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Title: A Mn Bipyrimidine Catalyst Predicted To Reduce CO 2 at Lower Overpotential

Abstract

Experimentally, [(L)Mn(CO) 3] - (where L = bis-alkyl-substituted bipyridine) has been observed to catalyze the electrochemical reduction of CO 2 to CO in the presence of trifluoroethanol (TFEH). Here we report the atomistic level mechanism of complete catalytic cycles for this reaction, on the basis of DFT calculations (B3LYP-D3 with continuum solvation) of the free energies of reaction and activation, as well as reduction potentials for all catalytically relevant elementary steps. The highly exergonic homoconjugation and carbonation of TFE– play critical roles in reaction thermodynamics and kinetics, the overall half-reaction being 3CO 2 + 2TFEH + 2e → CO + H 2O + 2[F 3CCH 2OCO 2] - (calculated standard reduction potential: -1.49 V vs SCE). In the catalytic cycle for CO formation, CO 2 coordinates to [(L)Mn(CO) 3] - (1a, L = bpy), and the adduct is then protonated to form [(L)Mn(CO) 3(CO 2H)] (3a). 3a subsequently reacts to form [(L)Mn(CO) 4] 0 (5a) via one of two pathways: (a) TFEH-mediated dehydroxylation to [(L)Mn(CO) 4] + (4a), followed by one-electron reduction to 5a, or (b) under more reducing potentials, one-electron reduction to [(L)Mn(CO)3(CO2H)]- (3'a), followed by dehydroxylation to 5a. Pathway b has a lower activation energy by 2.2more » kcal mol –1. Consequently, the maximum catalytic turnover frequency (TOFmax) is achieved at ~-1.75 V vs SCE (~0.25 V overpotential). For the analogous bipyrimidine compound (not yet studied experimentally), reduction of 3b to 3'b occurs at a potential 0.5 V more positive than that of 3a, and the overpotential required to achieve TOF max is predicted to be lower by ~0.25 V. This improvement is, however, achieved at the price of a lower TOF max, and we predict that 1b has superior TOF at potentials above ~-1.6 V vs SCE. In addition, the various factors contributing to product selectivity (CO over H 2) are discussed.« less

Authors:
 [1];  [1];  [1];  [1]
  1. California Inst. of Technology (CalTech), Pasadena, CA (United States)
Publication Date:
Research Org.:
California Inst. of Technology (CalTech), Pasadena, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1467633
Grant/Contract Number:  
SC0004993
Resource Type:
Accepted Manuscript
Journal Name:
ACS Catalysis
Additional Journal Information:
Journal Volume: 5; Journal Issue: 4; 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

Citation Formats

Lam, Yan Choi, Nielsen, Robert J., Gray, Harry B., and Goddard, William A. A Mn Bipyrimidine Catalyst Predicted To Reduce CO2 at Lower Overpotential. United States: N. p., 2015. Web. doi:10.1021/cs501963v.
Lam, Yan Choi, Nielsen, Robert J., Gray, Harry B., & Goddard, William A. A Mn Bipyrimidine Catalyst Predicted To Reduce CO2 at Lower Overpotential. United States. doi:10.1021/cs501963v.
Lam, Yan Choi, Nielsen, Robert J., Gray, Harry B., and Goddard, William A. Mon . "A Mn Bipyrimidine Catalyst Predicted To Reduce CO2 at Lower Overpotential". United States. doi:10.1021/cs501963v. https://www.osti.gov/servlets/purl/1467633.
@article{osti_1467633,
title = {A Mn Bipyrimidine Catalyst Predicted To Reduce CO2 at Lower Overpotential},
author = {Lam, Yan Choi and Nielsen, Robert J. and Gray, Harry B. and Goddard, William A.},
abstractNote = {Experimentally, [(L)Mn(CO)3]- (where L = bis-alkyl-substituted bipyridine) has been observed to catalyze the electrochemical reduction of CO2 to CO in the presence of trifluoroethanol (TFEH). Here we report the atomistic level mechanism of complete catalytic cycles for this reaction, on the basis of DFT calculations (B3LYP-D3 with continuum solvation) of the free energies of reaction and activation, as well as reduction potentials for all catalytically relevant elementary steps. The highly exergonic homoconjugation and carbonation of TFE– play critical roles in reaction thermodynamics and kinetics, the overall half-reaction being 3CO2 + 2TFEH + 2e– → CO + H2O + 2[F3CCH2OCO2]- (calculated standard reduction potential: -1.49 V vs SCE). In the catalytic cycle for CO formation, CO2 coordinates to [(L)Mn(CO)3]- (1a, L = bpy), and the adduct is then protonated to form [(L)Mn(CO)3(CO2H)] (3a). 3a subsequently reacts to form [(L)Mn(CO)4]0 (5a) via one of two pathways: (a) TFEH-mediated dehydroxylation to [(L)Mn(CO)4]+ (4a), followed by one-electron reduction to 5a, or (b) under more reducing potentials, one-electron reduction to [(L)Mn(CO)3(CO2H)]- (3'a), followed by dehydroxylation to 5a. Pathway b has a lower activation energy by 2.2 kcal mol–1. Consequently, the maximum catalytic turnover frequency (TOFmax) is achieved at ~-1.75 V vs SCE (~0.25 V overpotential). For the analogous bipyrimidine compound (not yet studied experimentally), reduction of 3b to 3'b occurs at a potential 0.5 V more positive than that of 3a, and the overpotential required to achieve TOFmax is predicted to be lower by ~0.25 V. This improvement is, however, achieved at the price of a lower TOFmax, and we predict that 1b has superior TOF at potentials above ~-1.6 V vs SCE. In addition, the various factors contributing to product selectivity (CO over H2) are discussed.},
doi = {10.1021/cs501963v},
journal = {ACS Catalysis},
number = 4,
volume = 5,
place = {United States},
year = {2015},
month = {2}
}

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