氮（氧）修饰在钼基材料电催化氮气还原反应中促进机制的理论研究

Ammonia is an important chemical raw material. At present, the industrial process of ammonia synthesis urgently needs to solve the high energy consumption and high CO2 emission issues, currently. The electrocatalytic process of ammonia synthesis is carbon-free and uses clean renewable energy at ambient conditions, which is considered a promising alternative to the industrial process. Mo-based electrocatalysts is currently considered as the potential of ammonia production materials, the nitrogen (oxygen) modified MoO3, MoO2 and Mo2N exhibited excellent performances, but the promotion mechanism is still not well understood. In this project, density functional theory calculations are employed on periodic MoNxOy models, which are constructed from the nitrogen modified MoO3 and MoO2, and oxygen modified Mo2N. (1) To study the computational method with a periodic boundary condition for the energy of H+ and the participation in ammonia production reaction process, combine with the solvation models. (2) To understand the role of different amount of nitrogen (oxygen) distribution at different sites for ammonia production reaction, and screen out the most superior MoNxOy model. (3)To investigate the regulation mechanism of nitrogen (oxygen) modification for ammonia production reaction activity and selectivity, and further clarify the active site, reaction mechanism and the microscopic mechanism to deter the competition hydrogen evolution reaction. Therefore, these will establish a universal, accurate and practical method for H+ calculation in case of periodic boundary conditions, and pave the way for the rational design and optimization of Mo-based materials for electrocatalytic ammonia synthesis.

氨是一种重要的化工原料，目前合成氨工艺亟待解决高能耗和高CO2排放问题。电催化合成氨可利用清洁可再生能源在常温常压下进行，且无碳排放，是合成氨工艺最有前景的替代者。Mo基催化剂是目前最有潜力的电催化合成氨材料，氮（氧）修饰形成的MoO3、MoO2和Mo2N等表现出优异的性能，但其促进机制尚不明确。本项目拟采用理论计算的方法，通过氮修饰的MoOy及氧修饰的MoNx构建MoNxOy周期性模型。（1）结合溶剂化模型，研究周期边界条件下H+能量及其参与合成氨反应过程的计算方法。（2）研究氮（氧）不同位置和浓度分布对合成氨反应的影响，从中筛选出性能最佳的MoNxOy模型。（3）探讨氮（氧）修饰对合成氨反应活性和选择性的调控，阐明反应活性位点、反应机理以及抑制竞争析氢反应的微观机制。从而提出普适、准确和实用的H+在周期边界条件下的计算方法，并为实验优化与合理设计Mo基电催化合成氨材料提供思路。

开发出高性能电化学合成氨催化剂是实现温和条件下合成氨，降低工业合成氨高能耗的最有效途径之一。本项目的研究内容如下：.（1）围绕电催化合成氨MoNxOy模型筛选与氮氧修饰的促进机制开展工作。构建不同浓度氮/氧修饰MoO2(001)模型，揭示活性位点结构特征，阐明反应机理，得到优异的MoNxOy模型：1)合成氨反应通过缔合路径进行，氧空位的引入促进MoO2(001)上N2加氢解离，但也会抑制解离氮的加氢，反应性能有所提升；2)调节氮掺杂氧空位修饰的浓度，能同时兼顾N2强吸附和解离N弱吸附，所筛选出的MoNxOy，使反应能垒下降56 %，且电位为-0.38 V可抑制析氢，提高选择性；3)氮氧修饰通过调节活性位点Mo的键价，得到高活性高选择性催化模型。.（2）探究氮空位修饰对Mo2N(001)电催化合成氨的调节机制，理清催化剂分解产氨机制。构建氮空位修饰Mo2N(001)，揭示氮空位是催化剂高活性前提，提出催化剂改性策略：Mo2N(001)通过缔合路径进行反应，虽电位低，但动力学能垒极高。引入氮空位后，反应是直接解离路径，电位低，同时动力学能垒下降。但空位易转移至体相，引发催化剂分解。在次表面引入Ti妨碍氮空位体相转移，稳定催化剂，提出MoTiN体系是潜在的合成氨催化剂。.（3）探究缺陷促进Fe基热催化合成氨的反应机制。构建台阶位缺陷Fe(211)D模型，计算揭示缺陷构筑新型C7活性位点，是反应高活性的关键，降温合成氨：反应是直接解离路径，Fe(211)D同时促进N2解离和后续加氢还原。COHP分析发现其N2活化度更高，促进N2解离。其结构更开放，Fe-N键价低，降低加氢能垒。动力学模拟发现，温度降低100 K，Fe(211)D仍比Fe(211)在高温下产率高6倍多。实现单一活性位点破解Scaling Relations对合成氨的制约。.上述研究为温和条件下高效合成氨提供思路和理论指导。