氨的低温催化合成

Low-temperature ammonia synthesis is one of the long-sought scientific goals. Extensive and intensive experimental and theoretical studies reveal that the development of highly efficient, low-temperature ammonia synthesis catalyst is restricted by the intrinsic kinetic scaling relations and adsorption-energy scaling relations of reacting species on transition metal surfaces. The introduction of second reactive center made of alkali or alkaline earth compounds may be the key in breaking those scaling relations. Alkali and alkaline earth oxides are indispensable catalyst components and have been regarded as electronic promoters in the catalytic NH3 synthesis and decomposition, although such a role has been under debate for nearly a century. Little investigation was given to alkali and alkaline amides. In our efforts of developing lithium amide for hydrogen storage, an accidental experimental phenomenon evidencing a strong interaction between lithium amide and transition metals was observed. More importantly, by compositing lithium amide and 3d transition metals, unprecedentedly high catalytic activities have been obtained showing strong promoting effect of lithium amide on transition metals. Our characterization results indicate that lithium amide plays a different role from that of alkali metal oxides. It synergizes with 3d metal nitrides, executes inductive effect to stabilize higher N-content intermediate (ternary transition metal nitrides with Li), alters reaction path and energetics, creates a step-wise chemical cycle that fulfills the abnormally high catalytic activity. According to the principle of microscopic reversibility, a good catalyst for ammonia decomposition will likely perform well in ammonia synthesis, which is indeed confirmed by our preliminary results. Therefore, a series of composites made of transition metals and alkali or alkaline earth metal amides will be synthesized and characterized, and their catalytic performances in ammonia synthesis will be evaluated. Based on the combined kinetic analyses and in situ spectroscopic characterizations, the relationships among the catalyst composition, structure and performance are to be established, and the mechanistic understanding will be pursued. It is our hope that through such a 5-year project, the promoting effects of alkali and alkaline earth amides in catalytic ammonia synthesis can be elucidated which may pave the way to the rational design and development of efficient catalysts for low-temperature ammonia synthesis.

合成氨工业关乎粮食生产和能源环保，而氨的低温合成是百余年来催化工作者努力探索的目标。在过渡金属表面进行的催化反应存在固有的动力学及吸附能线性限制关系，只有打破这些限制才有可能实现氨的低温高效合成。而对合成氨催化中不可或缺但又极具争议的碱（土）金属助剂的化学状态及其作用机制的深入认识则可能是打破该限制的切入点。申请者前期在研究氨基锂储氢性能的过程中偶然发现其与3d过渡金属间存在强的相互作用，二者复合后在氨的催化分解中展现出前所未有的高活性，且氨基锂是共催化剂而非电子助剂。近期研究初步表明该复合物在氨的催化合成中亦表现出独特的性能。本项申请拟考察多种碱（土）金属氨基化合物与过渡金属及其氮化物之间的相互作用，由此设计开发新型催化剂体系；结合动力学分析和现场表征探讨反应机制，优化催化剂组成和结构，以期打破线性限制，达到氨的低温高效催化合成。

氨不仅关乎粮食安全，近年来亦被认为是重要的能源载体。然而，目前Haber-Bosch合成氨工业严重依赖化石能源，是一高能耗、高碳排放的过程，开发温和条件下实施的绿色合成氨变革性技术是必然趋势，已成为当前化学化工领域的前沿研究课题。项目执行人在发现氢化锂的加入可大幅提高3d过渡金属合成氨催化性能的前期研究基础上，系统而深入研究了一系列碱（土）金属氢化物与过渡金属（包括钴、铬、锰、钌等）的相互作用。发展了一系列新型氢化物合成氨催化剂，包括氢化钡-钴、碱（土）金属氢化物-氮化锰、钡铬氮氢化合物、碱（土）金属-钌配位氢化物等催化剂体系。该催化剂体系在中低温条件下展现出优异的合成氨催化性能，显著优于文献报道的高活性钌基参比催化剂。这一研究大大丰富了合成氨催化剂家族，为下一代低温高效合成氨催化剂的设计与研发提供了坚实基础。在此基础上，通过对氢化物-3d过渡金属双中心协同催化机制的研究，促发了项目团队将合成氨反应解耦为固氮和加氢产氨两个分步骤，并由此构建了“氢化物-亚氨基化合物”介导的、全新的化学链合成氨路径。在常压、250℃下，该化学链过程的产氨速率较在高压下进行的热催化过程高出一个数量级；在常压和100℃下亦可缓慢实施氨的合成。这项研究为化学链合成氨拓展了材料研发空间，并有望实现可再生能源驱动的、小型化、分布式合成氨这一远景。上述“氢化物介导合成氨”的研究成果基于氢化物与N2、H2和NH3之间独特而丰富的化学作用，破解了Scaling Relations对合成氨的制约，为温和条件下氨的高效合成这一世纪难题打开了局面。作为“领头羊”式反应，合成氨研究的进展将对整个催化领域产生深刻的影响。