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Macrocyclic complexes and their use as electrocatalysts for the electrochemical reduction of carbon dioxide

机译：大环配合物及其作为电化学还原二氧化碳的电催化剂的用途

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摘要

The environmental problem of carbon dioxide pollution has caused considerable interest over recent years. Much work has concentrated on catalysts for the reduction of carbon dioxide. There are two main methods for the reduction, electrochemical and photochemical reduction. There are several catalysts reported in the literature but the most selective and efficient one for the electrochemical reduction of carbon dioxide to carbon monoxide is nickel(II) 1,4,8,11-tetraazacyclodecane perchlorate (more commonly known as [Ni(cyclam)]2+(ClO4)2). In order to understand how this catalysis occurs the complex was investigated using a variety of electrodes and solvents in order to determine the best system for catalysis to occur. The best system is an aqueous solution of the complex with sodium perchlorate as the supporting electrolyte and with a mercury working electrode. The effect of macrocycle ring size was investigated using [n]aneN4 complexes, n = 13 - 16. The highest current produced under carbon dioxide was for the n = 14 ring. Subsequent complexes investigated have a [14]-membered backbone. The other important effect in the macrocyclic structure is the nature of the conformation of the complex. Cyclam itself exists as five conformations, three of which are readily accessible. The other conformations do not reduce carbon dioxide to the same extent as the original [Ni(cyclam)]2+. The origin of this difference was shown to be related to the extent of adsorption of each conformer. Also, if the chelate sequence of the cyclam is changed from cyclam to isocyclam the catalytic efficiency is lowered but the potential at which the reduction occurs is more positive. Rotating disc electrochemistry was attempted in order to find a method for comparing the catalytic efficiency of different complexes. A mercury plated copper electrode was used but unfortunately the mercury deteriorated and left behind bare copper. Copper itself showed current plateaux for reduction under carbon dioxide; these current plateaux varied linearly with (rotation rate)1/2. On closer examination the current plateaux were due to the Ni(II/I) reduction and not carbon dioxide reduction. The adsorption of the catalyst onto the mercury is important as it is this adsorbed species that is catalytically active. To determine the nature of the catalytic species various electrochemical techniques such as chronocoulometry and a.c. impedance were employed to measure the extent of the adsorption. Chronocoulometry showed that adsorption occurred on the mercury, but adsorption onto the copper surface was inhibited in the presence of carbon dioxide. A.C. impedance spectroscopy showed that adsorption occurred at -1.20 V which is considerably more positive than the redox potential of the complex. A variety of unsaturated macrocycles were also investigated for catalytic behaviour. As the unsaturated macrocycles have lower redox potentials than the corresponding saturated macrocycle, the result should be that the potential for the reduction becomes more positive. However with these macrocycles no reduction of carbon dioxide is observed under the same conditions as [Ni(cyclam)]2+, A series of new catalysts has been identified. These complexes have the ability to reduce carbon dioxide at more positive potentials with a greater current efficiency, than [Ni(cyclam)]2+. The new catalysts are based on a [14aneN6 backbone. The extra nitrogen atoms appear to stabilise the intermediate hydrogen bonded NiI-CO2 adduct.
机译：近年来，二氧化碳污染的环境问题引起了人们的极大关注。许多工作集中在减少二氧化碳的催化剂上。还原有两种主要方法，电化学还原和光化学还原。文献中报道了几种催化剂，但用于将二氧化碳电化学还原为一氧化碳的最具选择性和最有效的催化剂是1,4,8,11-四氮杂环癸烷高氯酸镍（II）（通常称为[Ni（cyclam）） ] 2+（ClO4）2）。为了理解这种催化作用是如何发生的，使用各种电极和溶剂对络合物进行了研究，以确定最佳的催化体系。最好的系统是含有高氯酸钠作为支持电解质和汞工作电极的配合物水溶液。使用[n] aneN4配合物（n = 13-16）研究了大环环尺寸的影响。在二氧化碳下产生的最大电流是n = 14环。随后研究的复合物具有[14]元主链。大环结构中的另一个重要作用是配合物构象的性质。 Cyclam本身以五个构象存在，其中三个很容易获得。其他构象不能将二氧化碳还原到与原始[Ni（cyclam）] 2+相同的程度。结果表明，这种差异的根源与每个构象异构体的吸附程度有关。同样，如果将环素的螯合序列从环素改变为异环素，则催化效率会降低，但发生还原的可能性更高。为了找到比较不同配合物的催化效率的方法，尝试了转盘电化学。使用了镀汞的铜电极，但不幸的是汞变质并留下了裸铜。铜本身显示出在二氧化碳下还原的当前平稳状态。这些当前的平稳状态随（旋转速率）1/2线性变化。通过仔细检查，当前的平稳状态是由于Ni（II / I）还原而不是二氧化碳还原引起的。催化剂在汞上的吸附很重要，因为正是这种被吸附的物质才具有催化活性。为了确定催化物质的性质，各种电化学技术，例如计时库仑法和交流电。阻抗用于测量吸附程度。计时库仑法显示汞上发生了吸附，但是在二氧化碳的存在下，汞在铜表面的吸附被抑制。交流阻抗谱表明，吸附发生在-1.20V，比络合物的氧化还原电势强得多。还研究了各种不饱和大环化合物的催化行为。由于不饱和大环比相应的饱和大环具有更低的氧化还原电势，因此结果应为还原电势变得更正。然而，对于这些大环化合物，在与[Ni（cyclam）] 2+相同的条件下，未观察到二氧化碳的还原。已发现了一系列新催化剂。与[Ni（cyclam）] 2+相比，这些络合物具有在更大的正电势下以更高的电流效率还原二氧化碳的能力。新催化剂基于[14aneN6主链]。多余的氮原子似乎稳定了中间氢键结合的NiI-CO2加合物。

著录项

  • 作者

    Smith, Caroline.;

  • 作者单位

    University of St. Andrews (United Kingdom).;

  • 授予单位 University of St. Andrews (United Kingdom).;
  • 学科 Physical chemistry.
  • 学位 Ph.D.
  • 年度 1995
  • 页码 224 p.
  • 总页数 224
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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