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Why Morphology of CeO2 Matters in Catalysis:
Probing Defect Sites on CeO2 Nanocrystals with Well-Defined Surface Planes

Zili Wu1,2, Meijun Li1, Jane Howe3, Harry M. Meyer III3, Steven H. Overbury1,2
1, Chemical Science Division;  2, Center for Nanophase Materials Sciences;
3, Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831.

Research Achievements: 
Surface structure and structural defect sites on the surface play an essential role in catalysis. Using CeO2 nanocrystals specially synthesized to terminate on specific well-defined crystallographic planes, ORNL researchers have, for the first time, found experimentally that defect sites on ceria are controlled by the surface structure.  The defects in turn control the stability and activity of adsorbed oxygen and therefore directly affect reactivity in catalytic oxidation reactions. 
Ceria nanorods ({110} + {100}), nanocubes ({100}), and nano-octahedra ({111}) are employed to analyze the quantity and quality of defect sites on different ceria surfaces. Two types of defect sites on both oxidized and reduced ceria nanocrystals have been revealed by in situ Raman spectroscopy and O2 adsorption. The intrinsic defect sites on oxidized ceria, most likely the Frenkel-type oxygen defects, are the most abundant on nanorods, the least on nano-octahedra with nanocubes in the middle. The reduction-induced defect sites, O-vacancies and Ce3+ defects, are similar in surface density on nanorods and nanocubes while negligible on nano-octahedra. These O-vacancy sites are more clustered on nanorods than on nanocubes, in line with their surface terminations. The difference in defect sites result in different surface adsorbed oxygen species on the nanocrystals. Furthermore, the stability and reactivity of these oxygen species are also found to be surface-dependent, which has profound implications for ceria-catalyzed oxidation reactions.

Probe of sites

Significance:
Ceria nanocrystals with different morphologies display different catalytic performance in a variety of oxidation reactions. Our results provide direct insight into how the morphology of ceria can affect these reactions. The critical factor is the defect sites whose nature and amount are surface-dependent.  Our study implies that the catalytic properties of ceria could be controlled and tuned by controlling its shape and thus surface defects, which points to a strategy for both the improvement of current heterogeneous catalysts and the design of highly efficient catalyst without the change of the catalyst composition.

Credit:
This work was published online in Langmuir, 2010, DOI: 10.1021/la101723w. Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy.  A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory, by the Division of Scientific User Facilities, U. S. Department of Energy.

Surface Chemistry and Catalysis R&D Projects

Provided by Oak Ridge National Laboratory's Chemical Sciences Division
Rev:   August 2010