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

第1部分：非技术合成催化是一种有效地将一种化学物质转化为另一种化学物质的过程，是化学工业和石化工业的基础。据估计，催化作用约占全球国内生产总值的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(尺寸只有几个纳米)负载在非反应底物上进行化学转化的过程。由于这些颗粒是微小的，它们几乎所有的原子都在纳米颗粒的表面。表面原子的能量比纳米粒子块体中的原子高，因为它们没有完全成键。这种缺乏完整的键是至关重要的，因为它允许它们充当催化剂，但它也会导致问题。由于催化作用发生在高度反应的环境中(包括高温和侵蚀性环境)，表面的原子可以被驱离纳米颗粒。这些原子可以四处移动，并可能导致排列降低它们在进一步催化中有效的能力。该项目使用实时、高分辨率成像来直接观察这些过程，并将这些观察与计算机模拟紧密结合起来，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。该项目吸收了两所大学的本科生参与研究工作，以及波多黎各大学马亚圭兹分校等为少数群体服务的机构的学生。研究成果为科学家提供了必要的了解，以帮助稳定反应过程中的多相催化剂，潜在地导致成本和能源使用的大幅节省。第2部分：技术总结金属纳米颗粒最关键的应用之一是在多相催化领域，在多相催化领域，它们的小尺寸导致普遍存在不协调的表面中心，促进反应物转化为产品。然而，高度集中的欠配位表面位增加了总表面能，从而通过粗化、合并和蒸发来驱动粒子的演化。这些过程最终会导致整体催化活性的下降。虽然这些现象被普遍理解，但现有的理论描述都是平均场的，还在争论中。该项目使用高通量、定量的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“地面真相”之间的迭代，这项研究确定了1)粗糙表面如何影响接触线的动态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)表面能量各向异性的作用。除了提供对这些过程更好的基本理解外，这些研究还提出了新的途径来减轻技术相关系统中不必要的粗化。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.