构建基于石墨烯和钙铝石电子盐的钌基催化体系及其催化氨合成反应的协同机理研究

Ammonia synthesis is an artificial nitrogen fixation technique that supplies 40-60% of the nitrogen in human body. Furthermore, ammonia has been considered as a promising renewable fuel and hydrogen carrier recently. Developing an active catalyst for ammonia synthesis is theoretically and practically valuable from both the viewpoints of reducing energy consumption by the conventional Haber-Bosch process and shifting our society to renewable energies in the future. This project aims at designing a ruthenium catalyst having higher activity for ammonia synthesis at moderate temperature and pressure compared to conventional iron catalyst and investigating its synergetic catalysis mechanism. According to the fact that the ammonia synthesis activity of ruthenium catalysts can be significantly modified structurally and electronically by the supports and promoters, graphene and mayenite electride are supposed to be used respectively as support and promoter to tailor the ruthenium active sites structurally and electronically as following: the abundance of B5 sites of ruthenium crystallites is supposed to be enhanced by tuning the size and shape of ruthenium crystallites grown epitaxially on the surface of layered graphene support having high specific surface area, the epitaxial growth process is considered to be tailored by applying the approaches utilized in controllable synthesis of metallic crystallites; the turnover frequency of each ruthenium active site is expected to be improved via electron back donation from mayenite electride to ruthenium crystallites, the back donation is considered to be mediated by the graphene substrate having high conductivity. In addition, the synergetic catalysis mechanism in ammonia synthesis over ruthenium nanocrystallites promoted structurally and electronically by graphene and mayenite electride will be investigated by characterization with TEM, IR spectra of chemisorbed-N2, and other techniques. As a result, a theoretical basis for guiding the design of further better ruthenium catalytic system will be achieved.

合成氨是一种提供人体40-60%氮源的人工固氮手段。近年氨更被认为是潜在的可再生燃料和氢能载体。开发高活性合成氨催化剂对降低Haber-Bosch工艺的能耗和普及可再生能源利用都具有重要的理论和应用价值。本项目着眼于构建在低温低压下具有比铁性能更高的钌基合成氨催化体系并研究其协同催化机理。根据钌催化剂的结构和电子分布易受担体和助催化剂影响的特点，拟选用石墨烯和钙铝石电子盐分别作为担体和助催化剂对钌活性点进行结构和电子设计：利用具有高比表面积和层状结构的石墨烯作为担体，借助钌纳米颗粒可控合成技术，期望取向生长出具有高丰度B5活性位的钌晶体；利用具有强供电子能力的电子盐作为助催化剂，以高导电的石墨烯作为媒介将电子盐的电子传递给钌来增强单一钌活性点的转化频率。选择TEM和IR等手段研究石墨烯和电子盐对钌催化氨合成的结构和电子促进效应及其协同作用机制，为进一步优化钌基合成氨催化体系提供理论基础。

合成氨是一种提供人体40-60%氮源的人工固氮手段。近年氨更被认为是潜在的可再生燃料和氢能载体。开发高活性合成氨催化剂对降低Haber-Bosch工艺的能耗和普及可再生能源利用都具有重要的理论和应用价值。本项目着眼于构建在低温低压下具有比铁性能更高的钌基合成氨催化体系并研究其协同催化机理。根据钌催化剂的结构和电子分布易受担体和助催化剂影响的特点，选用石墨烯或层状复合氧化物和钙铝石电子盐分别作为担体和助催化剂对钌活性点进行结构和电子设计。层状结构的石墨烯或复合金属氧化物担体取向生长出具有高丰度B5活性位的钌晶体。完成的主要研究内容有：（1）建立了多元醇法控制Ru纳米颗粒粒径大小的方法，发现多元醇可以同时还原氧化石墨烯和Ru阳离子，得到石墨烯表面的高分散金属纳米颗粒。1，3-丙二醇-氧化石墨烯-过渡金属阳离子体系可以用于合成石墨烯负载金属纳米颗粒催化剂。（2）开发了一种石墨烯包裹钡掺杂钙铝石电子盐的溶胶凝胶一步合成法。证实了Ba-掺杂可以显著提升电子盐提供电子给石墨烯表面的Ru，使得Ru/G@Ba-(C12A7:e-)的活性达到5.3 mmol/gcat/h。（3）层状复合金属氧化物（铌酸锶，铌酸钡）是优良的Ru基合成氨催化剂载体。Cs-Ru/Sr2Nb2O7在400℃，0.1MPa的活性达到4.986 mmol/gcat/h，接近目前公开报道的最好催化剂Cs-Ru/MgO，Ru/(C12A7:e-)、Ru/Pr2O3的两倍。Cs-Ru/Ba5Nb4O15 在350℃，0.1MPa的活性达到4.9 mmol/gcat/h，因其最佳活性温度只有350℃，更适合用于将源自生物质和电解水的氢气与空气中的氮气转化成氨气，作为氢能载体使用。（4）发现了氧空位对Ru基合成氨热催化的贡献机制。我们通过制备具有钙钛矿结构的层状钽酸钡载体并将其应用于Ru合成氨催化体系。验证了部分还原过渡金属和氧空位（M-Ov）不但可以给Ru提供电子，还可以接受Ru表面的H原子溢流，既可以促进Ru对N≡N的解离，又可以缓解解离的氢原子占据氮气的活性位点。本研究结果提供了除电子盐、碱土金属等供电子体之外的另一个供电子途径，对合成氨热催化理论是一个重要突破。.