离子液体-双金属催化体系构筑及电催化N2还原合成氨的研究

Ammonia is considered an important chemical for both agriculture fertilizer and renewable energy. The conventional Haber-Bosh process to produce NH3 is energy intensive and leads to significant CO2 emission. Electrochemical synthesis of ammonia through nitrogen reduction reaction has recently attracted significant attention. The low solubility of nitrogen and competing reaction of hydrogen evolution reaction of current catalysts, leads to develop new catalytic system. Based on the unique physical and chemical properties of ionic liquids, such as solubility adjustable, overpotential controllable and superior conductivity, and the property of empty d orbital in the center of the transition metal absorbing lone pair electrons of nitrogen, investigation of electrochemical synthesis of ammonia from nitrogen and water catalyzed by ionic liquid-dual metal catalytic system is carried out. First, ionic liquid with strong adsorption and high solubility is designed as the electrolyte and ionic liquid-induced dual metal materials are served as electrode materials. The relationship between nitrogen activation and electrode material and electrolyte is clarified. The structure of transition state was achieved through the detection of in situ instrument and the calculation and density functional theory. The synergistic catalytic mechanism of ionic liquid electrode material and electrolyte is revealed by the charge transfer and binding free energy of N2H* and NH2* with electrode material. The above research aims to provide the scientific basis for the development of electrocatalytic nitrogen reduction to ammonia under mild conditions.

氨是农业肥料和可再生能源的重要化学品。传统的Haber-Bosch法制氨工艺能耗大。近年来，在常温常压条件下电催化氮气合成氨受到广泛关注。针对氮气难溶和析氢反应竞争的问题，基于离子液体具有溶解度可调、电位可控和导电性良好等独特性质和过渡金属中心空d轨道吸附氮气孤对电子的特性，本项目拟设计合成离子液体-双金属催化体系，研究电催化合成氨新过程及调控合成氨机制:设计合成阴阳离子具有强吸附力和高溶解度的新型离子液体，诱导合成具有空d轨道的双金属电极材料，明确离子液体电极材料和离子液体电解液与氮气活化之间的构效关系；结合原位仪器和密度泛函理论对N2过渡态进行监测和计算，获得过渡态产物结构，计算氮气吸附活化产物N2H*和氨脱附NH2*过渡态产物与催化材料之间的电荷转移和结合自由能，揭示离子液体电极材料和电解液协同催化机理，为温和条件下电催化氮气还原合成氨技术的开发提供科学基础。

氨（NH3）是化肥工业最基础的原料，对工农业生产和发展至关重要。当前Haber-Bosch法工业合成氨每年能耗占全球的1%，同时释放大量CO2。开发高效、低耗和绿色的合成氨方法迫在眉睫。本项目以氮气和水为原料，在常温常压下通过电催化氮气还原合成氨（N2RR），设计合成双金属和离子液体诱导催化体系，研究不同过渡金属对催化活性的影响规律，离子液体与催化性能之间的构效关系，揭示反应机理。同时，开发电催化N2和CO2合成尿素体系，为电催化N2还原开辟了新路径。研究结果表明，双金属催化体系NiCo2O4具有更多活性位点和更优的N2RR催化活性，表现出18.70%的法拉第效率，NH3产率最高达到30.63 μg h-1 mg-1，电流密度经多次循环和长时间电解后未见明显减小，表明通过双金属催化剂形貌和活性位点的共同调控是电催化合成氨的有效策略。通过水热法成功合成了不同含量IL诱导CuO催化体系，CuO-0.50 g IL催化体系展现出优异的电催化N2RR活性，法拉第效率最高达到23.21%。经过多次循环和长时间电解，电流密度、NH3产率和FE均没有明显的下降，说明CuO-0.50 g IL具有良好的选择性和稳定性，主要归因于CuO-0.50 g IL催化体系中离子液体的诱导效应，使电子发生了O→Cu的转移，产生了富电子Cu，有利于氢吸附到缺电子的氮上，进而促进N2分子的极化。开发了Bi-BiVO4 复合电极材料电催化N2和二氧化碳合成尿素实验，在[Bmim][BF4]-KHCO3电解液中表现出6.64 mmol h-1 g-1 的尿素产率及15.8%的法拉第效率。