Microscopic imaging provides new insights

The Titan Scanning/Transmission Electron Microscope.

As anyone who has watched high-definition TV or seen a 3-D movie knows, the conventional 2-D view just doesn't compare. The same higher resolution and detailed images that make for great viewing are also essential for scientists seeking atomic-level information about the structure and behavior of catalysts. Being able to view nanoparticles and their spatial distribution in 3-D provides a greater depth of information for clearer scientific understanding than could be obtained from a 2-D image. EMSL researchers have been able to capture 3-D images of nanoparticles to gain detailed information on the size, morphology, and location of small catalytic particles supported on a tubular structure using the Titan Scanning/Transmission Electron Microscope (S/TEM). The S/TEM was acquired through the American Recovery and Reinvestment Act.

"In many cases, seeing the 3-D distribution is essential for correlating a structure and its corresponding functional properties," said EMSL scientist Chongmin Wang. "Direct visualization of a multi-components system in 3-D offers us the chance to vividly observe how each component correlates and interacts spatially with other components. This information enables us to understand the properties and behavior of the system, which is hard to perceive from a 2-D image."

Schematic of S/TEM.

The research team used this 3-D method to analyze palladium nanoparticles loaded on hollow carbon nanofibers, which enables scientists to distinguish directly the particles inside versus outside of the hollow carbon nanofibers. Carbon nanofibers are materials composed of stacked graphene layers, and because of their unique structural, electronic, thermal and other properties they have shown promise for use in a variety of industrial applications. The location of metal catalyst particles in relation to the supporting nanofiber structure critically influences the catalytic properties of a system, such as those used in technologies for water purification or emissions control.

EMSL's new microscope is providing equally powerful information on another type of catalytic system. Scientists are seeking to understand how transition-metal catalytic clusters/nanoparticles are attached to oxide supports to gain new information about how to design new types of efficient low-cost catalysts. Transition metals are used as catalysts in a variety of clean technologies. For example, they can be used to help clean up syngas—a synthetic gas mixture that contains carbon monoxide and hydrogen derived from biomass (plant material or agricultural waste for example) used as an alternative fuel source. EMSL scientist Libor Kovarik and his team used the S/TEM to study platinum, palladium, rhodium, and iridium clusters/nanoparticles supported by a magnesium aluminate spinel (MgAl2O4) substrate.

a) Representative bright field image of the MgAl2O4 crystals supporting Ir clusters/nanoparticles. b) High-resolution transmission electron microscopy image of Ir nanoparticle attached to the (110) surface of the MgAl2O4. c) S/TEM high-angle annular dark field detail view of the Ir clusters and nanoparticles on the support oxide. d) Atomic model of the Ir nanoparticle anchored to the oxide support as derived on the basis of the high angle annular dark field observations.

"With the S/TEM, we can achieve sub-Angstrom resolution, which together with the excellent contrast allows us to resolve individul atoms. While resolving individual atoms is impresive, in the field of catalysis it is very important to characterize tiny catalytic clusters and nanoparticles at the atomic scale," said Kovarik. "Understanding the crystallographic and morphological nature of these small clusters and nanoparticles, and how they are attached to various substrates, enable us to rationalize the relationship between the structure and properties and provide the information needed to design better catalysts to clean the environment or produce new forms of energy" he added. The S/TEM also enables scientists to perform spectroscopy work to probe compositional and electronic properties at the atomic scale. This combination of imaging and spectroscpy makes the aberation-corrected S/TEM invaluable for research at the atomic level.

Scientific Impact

The insights gained about the relationship between the atomic-level microstructure and reactivity of these systems will enable new catalytic methods to be developed for clean technologies. The Titan S/TEM enables research involving energy materials, catalysis, interfacial phenomena, and nanostructured materials because of its superior structural imaging and chemical analysis on an atomic level. The scope of available experiments can also be extended to in situ studies of dynamic processes by using sample holders that can control temperature, electrical current, and gaseous environment. Like all of EMSL's experimental and computational tools, the Titan S/TEM capability is available at no cost to the global scientific community through EMSL's user proposal process.

Publications

S Danmeng, C Wang, A Genc, and CJ Werth. 2011. "A New Geometric Method Based on Two-Dimensional Transmission Electron Microscopy for Analysis of Interior versus Exterior Pd Loading on Hollow Carbon Nanofibers," The Journal of Physical Chemistry Letters 2: 1082-1087.

Bowker RH, MC Smith, ML Pease, KM Slenkamp, L Kovarik, and ME Bussell. 2011. "Synthesis and Hydrodeoxygenation Properties of Ruthenium Phosphide Catalysts." ACS Catalysis, 1: 917- 922.