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Selectivity for HCO_2^– over H_2 in the Electrochemical Catalytic Reduction of CO_2 by (POCOP)IrH_2

Johnson, Samantha I. and Nielsen, Robert J. and Goddard, William A., III (2016) Selectivity for HCO_2^– over H_2 in the Electrochemical Catalytic Reduction of CO_2 by (POCOP)IrH_2. ACS Catalysis, 6 (10). pp. 6362-6371. ISSN 2155-5435. http://resolver.caltech.edu/CaltechAUTHORS:20160902-085020686

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Abstract

It has been demonstrated experimentally that electrochemical CO_2 reduction catalyzed by (POCOP)IrH_2 ([C_6H_3-2,6-[OP(tBu)_2]_2]IrH_2) produces formate without significant H_2. We use first-principles density functional theory (M06) including Poisson–Boltzmann solvation to determine the detailed atomistic mechanism and illuminate strategies for designing formate-selective catalysts. A mechanism involving hydride transfer from Ir^(III) dihydride explains the selectivity for formate over H_2 and is corroborated by reduction potential (irreversible reduction of (POCOP)Ir(H)(NCMe)_2^+ at ca. −1.3 V vs NHE, in comparison to −1.31 V vs NHE calculated for one-electron reduction of Ir^(III)(H)(NCMe)_2^+) and turnover frequency. We find that several thermodynamically favorable pathways exist for the hydrogen evolution reaction (HER) from both Ir^(III)(H)_2 and Ir^I–H^– but are kinetically hindered, posing computed activation barriers above 25 kcal/mol at pH 7. However, with formate or bicarbonate acting as cocatalyst, the barriers are lowered to 18.8 kcal/mol. The preference of (POCOP)Ir to form a dihydride instead of a dihydrogen adduct also disfavors the HER and facilitates catalyst regeneration. In contrast, substituting cobalt for iridium raises the barrier for hydride transfer to CO_2 by 12.0 kcal/mol and lowers the required reduction potential to −1.65 V vs NHE. Calculated driving forces for hydride transfer from Ir^I and Ir^(III) intermediates illustrate different strategies for positioning the hydricity relative to the thermodynamic hydricities of H_2/H^+ and HCOO^–/CO_2. The data support an approach of selecting a hydricity that is just thermodynamically able to reduce CO_2. The effect of solvation on calculated driving forces for hydride transfer is also discussed.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1021/acscatal.6b01755DOIArticle
http://pubs.acs.org/doi/abs/10.1021/acscatal.6b01755PublisherArticle
http://pubs.acs.org/doi/suppl/10.1021/acscatal.6b01755PublisherSupporting Information
ORCID:
AuthorORCID
Nielsen, Robert J.0000-0002-7962-0186
Goddard, William A., III0000-0003-0097-5716
Additional Information:© 2016 American Chemical Society. Received: June 21, 2016; Revised: August 10, 2016; Publication Date (Web): August 15, 2016. This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993. S.I.J. is supported by a National Science Foundation Graduate Research Fellowship under Grant No. DGE-1144469 and by the generous support of the Resnick Sustainability Institute. The authors declare no competing financial interest.
Group:Resnick Sustainability Institute, JCAP
Funders:
Funding AgencyGrant Number
Department of Energy (DOE)DE-SC0004993
NSF Graduate Research FellowshipDGE-1144469
Resnick Sustainability Institute UNSPECIFIED
Subject Keywords:density functional theory, electrochemical CO2 reduction, reaction mechanism, chemoselectivity, hydricity, hydrogen evolution
Record Number:CaltechAUTHORS:20160902-085020686
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20160902-085020686
Official Citation:Selectivity for HCO2– over H2 in the Electrochemical Catalytic Reduction of CO2 by (POCOP)IrH2 Samantha I. Johnson, Robert J. Nielsen, and William A. Goddard, III ACS Catalysis 2016 6 (10), 6362-6371 DOI: 10.1021/acscatal.6b01755
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:70140
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:02 Sep 2016 16:29
Last Modified:21 Sep 2017 18:38

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