Oxygen reduction reactivity of cobalt(ii) hangman porphyrins

McGuire, Robert; Dogutan, Dilek K.; Teets, Thomas S.; Suntivich, Jin; Shao‐Horn, Yang; Nocera, Daniel G.

doi:10.1039/c0sc00281j

Cited by 239 publications

(263 citation statements)

References 29 publications

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“…S16 and S17. The greater kinetic facility of the Ni II/I reduction relative to the Co II/I reduction is attributed to the requisite dissociation of a solvent [k°E T = 0.01 cm/s (36)] ligand that is axially coordinated to the low-spin d 7 Co II molecule as observed in the solid-state (32). No axial ligand is expected from our theoretical computations nor observed in the solid-state structure ( Fig.…”

Section: Significancementioning

confidence: 74%

Role of pendant proton relays and proton-coupled electron transfer on the hydrogen evolution reaction by nickel hangman porphyrins

Bediako

Solis

Dogutan

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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189

204

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The hangman motif provides mechanistic insights into the role of pendant proton relays in governing proton-coupled electron transfer (PCET) involved in the hydrogen evolution reaction (HER). We now show improved HER activity of Ni compared with Co hangman porphyrins. Cyclic voltammogram data and simulations, together with computational studies using density functional theory, implicate a shift in electrokinetic zone between Co and Ni hangman porphyrins due to a change in the PCET mechanism. Unlike the Co hangman porphyrin, the Ni hangman porphyrin does not require reduction to the formally metal(0) species before protonation by weak acids in acetonitrile. We conclude that protonation likely occurs at the Ni(I) state followed by reduction, in a stepwise proton transfer-electron transfer pathway. Spectroelectrochemical and computational studies reveal that upon reduction of the Ni(II) compound, the first electron is transferred to a metal-based orbital, whereas the second electron is transferred to a molecular orbital on the porphyrin ring.renewable | solar fuels | electrocatalysis S olar-to-fuels conversions provide a path to harnessing the ubiquitous albeit intermittent renewable energy resource offered by the sun (1-6). Efficient catalysis of transformations of energy consequence (7-13) mandates the coupling of electron transfer (ET) to proton transfer (PT) in proton-coupled electron transfer (PCET) reactions (14)(15)(16)(17)(18)(19)(20). In the absence of PCET, intermediates possessing equilibrium potentials that are prohibitively large depreciate the storage capacity offered by the solarto-fuels conversion process. The coupling of protons to changes in electron equivalency offers the possibility of restricting the equilibrium potentials of the redox steps to a more narrow potential range, thereby minimizing the overpotential required to sustain catalysis at a desired turnover rate. Thus, the exploitation of PCET pathways to permit potential-leveling effects is a crucial prerequisite for the efficient catalytic conversion reactions of energy relevant molecules.PCET reactions may be classified into stepwise and concerted pathways (14,16,20,21). Stepwise PCET may involve ET first followed by PT (ETPT), or PT followed by ET (PTET). In concerted proton-electron transfers (CPET), the proton and electron traverse a common transition state. Whereas concerted pathways avoid the formation of thermodynamically costly intermediates, CPET reactions may incur kinetic penalties associated with the requirements for proton tunneling (19,20,22). The competition between these dynamics during catalysis determines the most efficient route of reaction. Studies that explore the interplay between these factors are crucial to designing catalytic reactions of high efficiency. Along these lines, the incorporation of proton relays in the second coordination sphere of molecular catalysts has emerged as a useful tool in optimizing PCET transformations (23-29). We have focused on the synthesis and mechanistic investigation of a class of me...

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Section: Significancementioning

confidence: 74%

Role of pendant proton relays and proton-coupled electron transfer on the hydrogen evolution reaction by nickel hangman porphyrins

Bediako

Solis

Dogutan

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

Self Cite

189

204

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“…They are believed to be responsible for the so-called "push effect", where the axial ligands act as strong internal electron donors. 16−20 There are also examples where proximal functional groups exert a beneficial effect on ORR catalyzed by metal macrocycles, e.g., acidic groups such as carboxyl 22 or even protonated pyridyl 23 acting as proton shuttles in ORR catalyzed by iron macrocycles. As the above examples illustrate, bringing together macrocyclic complexes of metals and suitable functional groups can promote a more effective ORR catalysis than that supported by the sole macrocyclic complexes.…”

Section: ■ Introductionmentioning

confidence: 99%

Effects of Axial Coordination of the Metal Center on the Activity of Iron Tetraphenylporphyrin as a Nonprecious Catalyst for Oxygen Reduction

Chlistunoff

Sansiñena

2014

J. Phys. Chem. C

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Effects of axial coordination of the metal center on oxygen reduction catalyzed by iron tetraphenylporphyrin have been studied using spectroelectrochemical and rotating ring disk electrode techniques. Among numerous ligands studied, the strongest complexation of the metal center was observed for imidazole and some substituted imidazole ligands. Around 300 mV positive shift of the oxygen reduction potential in 0.5 M H2SO4 was observed when the catalyst support (high surface area carbon Vulcan XC72) was impregnated with polyvinylimidazole. A smaller 200 mV shift of the reduction potential was recorded when graphene modified with 1-(3-aminopropyl)imidazole was used as the catalyst support instead of Vulcan. In both cases, the catalyst selectivity for four-electron reduction was also substantially improved. The effects of iron complexation on oxygen reduction kinetics and mechanism result from both a positive shift of the potential of the first porphyrin reduction and an increase of the electron density on the iron center through the so-called “push effect”. The observed phenomena are discussed in relation to heat-treated Fe/N/C catalysts for oxygen reduction.

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“…One suggested pathway would be to use 3D structures, which are able to differentiate between the intermediates e.g., by confining the OOH group. Hangman porphyrins have been suggested as possible candidates, which indeed have been shown to be good catalysts for OER [203][204][205][206][207]; however, there is no support of this claim from first-principles as of yet [208], see Figure 10. Scaling relation between the Gibbs free energies of adsorption of *OH, *O, and *OOH.…”

Section: Overpotentialmentioning

confidence: 93%

First-Principles View on Photoelectrochemistry: Water-Splitting as Case Study

Hellman

Wang

2017

Inorganics

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Abstract:Photoelectrochemistry is truly an interdisciplinary field; a natural nexus between chemistry and physics. In short, photoelectrochemistry can be divided into three sub-processes, namely (i) the creation of electron-hole pairs by light absorption; (ii) separation/transport on the charge carriers and finally (iii) the water splitting reaction. The challenge is to understand all three processes on a microscopic scale and, perhaps even more importantly, how to combine the processes in an optimal way. This review will highlight some first-principles insights to the above sub-processes, in particular as they occur using metal oxides. Based on these insights, challenges and future directions of first-principles methods in the field of photoelectrochemistry will be discussed.

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Oxygen reduction reactivity of cobalt(ii) hangman porphyrins

Cited by 239 publications

References 29 publications

Role of pendant proton relays and proton-coupled electron transfer on the hydrogen evolution reaction by nickel hangman porphyrins

Role of pendant proton relays and proton-coupled electron transfer on the hydrogen evolution reaction by nickel hangman porphyrins

Effects of Axial Coordination of the Metal Center on the Activity of Iron Tetraphenylporphyrin as a Nonprecious Catalyst for Oxygen Reduction

First-Principles View on Photoelectrochemistry: Water-Splitting as Case Study

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