Hydrogen evolution catalyzed by cobalt diimine-dioxime complexes.
Hydrogen evolution catalyzed by cobalt diimine-dioxime complexes.
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DOI：
10.1021/acs.accounts.5b00058
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发表时间：
2015-05-19
影响因子：
18.3
通讯作者：
Artero, Vincent
中科院分区：
文献类型：
--
作者：
Kaeffer, Nicolas;Chavarot-Kerlidou, Murielle;Artero, Vincent
模仿光合作用并生产太阳能燃料是一种以持久和可持续的方式储存来自太阳的大量可再生能源的有吸引力的方法。通过水分解制氢已被设定为人工光合作用的首要目标。追求这一目标需要开发高效稳定的催化系统，仅基于地球丰富的元素，用于将质子从水还原为分子氢。基于乙二肟配体的钴络合物，称为钴肟，十年前作为第一代此类催化剂出现。它们现在被广泛用于构建用于析氢的光催化系统。在本报告中，我们描述了我们对第二代催化剂钴二亚胺-二肟络合物的开发所做的贡献。虽然表现出与钴肟类似的催化活性，但由于二亚胺-二肟配体的四齿性质，这些催化剂证明在强酸性条件下对水解更稳定。重要的是，H2进化通过涉及肟桥作为质子化位点的质子耦合电子转移步骤进行，再现了在氢化酶活性位点发挥作用的机制。该特征允许H2在适度的过电位下析出，即在非水溶液中在宽范围的酸碱条件下接近热力学平衡。二亚胺-二肟配体在连接两个亚胺官能团的烃链处的衍生化使得能够以比母体双齿钴肟更方便的方式将络合物共价接枝到电极表面上。因此，我们将二亚胺-二肟钴催化剂附着到碳纳米管上，并证明了所得分子基电极对乙酸盐缓冲液中析氢的催化活性。固定化催化剂的稳定性被认为是数量级高于散装催化剂。这使我们有证据表明，这些钴络合物，如钴肟和其他钴盐，在周转条件下分解，其中它们在溶液中是自由的。值得注意的是，该过程在磷酸盐缓冲液水溶液中产生了由金属钴组成的纳米颗粒膜，该金属钴涂覆有与电解质接触的氧代/羟基磷酸钴层。这种新型材料，H2-CoCat，介导在低过电位下从中性水性缓冲液中放出H2。最后，在水/乙腈混合物和完全水溶液中，二亚胺-二肟钴络合物用于光驱动H2生成的潜力已经得到证实。总之，这些研究有望构建用于H2释放的分子基光电极，并进一步集成到能够实现整体水分解的染料敏化光电化学电池（DS-PEC）中。
Mimicking photosynthesis and producing solar fuels is an appealing way to store the huge amount of renewable energy from the sun in a durable and sustainable way. Hydrogen production through water splitting has been set as a first-ranking target for artificial photosynthesis. Pursuing that goal requires the development of efficient and stable catalytic systems, only based on earth abundant elements, for the reduction of protons from water to molecular hydrogen. Cobalt complexes based on glyoxime ligands, called cobaloximes, emerged ten years ago as a first generation of such catalysts. They are now widely utilized for the construction of photocatalytic systems for hydrogen evolution. In this Account, we describe our contribution to the development of a second generation of catalysts, cobalt diimine-dioxime complexes. While displaying similar catalytic activities as cobaloximes, these catalysts prove more stable against hydrolysis under strongly acidic conditions thanks to the tetradentate nature of the diimine-dioxime ligand. Importantly, H2 evolution proceeds via proton-coupled electron transfer steps involving the oxime bridge as a protonation site, reproducing the mechanism at play in the active sites of hydrogenase enzymes. This feature allows H2 to be evolved at modest overpotentials, i.e. close to the thermodynamic equilibrium over a wide range of acid-base conditions in non-aqueous solutions. Derivatization of the diimine-dioxime ligand at the hydrocarbon chain linking the two imine functions enables the covalent grafting of the complex onto electrode surfaces in a more convenient manner than for the parent bis-bidentate cobaloximes. Accordingly we attached diimine-dioxime cobalt catalysts onto carbon nanotubes and demonstrated the catalytic activity of the resulting molecular-based electrode for hydrogen evolution from aqueous acetate buffer. The stability of immobilized catalysts was found to be orders of magnitude higher than that of catalysts in the bulk. It led us to evidence that these cobalt complexes, as cobaloximes and other cobalt salts do, decompose under turnover conditions where they are free in solution. Of note this process generates in aqueous phosphate buffer a nanoparticulate film consisting of metallic cobalt coated with a cobalt-oxo/hydroxo-phosphate layer in contact with the electrolyte. This novel material, H2-CoCat, mediates H2 evolution from neutral aqueous buffer at low overpotentials. Finally, the potential of diimine-dioxime cobalt complexes for light-driven H2 generation has been attested both in water/acetonitrile mixtures and in fully aqueous solutions. All together, these studies hold promises for the construction of molecular-based photoelectrodes for H2 evolution and further integration in dye-sensitized photo-electrochemical cells (DS-PECs) able to achieve overall water splitting.
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