Collaborative Research: Quantifying the Coarsening Kinetics of Supported Metal Nanoparticles Using Time-resolved Electron Microscopy, Data Analytics and Simulations

催化是将一种化学品有效地转化为另一种化学品的过程，是化学和石化工业的基础。据估计，催化对全球国内生产总值的贡献约为35%。多相催化是催化的一个子类，是一种将非常小的金属纳米颗粒（只有几纳米大小）支撑在非反应性底物上用于化学转化的过程。因为这些粒子是微观的，几乎所有的原子都在纳米粒子的表面。表面原子比存在于纳米颗粒体中的原子具有更高的能量，因为它们没有完全结合。缺乏完整的键是必不可少的，因为它可以使它们发挥催化剂的作用，但它也会导致问题。因为催化作用发生在高度反应的环境中（包括高温和腐蚀性环境），表面的原子可以从纳米颗粒中被驱赶出来。这些原子可以四处迁移，并可能导致排列，降低他们的能力，有效的进一步催化。该项目使用实时、高分辨率成像技术直接观察这些过程，并将这些观察结果与计算机模拟紧密结合，以确定降低催化性能的基本物理机制。这项工作主要由宾夕法尼亚大学和西北大学的研究生完成。该项目包括两所大学的本科生，以及来自少数族裔服务机构（如波多黎各大学马亚圭斯分校）的学生。研究结果为科学家们提供了必要的理解，以帮助在反应过程中稳定非均相催化剂，从而可能大大节省成本和能源使用。金属纳米颗粒最重要的应用之一是在多相催化领域，它们的小尺寸导致了表面不协调位点的普遍存在，从而促进了反应物向产物的转化。然而，高浓度的不协调表面位点增加了总表面能，从而通过粗化、聚并和蒸发驱动粒子演化。这些过程最终导致整体催化活性的降低。虽然这些现象被普遍理解，但现有的理论描述是平均场的，并且存在争议。该项目使用高通量、定量图像分析来分析宾夕法尼亚大学的原位透射电子显微镜数据。这些数据与西北大学的大规模模拟密切相关。通过实验观察和模拟的“基本事实”之间的迭代，本研究正在确定1)粗糙表面如何影响接触线的动力学从而影响纳米颗粒的演化，2)颗粒大小和放置如何影响纳米颗粒的生长，以及3)表面能各向异性的作用。除了提供对这些过程的基本理解之外，这些研究还提出了在技术相关系统中减少不必要的粗化的新途径。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。

PART 1: NON-TECHNICAL SUMMARYCatalysis is the process used to efficiently convert one chemical to another, and is the basis of the chemical and petrochemical industries. Catalysis is estimated to contribute approximately 35% of the global gross domestic product. Heterogeneous catalysis is a sub-class of catalysis, and is a process by which very small metallic nanoparticles (of only a couple of nanometers in size) that are supported on non-reactive substrates are used for chemical conversions. Because these particles are microscopic, nearly all their atoms are on the nanoparticle's surface. Surface atoms have a higher energy than atoms that are present in the nanoparticle bulk because they are not fully bonded. This lack of full bond is essential as it allows them to act as catalysts, but it also leads to problems. Because catalysis occurs in highly reactive environments (including high temperatures and aggressive environments), the atoms at the surface can be driven off of the nanoparticle. These atoms can migrate around and can lead to arrangements that reduce their ability to be effective at further catalysis. This project uses real-time, high-resolution imaging to see these processes directly and tightly couple these observations with computer simulations to determine the fundamental physical mechanisms that degrade catalytic performance. The work is being performed primarily by graduate students at the University of Pennsylvania and Northwestern University. This project incorporates undergraduate students at both institutions in the research efforts, as well as students from minority-serving institutions such as the University of Puerto Rico at Mayaguez. The research outcomes provide scientists with the needed understanding to help stabilize heterogeneous catalysts during reactions, potentially leading to substantial savings in both cost and energy usage.PART 2: TECHNICAL SUMMARYOne of the most critical applications of metal nanoparticles is in the field of heterogeneous catalysis, where their small size leads to a prevalence of under-coordinated surface sites that facilitate the conversion of reactants to products. However, a high concentration of under-coordinated surface sites increases the total surface energy, which drives particle evolution via coarsening, coalescence, and evaporation. These processes eventually lead to a decrease in overall catalytic activity. While these phenomena are understood generally, existing theoretical descriptions are mean-field and are under debate. This project uses high-throughput, quantitative image analysis to analyze in-situ transmission electron microscopy data at the University of Pennsylvania. This data is tightly linked to large-scale simulations at Northwestern University. Through the resulting iteration between the ‘ground truth’ of experimental observations and simulations, this research is determining 1) how rough surfaces affect the dynamics of contact lines and thus nanoparticle evolution, 2) how particle size and placement affect nanoparticle growth, and 3) the role of surface energy anisotropy. In addition to providing an improved fundamental understanding of these processes, these studies suggest new routes to mitigate unwanted coarsening in technologically relevant systems.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.