New Model System Offers Fundamental Insight into Catalyst Structure and Behavior
John M. (Mike) White
Approximately 5.3 angstroms in width, the tungsten trioxide trimers exhibit a very stable chemical structure that, with further research, may provide an ideal platform for fundamental studies of catalytic reactions commonly used in fuels and value-added chemical production. Full Image
Like tiny nano-soldiers on parade, the cyclic tungsten trioxide clusters line up molecule-by-molecule on the titanium dioxide platform. One tungsten atom from each cluster is raised slightly, holding forth the potential to execute catalytic reactions. The new model system of nano-structures offers chemists a view into the structure and reaction mechanisms of metal oxides.
Developed by researchers from the U.S. Department of Energy's Pacific Northwest National Laboratory, the University of Texas - Austin, and Washington State University, formation of the model system was reported in the June 23 online issue of Angewandte Chemie, International Edition. Their discovery may offer a platform for fundamental reactivity studies of metal oxides used as catalysts in converting hydrocarbons into fuels and value-added chemicals.
There is a striking difference between commercial catalysts and the new model system. Variability in commercial catalyst size and chemical composition makes it difficult to accurately understand or describe the reactions taking place at a molecular level. Mike White, the UT professor leading the PNNL Institute for Interfacial Catalysis said, "Commercial catalysts are like a gravel pile with many sizes of rocks. Some rocks are purple. Some are blue. Some do one thing; some do another. But, our system has all the same size rocks."
The model system—where all the molecular clusters are the same size, are evenly dispersed, and are oriented in one of two directions on a single layer of titanium oxide crystals—holds promise as a platform for studying the behavior of early transition metal oxides. White continued, "While we have created the smallest nano-cluster of a uniform size you can imagine, it is a real oxide; the tungsten is in its normal oxide state. In principle, you have all the things needed to make bonds and break bonds. That's the scientific breakthrough here."
Though it appears simple, the model system was very challenging to develop. The collaborators employed specialized equipment available from the Environmental Molecular Sciences Laboratory, a DOE user facility located at PNNL, to prepare and characterize the platform as well as the clusters. Using a unique approach that changed the tungsten oxide directly from a solid to a gas, the collaborators successfully stabilized the molecular rings—or "trimers"—of tungsten on the titanium platform.
A scanning tunneling microscope imaged not only the trimers but also their consistent alignment with the single crystal structure of the platform. "A scanning tunneling microscope must be very stable; you have to vibrationally isolate the instrument. It cannot move even a small amount because we are using a stream of electrons to measure the distance from the microscope's tip to a small space between atoms," White explained. In addition to scanning tunneling microscopy, the collaborators characterized the cluster mass, determined the stoichiometry, and identified the tungsten oxidation state using X-ray photoelectron spectroscopy.
"This is a small piece of the basic science that could lead to control of chemical transformations for our energy future," White concluded. For the first time, researchers have created and imaged monodisperse oxide clusters on another oxide.
Work to develop the model system is part of the Early Transition Metals as Catalysts project at PNNL and was supported by the U.S. Department of Energy's Office of Basic Energy Sciences, Chemical & Materials Sciences, Geosciences, and Biosciences Division.
Images from the scanning tunneling microscope show the first-ever imaging of monodisperse oxide clusters on another oxide. The dark triangles show the center of the tungsten trioxide trimer, while the brighter side of the triangle indicates the raised tungsten atom. Consistent orientation of the atoms is shown in the graphic. Full Image
Oleksandr Bondarchuk, Xin Huang, Jooho Kim, Bruce D. Kay, Lai-Sheng Wang, J. M. White, and Zdenek Dohnálek. July 17, 2006. "Formation of Monodisperse (WO3)3 Clusters on TiO2(110)." Angewandte Chemie International Edition 45(29):4786-4789.