温和条件下电催化氮气合成氨用氮/氢复合催化位点的构筑及构效关系的研究

The study of electrocatalytic nitrogen synthesis of ammonia under mild conditions brings new opportunities for the “green nitrogen fixation” and the storage of renewable energy. However, the poor selectivity of the product, low efficiency and unclear reaction mechanism seriously restrict the development of electrocatalytic nitrogen synthesis of ammonia. Basing on the “volcano diagrams” of transition metal surfaces for reduction of nitrogen, this project will construct novel nitrogen/hydrogen composite catalytic sites. The catalytic sites not only break the linear restriction relationship of single transition metal, but also strengthen the nitrogen adsorption and product desorption on the catalysts. Then, taking the design of the catalysts as the main research highlight, gentle methods of stabilizing single atomic site metal by constructing defects on the surface of oxide supports are presented. Furthermore, three-dimensional porous integrated electrodes will be constructed, which is conducive to enhancing nitrogen molecules, electron and proton transfer at the electrode/electrolyte interface. By regulating the composition, exposed crystal planes, single atomic dispersion status of the catalysts, the most suitable catalytic centers will be constructed. And the atomic, defect and electronic structures of the single-atom catalysts will be characterized, and the interaction between nitrogen molecules with catalytic sites will also be in-situ monitored via advanced characterization methods. Finally, the structure-activity relationship between the microstructure of the catalytic sites and the catalytic performance will be revealed. In conclusion, the implementation of this project will promote the conversion of clean energy to chemical energy, and provide the scientific basis and preliminary exploration for the development of electrochemical nitrogen fixation.

电催化氮气合成氨的研究对温和条件下氨的“绿色”合成带来契机，为可再生电能的存储带来新途径。然而，产物选择性差、效率低、反应机制模糊等问题严重制约该方向的发展。本项目将基于过渡金属合成氨反应“火山图”，通过构筑氮/氢复合催化位点，打破单一过渡金属线性限制关系，增强氮气吸附及产物脱附能力；以催化剂的设计为研究内容，利用氧化物载体缺陷，限域合成金属单原子催化剂，设计三维多孔一体化电极，增强电极/电解液界面传质过程；通过调控催化剂的组成、暴露晶面、单原子分散度等与催化性能密切相关的微观结构，赋予最适宜的活性中心；利用先进的表征测试手段对单原子催化剂的原子、电子和缺陷结构进行表征，原位监测氮气与催化位点的作用情况；以分子动力学为指导，构建电催化反应界面，模拟氮气加氢过程，初步揭示催化位点微观结构与催化性能的构效关系。本项目的实施将促进清洁能源向化学能的转换，为电化学固氮的开发提供科学依据和前期探索。

目前，工业合成氨仍采用Haber-Bosch工艺在高温高压下将氢气和氮气催化合成氨，其吨氨能耗大约为1.52吨标准煤，每吨标煤排放二氧化碳约2620公斤、二氧化硫8.5公斤、氮氧化物7.4公斤，不符合“双碳”发展目标。电催化氮气合成氨是在温和条件下实现合成氨反应的新路径，以氮气和水为原料，一步法直接电催化合成，被公认为氨-氢循环的重要技术途径。.本项目针对电催化氮气还原反应特点，以密度泛函理论及量子化学方法为指导，设计合成一系列结构、组分可控的新型催化剂，调节氮气分子在催化剂表面的吸附行为，增强反应物传输能力，抑制氢析出竞争反应，提升催化反应的能量利用效率和电催化还原产物的选择性；通过活性组分理化性能表征、性能解析，揭示氮气电还原反应机理及电催化剂微观结构与催化性能的构效关系；结合前期研究经验，搭建了电催化氮气合成氨测试平台，提出标准测试流程、反应物净化及产物收集方法，利用核磁氢谱对氨产物进行定性检测与同位素定量分析。分别设计了缺陷态氧化钼纳米片、钙钛矿类异质元素掺杂催化剂、探索光电协同提升催化固氮效率及硝酸根合成氨，为电能向化学能的转化提供科学依据和前期探索。具体如下：（1）受自然界钼基生物固氮酶启发，一步法合成非晶态氧化钼纳米片，其含有大量配位不饱和位点和表面氧缺陷，有利于氮气的吸附以及活化，从而增强了其电催化氮还原性能，合成氨法拉第效率为7.06%；（2）通过过渡金属掺杂调节钙钛矿活性位点及电子结构，获得一系列钴掺杂镍酸镧粉体，材料的电催化氮还原催化性能与有效磁矩呈正相关性，合成氨法拉第效率为26.44%；（3）利用具备上转换效应稀土功能材料，制备具有宽太阳光谱吸收特性的复合催化剂，研究光电协同催化固氮合成氨；（4）电势诱导衍生纳米Cu用于高效硝酸根电还原合成氨，探究铜的化学态与硝酸盐还原产物选择性之间的关系。