| Abstract
| - Ionically dispersed Rh over CeO2 in Rh/CeO2 catalysts prepared by a single step solutioncombustion method is shown to improve the redox property and catalytic activity. The H2/Rh ratio obtained from hydrogen uptake measurement was 5.4, 2.4, and 2.1, respectively in0.5, 1, and 2% Rh/CeO2, indicating a significant contribution from the reduction of CeO2 inthe presence of Rh. In 1% Rh/CeO2, the light-off temperature for CO oxidation is about 80°C lower compared to Rh metal and 190 °C lower than that of Rh2O3. The enhanced redoxproperty and CO oxidation activity of the catalyst has been correlated with the structure.The X-ray diffraction (XRD) pattern could be refined to the fluorite structure with Rhsubstituting in the Ce site. Transmission electron microscopy (TEM) images show only CeO2crystallites of about 50 nm and no evidence of any metal particles up to 1 atom % Rh. X-rayphotoelectron spectroscopy (XPS) studies demonstrate that Rh is dispersed in the +3oxidation state on CeO2 with enhanced Rh ion concentration in the surface layers. An averagecoordination number of 2.5 at a distance of 2.05 Å in the first shell is obtained around Rhions from extended X-ray absorption fine structure (EXAFS) spectroscopy, indicating an oxideion vacancy around the Rh ion. The correlations at 2.72 and 3.16 Å correspond to Rh−Rhand Rh−Ce interactions, respectively. Thus, the enhanced catalytic activity of Rh/CeO2 isshown to be due to the formation of a Ce1-xRhxO2-δ type of solid solution with −□−Rh3+−O−Ce4+− kind of linkages on the surface.
- Ionic substitution of Rh3+ for Ce4+ in CeO2 crystallites occurs mostly in the surface layers, giving rise to −□−Rh3+−O−Ce4+− kind of linkages. Redox coupling of Rh3+/Rh0 with Ce4+/Ce3+ via an oxide ion vacancy is responsible for high catalytic activity of 1 atom % Rh/CeO2 compared to Rh metal, Rh2O3 and pure CeO2.
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