光催化合成氨水滑石基纳米催化材料的理性设计与性能调控

Industrial procedure for the production of ammonia under high energy consumption condition stimulates the research into the development of sustainable industrial catalytic systems with high efficiency. Photocatalysis featured by its high catalytic efficiency in activating chemical bonding intensively under mild reaction conditions, has become a green technology with promising technical prospect. The development of catalysts with high activity for photocatalytic conversion of N2 to NH3 has important scientific significance and wide application prospect. In this project, ultrathin nanosheet catalysts will be controllably synthesized by using ultrathin layered double hydroxides (LDHs) as the study subject, through intercalating photoactive elements into LDH layer. By adjusting the composition of metal elements, the thickness of nanosheets, etc., the bandgaps and surface defects of LDHs can be controlled to optimize the performance of photocatalytic NH3 formation. In order to prevent the aggregation of LDHs nanosheets and improve their dispersion, LDHs nanosheets will be complexed with suitable supports (MoS2 nanosheets, etc.) with good carrier transporting performance. The photocatalytic synthesis of NH3 performance can be further optimized by controlling the exposure of active facets and dispersion of LDHs nanosheets on the support. Highly efficient LDH-based photocatalysts will be finally obtained, exhibiting excellent photocatalytic performance in reduction of N2 to NH3 under mild condition. Through combination the theoretical calculation with experiment study, the relationships between the structures of photocatalysts and their photocatalytic performance, and the photocatalytic reaction mechanism will be systematically investigated, which may provide general rules for the rational design of highly efficient photocatalysts for NH3 synthesis in future.

工业合成氨技术仍存在高能耗等问题，开发新型高效可持续固氮路线成为该领域的研究重点和挑战。光催化技术具有常温常压深度反应的特点，已经成为一种有重要应用前景的绿色技术。发展新型高效光还原氮气制备氨的光催化材料具有重要的科学意义和应用价值。本项目拟以非贵金属水滑石（LDHs）纳米片为研究主体，通过在其层板中引入光催化活性元素，可控合成超薄纳米片催化材料，通过调控金属元素组成与纳米片厚度等，实现对LDHs能带结构和表面缺陷位等的调控，进而优化其光催化合成氨性能；为避免团聚，进一步提高LDHs的分散度，将其与具有良好载流子传输性能的载体材料（MoS2纳米片等）复合，通过调控LDHs纳米片的晶面取向和分散度，进一步优化其合成氨性能，最终获得高效光催化合成氨LDHs基纳米催化材料。采用理论与实验相结合的方法，系统深入地研究光催化材料的构效关系及光催化反应机理，为开发高效合成氨光催化材料提供设计准则。

光催化技术具有常温常压深度反应的特点，已经成为一种有重要应用前景的绿色技术。发展新型高效光还原氮气制备氨的光催化材料具有重要的科学意义和应用价值。本项目采用水热法、碱刻蚀等方法成功制备了一系列水滑石基纳米光催化剂，如Cu离子修饰ZnAl-LDH纳米片、碱刻蚀LDH纳米片、超小Cu2O纳米颗粒等。这些水滑石基光催化剂在光催化固氮合成氨方面表现出优异的催化活性，为开发和设计新型高效的水滑石基纳米光催化剂提供了新思路和新途径。代表性工作如下：1）发展了一种简便的铜掺杂方法实现了富含氧空位的ZnAl-LDH纳米片的可控制备，氧空位和衍生的富电子铜位点有助于光生电子-空穴对的分离/迁移和氮气吸附，最终使得光催化固氮合成氨速率大幅提升；2）利用Cu2+离子特殊的姜-泰勒效应，将Cu掺入TiO2晶胞中，促使Cu原子周围的氧原子丢失。通过控制进入晶胞Cu的含量，来实现TiO2中的氧空位浓度的精确调控，从而促进氮气吸附和活化，提高可见光催化固氮反应效率；3）利用水滑石的限域拓扑转变性质，采用抗坏血酸还原法制备了粒径尺寸均匀且小于三纳米的小尺寸氧化亚铜负载的水滑石纳米片催化剂。所制备的亚三纳米氧化亚铜含有丰富的捕获态不仅能加速光生载流子迁移速率，而且也能实现长寿命光生电子的形成。延长的光生载流子寿命和高度暴露的表面催化位点，能够和增强N2吸附/活化，从而展现了良好的可见光催化固氮合成氨性能；4）利用NaOH水溶液对ZnCr‐LDH、ZnAl‐LDH和NiAl‐LDH纳米片进行简单的预处理可以大大提高纳米片中的氧空位和低配位金属中心的浓度，从而显著提高了它们在UV-vis光照下将N2还原为NH3的光催化活性（无需添加牺牲剂或助催化剂）。这种简便高效的碱蚀刻策略有望在高性能LDH光催化剂的未来开发中被广泛采用；5）深刻分析了各种检测方法（靛酚蓝法、纳氏试剂法、离子色谱法和核磁法）在检测水性介质中痕量氨时的可靠性。基于一系列结果的分析，提出了精确定量氨的有效标准，避免了光/电催化氨合成中的不合理比较。