Photocatalytic N2 reduction utilizing the upconverted hot electron

氨（NH3）是工业规模生产的最重要的分子之一，因为它在农业和其他化学工业中起着关键作用。众所周知，目前用于制造NH3的Haber-Bosch工艺需要高压和高温，留下了非常大的碳足迹，消耗了全球总能源的1%以上。该项目探索了一种光催化替代化石燃料驱动的热Haber-Bosch工艺，有可能大幅减少碳足迹和能源消耗。虽然光催化方法从太阳获得能量，但在目前的技术水平上，太阳能利用效率太低，无法实现商业应用。因此，该项目研究了一种新的催化剂设计，这种设计可能会大大提高光催化NH3的制造效率，大大超过目前的最先进技术。该项目得到了针对K-12学生、教师和本科生的教育和推广活动的支持。人们正在探索光催化和电催化方法将N2转化为NH3，以解决Harbor-Bosch工艺的问题。然而，由于N2的高还原电位、高度稳定的三键和较弱的表面吸附亲和力，N2还原为NH3仍然是最具挑战性的光催化反应之一。该项目将开发一种新的光催化方法，通过利用mn掺杂半导体量子点（QDs）中激子到热电子上转换过程产生的热电子，将N2转化为NH3。这允许使用可见光来产生热电子，热电子在传导带以上具有非常高的多余能量，并表现出远程传输能力。这些热电子最近被证明可以增强光催化H2生成和CO2还原，并且有望(i)对N2到NH3的转化具有足够高的还原潜力，（ii）产生可以额外参与N2到NH3转化的溶剂化电子。具体来说，该研究将探索三种不同的方法，目标是将N2到NH3还原的整体量子效率显著提高到目前最先进的水平（~1%）。第一种方法旨在增强热电子和溶剂化电子还原N2和中间物质的动力学。这将通过采用二元溶剂体系来实现，该体系大大提高了N2和中间物质的浓度和稳定性。第二种方法使用QD/分子催化剂混合系统，其中远程热电子敏化将被利用，使分子N2还原催化剂的使用不需要共价附着在QD上。第三种方法旨在利用嵌入量子点光催化剂的氧化铟锡光子晶体同时提高热电子生成速率和氧化还原平衡，从而实现增强光吸收和空穴转移的双重功能。综上所述，本项目旨在建立一种热电子驱动的可见光光催化N2还原新方法，可以提高光催化N2还原效率。除了研究重点之外，该项目还将通过德克萨斯农工大学创新项目（Texas A&M Innovation [X] program）将本科教育与研究结合起来，该项目旨在通过解决现实问题的研究活动促进跨学科教育。此外，研究人员将继续参与全校范围的化学开放日和全国范围的美国晶体生长竞赛外展活动，这些活动将K-12学生、教师和公众带到STEM主题的讲座、参观和实践活动中。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Ammonia (NH3) is among the most important molecules produced at an industrial scale due to its critical role for agriculture and other chemical industries. The well-known Haber-Bosch process currently used to manufacture NH3 requires high pressure and operating temperatures leaving a very large carbon footprint consuming over 1% of the total energy produced globally. The project explores a photocatalytic alternative to the fossil fuel driven thermal Haber-Bosch process, potentially achieving drastic reductions in the carbon footprint and energy consumption. Although the photocatalytic approach derives energy from the sun, the solar utilization efficiency at the current level of technology is too low for commercial application. The project thus investigates a novel catalyst design that potentially can boost the photocatalytic NH3 manufacturing efficiency significantly beyond the current state-of-the-art. The project is bolstered by educational and outreach activities targeting K-12 students, teachers, and undergraduate students.Photocatalytic and electrocatalytic approaches are being explored for the conversion of N2 into NH3 to resolve the issues of the Harbor-Bosch process. However, because of the high reduction potential of N2, its highly stable triple bond, and weak surface adsorption affinity, the reduction of N2 to NH3 remains one of the most challenging photocatalytic reactions. The project will develop a new photocatalytic approach to convert N2 to NH3 by utilizing hot electrons that are produced via an exciton-to-hot electron upconversion process in Mn-doped semiconductor quantum dots (QDs). This allows for the use of visible light to generate hot electrons that possess very high excess energy above the conduction band and exhibit long-range transfer capability. These hot electrons have recently been shown to enhance photocatalytic H2 production as well as CO2 reduction, and are expected to (i) be of sufficiently high reduction potential for N2 to NH3 conversion and (ii) produce solvated electrons that can additionally participate in N2 to NH3 conversion. Specifically, the research will explore three different approaches with the goal of increasing the overall quantum efficiency of N2 to NH3 reduction significantly beyond the current state-of-the-art (~1%). The first approach aims at enhancing the kinetics of the reduction of N2 and intermediate species by hot electrons and solvated electrons. This will be accomplished by employing binary solvent systems that greatly increase the concentration and stability of N2 and intermediate species. The second approach uses QD/molecular catalyst hybrid systems in which the long-range hot electron sensitization will be exploited to enable the use of molecular N2 reduction catalysts without requiring covalent attachments to the QDs. The third approach aims at enhancing the rate of hot electron generation and the redox balance simultaneously by using indium tin oxide photonic crystals imbedded with QD photocatalysts leading to dual functionality of enhancing light absorption as well as hole transfer. In sum, the project aims to establish hot electron-driven visible light photocatalytic N2 reduction as a new approach that can bring much needed improvement in the photocatalytic N2 reduction efficiency. Beyond the research focus, the project will integrate undergraduate education with research via the Texas A&M Innovation [X] program designed to foster interdisciplinary education through research activities solving real-world problems. In addition, the investigators will continue to be involved in the university-wide Chemistry Open House and nation-wide US Crystal Growing Competition outreach activities that bring K-12 students, teachers and the general public to lectures, tours and hands-on activities on STEM subjects.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.