金纳米粒子/金属有机框架材料的构筑及其光催化还原二氧化碳的研究

The use of photocatalysts to reduce carbon dioxide into energy-dense carbon compounds can be considered as an effective way, by which a carbon neutral cycle as well as renewable energy source would be created without fossil fuel consumption, addressing the reduction of carbon dioxide emission into environment. As for metalloporphyrins, the dz2 orbital of the metal centrals can selectively coordinated with electrophilic carbon atom of carbon dioxide, accounting for its activation and being reduced into carbon monoxide and formic acid via proton coupled two-electron reduction process. Moreover, the hierarchical structures of porphyrin molecules can further upgrade the catalytic performance of metalloporphyrins. However, the challenge is the requirement of a photoinduced 2nd electron to produce catalytic active species with low valence state of metal sites under visible light irradiation, and thus metalloporphyrins generally exhibit weak photocatalytic activity towards carbon dioxide reduction. The project is to develop two synthetic routes to prepare core-shell composites comprised of Au nanoparticles (AuNPs) and metalloporphyrin-based metal-organic frameworks (MOFs) as photocatalysts to study proton-coupled catalytic reduction of carbon dioxide. On the one hand, the catalytic performance of the photocatalysts would be improved based on an increase of light-harvesting ability of Au@MOFs by localized surface plasmon resonance of AuNPs, facilitating the formation of catalytic active species. The effect of different AuNP-related surface plasmon-enhanced photocatalytic activity, including resonant energy transfer and plasmonic sensitization process, would be investigated in detail. On the other hand, the metalloprophyrins located in MOFs’ pores possessing high catalytic activity is attributed to the hierarchical molecular structures that can be modified with pyridine, hydroxyl group, and so on, and the modified structures can improve the efficiency of electron and proton transfer, as well as adsorption ability of carbon dioxide during the catalytic process. The project would provide a solid preliminary study and technical support for coupling the reduction of carbon dioxide and oxidation of water in a catalytic system under mild conditions in the future.

利用光催化剂将二氧化碳还原成具有高能量密度的含碳化合物，是减少二氧化碳排放、获得可再生能源、实现碳中性循环的有效途径。基于金属卟啉中金属的dz2轨道、以及卟啉分子的多层次结构，可以选择性配位、活化、催化还原二氧化碳，形成一氧化碳和甲酸，但由于金属卟啉在光照条件下较难形成两电子还原催化活性中心，常显示较低光催化还原二氧化碳的活性。本项目旨在以铁、钴、镍-金属卟啉作为桥连配体的金属-有机框架与金纳米粒子构筑的复合光催化剂，利用金纳米粒子局域表面等离子体共振增强金属卟啉光催化还原二氧化碳的性能。探究金纳米粒子通过共振能量转移、等离子体敏化方式改善金属卟啉的光吸收能力，增加催化活性物种的形成效率和浓度；考察卟啉配体中引入羟基、吡啶等官能团辅助提高催化剂富集二氧化碳能力、在催化过程中电子和质子转移效率。本项目为将来在温和条件下实现光催化还原二氧化碳和氧化水的耦合提供可靠的基础研究和技术支持。

探究CO2小分子的绿色催化转化路径，一方面为解决因CO2过度排放导致温室效应和海水酸化等环境问题提供思路，另一方面为获得碳中性循环、实现CO2的资源化打下基础。设计高性能CO2还原的复合纳米催化剂是CO2催化转化研究领域一个富有挑战性的重要研究方向。本项目利用液相法选择性合成了若干例无机纳米粒子负载的金属-有机框架（MOFs）复合纳米材料，实现在可见光辐照下CO2的高效催化转化，其中，Au纳米粒子负载的PPF-3纳米片复合光催化剂，展现出高的催化活性（HCOOH：42.7 mol g-1 h-1），是PPF-3纳米片的5倍，充分体现了利用局域表面等离子体共振效应增强MOFs光催化性能的设计思想；利用乙醇取代经典催化体系中的三乙胺/三乙醇胺作为电子牺牲剂和氢源，实现绿色、经济的催化转化过程；在此基础上，研究了具有水氧化性能、或者催化加氢性能的各种复合纳米材料，为进一步发展更加绿色的H2O-CO2体系提供可能，设计合成的Au纳米棒负载的PCN-222、MoS2负载的NH2-UiO-66等复合材料初步显示了一定的光催化还原CO2的活性。课题的实施是以MOFs的结构和组成的多样性、以及无机纳米材料独特的物理、化学性质为基础，设计、制备各种高性能的光催化还原CO2催化剂，探究绿色的CO2催化转化体系。