Selectivity for HCO2– over H2 in the Electrochemical Catalytic Reduction of CO2 by (POCOP)IrH2

doi:10.1021/acscatal.6b01755

Title: Selectivity for HCO₂^– over H₂ in the Electrochemical Catalytic Reduction of CO₂ by (POCOP)IrH₂

Abstract

It has been demonstrated experimentally that electrochemical CO₂ reduction catalyzed by (POCOP)IrH₂ ([C₆H₃-2,6-[OP(tBu)₂]₂]IrH₂) produces formate without significant H₂. 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 H₂ 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 CO₂ by 12.0 kcal/mol and lowers the required reduction potential to –1.65 V vs NHE. Calculated driving forces for hydride transfer from IrI and IrIII intermediates illustrate different strategies formore »« less

Authors:

Johnson, Samantha I. ^[1]; Nielsen, Robert J. ^[1]; Goddard, III, William A. ^[1]

California Inst. of Technology (CalTech), Pasadena, CA (United States)

Publication Date:: 2016-08-15

Research Org.:: California Institute of Technology (CalTech), Pasadena, CA (United States)

Sponsoring Org.:: USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:: 1467632

Grant/Contract Number:: SC0004993

Resource Type:: Accepted Manuscript

Journal Name:: ACS Catalysis

Additional Journal Information:: Journal Volume: 6; Journal Issue: 10; Journal ID: ISSN 2155-5435

Publisher:: American Chemical Society (ACS)

Country of Publication:: United States

Language:: English

Subject:: 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; chemoselectivity; density functional theory; electrochemical CO2 reduction; hydricity; hydrogen evolution; reaction mechanism

Citation Formats


                    Johnson, Samantha I., Nielsen, Robert J., and Goddard, III, William A. Selectivity for HCO2– over H2 in the Electrochemical Catalytic Reduction of CO2 by (POCOP)IrH2.  United States: N. p., 2016. 
        Web.  doi:10.1021/acscatal.6b01755.

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                    Johnson, Samantha I., Nielsen, Robert J., & Goddard, III, William A. Selectivity for HCO2– over H2 in the Electrochemical Catalytic Reduction of CO2 by (POCOP)IrH2.  United States.  doi:10.1021/acscatal.6b01755.

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                    Johnson, Samantha I., Nielsen, Robert J., and Goddard, III, William A. Mon .  
        "Selectivity for HCO2– over H2 in the Electrochemical Catalytic Reduction of CO2 by (POCOP)IrH2".  United States.  doi:10.1021/acscatal.6b01755.  https://www.osti.gov/servlets/purl/1467632.

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@article{osti_1467632,

  title        = {Selectivity for HCO2– over H2 in the Electrochemical Catalytic Reduction of CO2 by (POCOP)IrH2},

  author       = {Johnson, Samantha I. and Nielsen, Robert J. and Goddard, III, William A.},

  abstractNote = {It has been demonstrated experimentally that electrochemical CO2 reduction catalyzed by (POCOP)IrH2 ([C6H3-2,6-[OP(tBu)2]2]IrH2) produces formate without significant H2. 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 IrIII dihydride explains the selectivity for formate over H2 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 IrIII(H)(NCMe)2+) and turnover frequency. We find that several thermodynamically favorable pathways exist for the hydrogen evolution reaction (HER) from both IrIII(H)2 and IrI–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 CO2 by 12.0 kcal/mol and lowers the required reduction potential to –1.65 V vs NHE. Calculated driving forces for hydride transfer from IrI and IrIII intermediates illustrate different strategies for positioning the hydricity relative to the thermodynamic hydricities of H2/H+ and HCOO–/CO2. The data support an approach of selecting a hydricity that is just thermodynamically able to reduce CO2. In conclusion, the effect of solvation on calculated driving forces for hydride transfer is also discussed.},

  doi          = {10.1021/acscatal.6b01755},

  journal      = {ACS Catalysis},

  number       = 10,

  volume       = 6,

  place        = {United States},

  year         = {2016},

  month        = {8}

}

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DOI: 10.1021/acscatal.6b01755

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Title: Selectivity for HCO2– over H2 in the Electrochemical Catalytic Reduction of CO2 by (POCOP)IrH2

Abstract

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