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Title: In situ characterization of cofacial Co(IV) centers in Co 4O 4 cubane: Modeling the high-valent active site in oxygen-evolving catalysts

The Co 4O 4 cubane is a representative structural model of oxidic cobalt oxygen evolving catalysts (Co-OECs). The Co-OECs are active when residing at two oxidation levels above an all Co(III) resting state. This doubly oxidized Co(IV) 2 state may be captured in a Co(III) 2(IV) 2 cubane. We demonstrate that the Co(III) 2(IV) 2 cubane may be electrochemically generated and the electronic properties of this unique high-valent state may be probed by in situ spectroscopy. Intervalence charge transfer (IVCT) bands in the near-IR are observed for the Co(III) 2(IV) 2 cubane, and spectroscopic analysis together with electrochemical kinetics measurements reveal a larger reorganization energy and a smaller electron transfer rate constant for the doubly versus singly oxidized cubane. Spectroelectrochemical X-ray absorption data further reveal systematic spectral changes with successive oxidations from the cubane resting state. Electronic structure calculations correlated to experimental data suggest that this state is best represented as a localized, antiferromagnetically coupled Co(IV) 2 dimer. The exchange coupling in the cofacial Co(IV) 2 site allows for parallels to be drawn between the electronic structure of the Co 4O 4 cubane model system and the high valent active site of the Co-OEC, with specific emphasis on the manifestationmore » of a doubly oxidized Co(IV) 2 center on O–O bond formation.« less
Authors:
 [1] ;  [2] ;  [2] ;  [2] ;  [1] ;  [3] ; ORCiD logo [1]
  1. Harvard Univ., Cambridge, MA (United States). Dept. of Chemistry and Chemical Biology
  2. Argonne National Lab. (ANL), Argonne, IL (United States). Chemical Sciences and Engineering Division
  3. Argonne National Lab. (ANL), Argonne, IL (United States). Chemical Sciences and Engineering Division; Northwestern Univ., Evanston, IL (United States). Dept. of Chemistry
Publication Date:
Grant/Contract Number:
AC02-06CH11357
Type:
Accepted Manuscript
Journal Name:
Proceedings of the National Academy of Sciences of the United States of America
Additional Journal Information:
Journal Volume: 114; Journal Issue: 15; Journal ID: ISSN 0027-8424
Publisher:
National Academy of Sciences, Washington, DC (United States)
Research Org:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); National Science Foundation (NSF)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; renewable energy; Water-splitting; electrocatalysis; exchange coupling; mixed valency; oxygen evolution reaction; solar-to-fuels
OSTI Identifier:
1373381

Brodsky, Casey N., Hadt, Ryan G., Hayes, Dugan, Reinhart, Benjamin J., Li, Nancy, Chen, Lin X., and Nocera, Daniel G.. In situ characterization of cofacial Co(IV) centers in Co4O4 cubane: Modeling the high-valent active site in oxygen-evolving catalysts. United States: N. p., Web. doi:10.1073/pnas.1701816114.
Brodsky, Casey N., Hadt, Ryan G., Hayes, Dugan, Reinhart, Benjamin J., Li, Nancy, Chen, Lin X., & Nocera, Daniel G.. In situ characterization of cofacial Co(IV) centers in Co4O4 cubane: Modeling the high-valent active site in oxygen-evolving catalysts. United States. doi:10.1073/pnas.1701816114.
Brodsky, Casey N., Hadt, Ryan G., Hayes, Dugan, Reinhart, Benjamin J., Li, Nancy, Chen, Lin X., and Nocera, Daniel G.. 2017. "In situ characterization of cofacial Co(IV) centers in Co4O4 cubane: Modeling the high-valent active site in oxygen-evolving catalysts". United States. doi:10.1073/pnas.1701816114. https://www.osti.gov/servlets/purl/1373381.
@article{osti_1373381,
title = {In situ characterization of cofacial Co(IV) centers in Co4O4 cubane: Modeling the high-valent active site in oxygen-evolving catalysts},
author = {Brodsky, Casey N. and Hadt, Ryan G. and Hayes, Dugan and Reinhart, Benjamin J. and Li, Nancy and Chen, Lin X. and Nocera, Daniel G.},
abstractNote = {The Co4O4 cubane is a representative structural model of oxidic cobalt oxygen evolving catalysts (Co-OECs). The Co-OECs are active when residing at two oxidation levels above an all Co(III) resting state. This doubly oxidized Co(IV)2 state may be captured in a Co(III)2(IV)2 cubane. We demonstrate that the Co(III)2(IV)2 cubane may be electrochemically generated and the electronic properties of this unique high-valent state may be probed by in situ spectroscopy. Intervalence charge transfer (IVCT) bands in the near-IR are observed for the Co(III)2(IV)2 cubane, and spectroscopic analysis together with electrochemical kinetics measurements reveal a larger reorganization energy and a smaller electron transfer rate constant for the doubly versus singly oxidized cubane. Spectroelectrochemical X-ray absorption data further reveal systematic spectral changes with successive oxidations from the cubane resting state. Electronic structure calculations correlated to experimental data suggest that this state is best represented as a localized, antiferromagnetically coupled Co(IV)2 dimer. The exchange coupling in the cofacial Co(IV)2 site allows for parallels to be drawn between the electronic structure of the Co4O4 cubane model system and the high valent active site of the Co-OEC, with specific emphasis on the manifestation of a doubly oxidized Co(IV)2 center on O–O bond formation.},
doi = {10.1073/pnas.1701816114},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
number = 15,
volume = 114,
place = {United States},
year = {2017},
month = {3}
}

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In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co2+
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