新型异质结复合磁性光催化剂的功能化组装及其光催化还原CO2的研究

Nowaday the problem of global warming and energy crisis have aroused considerable public concern, which is dominantly due to the increasing levels of CO2 emissions from fossil fuel consumption. Managing CO2 emissions is one of the largest political and technical challenges for the sustainable development currently. One promising approach involves the photocatalytic reduction of CO2 into hydrocarbon fuel. One of the biggest challenges preventing the large scale development of photocatalysts for CO2 reduction is the problems such as the general photocatalyst in adsorption, agglomeration, and difficult to reuse. Hence, the new strategies are required to further promote the photocatalytic reduction of CO2. In this project, magnetic core-shell mesoporous material was synthesized by modified hydrothermal method. Heterostructural semiconductor with high dispersion and small particle size were deposited on mesoporous via layer by layer assembly. The photoreduction of CO2 was characterized to investigate the photoactivity of the materials. The photocatalytic activity, magnetic separability and stability are evaluated. The influence of mesoporous structure, chemical composition, physical and chemical properties on the photoactivity and adsorption stability is also studied. Based on the experimental results and theoretical calculation, we explore the relationship between the photocatalysts performance and its structure, to reveal the mechanism of photocatalytic CO2 reduction. Our work provides a novel strategy for designing of new functional efficient semiconductor-based photocatalysts for the reduction of CO2. This work can potentially be used at long term storage sites to convert CO2 into chemicals with industrial value, which ensure the long-term economic and social stability as well as the security and environmental sustainability of the world at large.

探索如何控制大气中CO2的总量和积极应对能源危机，已成为全球可持续发展的战略问题。在太阳能驱动下利用光催化材料将CO2转化为可再生碳氢燃料，实现碳循环利用，是理想的CO2资源化方案。但传统意义的半导体催化剂存在量子效率低、难以回收、吸附能力差等缺点，本项目以磁性核壳介孔氧化硅为载体，通过层层自组装的方法，将具有光催化活性组分与介孔组分相复合，构筑“磁性核-介孔硅-半导体”三元异质结复合磁性光催化剂，实现光催化剂在介孔孔道内的高分散和小粒径沉积。将该材料应用于光催化还原CO2合成甲醇反应中，评价光催化材料的光催化活性，磁可分离性和稳定性，获得高效、稳定的异质结复合光催化剂。在性能评价的基础上，研究异质结复合磁性光催化剂的结构、组成以及各组分对催化性质的影响，揭示光催化剂性能与结构之间的内在规律。本项目的研究为半导体材料在CO2的综合利用提供实验依据，也为环境治理中CO2的资源化提供新途径。

本项目以水热法制备的高磁饱和强度Fe3O4纳米颗粒为核，正硅酸乙酯(TEOS)为前驱体，采用改进的Stöber 法，制备介孔SiO2包覆Fe3O4磁性核壳复合微球。对制备样品的物相结构、形貌和磁性能进行了测试表征。研究结果表明，制备的复合材料呈球形，粒径分布均一，材料的比表面积和磁饱和强度分别为413 m2•g-1和68.93 emu•g-1。研究了TEOS的添加量对复合微球形貌的影响，随着TEOS添加量的增加，SiO2壳层增厚，复合粒子形貌均匀，饱和磁化强度有所下降，但仍具有良好的超顺磁性。在此基础上，制备合成了Fe3O4@SiO2@mSiO2@TiO2和SiO2-NH2。研究了Fe3O4@SiO2@mSiO2–NH2对水中重金属离子Cr(Ⅵ)的吸附性能。通过附动力学拟合，Fe3O4@SiO2@mSiO2–NH2对Cr(Ⅵ)的吸附过程准二级动力学模型占主导地位。探究了该材料对Cr(Ⅵ)的吸附过程和吸附机理。结果表明，其吸附机理及吸附容量与Cr(Ⅵ)的离子形态及–NH2有关，并通过吸附剂与吸附质之间的电子共用或静电吸附实现。通过紫外光下光催化还原CO2的性能来评价样品的光催化活性，研究发现Fe3O4@SiO2@mSiO2@TiO2的优异性能主要是由Fe3O4的磁核，介孔SiO2的中间层及TiO2粒子的协同作用决定的。由于磁性介孔氧化硅特殊的孔道结构，使得TiO2在结晶时被限制，在特殊的孔道内部生长为纳米颗粒，纳米尺寸的TiO2结构使得光生空穴-电子对的复合概率降低，从而提高了光生载流子浓度，进一步提高其光催化性能。当Ti/Si配比过小时，TiO2颗粒被大量的mSiO2包裹，导致其无法与反应物分子充分接触，失去小尺寸效应；当Ti/Si配比过大时，TiO2可能会造成某个方向孔道的堵塞，阻碍反应物分子与TiO2的接触，导致催化效果减弱。钛酸四丁酯(TBOT)的添加量为2.5mL 的Fe3O4@SiO2@mSiO2@TiO2光催化剂催化效率最高，在光照时间10 h后，生成CH3OH的产量为1.24mmol•g-1。此外，该光催化材料能够在外加磁场作用下，能实现快速分离，且在重复利用5次后，其催化活性仍没有明显下降。本项目的研究为三元“磁性核-介孔硅-半导体”复合材料的功能化构筑提供实验依据，也为CO2的资源化提供新途径。