氧空位-过渡金属双活性中心太阳光化学固氮材料合成及其催化机制研究

Although the industrial Haber-Bosch can fix N2 on a large scale, it has to be conducted under high temperature and pressure with huge amount of fossil fuels being consumed. Therefore, it is of vital importance to develop alternative mild processes for N2 fixation. Among the pioneering works, artificial photosynthesis imitated oxide semiconductor photocatalysis is a promising approach to harvest solar light for N2 fixation, while state-of-the-art catalytic efficiency is seriously hampered by the photocatalysts’ weak interaction with N2 and poor solar light utilization, especially the solar heating effect originated from the infrared radiation (accounting 53% of the solar photon energy). In contrast, industrial ammonia synthesis can achieve high catalytic efficiency via the thermally promoted N2 activation and hydrogenation on the heated Fe/Ru surface. In order to improve the photocatalytic N2 fixation efficiency by taking the advantage of both photocatalysis and thermocatalysis, this project aims to develop a series of novel solar N2 fixation catalysts (such as Ru/CeO2-x and Fe-TiO2-xHx) by the coupling of conventional Fe/Ru to the new active center of oxygen vacancies. The influences of oxygen vacancies, Fe and Ru nanoparticles on the catalysts’ optical and photochemical ammonia synthesis properties will be systematically studied. The nitrogen activation and reduction on the oxygen vacancies, Fe and Ru nanoparticles and their coupling effects will be unfolded. The design and synthesis of novel photochemical ammonia synthesis materials for basic applications will help to advance the fundamentals of photochemical N2 fixation, explore new routes for ammonia synthesis with high efficiency, push forward the development of new photochemical N2 fixation technology.

Haber-Bosch反应实现大规模人工固氮，但必须在高温高压条件下进行。如何发展温和固氮新策略是人工固氮面临的关键科学问题。模仿自然界光合作用的光催化固氮是目前备受关注的人工固氮策略，但传统光催化剂没有充分利用太阳光的热效应，其固氮效率仍有待提高；而利用热效应实现氮气的有效活化和还原是工业高效合成氨的关键。为了充分利用太阳光及其热效应促进光化学固氮，本项目提出以氧空位为新型固氮活性中心，耦合传统Fe/Ru纳米晶热催化活性中心，设计和制备氧空位-过渡金属双活性中心太阳光化学固氮催化新材料，考察材料表面氧空位和Fe/Ru纳米晶对太阳光化学合成氨效率的影响，阐明氧空位与Fe/Ru纳米晶双活性中心在太阳光化学合成氨过程中的耦合效应，揭示双活性中心太阳光化学合成氨催化材料的构效关系。项目成果将巩固和加深表面光催化固氮的基本理论，开辟太阳光化学合成氨的新途径，推动光化学固氮新技术的发展。

半导体光催化固氮有望突破工业Haber-Bosch过程苛刻的反应条件，从根本上解决能源和环境问题。传统光催化剂借助光电子模拟辅助中心输送电子活化氮气，但未能充分利用太阳光的热效应；工业高效合成氨的关键在于利用热效应有效还原活化氮气。新型固氮活性中心氧空位与传统热催化活性中心Fe/Ru纳米晶耦合使其能充分利用太阳光及热效应。阐明氧空位与Fe/Ru纳米晶双活性中心与其光化学合成氨活性之间的构效关系，不仅能加深表面光催化固氮基本理论的理解，推动光化学固氮新技术的发展，还有助于筛选和设计高效太阳光化学合成氨催化剂。本项目中，我们系统研究了氧空位与过渡金属耦合形成双活性中心促进太阳光化学合成氨机制，发现氧空位与载体H−耦合可以构筑合成氨第二活性中心，促进N2室温下活化加氢生成氨；通过研究Fe纳米项链-TiO2-xHy纳米颗粒特性与光热催化活性之间的构效关系，揭示了活性中心局域电场和热场优化反应热力学和动力学的规律，提供了解决合成氨“scaling relationship”难题新途径；通过比较热催化和光热催化合成氨的差别，利用等离子激元光热双温区催化突破了合成氨反应热力学平衡浓度限制，并证实了氧空位调控可以延长光生电子寿命，降低光致合成氨全路径能垒；我们还发展了Pt1-Ptn与载体氮空位、单原子Ru与CeO2载体氧空位-氧空位通道耦合等策略来促进表面氮物种加氢，抑制NHx表面累积导致的催化剂失活，显著提升了低温光热合成氨活性。