Lonely Atoms Work Harder on Palladium Catalyst
Scientists discover how to boost the metal's reactivity by 50 percent
Results: At Pacific Northwest National Laboratory, scientists discovered that the arrangement of palladium atoms can improve the metal's ability to speed reactions. The team designed a catalyst so that the palladium atoms on the catalyst surface had plenty of hydrogen atoms nearby but few other palladium atoms. When they tested the catalyst, it added hydrogen to 50 percent of the target hydrocarbons, an order-of-magnitude improvement over unmodified palladium.
"This paper goes against the traditional wisdom that palladium is a structure-insensitive catalyst," said Dr. Zdenek Dohnálek, one of the investigators on this research at PNNL.
Why It Matters: Palladium is an expensive industrial catalyst, costing about $180 an ounce. Designing the catalyst so that it performs better could mean less palladium is needed and more of the desired products are generated. This atom-by-atom design of the catalyst could open doors for controlling the physical and chemical properties of this important material.
Methods: In the Department of Energy's EMSL, a national scientific user facility at PNNL, the researchers began by depositing palladium atoms at -420 Fahrenheit. To deposit the atoms, they placed a substrate at an oblique angle to make an extremely porous palladium film. They characterized the surface morphology using a combination of scanning electron microscopy and low-temperature physisorption techniques afforded by EMSL's suite of state-of-the-art instrumentation. The team saw that the deposition caused the atoms to be arranged in peaks and valleys.
They tested the catalyst with a hydrogenation reaction, converting ethylene to ethane by adding hydrogen. They discovered that the tailored catalyst was more efficient than previous studies suggested. Typically, when palladium catalyzes the hydrogenation reaction, less than 1 percent of the ethylene is converted. Now, 50 percent of the ethylene was converted.
Based on their observations of high catalytic activity, the team embarked on additional experiments. They grew thin layers of palladium on a nonporous substrate, creating a smooth surface. Then, they added a few extra palladium atoms on top.
Again, the researchers saw the catalyst's improved efficiency. They analyzed the results and determined the catalyst worked for two reasons. First, the palladium atoms on the surface were unencumbered by nearby palladium atoms. Second, the thin films prevented the hydrogen atoms from diffusing into the bulk of the material. The thin films kept the hydrogen at the surface, where it could be used. Past studies did not see these results because thicker crystals soaked up the hydrogen.
What's Next: The researchers are continuing to provide new insights into the behavior of catalysts that are of value to industrial and energy applications.
Acknowledgments: DOE's Office of Basic Energy Sciences, Chemical Sciences Division funded all of the research done on this project. The work was done by Zdenek Dohnálek, Jooho Kim, and Bruce Kay. The research, including the scanning electron microscopy work, was done in EMSL.
The work, part of PNNL's Institute for Integrated Catalysis, supports the Laboratory's mission to strengthen U.S. scientific foundations for innovation by developing tools and understanding required to control chemical and physical processes in complex multiphase environments.
Reference: Dohnálek Z, J Kim, and BD Kay. 2008. "Understanding How Surface Morphology and Hydrogen Dissolution Influence Ethylene Hydrogenation on Palladium." Journal of Physical Chemistry Part C 112(40):15796-15801.