(0) diproton state. Proton reduction would involve a reverse symmetric bond formation.more » We report here an analysis that refutes the interpretation by these authors. We give, for the same model system, the structure of a heterolytic cleavage transition state consistent with the presence of the Ni(II) center acting as a Lewis acid and the pendant amines acting as Lewis bases. We present the associated intrinsic reaction coordinate (IRC) pathway connecting the dihydrogen (η 2-H 2 ) adduct and a hydride–proton state. We report also the transition state and associated IRC for the proton rearrangement from a hydride–proton state to a diproton state. Finally, we complete the characterization of the transition state reported by Vetere et al. through a determination of the corresponding IRC. In summary, H 2 oxidation/proton reduction with this class of catalysts involves a heterolytic bond breaking/formation.« less
  • The design of molecular electrocatalysts for H2 oxidation and production is important for the development of alternative renewable energy sources that are abundant, inexpensive, and environmentally benign. Recently nickel-based molecular electrocatalysts with pendant amines that act as proton relays for the nickel center were shown to effectively catalyze H2 oxidation and production. We developed a quantum mechanical approach for studying proton-coupled electron transfer processes in these types of molecular electrocatalysts. This theoretical approach is applied to a nickel-based catalyst in which phosphorous atoms are directly bonded to the nickel center and nitrogen atoms of the ligand rings act as protonmore » relays. The cataly c step of interest involves electron transfer between the nickel complex and the electrode as well as intramolecular proton transfer between the nickel and nitrogen atoms. This process can occur sequentially, with either the electron or proton transferring first, or concertedly, with the electron and proton transferring simultaneously without a stable intermediate. The heterogeneous rate constants are calculated as functions of overpotential for the concerted electron-proton transfer reaction and the two electron transfer reactions in the sequential mechanisms. Our calculations illustrate that the concerted electron-proton transfer standard rate constant will increase as the equilibrium distance between the nickel and nitrogen atoms decreases and as the nitrogen atoms become more mobile to facilitate the contraction of this distance. This approach assists in the identification of the favored mechanisms under various experimental conditions and provides insight into the qualitative impact of substituents on the nitrogen and phosphorous atoms. This research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under FWP 56073.« less