Nixing Noxious Nitrogen Oxides with a Better Support System
PNNL researchers study how barium oxide anchors itself onto alumina support materials
Results: A research team at Pacific Northwest National Laboratory observed, for the first time, the preferential anchoring of a barium oxide catalyst on the surface of γ-alumina. Barium oxide is used as an absorber of nitrogen oxides (aka NOx) products in automotive exhaust streams. The alumina is a common and relatively inexpensive support. The surface structure, formation, and thermal stability of γ-alumina have been and continue to be subjects of an enormous amount of research, but using traditional surface analysis techniques to study them is not feasible.
Why does it matter? The insights obtained by the research team provide fundamental knowledge that suggests the exact site where the catalytic materials, including barium oxide, begin to form or nucleate. This new knowledge could contribute to an atom-by-atom approach for building more efficient and less expensive catalysts.
Scientists observed, for the first time, the preferential anchoring of a barium oxide catalyst on the surface of γ-alumina using the 900-MHz NMR in DOE's Environmental Molecular Sciences Laboratory, a national scientific user facility, located at PNNL.
Method: Using the 900-MHz nuclear magnetic resonance spectrometer at the U.S. Department of Energy's Environmental Molecular Sciences Laboratory, a national scientific user facility located at PNNL, the team readily observed penta-coordinated aluminum ion sites. These sites represent about 10-15% of the surface of the alumina support material.
"The site is essentially an aluminum ion surrounded by five oxygen ions. One oxygen is underneath the aluminum, and the other four form a square around it," said PNNL's Dr. Janos Szanyi, a researcher on the project.
The structure of this site creates a logical landing spot or nucleation site for the barium oxide. In fact, changes in the NMR spectra showed a nearly atom-per-atom correlation between the sites filled in and the catalyst deposited.
Next steps: In future studies, the team will examine the interaction of γ-alumina with other metal and metal-oxide particles of catalytic interest to determine if penta-coordinated aluminum ions are nucleation sites for other catalysts.
Acknowledgments: This work was supported by U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences. The research was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE Office of Biological and Environmental Research, and located at PNNL.
Citation: Kwak, JH, JZ Hu, DH Kim, J Szanyi, and CHF Peden. 2007. "Penta-coordinated Al3+ Ions as Preferential Nucleation Sites for BaO on γ-Al2O3." Journal of Catalysis 251(1):189-194.