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

Northwestern University, Evanston IL

Investigators

Abstract

PART 1: NON-TECHNICAL SUMMARY Catalysis 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 SUMMARY One 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.