应用于CO2还原高效氮化碳光催化剂的设计合成

Design of high-efficient photocatalytic systems is one important research hotspot in the field of photocatalysis. Many studies mostly focus on promoting the light absorption and the separation of photogenerated charges of photocatalysts, however the adsorption and activation of CO2 were often ignored in the catalyst design. To achieve CO2 conversion efficiently, in principle, photocatalyst system should have abilities to produce enough electrons/holes and can convert these electrons and holes. In this project, we will design and create surface defects in graphitic carbon nitrides (C3N4) to promote CO2 adsorption, especially activation. And on this basis, we also promote the light absorption and the separation of photogenerated charges of C3N4 by thermal copolymerization of organic molecules and N-rich precursors (such as urea, thiourea and melamine). It is expected to obtain high-activity C3N4 photocatalysts via synchronously modulating light absorption, charges separation and CO2 adsorption and activation. We will explore the effect of surface defects in C3N4 on CO2 adsorption and activation, and uncover the detailed mechanism and the related kinetics of photocatalytic CO2 reduction from catalytic chemistry aspects. We will clarify the influence of light absorption, charges separation and CO2 adsorption and activation on CO2 reduction, which would provide theoretical supports for the rational designs of efficient photocatalyst systems. In addition, the effect of backward reactions on CO2 reduction will be investigated. We also design C3N4 photocatalysts with long-distance separation of oxidation/reduction sites. The long distance between the reduction and oxidation sites may block the backward reactions, thus further promoting the performance of CO2 reduction.

设计高效CO2还原光催化剂是光催化领域一个挑战性课题。当前多数研究只是关注提升催化剂的光吸收和电荷分离，而忽视对CO2吸附活化的调控。原理上，光催化系统需同时满足高效供给以及催化转化光生电荷的能力，方可实现CO2有效转化。本项目拟通过对氮化碳(C3N4)进行缺陷化设计，促进对CO2的吸附活化。在此基础上，耦合有机分子共聚和助催化剂修饰同时增强C3N4的光吸收、电荷分离以及对CO2的吸附活化，实现CO2高效转化。着重从催化化学层面认识C3N4表面缺陷对CO2吸附活化的影响及动力学规律。阐明C3N4光吸收、电荷分离以及CO2吸附活化对CO2还原的影响规律，为高效CO2还原催化剂理性设计提供一定理论支持。同时，研究CO2还原过程中逆反应的效应，拟通过氧化位和还原位空间分离抑制逆反应，进一步提升CO2还原效率。

氮化碳作为一种可见光响应的半导体光催化剂由于其合适的能带结构、制备过程简单以及易修饰等优点在光催化领域得到极大关注。研究已经证明氮化碳可以有效驱动水分解反应，然而其光催化CO2还原性能较差，其主要源于其弱的CO2吸附活化能力。本项目主要围绕对氮化碳进行结构调控以及助催化剂的设计，强化CO2的吸附活化过程，进而促进CO2的催化转化。同时，通过结构调变也实现了对氮化碳光吸收和电荷分离的优化。取得如下主要结果：（1）构建了一种低结晶度的新型氮化碳光催化剂，由于其丰富结构缺陷和氨基基团修饰，不仅有效的拓展了氮化碳的捕光范围，也有效促进了CO2的吸收，在长波长可见光驱动仍然可以驱动CO2还原反应。（2）利用水蒸气刻蚀，在氮化碳表面选择性刻蚀碳位点，实现碳空位的构建。发现碳空位修饰，有效促进CO2的吸收活化和光生电荷的分离。碳空位功能化的氮化碳光催化CO2还原效率相对于未修饰的氮化碳提高了45倍，420 nm波长下的量子效率达到4.8%。（3）构建了一种氮化钴/氮杂碳复合助催化剂，复合结构优化了催化中心的电子结构，显著促进CO2的吸附和活化，明显提升CO2的催化转化，复合催化剂中Co原子的转化频率为0.97 s-1。（4）通过Al金属调控了Co3O4助催化剂的电子结构，增加了Co3O4助催化剂表面的电子密度，提升了CO2的吸附，进而加速了CO2还原的动力学。（5）合成了一种富氮纳米碳材料，其氮含量高达27.6at%，发现该富氮纳米碳可以作为一种高效的助催化剂驱动CO2还原反应，反应性能接近现有报道金属体系的性能。通过本项目的资助，发表论文9篇，对氮化碳催化CO2还原过程中存在的科学问题有了较深刻的认识，对设计高效光催化CO2还原系统有重要借鉴意义。