pH-Dependent Reduction Potentials and Proton-Coupled Electron Transfer Mechanisms in Hydrogen-Producing Nickel Molecular Electrocatalysts
pH-Dependent Reduction Potentials and Proton-Coupled Electron Transfer Mechanisms in Hydrogen-Producing Nickel Molecular Electrocatalysts
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DOI：
10.1021/ic302056j
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发表时间：
2013-04-01
影响因子：
4.6
通讯作者：
Hammes-Schiffer, Sharon
中科院分区：
文献类型：
--
作者：
Horvath, Samantha;Fernandez, Laura E.;Hammes-Schiffer, Sharon
镍基(P2N2Bn)-N-Ph电催化剂由一个镍原子和两个1,5-二苯基-3,7-二苯基-1,5-二氮杂-3,7-二磷酸环辛烷配体组成，催化乙腈生成H-2。最近的电化学实验表明，Ni-II/I还原电位与pH呈线性关系，斜率为57 mV/pH单位，这意味着质子耦合电子转移（PCET）过程中转移了相同数量的电子和质子。本文结合理论和实验研究，在整体提出的催化机制的背景下，为这种pH依赖性提供了解释。在提出的机制中，催化循环开始于一系列分子间质子从酸转移到悬垂的胺配体，电化学电子转移到镍中心，产生双质子化的Ni-0，这是H-2进化的前体。计算得到的双质子化产物的Ni-II/I还原电位与实验观察到的强酸存在下的还原电位非常吻合，这表明在这些循环伏安（CV）实验中观察到的导致峰的催化活性物质是双质子化的。Ni-II/0还原电位略大于Ni-II/I还原电位，说明在Ni-II/I还原后，Ni-I/0还原是自发发生的，这与实验观察到的单个CV峰一致。这些结果表明，在CV实验中观察到的PCET过程是一个双电子/双质子过程，对应于最初的双质子化，然后是两次还原。在实验和理论数据的基础上，得到了该催化剂的完整热力学方案和Pourbaix图。Pourbaix图确定了在每个还原电位和pH值下最热力学稳定的物质，说明该催化剂在不同的pH值范围内经历了不同类型的PCET过程。这些热力学见解将有助于设计更有效的H-2生产分子催化剂。
The nickel-based (P2N2Bn)-N-Ph electrocatalysts comprised of a nickel atom and two 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane ligands catalyze H-2 production in acetonitrile. Recent electrochemical experiments revealed a linear dependence of the Ni-II/I reduction potential on pH with a slope of 57 mV/pH unit, implicating a proton-coupled electron transfer (PCET) process with the same number of electrons and protons transferred. The combined theoretical and experimental studies herein provide an explanation for this pH dependence in the context of the overall proposed catalytic mechanism. In the proposed mechanisms, the catalytic cycle begins with a series of intermolecular proton transfers from an acid to the pendant amine ligand and electrochemical electron transfers to the nickel center to produce the doubly protonated Ni-0 species, a precursor to H-2 evolution. The calculated Ni-II/I reduction potentials of the doubly protonated species are in excellent agreement with the experimentally observed reduction potential in the presence of strong acid, suggesting that the catalytically active species leading to the peak observed in these cyclic voltammetry (CV) experiments is doubly protonated. The Ni-II/0 reduction potential was found to be slightly more positive than the Ni-II/I reduction potential, indicating that the Ni-I/0 reduction occurs spontaneously after the Ni-II/I reduction, as implied by the experimental observation of a single CV peak. These results suggest that the PCET process observed in the CV experiments is a two-electron/two-proton process corresponding to an initial double protonation followed by two reductions. On the basis of the experimental and theoretical data, the complete thermodynamic scheme and the Pourbaix diagram were generated for this catalyst. The Pourbaix diagram, which identifies the most thermodynamically stable species at each reduction potential and pH value, illustrates that this catalyst undergoes different types of PCET processes for various pH ranges. These thermodynamic insights will aid in the design of more effective molecular catalysts for H-2 production.