Theoretical Analysis of the Sequential Proton-Coupled Electron Transfer Mechanisms for H2 Oxidation and Production Pathways Catalyzed by Nickel Molecular Electrocatalysts

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• 作者：Laura E. Fernandez ; Samantha Horvath ; Sharon Hammes-Schiffer
• 刊名：The Journal of Physical Chemistry C
• 出版年：2012
• 出版时间：February 2, 2012
• 年：2012
• 卷：116
• 期：4
• 页码：3171-3180
• 全文大小：936K
• 年卷期：v.116,no.4(February 2, 2012)
• ISSN：1932-7455

文摘

The design of electrocatalysts for the oxidation and production of H₂ is important for the development of alternative energy sources. This Article focuses on the [Ni(P₂^RN₂^{R鈥?/sup>)₂]2+ electrocatalysts, where P₂RN₂R鈥?/sup> denotes 1,5-diaza-3,7-diphosphacyclooctane ligands with substituent groups R and R鈥?covalently bound to the phosphorus and nitrogen atoms, respectively. Theoretical methods are used to investigate the mechanism of the step in the catalytic cycle corresponding to [HNiII(P₂N₂)₂]+ 鈥?e鈥?/sup> 鈫?[NiI(P₂HN₂)(P₂N₂)]2+ for H₂ oxidation and the reverse reaction for H₂ production. This step involves electron transfer (ET) between the Ni complex and the electrode as well as proton transfer (PT) between the Ni and the N. The sequential mechanisms, PT鈥揈T and ET鈥揚T, are investigated for the following (R,R鈥? substituents: (Me,Me), (Ph,Ph), and (Ph,Bz), where Me, Ph, and Bz denote methyl, phenyl, and benzyl substituents. Density functional theory is used to calculate reduction potentials, pK_a values, and PT pathways, and the inner- and outer-sphere reorganization energies for electrochemical ET are calculated within the framework of Marcus theory. For the (Ph,Ph) and (Ph,Bz) systems, the sequential PT鈥揈T mechanism for H₂ production would require surmounting a large free energy barrier for the initial PT step, followed by thermodynamically favorable ET. The sequential ET鈥揚T mechanism for these systems would require a moderate initial applied overpotential, followed by a PT reaction with a relatively low free energy barrier. Consistent with experimental data, the calculated overpotential required for the initial reduction in the ET鈥揚T mechanism is lower for the (Ph,Bz) system than for the (Ph,Ph) system for H₂ production. The concerted mechanism, in which the electron and proton transfer simultaneously without a stable intermediate, may be thermodynamically favorable and is a direction of future research.}