Selectivity for HCO₂^– over H₂ in the Electrochemical Catalytic Reduction of CO₂ by (POCOP)IrH₂
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.
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.Attached Files
Supplemental Material - cs6b01755_si_001.pdf
Supplemental Material - cs6b01755_si_002.xyz
Files
Name | Size | Download all |
---|---|---|
md5:4650245b147c1bd2d5c7c1341548f0c3
|
2.1 MB | Preview Download |
md5:941f5542c13f9fe4b8cbc31e75f83880
|
230.5 kB | Download |
Additional details
- Eprint ID
- 70140
- DOI
- 10.1021/acscatal.6b01755
- Resolver ID
- CaltechAUTHORS:20160902-085020686
- Department of Energy (DOE)
- DE-SC0004993
- NSF Graduate Research Fellowship
- DGE-1144469
- Resnick Sustainability Institute
- Created
-
2016-09-02Created from EPrint's datestamp field
- Updated
-
2021-11-11Created from EPrint's last_modified field
- Caltech groups
- Resnick Sustainability Institute, JCAP