The activity and lifetime of heterogeneous catalysts are intimately linked with their structural stability in reactive environments. However, it can be challenging to understand and predict how reactive environments lead to nanoparticle coarsening via center of mass motion and Ostwald ripening and how evaporation can lead to mass loss.
In the first part of the presentation, I will demonstrate how atmospheric pressure electron microscopy can be used to understand how a model automotive catalyst - Pt/Pd bimetallic nanoparticles supported on Al2O3 - responds to reduction and oxidation. Significant metal vaporization and metal diffusion were observed at temperatures above 600 °C, both in pure oxygen and air. This behavior implies that material transport through the vapor during typical catalyst aging processes for oxidation can play a more significant role in catalyst structural evolution than previously thought. The observation of significant evaporation and metal diffusion led us to explore these phenomena in a different model system, Au supported on non-reactive SiN thin films.
We developed and exploited advanced data analysis tools to track the temporal evolution of nanoparticles as a function of time, temperature, and reactive environment using transmission electron microscopy. We describe how a systematic investigation of dataset preparation, neural network architecture, and accuracy evaluation lead to a tool for determining the size and shape of nanoparticles in high pixel resolution TEM images. We use this algorithm to generate rich data regarding the complexities of nanoparticle coarsening, ripening, and evaporation.
We have developed an analytical model that describes this process, showing how local and long-range particle interactions through diffusive transport affect evaporation process. The extensive data of the evolution of several hundred particles allows us to determine physically reasonable values for the model parameters, quantify the particle size at which Gibbs-Thompson pressure accelerates the evaporation process, and explore how individual particle interactions deviate from the mean-field model.
Speaker: Eric Stach, Univ. of Pennsylvania
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