Mechanism of hydrogen evolution from hydridocobaloxime

CHAO, T.-H.; Espenson, James H.

doi:10.1021/ja00469a022

Cited by 126 publications

(100 citation statements)

References 5 publications

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“…[1][2][3][4] In the past decades, molecular approaches have mostly focused on one hand on the design of photosensitive systems displaying long-lived photo-induced charge separation states to permit further electron transfers 5-10 and, on the other hand, on catalysts able to use these photogenerated charges for achieving either oxygen [11][12][13][14][15][16][17][18] or hydrogen evolution. [19][20][21][22][23][24] As these two reactions are multi-electronic processes while photosensitizers deliver electrons and holes sequentially, the charges need to be directed to a charge accumulation site. 25 However, only a few molecular photoactive systems with a designed charge accumulation site have been described so far.…”

Section: Introductionmentioning

confidence: 99%

Charge photo-accumulation and photocatalytic hydrogen evolution under visible light at an iridium(iii)-photosensitized polyoxotungstate

Matt

Fize

Moussa

et al. 2013

Energy Environ. Sci.

145

115

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Steady-state irradiation under visible light of a covalent Ir(III)-photosensitized polyoxotungstate is reported. In the presence of a sacrificial electron donor, the photolysis leads to the very efficient photoreduction of the polyoxometalate. Successive formation of the one-electron and two-electron reduced species, which are unambiguously identified by comparison with spectroelectrochemical measurements, is observed with a significantly faster rate reaction for the formation of the oneelectron reduced species. The kinetics of the photoreduction, which are correlated to the reduction potentials of the polyoxometalate (POM), can be finely tuned by the presence of an acid. Indeed light-driven formation of the two-electron reduced POM is considerably facilitated in the presence of acetic acid. The system is also able to perform photocatalytic hydrogen production under visible light without significant loss of performance over more than 1 week of continuous photolysis and displays higher photocatalytic efficiency than the related multi-component system, outlining the decisive effect of the covalent bonding between the POM and the photosensitizer. This functional and modular system constitutes a promising step for the development of charge photoaccumulation devices and subsequent photoelectrocatalysts for artificial photosynthesis.Photoconversion of light into chemical fuels is emerging as a major scientific challenge. [1][2][3][4] In the past decades, molecular approaches have mostly focused on one hand on the design of photosensitive systems displaying long-lived photo-induced charge separation states to permit further electron transfers 5-10 and, on the other hand, on catalysts able to use these photogenerated charges for achieving either oxygen [11][12][13][14][15][16][17][18] or hydrogen evolution. [19][20][21][22][23][24] As these two reactions are multi-electronic processes while photosensitizers deliver electrons and holes sequentially, the charges need to be directed to a charge accumulation site. 25 However, only a few molecular photoactive systems with a designed charge accumulation site have been described so far. [26][27][28][29][30] Another requirement is crucial for efficient charge accumulation in such systems: when partially filled, the reservoir should not interfere with the photoactive moiety. Indeed, in classical donor-acceptor (D-A) systems, the electron acceptor, once reduced, potentially becomes an electron donor and often displays lightabsorbing properties. Thus it may act, in a subsequent light-driven process, as a deleterious 47,48 In the previously mentioned study, transient absorptions measurements only allowed for the characterization of the first photo-induced electron transfer. Charge photo-accumulation studies can be achieved in the presence of an additional electron donor in the solution that can irreversibly quench the charge separation state, regenerate the initial state of the photosensitizer and make a second photo-induced process possible. We herein provide unprecedente...

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Section: Introductionmentioning

confidence: 99%

Charge photo-accumulation and photocatalytic hydrogen evolution under visible light at an iridium(iii)-photosensitized polyoxotungstate

Matt

Fize

Moussa

et al. 2013

Energy Environ. Sci.

145

115

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“…Three issues can be addressed regarding the mechanism for H 2 evolution catalyzed by these compounds: (i) the oxidation state of the active species initiating the catalytic cycle, (ii) the nature of the hydrogen evolution step (20), where two pathways, either heterolytic and monometallic or homolytic and bimetallic, may account for hydrogen evolution (Scheme 2), and (iii) the specific role of the bridging link, H vs. BF 2 , as far as the electrocatalytic potential is concerned.…”

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confidence: 99%

Cobalt and nickel diimine-dioxime complexes as molecular electrocatalysts for hydrogen evolution with low overvoltages

Jacques

Artero

Pécaut

et al. 2009

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

398

423

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Hydrogen production through the reduction of water appears to be a convenient solution for the long-run storage of renewable energies. However, economically viable hydrogen production requests platinum-free catalysts, because this expensive and scarce (only 37 ppb in the Earth's crust) metal is not a sustainable resource [Gordon RB, Bertram M, Graedel TE (2006) Proc Natl Acad Sci USA 103:1209 -1214]. Here, we report on a new family of cobalt and nickel diimine-dioxime complexes as efficient and stable electrocatalysts for hydrogen evolution from acidic nonaqueous solutions with slightly lower overvoltages and much larger stabilities towards hydrolysis as compared to previously reported cobaloxime catalysts. A mechanistic study allowed us to determine that hydrogen evolution likely proceeds through a bimetallic homolytic pathway. The presence of a proton-exchanging site in the ligand, furthermore, provides an exquisite mechanism for tuning the electrocatalytic potential for hydrogen evolution of these compounds in response to variations of the acidity of the solution, a feature only reported for native hydrogenase enzymes so far.bio-inspired chemistry ͉ cobaloxime ͉ electrocatalysis ͉ hydrogen evolution reaction ͉ hydrogenase H ydrogen production, through the reduction of water, appears to be a solution to consider for the long-run storage of renewable energetic resources (1). The hydrogen evolution reaction (her) is apparently a very simple reaction but, as with most multielectronic processes, it is a slow process that should be catalyzed. Nickel-based electrodes can be used for that aim in strongly alkaline media, but this technology results in average energetic yields (Ϸ50-70%), suffers from corrosion issues, and can hardly be miniaturized due to the lack of polymer exchange membrane (PEM) material that is stable under these conditions. Actually, technological research now focuses on PEM electrolytic cells for which carbon-supported platinum nanoparticles are generally used as catalysts. This scarce and expensive noble metal is, however, not a renewable resource. Thus, progress to a mature hydrogen economy depends on breakthroughs in finding new catalytic materials based on earth-abundant elements (2). To that aim, the combination of a common electrode material with a coordination complex of a first-row transition metal, able to catalyze the reaction at a potential close to the standard potential of the H ϩ /H 2 couple, seems attractive. Macrocyclic cobalt complexes are very competitive in that respect (3). In the past four years, we and others reported on cobaloximes-cobalt complexes containing bridged bidentate oximato ligands (see Scheme 1)-(4-7) as a new class of such catalysts with good catalytic stability and Ϸ250-300 mV overvoltage-the difference between the observed catalytic potential and the thermodynamic H 2 /2H ϩ ϩ 2 e Ϫ equilibrium. To date, these catalysts are among the most efficient ones and, thus, are currently exploited for the construction of H 2 -evolving photocatalytic devices that actuall...

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“…Co-diglyoxime complexes have been shown to generate hydrogen from protic solutions at relatively modest overpotentials. Given the broad interest and potential application in artificial photosynthesis, the mechanism of hydrogen formation has been the subject of many experimental (16,18,(33)(34)(35)(36)(37)(38)(39)(40) and theoretical (41-43) investigations. Several pathways have been proposed (Fig.…”

mentioning

confidence: 99%

Molecular mechanisms of cobalt-catalyzed hydrogen evolution

Marinescu

Winkler

Gray

2012

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

246

217

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Several cobalt complexes catalyze the evolution of hydrogen from acidic solutions, both homogeneously and at electrodes. The detailed molecular mechanisms of these transformations remain unresolved, largely owing to the fact that key reactive intermediates have eluded detection. One method of stabilizing reactive intermediates involves minimizing the overall reaction free-energy change. Here, we report a new cobalt(I) complex that reacts with tosylic acid to evolve hydrogen with a driving force of just 30 meV/Co. Protonation of CoI produces a transient CoIII-H complex that was characterized by nuclear magnetic resonance spectroscopy. The CoIII-H intermediate decays by second-order kinetics with an inverse dependence on acid concentration. Analysis of the kinetics suggests that CoIII-H produces hydrogen by two competing pathways: a slower homolytic route involving two CoIII-H species and a dominant heterolytic channel in which a highly reactive CoII-H transient is generated by CoI reduction of CoIII-H.

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Mechanism of hydrogen evolution from hydridocobaloxime

Cited by 126 publications

References 5 publications

Charge photo-accumulation and photocatalytic hydrogen evolution under visible light at an iridium(iii)-photosensitized polyoxotungstate

Charge photo-accumulation and photocatalytic hydrogen evolution under visible light at an iridium(iii)-photosensitized polyoxotungstate

Cobalt and nickel diimine-dioxime complexes as molecular electrocatalysts for hydrogen evolution with low overvoltages

Molecular mechanisms of cobalt-catalyzed hydrogen evolution

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