铁、钴大环胺配合物光、电催化还原CO2体系研发及机理研究

CO2 is the main component of greenhouse gases, but also the carbon energy potentially. Mimicking photosynthesis is of great significance to explore innovative renewable energy. The aim of the project is to develop highly efficient system for photo- and electro-driven reduction of CO2 with earth-abundant transition metal complexes by studying the key reaction processes of photosynthesis. To reach these goals, the selected approach will combine design and synthesis of a series of macrocyclic-amine Fe and Co catalysts. By finely tuning the ligands and metal centers, the catalysis will be improved based on selectivity and reactivity in both solution and on modified electrode. The effect of the local protons in the ligand will be intensive studied in the proton-coupled-electron-transfer process which will promote CO2 reduction. Meanwhile, binuclear Fe and Co will be designed to improve electron transfer efficiency and lower the activation energy barrier for they can provide multiply redox properties which may hopefully get more advanced C1 products. Foot-of-the-wave method will be applied as evaluation method and benchmarking for molecular catalysts during electrocatalysis. Once the best catalysts were screened, electrode modification will be performed with them. Photocatalytic research will focus on the development of stable, earth-abundant three-component systems containing catalyst, sensitizer and sacrificial donor. Moreover, mechanistic investigation will be performed. We will decipher intrinsic characteristics of the most efficient ones so as to unravel the intrinsic factors that control both the reactivity and selectivity towards reduction products which will be contributed to the better design of catalysts, new fundamental insights and research accumulation in the catalytic reduction of CO2.

CO2为温室气体的主要成分，同时也是潜在的碳能源。模拟光合作用，对探索新型可再生能源具有重要的研究价值。本项目从光合作用的关键化学过程出发，利用廉价过渡金属配合物催化剂构建高效光、电催化还原CO2的体系。申请人拟设计与合成一系列大环Fe、Co配合物，通过修饰配体及改变金属中心来提高选择性与催化活性。配体的设计上主要考察原位质子在CO2还原反应中对质子耦合电子传递过程的影响。同时，本项目拟发展双核Fe、Co金属配合物，利用其多重氧化还原态来提高电子转移效率，降低反应能垒，期望得到更高级的C1产物。电催化研究中拟采用foot-of-the-wave法评价和筛选出具有重要研究价值的催化剂，并将其修饰在电极上。光催化研究侧重于研发稳定、廉价的催化剂/光敏剂/电子給体三组分体系。最后，通过机理研究阐明高活性催化剂的本质，揭示控制反应的内在因素，为设计更高效的催化剂提供理论指导和研究积累。

本项目合成了一系列大环吡啶胺类单核铁、钴配合物、四联吡啶单核钴、四联吡啶类双核钴、四联吡啶单核铁、五联吡啶单核铁、三联吡啶-二喹咛类单核铁配合物，总结了一些大环吡啶胺类、多吡啶类配体的合成方法学。所有化合物进行质谱表征并得到了部分化合物的单晶结构。在前期实验条件下，构建了简单的电催化还原二氧化碳和典型的由催化剂、光敏剂、电子给体/受体三组分组成的光催化还原二氧化碳的催化系统，与其催化二氧化碳还原与活性、产物选择性和稳定性之间的关系。研究发现，对于CoL1~CoL4四个配合物，含有四个N-H的CoL1的催化活性比之前（最初结果：22 h之后TON为267，仅有微量H2生成）提高了5倍，而且CoL1的活性和选择性也远远高于CoL2、CoL3和CoL4，说明了配体上的悬臂胺在催化还原CO2中起着非常重要的作用。但遗憾的是，在稳定性测试中，我们发现CoL1在优化的条件下反应22h后就失活了，稳定性较差。随后，我们进行配体改进工作，将吡啶胺配体改进为多吡啶配体并同样进行了光、电催化还原CO2测试，获得了一些非常好的结果，并分析推理了影响催化剂效率的因素，分析了质子的传递作用、多吡啶配体的平面性、配位能力和p电子的接受特性。通过实验和计算，提供了低价态金属及可能中间体的依据，探讨了催化机理。最后，将四联吡啶单核钴、铁配合物通过接枝到各种导电或半导体材料上，不仅有助于催化剂的循环利用，更是进一步提高了催化还原二氧化碳的选择性和稳定性。本项目的研究结果为开发出更为高效、稳定、对生成燃料小分子具有较高选择性的光、电催化二氧化碳还原提供实验依据和理论基础。