铁基双金属团簇对氮气活化制氨的催化反应机理研究

N2 activation is the essential and limiting step for ammonia synthesis. Industrial ammonia production is still dominated by the traditional H-B process, which requires high temperatures (350~500℃) and pressures (150~350 atm), consumes enormous energy resource. Meanwhile, massive CO2 will be released to the air which may aggravate the global warming. Nowadays, the society has noticed the importance of energy conservation and emission reduction. Therefore, it is principal and valuable to explore a new ammonia synthesis method which can decrease the reaction temperature and pressure, lower the energy consumption. The key to improve the ammonia production process is the development of an efficient catalyst and understanding the reactions on the catalyst surfaces. Herein, we combine the density functional theory method and electrocatalysis to investigate the N2 reduction reaction under mild conditions, even at the room temperature and ambient pressure. We selected Fe-based bimetal clusters as the main active catalysts and functionalized graphene as the support. By varying the size and element ratio of the transition metal atom doped Fe- clusters, we will delve the induced change of the cluster configurations and the electronic structures between clusters and supports. We will also analyze the adsorption, dissociation and reduction processes of N2 on the surface/interface active sites. The study is critical to understand the performance of Fe-based bimetal cluster catalysts for NH3 synthesis. Combining the electrocatalysis and theoretical study will provide further insight into the process which is necessary for exploring and developing an efficient catalyst to optimize the reaction conditions.

氮气活化是合成氨反应中最重要、最基本的一步。传统的合成氨工艺条件苛刻（350~500℃，150~350 atm），耗能巨大，且伴有CO2等温室气体产生；在提倡节能减排、绿色环保的今天，如何减少合成氨的能源消耗，降低反应温度和压力，具有重要的探究价值和现实意义。促进氮气活化的关键在于新型高效催化剂的开发以及对其表界面反应过程的调控。本项目基于密度泛函理论和电化学方法，开展低温低压金属团簇催化合成氨的探究。拟选取铁基双金属团簇作为主体催化剂，功能化石墨烯为载体，通过调变Fe与其他过渡金属元素的掺杂比例和团簇尺寸，考察团簇的几何构型和电子结构及其与载体界面之间的相互作用关系，分析氮气吸附、分解和加氢过程的反应机理，为寻找和设计高效合理的催化剂提供实验和理论支撑，对进一步开发低能低耗的合成氨工艺具有重要的指导意义。

实现氮气在温和条件下的高效活化与转化是缓解当前能源危机与环境问题的重要途径之一。促进氮气活化的关键在于新型高效催化剂的开发以及合成氨工艺的改进，所以从机理层面对催化过程进行深入探究和合理优化具有重要的意义。本项目以铁基金属团簇作为主体催化剂，运用密度泛函理论结合团簇反应实验，开展了温和条件下氮气活化制氨的系统研究。取得如下研究成果：.（1）以铁团簇Fe13为模型，通过掺杂3d过渡金属构筑异核双金属团簇，系统研究了掺杂团簇的结构稳定性，团簇/载体表界面电子排布规律，及其与N2等小分子的相互作用机制，为深入探究簇合物表面化学反应提供依据。.（2）将小尺寸铁团簇Fen(n=1-4)负载到不同载体表面，进行电化学合成氨的机理研究和尺寸效应考察，发现团簇具有的多活性位点可以增强与吸附氮气分子的相互作用，促使更多电荷转移到N2分子的反键轨道，从而有利于高效活化N≡N键，电化学合成氨的过电势显著降低。.（3）作为合成氨的逆反应，研究了石墨烯负载的五种典型Ptn团簇对氨分解的催化作用，深入诠释了金属团簇表面Lewis酸碱互补活性位点对极性分子催化活化过程的促进作用，为深入认识金属表面的析氢与氮还原机制提供理论基础。.（4）运用等离子体技术辅助Rh3团簇催化氮气活化，质谱直接观察到了奇数氮产物，表明温和条件下N2直接解离。结合理论计算揭示了氮气解离的机理，也说明了三原子团簇催化的优势和更广泛的应用前景。通过对一系列金属团簇上氮气活化反应的研究，发现了具有多活性位点的小尺寸团簇具有突出的催化性能，为高效活化氮气解离及合成氨反应提供了理论和实验依据。