News/Highlights

• S. Overbury and C.K. Narula are co-organizers of a symposium on "Catalysis in Fuel Chemistry" at 237th American Chemical Society National Meeting to be held on March 22-26, 2009 in Salt Lake City, Utah.

• University of Missouri-St. Louis researchers study fuel cell catalysts using HTML’s ACEM at ORNL: Under a new HTML project, Prof. Jimmy Liu and his student Jinfeng Wang are working with ORNL researcher Larry Allard using the HTML’s aberration-corrected electron microscope (ACEM) to study novel catalysts for on-board reforming of methanol to provide hydrogen for PEM fuel cells. An increase in the efficiency in such devices is needed to make it a favorable route for advanced fuel cell vehicles. The present study involves a new catalyst based on palladium on zinc oxide supports, where the zinc oxide is a synthesized material with a “nanobelt” microstructure. The support is an ideal model catalyst system for this study, as the nanobelts are single crystal of uniform thickness that provide an optimum structure for high resolution studies. The images show the overall structure of the material (left), where the bright contrast features are the Pd particles. The crystal lattice of the Pd relative to the ZnO nanobelt is shown in the high resolution image (right), indicating the kind of information yielded by the ACEM on the catalyst microstructure. Ultimately, this research can have a significant impact on understanding and predicting the general behavior of metal-oxide interactions and how these interactions affect the performance of nanostructured catalysts for fuel cell and other energy-production applications.

• University of Maine project analyzes structures of bimetallic fuel-cell catalysts: Prof. Howard Patterson from University of Maine and his graduate student Robert Gomez are studying the atomic structure of a series of zeolite-based monometallic and bimetallic catalysts designed to remove carbon impurities such as carbon monoxide from hydrogen that is supplied to fuel cells. Such fuel cells are being considering for powering electric vehicles in the future. Zeolites are aluminosilicate materials with a crystal structure that exhibits large “cages,” in which heavy metals such as cobalt and molybdenum can sit. The question was to try to determine if the different atoms actually infiltrated into the crystal structure, or simply agglomerated into larger particles sitting on the surface of the zeolite. ORNL researcher Larry Allard worked with Robert Gomez (Fig. 4) using the HTML’s aberration-corrected electron microscope (ACEM) to obtain dark-field images (in which high atomic number atoms appear in bright contrast) to characterize the catalyst microstructure and to identify the location of cobalt and molybdenum compounds. An example of the structure of a Mo/zeolite sample is shown in the dark-field image (Fig. 5 next page), which shows Mo(III) atoms in ZSM5 zeolite. The individual Mo(III) atoms (seen in bright contrast) appear to occupy the negatively-charged β-chambers of the zeolite, which are ordered in a regular grid-like pattern. An improvement in fuel cell efficiency with significant energy savings is expected as these catalysts are improved and effectively utilized to remove carbon impurities from hydrogen.

• In situ heating of Au-Pd nanoparticles reveals interesting phenomena in University of Texas-Austin catalyst project: University of Texas-Austin researcher Prof. Miguel Jose-Yacaman (below) and the HTML’s Dr. Larry Allard continued studies of catalyst nanoparticle structure and morphology on the HTML’s aberration-corrected electron microscope (ACEM), which allows the atomic imaging at sub-Ångström resolution that is imperative for such research. The work has been extended into the area of in situ heating to better understand both changes in particle structure and also the sintering behavior of particles at elevated temperatures. A unique heating capability that provides the ability to record ultra-high-resolution images at high temperature was applied to imaging the behavior of Au-Pd particles with a novel 3-layer structure (core with two surrounding shells). The images show the homogenization of the 3-layer particle (Pd-rich core, Au-rich first shell and Pd-rich outer shell) with time at temperature. Interestingly, it was observed that adjacent particles would sometimes not sinter together, or agglomerate, but instead one particle would gradually reduce in size while the grain in a closely adjacent particle concomitantly grew. This phenomenon is shown in the second and third images of Fig. 7. As a more complete understanding of in situ heating system is developed, systematic experiments will be conducted to further clarify behavior of catalyst materials to be used in fuel cells and other vehicle-related applications.

• PNNL continues collaborative electron microscopy studies of NOx trap catalysts: Using the HTML’s JEOL 2200FS-AC aberration-corrected electron microscope (ACEM), both Ba-alumina and Pt-alumina compositions of lean NOx trap catalysts are being studied by PNNL’s Dr. Chuck Peden and HTML researcher Dr. Larry Allard. In recent follow-up work to a user visit in 2007, Allard imaged Pt/alumina samples calcined at both 300°C and 600°C, to determine the structure and morphology of the Pt species at atomic resolution. Because of a relatively high loading (10% Pt by weight), the sample at 300°C showed some discrete crystals in the 3-5nm size range, but also a uniform dispersion of single atoms, small clusters, and “rafts” of atoms with irregular periodicity. The 600°C sample, however, showed a bimodal distribution of species, with some large perfect Pt crystals 10nm or larger, although with some remaining clusters and rafts of atoms. Work is continuing with new samples having significantly lower Pt loadings.

Highlights:

• ORNL researchers presented following papers at 2008 Society of Automotive Engineers World Congress held at COBO Hall, Detroit, MI on April 14-18, 2008.
• Development and Validation of a Reduced Reaction Mechanism for Biodiesel Fueled Engine Simulations, Brakora, J.L.; Ra, Y.; Reitz, R.D.; McFarlane, J.; Daw, C.S.; 2008-01-1378.
• Effects of Fuel Physical Properties on Diesel Engine Combustion using Diesel and Bio-diesel Fuels, Ra, Y.; Reitz, R.D.; McFarlane, J.; 2008-01-1379.
• Bridging the Gap between Theory and Experiments - Nanostructural Changes in Supported Catalysts under Operating Conditions, Narula, C.K.; Allard, L.F.; Blom, D.A.; DeBusk, M.M.; 2008-01-0416.