(710g) Shape Effect of Nanoscale Oxides Used as Metal Supports in Redox Catalysis

During the last decade, nanoscale functional materials have attracted much interest due to their remarkable size- and shape-dependent physical properties. In catalysis, the size effect of diverse metal or metal oxide nanocrystals has been widely investigated for various catalytic reactions. However, to date, reports on the shape effect of nanoscale oxides in catalysis are rather limited.^1,2 Recently, we reported a strong shape/crystal plane effect of nanoscale ceria on the activity of Au-CeO2 catalysts for the water-gas shift (WGS) reaction.¹ The same reaction on other nanoscale oxide supports is under investigation. In this paper, we will present new results on shape effect of metal oxide (cerium oxide, cobalt oxide, and zinc oxide) nanostructures for different redox reactions, including the water-gas shift, methanol steam reforming, CO oxidation and the oxidation of n-hexane.

For cerium oxide (CeO₂), the strong shape effect on the activity of Au-CeO₂ catalysts has been confirmed for the CO oxidation reaction. 1at.%Au on rod-like ceria support exhibits better catalytic performance than that on cube-like ceria. For cobalt oxide (Co₃O₄), controlled hydrolysis methods, together with hydrothermal treatment with/without the aid of organic surfactant, have been applied to prepare spinel cobalt oxide.^2,3 The obtained Co₃O₄ nanocrystals with the addition of oleylamine are of 5-10 nm in size and a polyhedron in shape. The as-washed products can be redispersed in the oil phase. Activity data confirm that the 400 ^oC calcined sample is an excellent catalyst for the oxidation of n-hexane. Surfactant-free Co₃O₄ with tunable sizes from 3 to 20 nm and shapes of polyhedron or cube were also prepared; these are easily suspended in polar solvents (water, ethanol, etc.). This hydrophilic oxide support can be doped with metals, such as gold, platinum or copper, in one step. The activity of metal-cobalt oxide catalysts for the WGS reaction will be discussed. For zinc oxide (ZnO), cotrollable synthesis of different nanoshapes, such as polyhedron, rod, wire, belt, can be approached by tuning the water-alcohol ratio during the synthesis step.⁴ Activity-structure correlations from nanoscale ZnO- supported Au catalysts will be presented and compared to Au/nanoscale ceria for the water-gas shift and methanol steam reforming reactions.

References:

1. R. Si, M. Flytzani-Stephanopoulos, Angew. Chem., Int. Ed. 2008, 47, 2884.

2. L. Hu, Q. Peng, Y. Li, J. Am. Chem. Soc. 2008, 130, 16136.

3. Y. Dong, K. He, L. Yin, A. Zhang, Nanotechnology 2007, 18, 435602.

4. B. Cheng, W. Shi, J. M. Russell-Tanner, L. Zhang, E. T. Samulski, Inorg. Chem. 2006, 45, 1208.