Dynamics of Supported Metal Nanoparticles Observed in a CS Corrected Environmental Transmission Electron Microscope

In catalyst design, high performance and stability are key issues. Many catalysts consist of metals or metal alloys deposited onto a support material as nanoparticles in order to optimize the exposed surface area. When exposed to the environment in a catalytic reactor, the particles tend to sinter resulting in the formation of larger particles and a loss of catalytic performance. Several models of sintering in different systems have been put forward [1,2]. However, most investigations have been post mortem studies, revealing only the final state of the catalyst. Transmission electron microscopy (TEM) has been used extensively in catalysis research [3]. However, in contrast to chemical reactors, a conventional TEM is a high vacuum tool. Thus, observations do not always reflect the active state of materials. Environmental TEM (ETEM) provides the capability to expose samples to a gas atmosphere during imaging and analysis. Even though the gap between reactor and high-vacuum pressures has not been fully bridged, progress has been made towards observing materials in their working environment. The surface structures of catalytic materials are highly dependent on the surrounding atmosphere. The combined capabilities of ETEM and image CS correction provide unique possibilities to study this relationship. However, in order to fully quantify image contrast from such experiments, a deeper understanding of the scattering of fast electrons in the presence of gas molecules in the pole piece gap of the microscope is needed. As industrial catalysts are usually complex high surface area materials, they are often not suited for fundamental studies. For this purpose, model systems consisting of gold nanoparticles on sheets of low surface area boron nitride and graphite supports were produced. Sheets of the support were deposited onto an amorphous carbon film on a 3mm copper TEM grid and sputter coated with a thin film of gold. The Au film readily formed nanoparticles ranging from a few nm up to 20nm in size. The samples were exposed to oxidizing and reducing environments at various temperatures and the behavior of the nanoparticles were recorded. Under these conditions, mobility of the particles was clearly visible, while maintaining lattice resolution of both the BN support and the Au particles. Some particles remained immobile during observation while others behaved dynamically on the support. Some sintered by migration and coalescence while others were observed to shrink in size and finally disappear as neighboring particles gradually grew by Ostwald ripening. These observations indicate that several mechanisms may occur simultaneously. Particles on steps were significantly smaller than those on terraces, indicating a stronger interaction of the metal and support at these sites. By quantifying these observations, fundamental insight into activation energies and energy barriers for sintering processes can be studied. References [3] J.T. Richardson and J.G. Crump, J. Catal. 56 (1979) 417. [4] C. H. Bartholomew, Appl. Catal. A. 107 (1993) 1. [5] A.K. Datye, J. Catal. 216 (2003) 144.