Title: Mechanism of Catalytic O₂ Reduction by Iron Tetraphenylporphyrin

Abstract

The catalytic reduction of O₂ to H₂O is important for energy transduction in both synthetic and natural systems. Herein, we report a kinetic and thermochemical study of the oxygen reduction reaction (ORR) catalyzed by iron tetraphenylporphyrin (Fe(TPP)) in N,N'-dimethylformamide using decamethylferrocene as a soluble reductant and para-toluenesulfonic acid (pTsOH) as the proton source. This work identifies and characterizes catalytic intermediates and their thermochemistry, providing a detailed mechanistic understanding of the system. Specifically, reduction of the ferric porphyrin, [Fe^III(TPP)]⁺, forms the ferrous porphyrin, Fe^II(TPP), which binds O₂ reversibly to form the ferric-superoxide porphyrin complex, Fe^III(TPP)(O₂^•–). The temperature dependence of both the electron transfer and O₂ binding equilibrium constants has been determined. Kinetic studies over a range of concentrations and temperatures show that the catalyst resting state changes during the course of each catalytic run, necessitating the use of global kinetic modeling to extract rate constants and kinetic barriers. The rate-determining step in oxygen reduction is the protonation of Fe^III(TPP)(O₂^•–) by pTsOH, which proceeds with a substantial kinetic barrier. Computational studies indicate that this barrier for proton transfer arises from an unfavorable preassociation of the proton donor with the superoxide adduct and a transition state that requires significant desolvation of the protonmore » donor. Together, these results are the first example of oxygen reduction by iron tetraphenylporphyrin where the pre-equilibria among ferric, ferrous, and ferric-superoxide intermediates have been quantified under catalytic conditions. This work gives a generalizable model for the mechanism of iron porphyrin-catalyzed ORR and provides an unusually complete mechanistic study of an ORR reaction. More broadly, this study also highlights the kinetic challenges for proton transfer to catalytic intermediates in organic media.« less

Authors:

Pegis, Michael L.  ^[1];

• Search DOE PAGES for author "Pegis, Michael L."
• Search DOE PAGES for ORCID "0000-0001-6686-1717"

Martin, Daniel J. ^[2]; Wise, Catherine F. ^[2]; Brezny, Anna C. ^[2]; Johnson, Samantha I. ^[3]; Johnson, Lewis E.  ^[4];

• Search DOE PAGES for author "Johnson, Lewis E."
• Search DOE PAGES for ORCID "0000-0002-7412-073X"

Kumar, Neeraj  ^[5];

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• Search DOE PAGES for ORCID "0000-0001-6713-2129"

Raugei, Simone  ^[6];

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• Search DOE PAGES for ORCID "0000-0001-9118-8480"

Mayer, James M.  ^[2]

• Search DOE PAGES for author "Mayer, James M."
• Search DOE PAGES for ORCID "0000-0002-3943-5250"

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+ Show Author Affiliations

1. Yale Univ., New Haven, CT (United States). Dept. of Chemistry; Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Dept. of Chemistry
2. Yale Univ., New Haven, CT (United States). Dept. of Chemistry
3. Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Center for Molecular Electrocatalysts and Biological Sciences Division
4. Univ. of Washington, Seattle, WA (United States). Dept. of Chemistry
5. Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Biological Sciences Division
6. Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Center for Molecular Electrocatalysts

Publication Date:

Wed May 01 00:00:00 EDT 2019

Research Org.:

Energy Frontier Research Centers (EFRC) (United States). Center for Molecular Electrocatalysis (CME); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center (NERSC)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:

1527026

Grant/Contract Number:  

AC02-05CH11231

Resource Type:

Accepted Manuscript

Journal Name:

Journal of the American Chemical Society

Additional Journal Information:

Journal Volume: 141; Journal Issue: 20; Journal ID: ISSN 0002-7863

Publisher:

American Chemical Society (ACS)

Country of Publication:

United States

Language:

English

Subject:

36 MATERIALS SCIENCE