Direct Imaging of Catalyst Evolution with Atomic Resolution
Researchers at ORNL, in collaboration with Tufts University, have imaged gold (Au) catalysts with sufficient resolution to see single atoms within the catalyst, using an aberration-corrected electron microscope. Most importantly, they have succeeded in obtaining images while heating the catalyst and have been able to observe directly the effects of exposure to the environment of a catalytic reactor. The goal of the study was to learn how the catalyst works and how it transforms during preparation steps or during catalytic operation. Correlation of the images of the differently treated catalysts with corresponding catalytic studies at Tufts University proved the active sites for two reactions important for energy and environmental applications. Small (~2-3nm) metallic Au particles were determined to be the active agent for catalyzing CO oxidation reactions while highly dispersed cationic Au species are the active agent for water gas shift reactions. A crucial aspect is that the starting catalyst was leached to produce a state in which no discrete Au nanoparticles were present on the surface of the supporting iron oxide crystallites. Subsequent microscopy was able to detect Au cations and 1-2 nm sized Au nanoparticles (NPs) embedded within the oxide support crystallites, revealing the phase interpenetration that results from co-impregnation methods of catalyst preparation. Voids within the iron oxide support particles were observed, with surfaces decorated with Au atoms. A technique of through-particle focusing permitted analysis of the 3-d spatial relationship of the Au NPs. Interestingly, embedded NPs had decreased lattice spacings, compared to bulk Au or compared to comparably sized particles on the external surfaces. Thermal treatments resulted in the collapse of the voids and the migration of Au to the external surfaces of the iron oxide, a process captured by sequences of images recorded with time at temperature. Depending upon temperature, this Au exists either as individually dispersed Au atoms (cations) or is aggregated to form metallic Au only a few nanometers across.
In-situ microscopy now permits direct observation of the evolution of catalyst structure with atomic resolution, as a result of elevated temperature treatments. This approach has been applied to determine the active sites in supported Au catalysts, potential catalysts for fuel cells, emission control and biofuel conversion.
This research was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy.
Lawrence Allard, Albina Borisevich, Weiling Deng, Rui Si, Maria Flytzani-Stephanopoulos, and Steven Overbury
Journal of Electron Microscopy 2009, 58 (3), 199-212.