钛基金属有机框架材料(MOF)光催化还原CO2的研究

The capture and efficient utilization of CO2 is an important issue since CO2 released by burning of fossil fuels is a primary cause of global warming. One of the best solutions is to convert CO2 into valuable organic products by means of solar energy. The key to the effcient utilization of CO2 is the development of high efficient photocatalyst for CO2 reduction. The discovery that Ti-incorporated zeolites show superior photocatalytic performance for CO2 reduction than dense TiO2 implies that the single-center metal oxide photocatalyst approach is a promising direction for the development of new photocatalysts for CO2 reduction. However, the Ti-modified zeolite photocatalysts are only active in the UV light region and their efficiency for CO2 reduction is still quite low. The emerging of the metal-organic framework (MOF) has provided a new dimension to develop new photocatalysts for CO2 reduction since MOF materials have structures similar to that of metal-incorporated zeolites. Most importantly, the composition, structure and the light absorption ability of the MOF materials can be facilely tuned by modifications on the metal ions or the organic ligands, which make MOF materials more appealing as photocatalysts as compared to the metal-incorporated zeolites. Motivated by the discovery on the Ti-incorporated zeolites, we will initiate a research project to target the photactive Ti-based MOF for photocatalytic CO2 reduction. Our preliminary result showed that NH2-MIL-125(Ti), a Ti-based MOF material prepared by ligand substitution on UV responsive MIL-125(Ti) exhibits photocatalytic activity for CO2 reduction to give HCOO- under visible light irradiations, although its activity is still quite low. With an aim to develop high efficient Ti-based MOF photocatalyst for CO2 reduction, this project intends to synthesis and characterize different Ti-based MOF materials with a variety of organic linkers and study their photocatalytic performance for CO2 reduction. Based on these results, hopefully we can elucidate how the composition and the structure of these Ti-based MOF materials can influence their photocatalytic activity and the photocatalytic mechanism of MOF-based photocatalysts. This results not only can provide us with some useful information in the photocatalytic CO2 fixation, but also can enrich the theory of the photocatalysis and expand the application of the MOF-based materials.

CO2的固定和转化是环境和能源领域中的一个研究热点。利用光催化还原CO2来人工模拟光合作用是一种绿色的固碳技术，然而目前光催化还原CO2的效率很低，开发可还原CO2的新型高效光催化材料是实现这一绿色固碳技术的关键。基于NH2-MIL-125(Ti)这一含Ti的MOF材料在可见光下可以把CO2还原成甲酸根离子这一前期发现，本项目提出了开发Ti基MOF材料作为还原CO2光催化材料这一新思路。拟设计合成含有不同羧酸连接配体的Ti基MOF材料，考察其光催化还原CO2的性能，研究MOF材料的组成、结构与其性能之间的关系，揭示影响其光催化还原CO2性能的本质因素，并阐明其在光催化还原CO2上与传统半导体以及分子筛光催化剂上的共性与差异，从而开发出可还原CO2的Ti基MOF高效光催化材料。该研究不仅对于CO2的固定具有很好的理论指导意义，而且还可以丰富光催化理论，拓展MOF材料的实际应用。

该项目系统地研究了含Ti的NH2-MIL-125(Ti)，含Zr的NH2-Uio-66(Zr)以及三个含有相同配体但具有不同结构的Fe基MOF材料MIL-101(Fe)、MIL-53(Fe)和MIL-88B(Fe)以及它们的胺基衍生物在可见光下还原CO2的性能，发现组成MOF材料的金属中心、配体及结构对MOF材料光催化性能具有很大的影响，尤其是MOF材料中的配位不饱和金属位点在光还原CO2中起了重要作用。我们还提出了在胺基修饰的Fe基MOF材料中所存在的两种光激发途径，一种为金属簇的直接激发，另一个途径为配体的激发然后电子从配体到金属中心发生电荷转移跃迁。在这些研究基础上，我们提出了提高MOF基材料光催化还原CO2性能的三种途径，即（1）通过配体的取代来提高MOF材料对光的吸收及其对CO2的吸附能力；（2）通过后修饰金属离子的取代引入电子传输的中继体；（3）利用贵金属的负载来提高MOF材料中电荷的分离效率并促进氢的溢流。除了在光还原CO2中的应用外， 我们还发现MOF材料在光催化有机合成方面具有巨大的潜在应用，尤其是结合MOF材料的光催化性能及其结构中丰富的催化活性位，有可能使MOF材料作为多功能的催化材料用于光诱导有机反应。这些研究为更好地理解光催化还原CO2机理，设计合成基于MOF的还原CO2的新型光催化材料提供了一定的理论指导。我们的研究也表明MOF材料是光诱导有机反应的一类非常有潜力的催化剂，这一发现也拓展了MOF材料的应用领域。基于该课题我们在国际重要SCI期刊上共发表学术论文27篇，受邀在Springer出版社出版的“Nanostructured Photocatalysts, Advanced Functional Materials”中撰写一章 “Metal-Organic Frameworks (MOFs) for Photocatalytic Organic Transformations”，在Wiley出版社出版的“Encyclopedia of Inorganic and Bioinorganic Chemistry”的 “Sustainable Inorganic Chemistry” 中撰写一章“Multifunctional MOF-Based Photocatalysis”，获授权发明专利5项。