4 research outputs found
Mechanistic details for cobalt catalyzed photochemical hydrogen production in aqueous solution: Efficiencies of the photochemical and non-photochemical steps
A detailed examination of each step of the reaction sequence in the photochemical sacrificial hydrogen generation system consisting of [Ru(bpy) 3]2+/ascorbate/[Co(DPA-bpy)OH2]3+ was conducted. By clearly defining quenching, charge separation, and back electron transfer in the [Ru(bpy)3]2+/ascorbate system, the details necessary for evaluation of the efficiency of water reduction with various catalysts are provided. In the particular Co(III) catalyst investigated, it is clear that the light induced catalytic process is considerably less efficient than the electrocatalytic process. A potential source of catalyst inefficiency in this system is reduction of the products formed in oxidation of the sacrificial electron donor. The data provided for excited state quenching and charge separation in this particular aqueous system are meant to be used by others for thorough investigation of the quantum efficiencies of other aqueous homogeneous and nanoheterogeneous catalysts for water reduction. © 2013 American Chemical Society
Mechanistic studies of hydrogen evolution in aqueous solution catalyzed by a tertpyridine-amine cobalt complex
The ability of cobalt-based transition metal complexes to catalyze electrochemical proton reduction to produce molecular hydrogen has resulted in a large number of mechanistic studies involving various cobalt complexes. While the basic mechanism of proton reduction promoted by cobalt species is well-understood, the reactivity of certain reaction intermediates, such as CoI and CoIII-H, is still relatively unknown owing to their transient nature, especially in aqueous media. In this work we investigate the properties of intermediates produced during catalytic proton reduction in aqueous solutions promoted by the [(DPA-Bpy)Co(OH2)]n+ (DPA-Bpy = N,N-bis(2-pyridinylmethyl)-2,20-bipyridine-6-methanamine) complex ([Co(L)(OH2)]n+ where L is the pentadentate DPA-Bpy ligand or [Co(OH2)]n+ as a shorthand). Experimental results based on transient pulse radiolysis and laser flash photolysis methods, together with electrochemical studies and supported by density functional theory (DFT) calculations indicate that, while the water ligand is strongly coordinated to the metal center in the oxidation state 3+, one-electron reduction of the complex to form a CoII species results in weakening the Co-O bond. The further reduction to a CoI species leads to the loss of the aqua ligand and the formation of [CoI-VS)]+ (VS = vacant site). Interestingly, DFT calculations also predict the existence of a [CoI(κ4-L)(OH2)]+ species at least transiently, and its formation is consistent with the experimental Pourbaix diagram. Both electrochemical and kinetics results indicate that the CoI species must undergo some structural change prior to accepting the proton, and this transformation represents the rate-determining step (RDS) in the overall formation of [CoIII-H]2+. We propose that this RDS may originate from the slow removal of a solvent ligand in the intermediate [CoI(κ4-L)(OH2)]+ in addition to the significant structural reorganization of the metal complex and surrounding solvent resulting in a high free energy of activation
Mechanistic Details for Cobalt Catalyzed Photochemical Hydrogen Production in Aqueous Solution: Efficiencies of the Photochemical and Non-Photochemical Steps
A detailed examination of each step
of the reaction sequence in
the photochemical sacrificial hydrogen generation system consisting
of [RuÂ(bpy)<sub>3</sub>]<sup>2+</sup>/ascorbate/[CoÂ(DPA-bpy)ÂOH<sub>2</sub>]<sup>3+</sup> was conducted. By clearly defining quenching,
charge separation, and back electron transfer in the [RuÂ(bpy)<sub>3</sub>]<sup>2+</sup>/ascorbate system, the details necessary for
evaluation of the efficiency of water reduction with various catalysts
are provided. In the particular CoÂ(III) catalyst investigated, it
is clear that the light induced catalytic process is considerably
less efficient than the electrocatalytic process. A potential source
of catalyst inefficiency in this system is reduction of the products
formed in oxidation of the sacrificial electron donor. The data provided
for excited state quenching and charge separation in this particular
aqueous system are meant to be used by others for thorough investigation
of the quantum efficiencies of other aqueous homogeneous and nanoheterogeneous
catalysts for water reduction
Electrocatalytic and photocatalytic hydrogen production in aqueous solution by a molecular cobalt complex
A mononuclear cobalt complex supported by a new type of pentadentate ligand catalyzed the evolution of hydrogen in purely aqueous solution (see picture). This cobalt complex could serve as both electrocatalyst and photocatalyst for efficient evolution of hydrogen. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim