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Title: Comprehensive Mechanisms of Electrocatalytic CO 2 Reduction by [Ir(bip)(ppy)(CH 3CN)](PF 6) 2

Abstract

Examples of transition metal complexes capable of the dual roles of light harvesting and catalysis of CO 2 reduction are rare. This self-sensitized approach simplifies systems and efficiencies; therefore complete understanding of mechanistic principles is essential for improving catalysts. Here we present a comprehensive study of dark reactions using electrochemical techniques to understand the multiple pathways for the selective reduction of CO 2 to CO by an example self-sensitized photocatalyst: [Ir(bip)(ppy)(CH 3CN)] 2+. (bip = 2,6-bis(benzimidazole)pyridine, ppy = 2-phenylpyridine). Cyclic voltammetry (CV) in acetonitrile under anhydrous conditions reveals electrocatalysis by a two electron cycle at -1.7 V vs Fc +/0 (denoted the cat-1 region) in which the metallocarboxylate formed by binding of Ir(I) to CO 2 is cleaved by CO 2 as the oxide acceptor. At -1.9 V (denoted the cat-2 region) the Ir(CO 2) intermediate is reduced and catalysis is accelerated. In the presence of water, the Ir(CO 2) is protonated to Ir(CO 2H) which is reduced at a potential less negative than -1.7 V and then the oxide acceptor is either CO 2 to form HCO 3- or protons to release H 2O and the conjugate base of the acid source. Further reduction of Ir(CO 2H) at cat-2more » again accelerates catalysis. Rates vary widely in these various regimes with the minimum kobs of 0.3 s -1 for anhydrous cat-1 to a maximum cat-2 rate of 2100 s -1 with 1% water. Competitive deactivation pathways were discovered as Ir-Ir dimerization without reacting with CO 2 or the formation of a hydride-bridged dinuclear complex during extended electrolysis at high water concentration. The Ir-Ir dimer was characterized by high-resolution mass spectrometry and X-ray absorption spectroscopy (XAS).« less

Authors:
ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]
  1. Brookhaven National Lab. (BNL), Upton, NY (United States)
Publication Date:
Research Org.:
Brookhaven National Lab. (BNL), Upton, NY (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES)
OSTI Identifier:
1637482
Report Number(s):
BNL-216111-2020-JAAM
Journal ID: ISSN 2155-5435
Grant/Contract Number:  
SC0012704
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
ACS Catalysis
Additional Journal Information:
Journal Volume: 10; Journal Issue: 11; 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

Manbeck, Gerald F., Polyansky, Dmitry E., and Fujita, Etsuko. Comprehensive Mechanisms of Electrocatalytic CO2 Reduction by [Ir(bip)(ppy)(CH3CN)](PF6)2. United States: N. p., 2020. Web. doi:10.1021/acscatal.9b04371.
Manbeck, Gerald F., Polyansky, Dmitry E., & Fujita, Etsuko. Comprehensive Mechanisms of Electrocatalytic CO2 Reduction by [Ir(bip)(ppy)(CH3CN)](PF6)2. United States. doi:10.1021/acscatal.9b04371.
Manbeck, Gerald F., Polyansky, Dmitry E., and Fujita, Etsuko. Wed . "Comprehensive Mechanisms of Electrocatalytic CO2 Reduction by [Ir(bip)(ppy)(CH3CN)](PF6)2". United States. doi:10.1021/acscatal.9b04371.
@article{osti_1637482,
title = {Comprehensive Mechanisms of Electrocatalytic CO2 Reduction by [Ir(bip)(ppy)(CH3CN)](PF6)2},
author = {Manbeck, Gerald F. and Polyansky, Dmitry E. and Fujita, Etsuko},
abstractNote = {Examples of transition metal complexes capable of the dual roles of light harvesting and catalysis of CO2 reduction are rare. This self-sensitized approach simplifies systems and efficiencies; therefore complete understanding of mechanistic principles is essential for improving catalysts. Here we present a comprehensive study of dark reactions using electrochemical techniques to understand the multiple pathways for the selective reduction of CO2 to CO by an example self-sensitized photocatalyst: [Ir(bip)(ppy)(CH3CN)]2+. (bip = 2,6-bis(benzimidazole)pyridine, ppy = 2-phenylpyridine). Cyclic voltammetry (CV) in acetonitrile under anhydrous conditions reveals electrocatalysis by a two electron cycle at -1.7 V vs Fc+/0 (denoted the cat-1 region) in which the metallocarboxylate formed by binding of Ir(I) to CO2 is cleaved by CO2 as the oxide acceptor. At -1.9 V (denoted the cat-2 region) the Ir(CO2) intermediate is reduced and catalysis is accelerated. In the presence of water, the Ir(CO2) is protonated to Ir(CO2H) which is reduced at a potential less negative than -1.7 V and then the oxide acceptor is either CO2 to form HCO3- or protons to release H2O and the conjugate base of the acid source. Further reduction of Ir(CO2H) at cat-2 again accelerates catalysis. Rates vary widely in these various regimes with the minimum kobs of 0.3 s-1 for anhydrous cat-1 to a maximum cat-2 rate of 2100 s-1 with 1% water. Competitive deactivation pathways were discovered as Ir-Ir dimerization without reacting with CO2 or the formation of a hydride-bridged dinuclear complex during extended electrolysis at high water concentration. The Ir-Ir dimer was characterized by high-resolution mass spectrometry and X-ray absorption spectroscopy (XAS).},
doi = {10.1021/acscatal.9b04371},
journal = {ACS Catalysis},
issn = {2155-5435},
number = 11,
volume = 10,
place = {United States},
year = {2020},
month = {5}
}

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