经济发展和人口增长导致了严重的温室效应和能源短缺等社会问题，该现状迫切需要发展CO₂催化转化为燃料或化学品技术，尤其是具有高能量密度和高市场价值的CH₃CH₂OH的合成。光催化CO₂还原高效合成C_nH_2n+1OH(n=1, 2)是实现2060年碳中和目标最有希望的途径之一。然而，多个电子耦合的多质子转移、低效的载流子分离能力和化学稳定的CO₂分子使得该反应在产率和选择性方面面临巨大挑战。近年来，科研工作者提出了多种用于改善光还原CO₂性能的催化策略以促进该领域的发展，如助催化剂负载、构建异质结工程、光敏剂修饰、金属有机框架(MOF)合成、单原子(SAs)设计和反应介质引入等。
在本论文中，为解决光催化还原CO₂合成C_nH_2n+1OH(n=1, 2)技术中载流子分离能力低和C–C耦合困难等关键科学问题，我们通过在g-C₃N₄中分别引入酞菁分子和稀土SAs合成了酞菁钴(CoPc)/g-C₃N₄和Er-Nd/g-C₃N₄复合材料。受工作一中离子液体(ILs)活化CO₂和稳定^*C₁中间体启发，采用多步水热法合成了ILs修饰的多孔MOF材料以进一步提高合成CH₃CH2OH的能力。合成的三种光催化剂在CO₂还原性能评估中展现出了优异的CH₃OH和CH₃CH₂OH生成能力。基于系列表征和理论计算，我们提出ILs赋予CO₂经历^*HCOO途径高效合成CH₃OH的能力，并能通过稳定^*C₁中间体改善CH₃CH₂OH的选择性；从^*CO中间体吸附角度来看，Er和Nd SAs在合成CH₃OH和CH₃CH₂OH过程中存在协同效应；填充在有序MOF结构中的ILs通过稳定*C1中间体显著地改善CH₃CH₂OH的选择性。本文研究内容如下：
(1) 首先，以盐酸预处理的尿素为原料采用热缩聚法合成超薄多孔g-C₃N₄纳米片；然后，在溶剂蒸发作用下合成了CoPc修饰的g-C₃N₄纳米复合物(CoPc/CN)。CoPc/CN表现出增强的光生电荷分离能力，通过充当化学吸附剂和电解质的离子液体的双重作用，实现CO₂向CH₃OH和CH₃CH₂OH的有效转化。差分电荷密度显示，因大量电子从[BMIM]⁺转移到Co表面而引起了强烈的局部场效应，这有利于CO₂活化和^*HCOO中间体的稳定。原位红外光谱(FT-IR)表明ILs能够驱动CO₂向^*HCOO转化，这是CH₃OH生成的关键中间体。DFT计算表明在ILs介质中C–C耦合反应的吉布斯自由能从4.94 eV下降到2.21 eV。这项工作开发了一种ILs辅助的光电催化策略以获得优异的CH3OH产率，并为ILs辅助CO₂还原中的C–C偶联机制提供了新的见解。
(2) 基于理论模拟和原子配位-限域的方法，以三聚氰胺泡沫、尿素、(Er(NO₃)₃·5H₂O和(Nd(NO₃)₃·6H₂O为原料，我们设计并制备了具有ErN₆和NdN₆单元的稀土SAs催化剂(Er₁Nd₃/CN)。合成的样品对CH₃CH₂OH和CH₃OH分别表现出破纪录的生成速率，分别是1761.4 μmol g^–1 h^–1和987.7 μmol g^–1 h^–1。通过DFT计算、X射线吸收近边结构分析、像差校正透射电子显微镜和原位FT-IR光谱的结合，我们证明了Er SAs不仅能促进电荷转移，还作为C−C键形成的活性中心实现CO₂还原，而Nd SAs在可见光下为C₂H₅OH合成中的C−C耦合提供了必要的^*CO。此外，吉布斯自由能表明这种催化策略稳定了^*CO，同时促进了^*CHO的产生，并提高了CH₃OH的产量。本工作揭示了^*CO吸附在实现从CO₂到C_nH_2n+1OH(n=1, 2)的高效转化中的关键作用，为SAs催化剂的设计和合成提供了新的设计思路。
(3) 首先，以苯甲酸和微量水为调节剂，四(4-羧基苯基)-卟啉(TCPP)为配体，Zr簇为节点，采用溶剂热制备了具有多孔结构的卟啉基MOF；然后，通过金属配位的方式合成了原子级Ni分散的PMOF光催化剂(Ni ADMS/PMOF)；最后，通过真空浸渍法合成了用于光催化CO₂还原的(ILs@Ni ADMS/PMOF)。在没有任何空穴牺牲剂下，PMOF基样品展现了优异的全光谱响应的C₂H₅OH合成能力。在配备420 nm和765 nm带通滤光片的氙灯辐照下，ILs@Ni ADMS/PMOF催化的CO2还原生成的C₂H₅OH速率分别是17.39 mmol h⁻¹ g⁻¹和8.89 mmol h⁻¹ g⁻¹。表征和理论计算证明了多孔PMOF结构赋予ILs稳定^*·CH₃和^*·CH₂OH中间体的能力，进而促进C−C耦合。此外，差分电荷密度显示ILs调控了Ni位点周围的电子密度，这为CO₂活化和稳定C₁中间体创造了丰富的电子环境。时间分辨瞬态吸收光谱表明，电子从配体到Ni发生了转移，并在可见-近红外激发下展现了延长的电子寿命，这有利于实现CO₂深度还原。这项工作揭示了光还原CO₂高效合成C₂H₅OH的新机制，为全光谱响应型催化剂的设计和合成提供了新思路。

The greenhouse effect and energy shortage caused economic development and population growth, which need to promote the development of catalytic CO₂ conversion into fuel or chemicals, especially the generation of CH₃CH₂OH with high energy density and satisfactory market value. The efficient synthesis of C_nH_2n+1OH (n=1, 2) through photocatalytic CO₂ reduction is one of the most promising ways to achieve carbon neutrality goals in 2060. However, the complex electron/proton transfer process, inefficient carrier separation ability, and chemically stable CO₂ molecule make photocatalytic CO₂ reduction reactions still challenging in terms of yield and selectivity. Recently, researchers have proposed various catalytic strategies to improve the photocatalytic CO₂ reduction performance to boosted this field, such as co-catalyst loading, heterojunction engineering, photosensitizer modification, metal organic framework (MOF) synthesis, single atoms (SAs) design, and introduction of reaction media, etc.
In this thesis, to address key scientific issues such as low carrier separation ability and difficulties in C−C coupling in the photocatalytic CO₂ reduction to C_nH_2n+1OH (n=1, 2), we synthesized cobalt phthalocyanine (CoPc)/g-C₃N₄ and Er Nd/g-C₃N₄ composite materials by introducing phthalocyanine molecules and rare earth SAs into g-C₃N₄, respectively. Inspired by the activation of CO₂ and stable ^*C₁ intermediates by ionic liquids (ILs) in work one, ILs modified porous MOF materials were synthesized using a multi-step hydrothermal method to further enhance the ability to synthesize CH₃CH₂OH. The three photocatalysts demonstrate excellent CH₃OH and CH₃CH₂OH generation abilities in CO₂ reduction performance evaluation. Based on characterizations and theoretical calculations, we propose that ILs endow CO₂ with the ability to efficiently synthesize CH₃OH through the ^*HCOO pathway, and could improve CH₃CH₂OH selectivity by stabilizing ^*C₁ intermediates; there is a synergistic effect between Er and Nd SAs in the synthesis of CH₃OH and CH₃CH₂OH from the perspective of ^*CO species adsorption; and in an ordered MOF structure; ILs filled in MOF pores can significantly improve the selectivity of CH₃CH₂OH by stabilizing ^*C₁ intermediates. The research content of this thesis is as follows:
(1) Firstly, ultra-thin porous g-C₃N₄ nanosheets were synthesized using urea pretreated with hydrochloric acid as the raw material through thermal polymerization method; then, the g-C₃N₄ nanocomposites modified with CoPc (CoPc/CN) were synthesized under action of solvent evaporation. The CoPc/CN exhibits enhanced photogenerated charge separation ability, achieving efficient conversion of CO₂ to CH₃OH and CH₃CH₂OH through the dual action of ILs acting as chemical adsorbents and electrolytes. The differential charge density shows that a strong local field effect is caused by the transfer of a large number of electrons from [BMIM]⁺ to the Co surface, which is beneficial for the CO₂ activation and the stability of ^*HCOO. In-situ infrared spectroscopy (FT-IR) indicates that ILs can drive the conversion of CO₂ to ^*HCOO, which is a key intermediate in the generation of CH₃OH. DFT indicates that the Gibbs free energy of the C−C coupling in ILs medium decreases from 4.94 eV to 2.21 eV. This work developed an ILs assisted photoelectrocatalytic strategy to obtain excellent CH₃OH yield, and provided new insights into C−C coupling in ILs assisted CO₂ reduction.
(2) Using melamine foam, urea, (Er(NO₃)₃·5H₂O and (Nd(NO₃)₃·6H₂O as raw materials, we synthesized rare earth SAs catalyst (Er₁Nd₃/CN) with ErN6 and NdN6 units based on theoretical simulation and atomic coordination-confinement methods. The synthesized samples showed unprecedented rate of 1761.4 μmol g^–1 h^–1 and 987.7 μmol g^–1 h^–1 for CH₃CH₂OH and CH₃OH, respectively. Through a combination of DFT calculation, X-ray absorption near edge structural analysis, aberration-corrected transmission electron microscopy, and in situ FT-IR spectroscopy, we demonstrate that the Er SAs facilitate charge transfer, serving as active centers for C−C bond formation, while Nd SAs metal provide the necessary ^*CO for C−C coupling in C₂H₅OH synthesis under visible light. Furthermore, the calculated Gibbs free energy indicates that this catalytic strategy stabilizes ^*CO and promotes ^*CHO generation concurrently, enhancing CH₃OH production. This work reveals the key role of CO adsorption in achieving efficient conversion from CO₂ to C_nH_2n+1OH (n=1, 2), providing new research ideas for the design and synthesis of SAs catalysts.
(3) The PMOF with porous structure was first prepared by solvothermal method using benzoic acid and trace water as regulators, Tetrakis(4-carboxyphenyl)-porphyrin (TCPP) as ligands, and Zr clusters as nodes; and then atomic Ni dispersed PMOF photocatalysts (Ni ADMS/PMOF) were synthesized through metal coordination method; finally, the ILs@Ni ADMS/PMOF was synthesized by vacuum impregnation method for photocatalytic CO₂ reduction. The PMOF-based samples exhibit excellent full spectrum response to C₂H₅OH synthesis without any hole sacrificial agent. Under xenon lamp irradiation equipped with 420 nm and 765 nm bandpass filters, ILs@Ni ADMS/PMOF showed the rates of C₂H₅OH are 17.39 mmol h⁻¹ g⁻¹ and 8.89 mmol h⁻¹ g⁻¹, respectively. Characterization and theoretical calculations have demonstrated that the porous PMOF structure endows ILs with the ability to stabilize ^*·CH₃ and ^*·CH₂OH intermediates, thereby promoting C−C coupling. In addition, differential charge density shows that ILs regulate the electron density around Ni sites, creating a rich electronic environment for CO₂ activation and stabilization of C₁ intermediates. Time resolved transient absorption spectroscopy indicates that electrons have transferred from the ligand to Ni and exhibit extended electron lifetime under visible near-infrared excitation, which is beneficial for achieving deep CO₂ reduction. This work reveals a new mechanism for the efficient synthesis of C₂H₅OH through photo-reduction of CO₂, and provides new ideas for the design and synthesis of full spectrum response-photocatalysts.